U.S. patent application number 14/838254 was filed with the patent office on 2015-12-24 for magnetoresistive device and a writing method for a magnetoresistive device.
This patent application is currently assigned to AGENCY FOR SCIENCE, TECHNOLOGY AND RESEARCH. The applicant listed for this patent is AGENCY FOR SCIENCE, TECHNOLOGY AND RESEARCH. Invention is credited to Yan Hwee Sunny LUA, Yuanhong LUO, Rachid SBIAA.
Application Number | 20150371697 14/838254 |
Document ID | / |
Family ID | 47911145 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150371697 |
Kind Code |
A1 |
LUO; Yuanhong ; et
al. |
December 24, 2015 |
Magnetoresistive Device and a Writing Method for a Magnetoresistive
Device
Abstract
According to embodiments of the present invention, a
magnetoresistive device is provided. The magnetoresistive device
includes at least two ferromagnetic soft layers, wherein the at
least two ferromagnetic soft layers have different ranges of
magnetization switching frequencies. Further embodiments provide a
magnetoresistive device including at least two oscillating
ferromagnetic structures, wherein ranges of operating current
amplitudes at which oscillations are induced for the at least two
oscillating ferromagnetic structures are different. According to
further embodiments of the present invention, writing methods for
the magnetoresistive devices are provided.
Inventors: |
LUO; Yuanhong; (Singapore,
SG) ; SBIAA; Rachid; (Singapore, SG) ; LUA;
Yan Hwee Sunny; (Singapore, SG) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AGENCY FOR SCIENCE, TECHNOLOGY AND RESEARCH |
Singapore |
|
SG |
|
|
Assignee: |
AGENCY FOR SCIENCE, TECHNOLOGY AND
RESEARCH
Singapore
SG
|
Family ID: |
47911145 |
Appl. No.: |
14/838254 |
Filed: |
August 27, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13623741 |
Sep 20, 2012 |
9123884 |
|
|
14838254 |
|
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|
61537591 |
Sep 22, 2011 |
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Current U.S.
Class: |
365/158 |
Current CPC
Class: |
G11C 11/161 20130101;
G11C 11/1675 20130101; H01L 43/08 20130101 |
International
Class: |
G11C 11/16 20060101
G11C011/16 |
Claims
1. A writing method for a magnetoresistive device having a first
ferromagnetic soft layer and a second ferromagnetic soft layer, the
method comprising: applying a signal with a magnetization switching
frequency which is within either a range of magnetization switching
frequencies of the first ferromagnetic soft layer or a range of
magnetization switching frequencies of the second ferromagnetic
soft layer.
2. A writing method for a magnetoresistive device having a first
oscillating ferromagnetic structure and a second oscillating
ferromagnetic structure, the method comprising: applying a signal
with an operating current amplitude which is within either a range
of operating current amplitudes at which oscillations are induced
for the first oscillating ferromagnetic structure or a range of
operating current amplitudes at which oscillations are induced for
the second oscillating ferromagnetic structure.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 13/623,741, filed Sep. 20, 2012, which claims priority to U.S.
provisional application No. 61/537,591, filed 22 Sep. 2011.
TECHNICAL FIELD
[0002] Various embodiments relate to a magnetoresistive device and
a writing method for a magnetoresistive device.
BACKGROUND
[0003] Until now, hard disk drive (HDD) offers an advantage of
storing data at low cost. At the same time, other types of memories
such as flash memory caught up and now represent a threat to HDD.
Flash memory belongs to a category of non-volatile memories (NVM).
It allows the data to be stored even when power is down or off.
[0004] The flash memory market is getting bigger but the cost per
gigabit (Gbit) is higher than that of HDD. HDD technology is moving
towards patterned media where bits are made by lithography
processes. The cost per Gbit for HDD should not be increased by
more than 10% or 20% in order to remain competitive. This is one of
the major challenges facing the HDD technology.
[0005] A current trend is to develop non-volatile memories (NVM)
beyond flash memory, which is cheaper and has a high performance.
Magnetoresistive random access memory (MRAM) and phase change
random access memory (PC-RAM) represent good candidates for future
NVM. It is expected that spin transfer torque magnetoresistive
random access memory (STT-MRAM) devices can have a cell less than
10 nm based on high magnetic anisotropy materials used for the
storage layer, such as ordered FePt-Ll.sub.0. However, this is not
possible for flash memory.
[0006] In order for MRAM to be industrially viable, there is a need
to increase the memory storage density of MRAM devices.
SUMMARY
[0007] According to an embodiment, a magnetoresistive device is
provided. The magnetoresistive device may include at least two
ferromagnetic soft layers, wherein the at least two ferromagnetic
soft layers have different ranges of magnetization switching
frequencies.
[0008] According to an embodiment, a writing method for a
magnetoresistive device having a first ferromagnetic soft layer and
a second ferromagnetic soft layer is provided. The writing method
may include applying a signal with a magnetization switching
frequency which is within either a range of magnetization switching
frequencies of the first ferromagnetic soft layer or a range of
magnetization switching frequencies of the second ferromagnetic
soft layer.
[0009] According to an embodiment, a magnetoresistive device is
provided. The magnetoresistive device may include at least two
oscillating ferromagnetic structures, wherein ranges of operating
current amplitudes at which oscillations are induced for the at
least two oscillating ferromagnetic structures are different.
[0010] According to an embodiment, a writing method for a
magnetoresistive device having a first oscillating ferromagnetic
structure and a second oscillating ferromagnetic structure is
provided. The writing method may include applying a signal with an
operating current amplitude which is within either a range of
operating current amplitudes at which oscillations are induced for
the first oscillating ferromagnetic structure or a range of
operating current amplitudes at which oscillations are induced for
the second oscillating ferromagnetic structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] In the drawings, like reference characters generally refer
to the same parts throughout the different views. The drawings are
not necessarily to scale, emphasis instead generally being placed
upon illustrating the principles of the invention. In the following
description, various embodiments of the invention are described
with reference to the following drawings, in which:
[0012] FIG. 1A shows a schematic block diagram of a
magnetoresistive device, according to various embodiments;
[0013] FIG. 1B illustrates a writing method for a magnetoresistive
device, according to various embodiments;
[0014] FIG. 1C shows a schematic block diagram of a
magnetoresistive device, according to various embodiments;
[0015] FIG. 1D illustrates a writing method for a magnetoresistive
device, according to various embodiments;
[0016] FIG. 2A shows a schematic cross-sectional view of a
magnetoresistive device, according to various embodiments;
[0017] FIG. 2B shows a schematic cross-sectional view of a
magnetoresistive device, according to various embodiments;
[0018] FIG. 2C shows a schematic diagram illustrating the concept
of frequency bandwidth for various embodiments;
[0019] FIGS. 3A to 3D show respective schematic cross-sectional
views of an oscillating ferromagnetic structure, according to
various embodiments;
[0020] FIG. 4 shows different configurations of magnetization and
polarization during a switching process;
[0021] FIG. 5 shows a plot of macrospin simulation results of the
critical current for P.fwdarw.AP and AP.fwdarw.P respectively when
a 3 GHz oscillation is applied, according to various
embodiments;
[0022] FIG. 6 shows a plot of macrospin simulation results of the
relationship between the frequency bandwidth and the applied
current density for P.fwdarw.AP and AP.fwdarw.P switchings,
according to various embodiments;
[0023] FIG. 7 shows a plot of macrospin simulation results of the
relationship between the optimum magnetization switching frequency
and the perpendicular anisotropy field for P.fwdarw.AP and
AP.fwdarw.P switchings, according to various embodiments;
[0024] FIG. 8 shows a plot of macrospin simulation results of the
relationship between the switching time and the applied oscillation
frequency for P.fwdarw.AP and AP.fwdarw.P switchings for different
perpendicular anisotropy fields, according to various
embodiments;
[0025] FIGS. 9A and 9B show plots of macrospin simulation results
of the respective perpendicular magnetization components, Mz, of
two ferromagnetic soft layers for a 2-bit STT-MRAM device, as a
function of time, according to various embodiments;
[0026] FIG. 10 shows a plot of an oscillator's frequency as a
function of applied current density;
[0027] FIGS. 11A and 11B show schematic cross-sectional views of
the magnetoresistive device of FIG. 2A employing OSSS, in
operation;
[0028] FIGS. 12A and 12B show plots of macrospin simulation results
of the respective perpendicular magnetization components, Mz, of
two ferromagnetic soft layers for a 2-bit STT-MRAM device, as a
function of time, according to various embodiments;
[0029] FIGS. 13A and 13B show schematic cross-sectional views of a
magnetoresistive device, according to various embodiments; and
[0030] FIG. 14 shows a schematic cross-sectional view of a
magnetoresistive device, according to various embodiments.
DETAILED DESCRIPTION
[0031] The following detailed description refers to the
accompanying drawings that show, by way of illustration, specific
details and embodiments in which the invention may be practiced.
These embodiments are described in sufficient detail to enable
those skilled in the art to practice the invention. Other
embodiments may be utilized and structural, logical, and electrical
changes may be made without departing from the scope of the
invention. The various embodiments are not necessarily mutually
exclusive, as some embodiments can be combined with one or more
other embodiments to form new embodiments.
[0032] Embodiments described in the context of one of the methods
or devices are analogously valid for the other method or device.
Similarly, embodiments described in the context of a method are
analogously valid for a device, and vice versa.
[0033] Features that are described in the context of an embodiment
may correspondingly be applicable to the same or similar features
in the other embodiments. Features that are described in the
context of an embodiment may correspondingly be applicable to the
other embodiments, even if not explicitly described in these other
embodiments. Furthermore, additions and/or combinations and/or
alternatives as described for a feature in the context of an
embodiment may correspondingly be applicable to the same or similar
feature in the other embodiments.
[0034] In the context of various embodiments, the phrase "at least
substantially" may include "exactly" and a variance of +/-5%
thereof. As an example and not limitations, "A is at least
substantially same as B" may encompass embodiments where A is
exactly the same as B, or where A may be within a variance of
+/-5%, for example of a value, of B, or vice versa.
[0035] In the context of various embodiments, the term "about" or
"approximately" as applied to a numeric value encompasses the exact
value and a variance of +/-5% of the value.
[0036] As used herein, the term "and/or" includes any and all
combinations of one or more of the associated listed items.
[0037] Various embodiments relate to a non-volatile magnetic memory
device, for example a spin transfer torque magnetoresistive random
access memory (STT-MRAM) multi-bit per cell device.
[0038] Various embodiments may provide multi-bit per cell for spin
transfer torque magnetic memory with frequency selection switching
scheme (FSSS) or oscillator selection switching scheme (OSSS).
[0039] Various embodiments may provide multi-bit per cell designs
or configurations as an approach for increasing the memory storage
density in STT-MRAM devices. Various embodiments may provide a
multi-bit per cell magnetoresistive device (e.g. a magnetic memory
element) with perpendicular magnetization or anisotropy, and spin
torque switching.
[0040] Various embodiments may provide a magnetoresistive device
that may enable switching magnetization by a spin torque effect in
perpendicular anisotropy, and a method for switching magnetization
by the spin torque effect. The spin torque effect enables the
magnetization orientation, for example of a ferromagnetic soft
layer, to be switched by using a spin-polarized current or a spin
transfer current.
[0041] Various embodiments may provide a magnetoresistive device
(e.g. a magnetic memory element). The magnetoresistive device may
be a giant magnetoresistive (GMR) device or a tunnel
magnetoresistive (TMR) device, with a current flowing perpendicular
to the plane (CPP) direction.
[0042] Various embodiments may provide a multi-bit per cell spin
transfer torque magnetoresistive random access memory (STT-MRAM)
including at least two soft layers or ferromagnetic soft layers
that have different resonance frequencies, and at least an
oscillating ferromagnetic structure at the side of each
ferromagnetic soft layer to assist the switching of the
magnetization orientation of the ferromagnetic soft layer by
Frequency Induced Spin Torque Switching (FISTS). Each of the
ferromagnetic soft layers has a perpendicular anisotropy component.
The STT-MRAM further includes at least one reference layer or
ferromagnetic hard layer, with a perpendicular anisotropy
component, arranged on one side of the ferromagnetic soft layers.
In various embodiments, the oscillating ferromagnetic structure
arranged on the other side of each ferromagnetic soft layer may
generate an additional effective spin torque field or energy to
assist the switching of the ferromagnetic soft layer magnetization.
In various embodiments, the properties of each ferromagnetic soft
layer and the corresponding oscillating ferromagnetic structure are
chosen such that the switching of the ferromagnetic soft layer
magnetization orientation may be performed only in a range of
frequencies close to the resonance frequency of the ferromagnetic
soft layer.
[0043] In various embodiments, through matching of the respective
resonance frequencies of the ferromagnetic soft layers and the
oscillation frequencies of the corresponding oscillating
structures, the magnetization orientations of the ferromagnetic
soft layers may be switched independently. Such a Frequency
Selection Switching Scheme (FSSS) may help to address any related
overwriting issue that may occur in multi-bit per cell STT-MRAM.
For example, the overwriting issue may arise from delivering more
current than required for the switching of the ferromagnetic soft
layers during the writing process. This oversupply of the write
current, over time, may cause or accelerate the degradation of the
performance of the device, thereby affecting its reliability.
[0044] In various embodiments, a writing scheme using spin torque
effect with selected frequencies to switch the magnetizations of
the magnetoresistive devices having multi-bit per cell capability
may be provided. The writing scheme may be applied to embodiments
having two or more ferromagnetic soft layers, with associated
additional ferromagnetic hard layer(s).
[0045] Various embodiments may provide a multi-bit per cell spin
transfer torque magnetoresistive random access memory (STT-MRAM)
including at least two soft layers or ferromagnetic soft layers
that have different resonance frequencies, and at least an
oscillating ferromagnetic structure at the side of each
ferromagnetic soft layer to assist the switching of the
magnetization of the ferromagnetic soft layer by an Oscillator
Selection Switching Scheme (OSSS). Each of the ferromagnetic soft
layers has a perpendicular anisotropy component. The STT-MRAM
further includes at least one reference layer or ferromagnetic hard
layer, with a perpendicular anisotropy component, arranged on one
side of the ferromagnetic soft layers. In various embodiments, the
oscillating ferromagnetic structure arranged on the other side of
each ferromagnetic soft layer may generate an additional effective
spin torque field or energy to assist the switching of the
ferromagnetic soft layer magnetization. In various embodiments, the
oscillating ferromagnetic structures may be designed or engineered
differently so that they have different critical current densities
or current amplitudes.
[0046] In various embodiments, the Oscillator Selection Switching
Scheme (OSSS) may also be employed even if the at least two
ferromagnetic soft layers have the same resonance frequency. By
design, the oscillating structures are not "ON" at the same time,
and therefore, the magnetization orientation of only one
ferromagnetic soft layer is switched at any one time.
[0047] In various embodiments, through selection of the respective
critical current densities of the oscillating ferromagnetic
structures, the magnetizations orientation of the corresponding
ferromagnetic soft layers may be switched independently. Such an
Oscillator Selection Switching Scheme (OSSS) may help to address
any related overwriting issue that may occur in multi-bit per cell
STT-MRAM. For example, the overwriting issue may arise from
delivering more current than required for the switching of the
ferromagnetic soft layers during the writing process. This
oversupply of the write current, over time, may cause or accelerate
the degradation of the performance of the device, thereby affecting
its reliability.
[0048] In various embodiments, a writing scheme using spin torque
effect with selected critical current densities to switch the
magnetizations of the magnetoresistive devices having multi-bit per
cell capability may be provided. The writing scheme may be applied
to embodiments having two or more ferromagnetic soft layers, with
associated additional ferromagnetic hard layer(s).
[0049] FIG. 1A shows a schematic block diagram of a
magnetoresistive device 100, according to various embodiments. The
magnetoresistive device 100 includes at least two ferromagnetic
soft layers 102 (e.g. 204a, 204b, FIGS. 2A, 13A, 13B, 14), wherein
the at least two ferromagnetic soft layers 102 have different
ranges of magnetization switching frequencies. The range of
magnetization switching frequencies of each ferromagnetic soft
layer 102 may be a frequency bandwidth of the ferromagnetic soft
layer 102.
[0050] In other words, the magnetoresistive device 100 may have two
or more ferromagnetic soft layers 102, e.g. two, three, four, five
or any higher number of ferromagnetic soft layers 102. These
ferromagnetic soft layers 102 may form part of a magnetic junction
of the magnetoresistive device 100.
[0051] Each ferromagnetic soft layer 102 has a range of
magnetization switching frequencies. In various embodiments, the
range of magnetization switching frequencies of each ferromagnetic
soft layer 102 may be non-overlapping, e.g. the range of
magnetization switching frequencies of a ferromagnetic soft layer
102 (e.g. 204a) occurs at a frequency range different from that of
another ferromagnetic soft layer 102 (e.g. 204b). In various
embodiments, the range of magnetization switching frequencies of
each ferromagnetic soft layer 102 may be substantially
non-overlapping, e.g. the range of magnetization switching
frequencies of each ferromagnetic soft layer 102 has a tolerable
overlapping region. This may mean that a small overlap, for example
an overlap of a small range between the respective ranges of the
magnetization switching frequencies of the ferromagnetic soft
layers 102, may be tolerated as long as there is a portion or
region of the range of magnetization switching frequencies of each
ferromagnetic soft layer 102 for which independent or individual
switching of each ferromagnetic soft layer 102 is possible.
[0052] It should be appreciated that the at least two ferromagnetic
soft layers 102 may have one or more ferromagnetic soft layers 102
that may have non-overlapping ranges of magnetization switching
frequencies, and/or one or more ferromagnetic soft layers 102 that
may have substantially non-overlapping ranges of magnetization
switching frequencies.
[0053] In various embodiments, the range of magnetization switching
frequencies of each ferromagnetic soft layer 102 may be close to a
resonance frequency of the ferromagnetic soft layer 102. Each
ferromagnetic soft layer 102 may be associated with an optimum
magnetization switching frequency that is within the range of
magnetization switching frequencies of the ferromagnetic soft layer
102. The optimum magnetization switching frequency of each
ferromagnetic soft layer 102 may be close to or equal to a
resonance frequency of the ferromagnetic soft layer 102.
[0054] In various embodiments, the range of magnetization switching
frequencies of each ferromagnetic soft layer 102 may be dependent
on a current density applied to the magnetoresistive device
100.
[0055] In various embodiments, each ferromagnetic soft layer 102
may have a corresponding oscillating ferromagnetic structure
disposed on one side of the ferromagnetic soft layer 102. In other
words, the magnetoresistive device 100 may have an oscillating
ferromagnetic structure corresponding to each ferromagnetic soft
layer 102, and disposed on one side of the ferromagnetic soft layer
102. These oscillating ferromagnetic structures (e.g. 206a, 206b),
together with the ferromagnetic soft layers 102, may form part of a
magnetic junction of the magnetoresistive device 100.
[0056] In various embodiments, the optimum magnetization switching
frequency of each ferromagnetic soft layer 102 may be the
magnetization switching frequency at which magnetization of the
ferromagnetic soft layer 102 may switch in the fastest time (or
shortest time) for a current pulse of a particular amplitude and
time width. The optimum magnetization switching frequency of each
ferromagnetic soft layer 102 may be dependent on a perpendicular
magnetic anisotropy field of the ferromagnetic soft layer 102. Each
ferromagnetic soft layer 102 may have a different perpendicular
magnetic anisotropy field. In various embodiments, the optimum
magnetization switching frequency of each ferromagnetic soft layer
102 may be close to an oscillation frequency of the corresponding
oscillating ferromagnetic structure.
[0057] In various embodiments, the optimum magnetization switching
frequency of each ferromagnetic soft layer 102 may be dependent on
one or more of a saturation magnetization of the ferromagnetic soft
layer 102 and a thickness of the ferromagnetic soft layer 102. Each
ferromagnetic soft layer 102 may have a different saturation
magnetization and/or a different thickness.
[0058] In the context of various embodiments, the magnetoresistive
device 100 may include a first stack arrangement (e.g. 200, FIGS.
2A, 13A, 13B, 14) including a ferromagnetic hard layer (e.g. 202),
a first ferromagnetic soft layer (e.g. 204a), a second
ferromagnetic soft layer (e.g. 204b), a first oscillating
ferromagnetic structure (e.g. 206a), and a second oscillating
ferromagnetic structure (e.g. 206b), wherein the first oscillating
ferromagnetic structure is disposed at one side of the first
ferromagnetic soft layer and the second oscillating ferromagnetic
structure is disposed at one side of the second ferromagnetic soft
layer, and wherein the ferromagnetic hard layer is disposed at the
other side of each of the first ferromagnetic soft layer and the
second ferromagnetic soft layer such that the ferromagnetic hard
layer is disposed between the first ferromagnetic soft layer and
the second ferromagnetic soft layer. In other words, the at least
two ferromagnetic soft layers 102 may include the first
ferromagnetic soft layer and the second ferromagnetic soft layer as
described above. The corresponding oscillating ferromagnetic
structure disposed on one side of the ferromagnetic soft layer 102
may include the first oscillating ferromagnetic structure and the
second oscillating ferromagnetic structure as described above.
[0059] The first stack arrangement may further include a first
separating layer (e.g. 208a) disposed between the ferromagnetic
hard layer and the first ferromagnetic soft layer, a second
separating layer (e.g. 208b) disposed between the ferromagnetic
hard layer and the second ferromagnetic soft layer, a third
separating layer (e.g. 210a) disposed between the first oscillating
ferromagnetic structure and the first ferromagnetic soft layer, and
a fourth separating layer (e.g. 210b) disposed between the second
oscillating ferromagnetic structure and the second ferromagnetic
soft layer.
[0060] In the context of various embodiments, the first separating
layer, the second separating layer, the third separating layer and
the fourth separating layer include non-magnetic materials.
[0061] In the context of various embodiments, the first separating
layer and the second separating layer include non-conductive
materials (e.g. an insulator), e.g. magnesium oxide (MgO). By
arranging insulators as separating layers between the ferromagnetic
hard layer with the first ferromagnetic soft layer and the second
ferromagnetic soft layer, the magnetoresistive device 100 may be
configured as a tunnel magnetoresistive (TMR) device.
[0062] However, it should be appreciated that the first separating
layer and the second separating layer may include conductive
materials (e.g. a conductor), e.g. copper (Cu). By arranging
conductors as separating layers between the ferromagnetic hard
layer with the first ferromagnetic soft layer and the second
ferromagnetic soft layer, the magnetoresistive device 100 may be
configured as a giant magnetoresistive (GMR) device.
[0063] In the context of various embodiments, the third separating
layer and the fourth separating layer include conductive materials
(e.g. a conductor), e.g. copper (Cu).
[0064] In the context of various embodiments, one of or each of the
first separating layer and the second separating layer may have a
thickness ranging from about 0.4 nm to about 1.5 nm, e.g. about 0.4
nm to about 1.0 nm, about 0.4 nm to about 0.8 nm, about 0.8 nm to
about 1.5 nm or about 0.6 nm to about 1.2 nm.
[0065] In the context of various embodiments, the first stack
arrangement may be or may form part of a 2-bit per cell spin
transfer torque magnetoresistive random access memory (STT-MRAM).
In other words, the magnetoresistive device 100 with the first
stack arrangement may provide one, two, three, or four resistance
states, which may enable data storage of up to two bits of
information, thereby allowing multi-state or multi-bit storage.
[0066] In the context of various embodiments, the magnetoresistive
device 100 may further include a second stack arrangement (e.g.
220, FIGS. 13A, 13B) including a further ferromagnetic hard layer
(e.g. 222, FIGS. 13A, 13B), a third ferromagnetic soft layer (e.g.
224), and a third oscillating ferromagnetic structure (e.g. 226),
wherein the third ferromagnetic soft layer is disposed between the
further ferromagnetic hard layer and the third oscillating
ferromagnetic structure. The second stack arrangement may further
include a fifth separating layer (e.g. 228) disposed between the
further ferromagnetic hard layer and the third ferromagnetic soft
layer, and a sixth separating layer (e.g. 230) disposed between the
third ferromagnetic soft layer and the third oscillating
ferromagnetic structure.
[0067] In the context of various embodiments, the fifth separating
layer and the sixth separating layer include non-magnetic
materials.
[0068] In the context of various embodiments, the fifth separating
layer includes non-conductive materials (e.g. as insulator), e.g.
magnesium oxide (MgO). By arranging an insulator as a separating
layer between the further ferromagnetic hard layer and the third
ferromagnetic soft layer, the magnetoresistive device 100 may be
configured as a tunnel magnetoresistive (TMR) device.
[0069] However, it should be appreciated that the fifth separating
layer may include conductive materials (e.g. as conductor), e.g.
copper (Cu). By arranging a conductor as a separating layer between
the further ferromagnetic hard layer and the third ferromagnetic
soft layer, the magnetoresistive device 100 may be configured as a
giant magnetoresistive (GMR) device.
[0070] In the context of various embodiments, the sixth separating
layer includes conductive materials (e.g. a conductor), e.g. copper
(Cu).
[0071] In the context of various embodiments, the second stack
arrangement may be or may form part of a 1-bit per cell spin
transfer torque magnetoresistive random access memory
(STT-MRAM).
[0072] In the context of various embodiments, the second stack
arrangement may be disposed above or below the first stack
arrangement. In other words, the first stack arrangement and the
second stack arrangement may be arranged one over the other. The
magnetoresistive device 100 may further include a seventh
separating layer (e.g. 1302) disposed between the second stack
arrangement and the first stack arrangement. The seventh separating
layer may include non-magnetic materials. The seventh separating
layer may include conductive materials, e.g. copper (Cu).
[0073] In the context of various embodiments, the first stack
arrangement and the second stack arrangement may be or may form
part of a 3-bit per cell spin transfer torque magnetoresistive
random access memory (STT-MRAM). In other words, the
magnetoresistive device 100 with the first stack arrangement and
the second stack arrangement may provide one, two, three, four,
five, six, seven or eight resistance states, which may enable data
storage of up to three bits of information, thereby allowing
multi-state or multi-bit storage.
[0074] In the context of various embodiments, the magnetoresistive
device 100 including the first stack arrangement may further
include a further first stack arrangement (e.g. 1401, FIG. 14),
e.g. a further stack arrangement similar to the first stack
arrangement, wherein the further first stack arrangement is
disposed above or below the first stack arrangement. In other
words, the two first stack arrangements may be arranged one over
the other. The magnetoresistive device 100 may further include an
eighth separating layer (e.g. 1412) disposed between the first
stack arrangement and the further first stack arrangement. The
eight separating layer may include non-magnetic materials. The
eight separating layer may include conductive materials, e.g.
copper (Cu).
[0075] In the context of various embodiments, the first stack
arrangement and the further first stack arrangement may be or may
form part of a 4-bit per cell spin transfer torque magnetoresistive
random access memory (STT-MRAM). In other words, the
magnetoresistive device 100 with the first stack arrangement and
the further first stack arrangement may provide between one to
sixteen resistance states, which may enable data storage of up to
four bits of information, thereby allowing multi-state or multi-bit
storage.
[0076] In the context of various embodiments, it should be
appreciated that any n-bit per cell spin transfer torque
magnetoresistive random access memory (STT-MRAM) may be formed by
providing a combination of any number of the first stack
arrangement, where each first stack arrangement may provide 2-bit
per cell STT-MRAM, and/or any number of the second stack
arrangement, where each second stack arrangement may provide 1-bit
per cell STT-MRAM. In the context of various embodiments, a
conductive and non-magnetic separating layer may be disposed
between any two stack arrangements.
[0077] FIG. 1B illustrates a writing method 110 for a
magnetoresistive device, according to various embodiments, where
the magnetoresistive device may have a first ferromagnetic soft
layer and a second ferromagnetic soft layer. For the writing method
110, a signal with a magnetization switching frequency which is
within either a range of magnetization switching frequencies of the
first ferromagnetic soft layer or a range of magnetization
switching frequencies of the second ferromagnetic soft layer is
applied.
[0078] When the magnetization switching frequency of the signal is
within the range of magnetization switching frequencies of the
first ferromagnetic soft layer, the magnetization of the first
ferromagnetic soft layer is switched, and when the magnetization
switching frequency of the signal is within the range of
magnetization switching frequencies of the second ferromagnetic
soft layer, the magnetization of the second ferromagnetic soft
layer is switched. In various embodiments, the range of
magnetization switching frequencies of the first ferromagnetic soft
layer may be non-overlapping or substantially non-overlapping with
the range of magnetization switching frequencies of the second
ferromagnetic soft layer, e.g. the ranges of the magnetization
switching frequencies of the first ferromagnetic soft layer and the
second ferromagnetic soft layer may be different.
[0079] FIG. 1C shows a schematic block diagram of a
magnetoresistive device 120, according to various embodiments. The
magnetoresistive device 120 includes at least two oscillating
ferromagnetic structures 122 (e.g. 206a, 206b, FIGS. 2A, 13A, 13B,
14), wherein ranges of operating current amplitudes (or current
densities) at which oscillations are induced for the at least two
oscillating ferromagnetic structures 122 are different.
[0080] In other words, the magnetoresistive device 120 may have two
or more oscillating ferromagnetic structures 122, e.g. two, three,
four, five or any higher number of oscillating ferromagnetic
structures 122. These oscillating ferromagnetic structures 122 may
form part of a magnetic junction of the magnetoresistive device
120.
[0081] The magnetoresistive device 120 may further include at least
two ferromagnetic soft layers (e.g. 204a, 204b) wherein each
oscillating ferromagnetic structure 122 may be disposed at one side
of a corresponding ferromagnetic soft layer. In other words, the
magnetoresistive device 120 may have a ferromagnetic soft layer
corresponding to each oscillating ferromagnetic structure 122, and
disposed on one side of the oscillating ferromagnetic structure
122. These ferromagnetic soft layers, together with the oscillating
ferromagnetic structures 122, may form part of a magnetic junction
of the magnetoresistive device 100.
[0082] Each oscillating ferromagnetic structure 122 has a range of
operating current amplitudes at which oscillations are induced. In
various embodiments, the ranges of operating current amplitudes at
which oscillations are induced for the at least two oscillating
ferromagnetic structures 122 may be non-overlapping, e.g. the range
of operating current amplitudes of an oscillating ferromagnetic
structures 122 occurs at a current amplitude range different from
that of another oscillating ferromagnetic structures 122. In
various embodiments, the ranges of operating current amplitudes at
which oscillations are induced for the at least two oscillating
ferromagnetic structures 122 may be substantially non-overlapping,
e.g. the ranges of operating current amplitudes may have a
tolerable overlapping region respectively. This may mean that a
small overlap, for example an overlap of a small range between the
respective ranges of operating current amplitudes, may be tolerated
as long as there is a portion or region of the operating current
amplitudes of each oscillating ferromagnetic structures 122 for
which independent or individual switching of the corresponding
ferromagnetic soft layer is possible.
[0083] It should be appreciated that the at least two oscillating
ferromagnetic structures 122 may have one or more oscillating
ferromagnetic structures 122 that may have non-overlapping ranges
of operating current amplitudes, and/or one or more oscillating
ferromagnetic structures 122 that may have substantially
non-overlapping ranges of operating current amplitudes.
[0084] In various embodiments, each ferromagnetic soft layer may
configured to switch magnetization from a parallel state to an
anti-parallel state (P.fwdarw.AP) or from an anti-parallel state to
a parallel state (AP.fwdarw.P) when an operating current amplitude
is within the range of operating current amplitudes at which
oscillations are induced for the corresponding oscillating
ferromagnetic structure 122.
[0085] In various embodiments, each oscillating ferromagnetic
structure 122 has an oscillation frequency. The range of operating
current amplitudes at which oscillations are induced for each
oscillating ferromagnetic structure 122 and the oscillation
frequency of each oscillating ferromagnetic structure 122 may be
dependent on one or more of a group of parameters consisting of
Gilbert damping constant of the oscillating ferromagnetic
structure, saturation magnetization of the oscillating
ferromagnetic structure, perpendicular anisotropy field of the
oscillating ferromagnetic structure and thickness of the
oscillating ferromagnetic structure.
[0086] In the context of various embodiments, the magnetoresistive
device 120 may include a first stack arrangement e.g. 200, FIGS.
2A, 13A, 13B, 14) including a ferromagnetic hard layer (e.g. 202),
a first ferromagnetic soft layer (e.g. 204a), a second
ferromagnetic soft layer (e.g. 204b), a first oscillating
ferromagnetic structure (e.g. 206a), and a second oscillating
ferromagnetic structure (e.g. 206b), wherein the first oscillating
ferromagnetic structure is disposed at one side of the first
ferromagnetic soft layer and the second oscillating ferromagnetic
structure is disposed at one side of the second ferromagnetic soft
layer, and wherein the ferromagnetic hard layer is disposed at the
other side of each of the first ferromagnetic soft layer and the
second ferromagnetic soft layer such that the ferromagnetic hard
layer is disposed between the first ferromagnetic soft layer and
the second ferromagnetic soft layer. In other words, the at least
two oscillating ferromagnetic structures 122 may include the first
oscillating ferromagnetic structure and the second oscillating
ferromagnetic structure as described above. The corresponding
ferromagnetic soft layer disposed on one side of the oscillating
ferromagnetic structure 122 may include the first ferromagnetic
soft layer and the second ferromagnetic soft layer as described
above.
[0087] The first stack arrangement may further include a first
separating layer (e.g. 208a) disposed between the ferromagnetic
hard layer and the first ferromagnetic soft layer, a second
separating layer (e.g. 208b) disposed between the ferromagnetic
hard layer and the second ferromagnetic soft layer, a third
separating layer (e.g. 210a) disposed between the first oscillating
ferromagnetic structure and the first ferromagnetic soft layer, and
a fourth separating layer (e.g. 210b) disposed between the second
oscillating ferromagnetic structure and the second ferromagnetic
soft layer.
[0088] In the context of various embodiments, the first separating
layer, the second separating layer, the third separating layer and
the fourth separating layer include non-magnetic materials.
[0089] In the context of various embodiments, the first separating
layer and the second separating layer include non-conductive
materials (e.g. an insulator), e.g. magnesium oxide (MgO). By
arranging insulators as separating layers between the ferromagnetic
hard layer with the first ferromagnetic soft layer and the second
ferromagnetic soft layer, the magnetoresistive device 120 may be
configured as a tunnel magnetoresistive (TMR) device.
[0090] However, it should be appreciated that the first separating
layer and the second separating layer may include conductive
materials (e.g. a conductor), e.g. Cu. By arranging conductors as
separating layers between the ferromagnetic hard layer with the
first ferromagnetic soft layer and the second ferromagnetic soft
layer, the magnetoresistive device 120 may be configured as a giant
magnetoresistive (GMR) device.
[0091] In the context of various embodiments, the third separating
layer and the fourth separating layer include conductive materials
(e.g. a conductor), e.g. copper (Cu).
[0092] In the context of various embodiments, one of or each of the
first separating layer and the second separating layer may have a
thickness ranging from about 0.4 nm to about 1.5 nm, e.g. about 0.4
nm to about 1.0 nm, about 0.4 nm to about 0.8 nm, about 0.8 nm to
about 1.5 nm or about 0.6 nm to about 1.2 nm.
[0093] In the context of various embodiments, the first stack
arrangement may be or may form part of a 2-bit per cell spin
transfer torque magnetoresistive random access memory (STT-MRAM).
In other words, the magnetoresistive device 120 with the first
stack arrangement may provide one, two, three, or four resistance
states, which may enable data storage of up to two bits of
information, thereby allowing multi-state or multi-bit storage.
[0094] In the context of various embodiments, the magnetoresistive
device 120 may further include a second stack arrangement (e.g.
220, FIGS. 13A, 13B) including a further ferromagnetic hard layer
(e.g. 222), a third ferromagnetic soft layer (e.g. 224), and a
third oscillating ferromagnetic structure (e.g. 226), wherein the
third ferromagnetic soft layer is disposed between the further
ferromagnetic hard layer and the third oscillating ferromagnetic
structure. The second stack arrangement may further include a fifth
separating layer (e.g. 228) disposed between the further
ferromagnetic hard layer and the third ferromagnetic soft layer,
and a sixth separating layer (e.g. 230) disposed between the third
ferromagnetic soft layer and the third oscillating ferromagnetic
structure.
[0095] In the context of various embodiments, the fifth separating
layer and the sixth separating layer include non-magnetic
materials.
[0096] In the context of various embodiments, the fifth separating
layer includes non-conductive materials (e.g. as insulator), e.g.
magnesium oxide (MgO). By arranging an insulator as a separating
layer between the further ferromagnetic hard layer and the third
ferromagnetic soft layer, the magnetoresistive device 120 may be
configured as a tunnel magnetoresistive (TMR) device.
[0097] However, it should be appreciated that the fifth separating
layer may include conductive materials (e.g. as conductor), e.g.
copper (Cu). By arranging a conductor as a separating layer between
the further ferromagnetic hard layer and the third ferromagnetic
soft layer, the magnetoresistive device 120 may be configured as a
giant magnetoresistive (GMR) device.
[0098] In the context of various embodiments, the sixth separating
layer includes conductive materials (e.g. a conductor), e.g. copper
(Cu).
[0099] In the context of various embodiments, the second stack
arrangement may be or may form part of a 1-bit per cell spin
transfer torque magnetoresistive random access memory
(STT-MRAM).
[0100] In the context of various embodiments, the second stack
arrangement may be disposed above or below the first stack
arrangement. In other words, the first stack arrangement and the
second stack arrangement may be arranged one over the other. The
magnetoresistive device 120 may further include a seventh
separating layer (e.g. 1302) disposed between the second stack
arrangement and the first stack arrangement. The seventh separating
layer may include non-magnetic materials. The seventh separating
layer may include conductive materials, e.g. copper (Cu).
[0101] In the context of various embodiments, the first stack
arrangement and the second stack arrangement may be or may form
part of a 3-bit per cell spin transfer torque magnetoresistive
random access memory (STT-MRAM). In other words, the
magnetoresistive device 120 with the first stack arrangement and
the second stack arrangement may provide one, two, three, four,
five, six, seven or eight resistance states, which may enable data
storage of up to three bits of information, thereby allowing
multi-state or multi-bit storage.
[0102] In the context of various embodiments, the magnetoresistive
device 120 including the first stack arrangement may further
include a further first stack arrangement (e.g. 1401, FIG. 14),
e.g. a further stack arrangement similar to the first stack
arrangement, wherein the further first stack arrangement is
disposed above or below the first stack arrangement. In other
words, the two first stack arrangements may be arranged one over
the other. The magnetoresistive device 120 may further include an
eighth separating layer (e.g. 1412) disposed between the first
stack arrangement and the further first stack arrangement. The
eight separating layer may include non-magnetic materials. The
eight separating layer may include conductive materials, e.g.
copper (Cu).
[0103] In the context of various embodiments, the first stack
arrangement and the further first stack arrangement may be or may
form part of a 4-bit per cell spin transfer torque magnetoresistive
random access memory (STT-MRAM). In other words, the
magnetoresistive device 120 with the first stack arrangement and
the further first stack arrangement may provide between one to
sixteen resistance states, which may enable data storage of up to
four bits of information, thereby allowing multi-state or multi-bit
storage.
[0104] In the context of various embodiments, it should be
appreciated that any n-bit per cell spin transfer torque
magnetoresistive random access memory (STT-MRAM) may be formed by
providing a combination of any number of the first stack
arrangement, where each first stack arrangement may provide 2-bit
per cell STT-MRAM, and/or any number of the second stack
arrangement, where each second stack arrangement may provide 1-bit
per cell STT-MRAM. In the context of various embodiments, a
conductive and non-magnetic separating layer may be disposed
between any two stack arrangements.
[0105] In the context of various embodiments, a combination of any
number of the first stack arrangement, where each first stack
arrangement may provide 2-bit per cell STT-MRAM, and/or any number
of the second stack arrangement, where each second stack
arrangement may provide 1-bit per cell STT-MRAM, may be provided as
part of a magnetic junction of a magnetoresistive device.
[0106] FIG. 1D illustrates a writing method 130 for a
magnetoresistive device, according to various embodiments, where
the magnetoresistive device may have a first oscillating
ferromagnetic structure and a second oscillating ferromagnetic
structure. For the writing method 130, a signal with an operating
current amplitude which is within either a range of operating
current amplitudes at which oscillations are induced for the first
oscillating ferromagnetic structure or a range of operating current
amplitudes at which oscillations are induced for the second
oscillating ferromagnetic structure is applied.
[0107] When the operating current amplitude of the signal is within
the range of operating current amplitudes at which oscillations are
induced for the first oscillating ferromagnetic structure, the
magnetization of the first oscillating ferromagnetic structure
oscillates, which may cause the magnetization of the corresponding
first ferromagnetic soft layer to switch, and when the operating
current amplitude of the signal is within the range of operating
current amplitudes at which oscillations are induced for the second
oscillating ferromagnetic structure, the magnetization of the
second oscillating ferromagnetic structure oscillates, which may
cause the magnetization of the corresponding second ferromagnetic
soft layer to switch. In various embodiments, the range of
operating current amplitudes at which oscillations are induced for
the first oscillating ferromagnetic structure may be
non-overlapping or substantially non-overlapping with the range of
operating current amplitudes at which oscillations are induced for
the second oscillating ferromagnetic structure, e.g. the ranges of
the operating current amplitudes at which oscillations are induced
for the first oscillating ferromagnetic structure and the second
oscillating ferromagnetic structure may be different.
[0108] In the context of various embodiments, the writing method
110 and/or the writing method 130 may be applied to the
magnetoresistive device 100 and/or the magnetoresistive device 120
of various embodiments.
[0109] Features or components of the magnetoresistive device 120
that are similarly present in the magnetoresistive device 100 may
be as described in the context of the magnetoresistive device 100,
and vice versa.
[0110] In the context of various embodiments, the magnetoresistive
device 100 and/or the magnetoresistive device 120 may be or may
form part of a spin transfer torque magnetoresistive random access
memory (STT-MRAM) with perpendicular anisotropy (p-STT-MRAM).
[0111] The magnetization orientations or directions of the
respective magnetic layers, e.g. the ferromagnetic soft layers and
the ferromagnetic hard layers, are configured to orient in a
direction at least substantially perpendicular to a plane defined
by an interface between the magnetic layers. In other words, the
ferromagnetic soft layers and the ferromagnetic hard layers have
their magnetic easy axis (e.g. magnetization orientation or
direction) aligned in a perpendicular direction (i.e. perpendicular
anisotropy), for example in a direction at least substantially
perpendicular to a plane defined by an interface between the
magnetic layers.
[0112] In the context of various embodiments, the term "easy axis"
as applied to magnetism may mean an energetically favorable
direction of spontaneous magnetization as a result of magnetic
anisotropy. The magnetization orientation may be either of two
opposite directions along the easy axis.
[0113] In the context of various embodiments, a ferromagnetic hard
layer may mean a magnetic layer having a fixed magnetization
orientation or direction. The ferromagnetic hard layer may include
a hard ferromagnetic material, which may be resistant to
magnetization and demagnetization (i.e. not easily magnetized and
demagnetized), and may have a high hysteresis loss and a high
coercivity. In the context of various embodiments, a ferromagnetic
hard layer may also be referred to as a fixed magnetic layer. In
the context of various embodiments, the ferromagnetic hard layer
may act as a reference layer.
[0114] In the context of various embodiments, a ferromagnetic hard
layer may include at least one of iron (Fe), cobalt (Co) or nickel
(Ni). In various embodiments, the ferromagnetic hard layer may
include cobalt-iron-boron (CoFeB), a (Co/Ni) bilayer structure, or
a bilayer structure including a first layer of material selected
from the group consisting of cobalt (Co), cobalt-iron (CoFe) and
cobalt-iron-boron (CoFeB), and a second layer of material selected
from the group consisting of palladium (Pd), platinum (Pt),
iron-platinum (FePt) alloy, cobalt-platinum (CoPt) alloy,
cobalt-iron (CoFe) and any combination thereof. For example, the
ferromagnetic hard layer may include a bilayer or a multilayer of
(Co/X), (CoFe/X) or (CoFeB/X) where X is palladium (Pd), platinum
(Pt), FePt alloy, CoPt alloy, CoFe or any combination of these
materials. Any combination of cobalt-iron-boron (CoFeB), (Co/Ni)
multilayer, (Co/X) multilayer, (CoFe/X) multilayer and (CoFeB/X)
multilayer may also be provided. As a non-limiting example, the
ferromagnetic hard layer may include (CoFe/Pd).sub.5 of 5 layers of
CoFe arranged alternately with 5 layers of Pd, i.e.
(CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd). In embodiments with a
multilayer structure, the number of repeats of bilayer structures
may be more than or equal to 2, e.g. 3, 4, 5 or any higher
number.
[0115] In the context of various embodiments, a ferromagnetic hard
layer may have a thickness in a range of between about 3 nm and
about 50 nm, e.g. between about 3 nm and about 20 nm, between about
3 nm and about 10 nm, between about 10 nm and about 50 nm, between
about 30 nm and about 50 nm or between about 5 nm and about 20
nm.
[0116] In the context of various embodiments, a ferromagnetic soft
layer may mean a magnetic layer having a varying or variable
magnetization orientation or direction. In other words, the
magnetization orientation may be changed or varied, for example by
applying a current, such as a spin-polarized current. The
ferromagnetic soft layer may include a soft ferromagnetic material,
which may be receptive to magnetization and demagnetization (i.e.
easily magnetized and demagnetized), and may have a small
hysteresis loss and a low coercivity. In the context of various
embodiments, a ferromagnetic soft layer may also be referred to as
a free magnetic layer. In the context of various embodiments, the
ferromagnetic soft layer may act as a storage layer.
[0117] In the context of various embodiments, a ferromagnetic soft
layer may include at least one of iron (Fe), cobalt (Co) or nickel
(Ni). In various embodiments, the ferromagnetic soft layer may
include cobalt-iron-boron (CoFeB), a (Co/Ni) bilayer structure, or
a bilayer structure including a first layer of material selected
from the group consisting of cobalt (Co), cobalt-iron (CoFe) and
cobalt-iron-boron (CoFeB), and a second layer of material selected
from the group consisting of palladium (Pd), platinum (Pt),
iron-platinum (FePt) alloy, cobalt-platinum (CoPt) alloy,
cobalt-iron (CoFe) and any combination thereof. For example, the
ferromagnetic soft layer may include a bilayer or a multilayer of
(Co/X), (CoFe/X) or (CoFeB/X) where X is palladium (Pd), platinum
(Pt), FePt alloy, CoPt alloy, CoFe or any combination of these
materials. Any combination of cobalt-iron-boron (CoFeB), (Co/Ni)
multilayer, (Co/X) multilayer, (CoFe/X) multilayer and (CoFeB/X)
multilayer may also be provided. As a non-limiting example, the
ferromagnetic soft layer may include (CoFe/Pd).sub.5, of 5 layers
of CoFe arranged alternately with 5 layers of Pd, i.e.
(CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd). In embodiments with a
multilayer structure, the number of repeats of bilayer structures
may be more than or equal to 2, e.g. 3, 4, 5 or any higher
number.
[0118] In the context of various embodiments, a ferromagnetic soft
layer may have a thickness in a range of between about 0.3 nm and
about 4 nm, for example between about 0.3 nm and about 2 nm,
between about 0.3 nm and about 1 nm or between about 1 nm and about
4 nm.
[0119] In the context of various embodiments, the magnetization
orientation of a ferromagnetic soft layer may be in one of two
directions. The ferromagnetic soft layer may be in a parallel state
(P) or an anti-parallel state (AP) with respect to a ferromagnetic
hard layer. In the parallel state, the magnetization orientation of
the ferromagnetic soft layer is parallel to the magnetization
orientation of the ferromagnetic hard layer, such that the two
magnetization orientations are in the same direction. In the
anti-parallel state, the magnetization orientation of the
ferromagnetic soft layer is anti-parallel to the magnetization
orientation of the ferromagnetic hard layer, such that the two
magnetization orientations are in opposite directions.
[0120] In the context of various embodiments, an oscillating
ferromagnetic structure or layer may mean a magnetic layer with a
magnetization orientation or direction which is configured to
oscillate or precess in a direction in response to a current or a
voltage applied across the magnetoresistive device. The oscillating
ferromagnetic structure may change the magnetization orientation of
the corresponding ferromagnetic soft layer as a result of the
oscillation or precession, for example due to spin transfer torque.
The oscillation may be in a circular direction or resembling a cone
direction, and for example may be a clockwise direction or an
anti-clockwise direction.
[0121] In the context of various embodiments, a separating layer
may be of a conductive and non-magnetic material, or a
non-conductive and non-magnetic material. In various embodiments, a
separating layer having or of a conductive and non-magnetic
material (e.g. an electrical conductor) may include but not limited
to any one of or any combination of copper (Cu), silver (Ag), gold
(Au), tantalum (Ta), chromium (Cr), palladium (Pd), platinum (Pt),
iridium (Ir), rhodium (Rh) or ruthenium (Ru). In various
embodiments, a separating layer having or of a non-conductive and
non-magnetic material (e.g. an insulator) may include but not
limited to any one of or any combination of magnesium oxide (MgO),
alumina (AlO.sub.x) or titanium oxide (TiO.sub.x). In the context
of various embodiments, a separation layer may also be referred to
as a spacer layer.
[0122] In the context of various embodiments, where a separating
layer is of a conductive and non-magnetic material, the separating
layer may have a thickness of between about 1 nm and about 20 nm,
e.g. between about 1 nm and about 10 nm, between about 1 nm and
about 5 nm, between about 5 nm and about 20 nm, between about 5 nm
and about 10 nm, or between about 1.5 nm and about 5 nm.
[0123] In the context of various embodiments, where a separating
layer is of a non-conductive and non-magnetic material, the
separating layer may have a thickness ranging from about 0.3 nm to
about 2.0 nm, e.g. about 0.3 nm to about 1.5 nm, about 0.3 nm to
about 0.8 nm, about 0.8 nm to about 2.0 nm, about 0.8 nm to about
1.5 nm or about 0.6 nm to about 1.2 nm.
[0124] In the context of various embodiments, the terms "first" and
"second" with respect to a feature (e.g. ferromagnetic soft layer)
refer to separate but similar features. The terms may be
interchangeable, for example depending on the arrangement of the
magnetoresistive device. For example, where two ferromagnetic soft
layers are arranged one above the other, the bottom ferromagnetic
soft layer may be termed as "first ferromagnetic soft layer" while
the top ferromagnetic soft layer may be termed as "second
ferromagnetic soft layer", or vice versa.
[0125] In the context of various embodiments, the term
"magnetization switching frequencies" with respect to a
ferromagnetic soft layer may mean a range of frequencies at which
the ferromagnetic soft layer may switch its magnetization
orientation.
[0126] In the context of various embodiments, the term "optimum
magnetization switching frequency" with respect to a ferromagnetic
soft layer may mean a particular frequency at which the
ferromagnetic soft layer may switch its magnetization orientation,
close to or equal to a resonance frequency of the ferromagnetic
soft layer.
[0127] In the context of various embodiments, the term "resonance
frequency" with respect to a ferromagnetic soft layer may refer to
the frequency of the ferromagnetic soft layer at which the
ferromagnetic soft layer may switch with the smallest critical
current for a given pulse length and amplitude, or equivalently may
switch in the fastest time for a given current pulse time width and
amplitude. The term "resonance frequency" with respect to a
ferromagnetic soft layer may also refer to the oscillating
frequency of the oscillating ferromagnetic structure at which the
corresponding ferromagnetic soft layer may switch with the smallest
critical current for a given pulse length and amplitude, or
equivalently may switch in the fastest time for a given current
pulse time width and amplitude.
[0128] In the context of various embodiments, the term
"perpendicular magnetic anisotropy field" may mean a magnetic field
in the perpendicular direction of a ferromagnetic layer, e.g.
perpendicular to a surface of the ferromagnetic layer, which may
interface with another ferromagnetic layer.
[0129] In the context of various embodiments, the term "saturation
magnetization" may mean the maximum induced magnetic moment that
may be obtained for a material in a magnetic field, which beyond
this field no further increase in magnetization occurs.
[0130] In the context of various embodiments, the term "disposed"
may be interchangeably used with the terms "arranged" and/or
"formed".
[0131] FIG. 2A shows a schematic cross-sectional view of a
magnetoresistive device 200, according to various embodiments,
illustrating a non-limiting example of a
2-bit spin transfer torque magnetoresistive random access memory
(STT-MRAM), for implementation using the Frequency Selection
Switching Scheme (FSSS) or the Oscillator Selection Switching
Scheme (OSSS).
[0132] The magnetoresistive device 200 may be a giant
magnetoresistive (GMR) device or a tunneling magnetoresistive (TMR)
device, e.g. a spin transfer torque magnetic random access memory
(STT-MRAM) with perpendicular anisotropy. The magnetoresistive
device 200 has a stack structure, having for example a plurality of
ferromagnetic layers.
[0133] The magnetoresistive device 200 includes a ferromagnetic
hard layer 202 as a reference layer, two (2) ferromagnetic soft
layers, e.g. a first ferromagnetic soft layer 204a and a second
ferromagnetic soft layer 204b, and two (2) oscillating
ferromagnetic structures, e.g. a first oscillating ferromagnetic
structure 206a and a second oscillating ferromagnetic structure
206b, arranged one over the other. The ferromagnetic hard layer 202
may have a fixed magnetization orientation, the ferromagnetic soft
layers 204a, 204b may have a variable magnetization orientation,
i.e. the magnetization orientation is changeable or switchable
between different orientations or states, while the oscillating
ferromagnetic structures, 206a, 206b may have a magnetization
orientation which may oscillate or precess in a direction in
response to a current or a voltage applied across the
magnetoresistive device 200.
[0134] The ferromagnetic hard layer 202, the first ferromagnetic
soft layer 204a, the second ferromagnetic soft layer 204b, the
first oscillating ferromagnetic structure 206a and the second
oscillating ferromagnetic structure 206b have their magnetic easy
axis (e.g. magnetization orientation or direction) aligned in a
perpendicular direction (i.e. perpendicular anisotropy), for
example in a direction at least substantially perpendicular to a
plane defined by an interface, for example an interface between the
first ferromagnetic soft layer 204a and the first oscillating
ferromagnetic structure 206a or between the ferromagnetic hard
layer 202 and the first ferromagnetic soft layer 204a.
[0135] As shown in FIG. 2A, the arrow shown within the
ferromagnetic hard layer 202 illustrates the direction of
magnetization orientation of the ferromagnetic hard layer 202.
While the arrow is shown pointing in a downward direction, it
should be appreciated that the arrow may be illustrated as pointing
in an upward direction, such that a magnetization orientation in
the opposite direction to that of the embodiment of FIG. 2A may be
provided for the ferromagnetic hard layer 202.
[0136] The magnetization orientation or direction of each of the
first ferromagnetic soft layer 204a and the second ferromagnetic
soft layer 204b may be oriented parallel to and in the same
direction (parallel state) as the magnetization orientation of the
ferromagnetic hard layer 202, or oriented parallel to and in the
opposite direction (anti-parallel state) as the magnetization
orientation of the ferromagnetic hard layer 202. The respective
arrows shown within the first ferromagnetic soft layer 204a and the
second ferromagnetic soft layer 204b illustrate the two directions
(upwards and downwards) of the magnetization orientation, such that
the magnetization orientation of each of the first ferromagnetic
soft layer 204a and the second ferromagnetic soft layer 204b may be
in either of these two directions.
[0137] Furthermore, in various embodiments, the magnetization
orientations or directions of the first oscillating ferromagnetic
structure 206a and the second oscillating ferromagnetic structure
206b are oriented in an opposite direction with respect to the
magnetization orientation of the ferromagnetic hard layer 202. For
illustration purposes, each of the magnetization orientations or
directions of the first oscillating ferromagnetic structure 206a,
the second oscillating ferromagnetic structure 206b and the
ferromagnetic hard layer 202, as represented by the respective
arrows within the respective layers in FIG. 2A, may point in an
upward direction or a downward direction.
[0138] The magnetization orientations of the first oscillating
ferromagnetic structure 206a and the second oscillating
ferromagnetic structure 206b may oscillate or precess in a
direction, such as a cone direction or a direction resembling a
cone, in response to a current or a voltage applied across the
magnetoresistive device 200 so as to change the magnetization
orientation of the corresponding first ferromagnetic soft layer
204a and second ferromagnetic soft layer 204b respectively, for
example due to spin transfer torque. For illustration purposes, the
magnetization orientations of the first oscillating ferromagnetic
structure 206a and the second oscillating ferromagnetic structure
206b are shown as oscillating in a cone direction, which may occur
during operation, or in-motion or oscillating mode (e.g. when a
current or a voltage is applied across the magnetoresistive device
200). As illustrated in FIG. 2A, the cone direction may be a
direction as represented by the arrow 207a (e.g. clockwise
direction) or the arrow 207b (e.g. anti-clockwise direction).
[0139] In various embodiments, the magnetization orientations of
the first oscillating ferromagnetic structure 206a and the second
oscillating ferromagnetic structure 206b may oscillate or precess
in the same or opposite directions. In various embodiments, the
magnetization orientations of the first oscillating ferromagnetic
structure 206a and the second oscillating ferromagnetic structure
206b may oscillate or precess at the same or different
amplitudes.
[0140] In various embodiments, the first ferromagnetic soft layer
204a is arranged between the ferromagnetic hard layer 202 and the
first oscillating ferromagnetic structure 206a, while the second
ferromagnetic soft layer 204b is arranged between the ferromagnetic
hard layer 202 and the second oscillating ferromagnetic structure
206b.
[0141] The magnetoresistive device 200 has a stack arrangement
having different ferromagnetic layers in the order of the first
oscillating ferromagnetic structure 206a, the first ferromagnetic
soft layer 204a, the ferromagnetic hard layer 202, the second
ferromagnetic soft layer 204b and the second oscillating
ferromagnetic structure 206b. It should be appreciated that the
positions of the first ferromagnetic soft layer 204a and the second
ferromagnetic soft layer 204b may be interchangeable, and/or the
positions of the first oscillating ferromagnetic structure 206a and
the second oscillating ferromagnetic structure 206b may be
interchangeable. The first oscillating ferromagnetic structure
206a, the first ferromagnetic soft layer 204a, the ferromagnetic
hard layer 202, the second ferromagnetic soft layer 204b and the
second oscillating ferromagnetic structure 206b may form part of a
magnetic junction of the magnetoresistive device 200.
[0142] As shown in FIG. 2A, each of the first oscillating
ferromagnetic structure 206a, the first ferromagnetic soft layer
204a, the ferromagnetic hard layer 202, the second ferromagnetic
soft layer 204b and the second oscillating ferromagnetic structure
206b are separated from each other by a separating layer, for
example a spacer layer.
[0143] The magnetoresistive device 200 includes a separating layer
208a arranged in between the ferromagnetic hard layer 202 and the
first ferromagnetic soft layer 204a. The magnetoresistive device
200 further includes a separating layer 208b arranged in between
the ferromagnetic hard layer 202 and the second ferromagnetic soft
layer 204b.
[0144] Each of the separating layers 208a, 208b may be of a
non-conductive and non-magnetic material (e.g. an insulator). For
example, each of the separating layers 208a, 208b may include but
not limited to one or more of magnesium oxide (MgO), aluminium
oxide (AlO.sub.x), or titanium oxide (TiO.sub.x).
[0145] The magnetoresistive device 200 includes a separating layer
210a arranged in between the first oscillating ferromagnetic
structure 206a and the first ferromagnetic soft layer 204a. The
magnetoresistive device 200 further includes a separating layer
210b arranged in between the second ferromagnetic soft layer 204b
and the second oscillating ferromagnetic structure 206b.
[0146] Each of the separating layers 210a, 210b may be of a
conductive and non-magnetic material (e.g. a conductor). For
example, each of the separating layers 210a, 210b may include but
not limited to one or more of copper (Cu), silver (Ag), gold (Au),
tantalum (Ta), chromium (Cr), palladium (Pd), platinum (Pt),
iridium (Ir), rhodium (Rh) or ruthenium (Ru). The magnetoresistive
device 200 may therefore be configured as a tunneling
magnetoresistive (TMR) device.
[0147] However, it should be appreciated that the magnetoresistive
device 200 may be configured as a giant magnetoresistive (GMR)
device, with both the separating layers 208a, 208b of a conductive
and non-magnetic material (e.g. a conductor), e.g. one or more of
Cu, Ag, Au, Ta, Cr, Pd, Pt, Ir, Rh or Ru.
[0148] In addition, it should be appreciated that any one of or
both separating layers 210a, 210b may be of a non-conductive and
non-magnetic material (e.g. an insulator) including but not limited
to one or more of magnesium oxide (MgO), aluminium oxide
(AlO.sub.x), or titanium oxide (TiO.sub.x). The thickness of the
insulating layer may be between about 0.3 nm and about 2.0 nm, e.g.
between about 0.3 nm and about 1.5 nm, between about 0.3 nm and
about 0.8 nm, between about 0.8 nm and about 2.0 nm, between about
0.8 nm and about 1.5 nm or between about 0.6 nm and about 1.2
nm.
[0149] While FIG. 2A illustrates a common ferromagnetic hard layer
202 shared by the first ferromagnetic soft layer 204a and the
second ferromagnetic soft layer 204b, it should be appreciated that
a respective corresponding ferromagnetic hard layer may be provided
for each of the first ferromagnetic soft layer 204a and the second
ferromagnetic soft layer 204b.
[0150] It should be appreciated that the magnetoresistive device
200 may have any stack structure or arrangement with any number of
ferromagnetic layers or structures, and/or arranged in any
configurations, as long as a ferromagnetic soft layer has a
corresponding oscillating ferromagnetic structure arranged on one
side of the ferromagnetic soft layer and a ferromagnetic hard layer
arranged on the other side of the ferromagnetic soft layer,
opposite to the oscillating ferromagnetic structure.
[0151] FIG. 2B shows a schematic cross-sectional view of a
magnetoresistive device 220, according to various embodiments,
illustrating a non-limiting example of a 1-bit spin transfer torque
magnetoresistive random access memory (STT-MRAM), for
implementation using the Frequency Selection Switching Scheme
(FSSS) or the Oscillator Selection Switching Scheme (OSSS).
[0152] The magnetoresistive device 220 may be a giant
magnetoresistive (GMR) device or a tunneling magnetoresistive (TMR)
device, e.g. a spin transfer torque magnetic random access memory
(STT-MRAM) with perpendicular anisotropy. The magnetoresistive
device 220 has a stack structure, having for example a plurality of
ferromagnetic layers.
[0153] The magnetoresistive device 220 includes a ferromagnetic
hard layer 222 as a reference layer, a ferromagnetic soft layer
224, and an oscillating ferromagnetic structure 226, arranged one
over the other. In various embodiments, the ferromagnetic soft
layer 224 is arranged between the ferromagnetic hard layer 222 and
the oscillating ferromagnetic structure 226.
[0154] The ferromagnetic hard layer 222 may have a fixed
magnetization orientation, the ferromagnetic soft layer 224a may
have a variable magnetization orientation, i.e. the magnetization
orientation is changeable or switchable between different
orientations or states, while the oscillating ferromagnetic
structure 226 may have a magnetization orientation which may
oscillate or precess in a direction in response to a current or a
voltage applied across the magnetoresistive device 220.
[0155] The ferromagnetic hard layer 222, the ferromagnetic soft
layer 224 and the oscillating ferromagnetic structure 226 have
their magnetic easy axis (e.g. magnetization orientation or
direction) aligned in a perpendicular direction (i.e. perpendicular
anisotropy), for example in a direction at least substantially
perpendicular to a plane defined by an interface, for example an
interface between the ferromagnetic hard layer 222 and the
ferromagnetic soft layer 224 or between the oscillating
ferromagnetic structure 226 and the ferromagnetic soft layer
224.
[0156] As shown in FIG. 2B, the arrow shown within the
ferromagnetic hard layer 222 illustrates the direction of
magnetization orientation of the ferromagnetic hard layer 222.
While the arrow is shown pointing in a downward direction, it
should be appreciated that the arrow may be illustrated as pointing
in an upward direction, such that a magnetization orientation in
the opposite direction to that of the embodiment of FIG. 2B may be
provided for the ferromagnetic hard layer 222.
[0157] The magnetization orientation or direction of the
ferromagnetic soft layer 224 may be oriented parallel to and in the
same direction (parallel state) as the magnetization orientation of
the ferromagnetic hard layer 222, or oriented parallel to and in
the opposite direction (anti-parallel state) as the magnetization
orientation of the ferromagnetic hard layer 222. The respective
arrows shown within the ferromagnetic soft layer 224 illustrate the
two directions (upwards and downwards) of the magnetization
orientation, such that the magnetization orientation of the
ferromagnetic soft layer 224 may be in either of these two
directions.
[0158] Furthermore, in various embodiments, the magnetization
orientation of the oscillating ferromagnetic structure 226 is
oriented in an opposite direction with respect to the magnetization
orientation of the ferromagnetic hard layer 222. For illustration
purposes, the magnetization orientations of the oscillating
ferromagnetic structure 226 and the ferromagnetic hard layer 222,
as represented by the respective arrows within the respective
layers in FIG. 2B, may point in an upward direction or a downward
direction.
[0159] The magnetization orientations of the oscillating
ferromagnetic structure 226 may oscillate or precess in a
direction, such as a cone direction or a direction resembling a
cone, in response to a current or a voltage applied across the
magnetoresistive device 220 so as to change the magnetization
orientation of the ferromagnetic soft layer 224, for example due to
spin transfer torque. For illustration purposes, the magnetization
orientation of the oscillating ferromagnetic structure 226 is shown
as oscillating in a cone direction, which may occur during
operation, or in-motion or oscillating mode (e.g. when a current or
a voltage is applied across the magnetoresistive device 220). As
illustrated in FIG. 2B, the cone direction may be a direction as
represented by the arrow 227a (e.g. clockwise direction) or the
arrow 227b (e.g. anti-clockwise direction).
[0160] The magnetoresistive device 220 has a stack arrangement
having different ferromagnetic layers in the order of the
oscillating ferromagnetic structure 226, the ferromagnetic soft
layer 224 and the ferromagnetic hard layer 222. It should be
appreciated that the positions of the oscillating ferromagnetic
structure 226 and the ferromagnetic hard layer 222 may be
interchangeable. The oscillating ferromagnetic structure 226, the
ferromagnetic soft layer 224 and the ferromagnetic hard layer 222
may form part of a magnetic junction of the magnetoresistive device
220.
[0161] As shown in FIG. 2B, each of the oscillating ferromagnetic
structure 226, the ferromagnetic soft layer 224 and the
ferromagnetic hard layer 222 are separated from each other by a
separating layer, for example a spacer layer.
[0162] The magnetoresistive device 220 includes a separating layer
228 arranged in between the ferromagnetic hard layer 222 and the
ferromagnetic soft layer 224. The separating layer 228 may be of a
non-conductive and non-magnetic material (e.g. an insulator). For
example, the separating layer 228 may include but not limited to
one or more of magnesium oxide (MgO), aluminium oxide (AlO.sub.x),
or titanium oxide (TiO.sub.x).
[0163] The magnetoresistive device 220 further includes a
separating layer 230 arranged in between the oscillating
ferromagnetic structure 226 and the ferromagnetic soft layer 224.
The separating layer 230 may be of a conductive and non-magnetic
material (e.g. a conductor). For example, the separating layer 230
may include but not limited to one or more of copper (Cu), silver
(Ag), gold (Au), tantalum (Ta), chromium (Cr), palladium (Pd),
platinum (Pt), iridium (Ir), rhodium (Rh) or ruthenium (Ru). The
magnetoresistive device 220 may therefore be configured as a
tunneling magnetoresistive (TMR) device.
[0164] However, it should be appreciated that the magnetoresistive
device 220 may be configured as a giant magnetoresistive (GMR)
device, with the separating layer 228 of a conductive and
non-magnetic material (e.g. a conductor), e.g. one or more of Cu,
Ag, Au, Ta, Cr, Pd, Pt, Ir, Rh or Ru.
[0165] It should be appreciated that any number of the stack
arrangements illustrated in FIG. 2A and/or FIG. 2B may be arranged
to provide a multi-bit per cell spin transfer torque
magnetoresistive random access memory (STT-MRAM).
[0166] In the context of various embodiments of the
magnetoresistive devices 200, 220, one or more of the first
oscillating ferromagnetic structure 206a, the second oscillating
ferromagnetic structure 206b and the oscillating ferromagnetic
structure 226 may include one or more ferromagnetic materials,
and/or one or more ferromagnetic layers. Each ferromagnetic layer
may include at least one of iron (Fe), cobalt (Co) or nickel (Ni).
In various embodiments, one or more of the ferromagnetic layers may
include cobalt-iron-boron (CoFeB), a bilayer structure of
cobalt-nickel (Co/Ni), or a bilayer structure including a first
layer of material selected from the group consisting of cobalt
(Co), cobalt-iron (CoFe) and cobalt-iron-boron (CoFeB), and a
second layer of material selected from the group consisting of
palladium (Pd), platinum (Pt), iron-platinum (FePt) alloy,
cobalt-platinum (CoPt) alloy, cobalt-iron (CoFe) and any
combination thereof. For example, the ferromagnetic layer may
include a bilayer or a multilayer of (Co/Ni), (Co/X), (CoFe/X) or
(CoFeB/X). Any combination of cobalt-iron-boron (CoFeB), (Co/Ni),
(Co/X) multilayer, (CoFe/X) multilayer and (CoFeB/X) multilayer may
also be provided. As a non-limiting example, the ferromagnetic
layer may include (CoFe/Pd).sub.5, of 5 layers of CoFe arranged
alternately with 5 layers of Pd, i.e.
(CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd)
[0167] In various embodiments, the thickness of each ferromagnetic
layer in the first oscillating ferromagnetic structure 206a, the
second oscillating ferromagnetic structure 206b and the oscillating
ferromagnetic structure 226 may be in a range of between about 0.2
nm and about 4 nm, for example between about 0.2 nm and about 2 nm,
between about 0.2 nm and about 1 nm, between about 1 nm and about 4
nm, between about 2 nm and about 4 nm, or between about 1 nm and
about 3 nm.
[0168] In various embodiments, where any of the first oscillating
ferromagnetic structure 206a, the second oscillating ferromagnetic
structure 206b and the oscillating ferromagnetic structure 226 has
a multilayer structure, the multilayer structure may include any
number of a bilayer structure, e.g. (CoFe/Pd), of 2 (e.g.
(CoFe/Pd).sub.2) or more (e.g. 3, 4, 5, 6 or any higher number)
bilayer structure. In other words, the bilayer structure, e.g.
(CoFe/Pd), may be repeated twice or any higher number.
[0169] In the context of various embodiments of the
magnetoresistive devices 200, 220, where any of the first
oscillating ferromagnetic structure 206a, the second oscillating
ferromagnetic structure 206b and the oscillating ferromagnetic
structure 226 includes more than one ferromagnetic layer, e.g. 2,
3, 4, 5 or any higher number, one or more non-magnetic layer may be
deposited or arranged between any two adjacent ferromagnetic
layers. The non-magnetic layer may be ruthenium (Ru), rhodium (Rh),
iridum (Ir), copper (Cu), silver (Ag), or chromium (Cr), with a
thickness of between about 0.2 nm and about 3 nm, for example
between about 0.2 nm and about 2 nm, between about 0.2 nm and about
1 nm, between about 1 nm and about 3 nm, or between about 0.5 nm
and about 2 nm.
[0170] In embodiments where there are two or more oscillating
ferromagnetic structures, the oscillating ferromagnetic structures
may be of the same or different configurations and/or materials
and/or number of layers and/or materials per layer and/or number of
repeating bilayer structures. In various embodiments, where each of
the two or more oscillating ferromagnetic structures includes a
single ferromagnetic layer, the oscillating ferromagnetic
structures may be of different materials.
[0171] In the context of various embodiments of the
magnetoresistive devices 200, 220, the first oscillating
ferromagnetic structure 206a, the second oscillating ferromagnetic
structure 206b and the oscillating ferromagnetic structure 226 may
assist the switching of the magnetization of the corresponding
ferromagnetic soft layers 204a, 204b, 224, by Frequency Induced
Spin Torque Switching (FISTS), which will be described later.
[0172] For embodiments of the Frequency Selection Switching Scheme
(FSSS), the properties of the first ferromagnetic soft layer 204a,
the second ferromagnetic soft layer 204b and the ferromagnetic soft
layer 224, and its respective corresponding first oscillating
ferromagnetic structure 206a, second oscillating ferromagnetic
structure 206b and oscillating ferromagnetic structure 226 may be
determined or chosen such that switching of each of the first
ferromagnetic soft layer 204a, the second ferromagnetic soft layer
204b and the ferromagnetic soft layer 224 may be carried out only
in a range of frequencies close to a resonance frequency of the
respective ferromagnetic soft layer. This may allow each of the
first ferromagnetic soft layer 204a, the second ferromagnetic soft
layer 204b and the ferromagnetic soft layer 224 to switch at
different frequencies independently, e.g. each ferromagnetic soft
layer may be switched independently within a particular range of
magnetization switching frequencies.
[0173] FIG. 2C shows a schematic diagram 240 illustrating the
concept of frequency bandwidth 242 for various embodiments. The
frequency bandwidth 242 may be defined as the range of frequencies
where switching (e.g. a range of magnetization switching
frequencies), for example of a ferromagnetic soft layer, occurs for
a particular applied current density and pulse width (e.g. 2
ns).
[0174] For embodiments of the Oscillator Selection Switching Scheme
(OSSS), the properties of the respective first oscillating
ferromagnetic structure 206a, second oscillating ferromagnetic
structure 206b and oscillating ferromagnetic structure 226
corresponding to each of the first ferromagnetic soft layer 204a,
the second ferromagnetic soft layer 204b and the ferromagnetic soft
layer 224 may be determined or chosen such that the first
oscillating ferromagnetic structure 206a, the second oscillating
ferromagnetic structure 206b and the oscillating ferromagnetic
structure 226 have different critical current densities (or ranges
of operating current amplitudes) at which oscillations are induced,
so that FISTS is in effect for only one ferromagnetic soft layer
for a particular applied current density. This may allow each of
the first ferromagnetic soft layer 204a, the second ferromagnetic
soft layer 204b and the ferromagnetic soft layer 224 to switch
independently at different current densities, e.g. each
ferromagnetic soft layer may be switched independently at a
particular current density or a particular range of operating
current amplitudes.
[0175] Non-limiting examples of an oscillating ferromagnetic
structure are now described with reference to FIGS. 3A to 3D.
[0176] FIG. 3A shows a schematic cross-sectional view of an
oscillating ferromagnetic structure 300, according to various
embodiments, during operation, or in-motion or oscillating mode
(e.g. when a current or a voltage is applied across the oscillating
ferromagnetic structure 300). The oscillating ferromagnetic
structure 300 includes a ferromagnetic layer 302 with a
perpendicular anisotropy component, a ferromagnetic layer 304 with
an in-plane anisotropy component (i.e. in an initial state or rest
state, the magnetization orientation of the ferromagnetic layer 304
points in a left or right direction) and a ferromagnetic hard layer
306 with a perpendicular anisotropy component, the ferromagnetic
layer 302, the ferromagnetic layer 304 and the ferromagnetic hard
layer 306 arranged one over the other. In other words, the
ferromagnetic layer 302 and the ferromagnetic hard layer 306 have
respective magnetization orientations configured to orient in a
direction at least substantially perpendicular to a plane defined
by an interface, for example an interface between the ferromagnetic
layer 302 and the ferromagnetic layer 304 or between the
ferromagnetic layer 302 and the ferromagnetic hard layer 306. The
ferromagnetic layer 304 has a magnetization orientation configured
to orient in a direction at least substantially parallel to a plane
defined by an interface, for example an interface between the
ferromagnetic layer 302 and the ferromagnetic layer 304 or between
the ferromagnetic layer 302 and the ferromagnetic hard layer 306.
As a non-limiting example, FIG. 3A illustrates that the
ferromagnetic layer 302 is arranged or stacked on top of the
ferromagnetic layer 304, which in turn in arranged or stacked on
top of the ferromagnetic hard layer 306.
[0177] As shown in FIG. 3A, during operation or during in-motion or
oscillating mode, the respective magnetization orientations of the
ferromagnetic layer 302 and the ferromagnetic layer 304 may
oscillate or precess in a direction, such as a cone direction or a
direction resembling a cone, in response to a current or a voltage
applied across the oscillating ferromagnetic structure 300 so as to
change the magnetization orientation of a corresponding
ferromagnetic soft layer, for example due to spin transfer torque.
As a non-limiting example, with reference to FIGS. 2A and 3A, the
structure of the first oscillating ferromagnetic structure 206a may
be that of the oscillating ferromagnetic structure 300. Where the
first oscillating ferromagnetic structure 206a is configured such
that the magnetization orientation of the first oscillating
ferromagnetic structure 206a oscillates in a direction, e.g. the
cone direction as represented by the arrow 207a (e.g. clockwise
direction), the magnetization orientation of the ferromagnetic
layer 302 oscillates in a similar clockwise direction, as
represented by the arrow 303a, while the magnetization orientation
of the ferromagnetic layer 304 may oscillate in a clockwise
direction or anti-clockwise direction, as represented respectively
by the arrows 305a and 305b. Where the first oscillating
ferromagnetic structure 206a is configured such that the
magnetization orientation of the first oscillating ferromagnetic
structure 206a oscillates in a direction, e.g. the cone direction
as represented by the arrow 207b (e.g. anti-clockwise direction),
the magnetization orientation of the ferromagnetic layer 302
oscillate in a similar anti-clockwise direction, as represented by
the arrow 303b, while the magnetization orientation of the
ferromagnetic layer 304 may oscillate in a clockwise direction or
anti-clockwise direction, as represented respectively by the arrows
305a and 305b. While the ferromagnetic layer 304 has an in-plane
anisotropy component in the initial state or rest state, the
magnetization orientation of the ferromagnetic layer 304 may be
configured to oscillate or precess in the direction 305a or 305b
(similar to directions 303a or 303b for the ferromagnetic layer 302
having a perpendicular anisotropy component) due to the effect of
the ferromagnetic hard layer 306. The electrons acquire
polarization in the polarization direction of the ferromagnetic
hard layer 306, and the spin torque from these electrons act on the
ferromagnetic layer 304. Due to the in-plane anisotropy of the
ferromagnetic layer 304, the ferromagnetic layer 304 may respond to
the spin torque in such a way that it may oscillate or precess in a
cone direction (e.g. 305a or 305b) whose axis is perpendicular to
the plane by an interface, for example an interface between the
ferromagnetic layer 302 and the ferromagnetic layer 304 or between
the ferromagnetic layer 302 and the ferromagnetic hard layer 306,
similar to that for the ferromagnetic layer 302.
[0178] In various embodiments, the ferromagnetic layer 302 and the
ferromagnetic layer 304 may be `softer` (i.e. ferromagnetically
softer) than the ferromagnetic hard layer 306. In various
embodiments, the ferromagnetic layer 302 and the ferromagnetic
layer 304 may or may not be `softer` (i.e. ferromagnetically
softer) than the ferromagnetic soft layers (e.g. 204a, 204b,
224).
[0179] The oscillating ferromagnetic structure 300 further includes
a separating layer 308 arranged in between the ferromagnetic layer
302 and the ferromagnetic layer 304. The oscillating ferromagnetic
structure 300 further includes a separating layer 310 arranged in
between the ferromagnetic layer 304 and the ferromagnetic hard
layer 306.
[0180] In various embodiments, each of the separating layers 308,
310 may be of a conductive and non-magnetic material (e.g. Cu, Ag
or Au). In various embodiments, each of the separating layers 308,
310 may have a thickness of at least 1.5 nm, for example a
thickness of between about 1.5 nm and 20 nm, e.g. between about 1.5
nm and 10 nm, between about 1.5 nm and 5 nm, between about 5 nm and
20 nm or between about 5 nm and 10 nm.
[0181] In various embodiments, each of the ferromagnetic layer 302
and the ferromagnetic layer 304 may include a material, including
but not limited to cobalt (Co), cobalt-iron (CoFe),
cobalt-iron-boron (CoFeB) or any combinations thereof.
[0182] In various embodiments, each of the ferromagnetic layer 302
and the ferromagnetic layer 304 may have a thickness in a range of
between about 0.5 nm and about 4 nm, for example between about 0.5
nm and about 2 nm, between about 0.5 nm and about 1 nm or between
about 1 nm and about 4 nm.
[0183] In various embodiments, the ferromagnetic hard layer 306 may
include a material having at least one of iron (Fe), cobalt (Co)
and nickel (Ni) elements, for example cobalt-iron-boron (CoFeB),
(Co/Ni) multilayer, (Co/X) multilayer, (CoFe/X) multilayer,
(CoFeB/X) multilayer where X is palladium (Pd) and/or platinum
(Pt). Any combination of cobalt-iron-boron (CoFeB), (Co/Ni)
multilayer, (Co/X) multilayer, (CoFe/X) multilayer and (CoFeB/X)
multilayer may also be provided. The (Co/X) multilayer, the
(CoFe/X) multilayer and the (CoFeB/X) multilayer may include a
plurality of a bilayer structure having a first layer of material
(Co, CoFe and CoFeB respectively) and a second layer of Pd and/or
Pt. As a non-limiting example, the ferromagnetic hard layer 306 may
include (CoFe/Pd).sub.5, of 5 layers of CoFe arranged alternately
with 5 layers of Pd, i.e.
(CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd).
[0184] In various embodiments, the ferromagnetic hard layer 306 may
have a thickness of between about 3 nm and about 50 nm, e.g.
between about 3 nm and about 20 nm, between about 3 nm and about 10
nm, between about 10 nm and about 50 nm, between about 30 nm and
about 50 nm or between about 5 nm and about 20 nm.
[0185] While FIG. 3A illustrates that the ferromagnetic hard layer
306 is arranged beneath the ferromagnetic layer 302 and the
ferromagnetic layer 304, it should be appreciated that other
arrangements of the ferromagnetic layer 302, the ferromagnetic
layer 304 and the ferromagnetic hard layer 306 may be possible.
[0186] FIG. 3B shows a schematic cross-sectional view of an
oscillating ferromagnetic structure 320, according to various
embodiments. The oscillating ferromagnetic structure 320 includes a
ferromagnetic layer 302 with a perpendicular anisotropy component,
a ferromagnetic layer 304 with an in-plane anisotropy component and
a ferromagnetic hard layer 306 with a perpendicular anisotropy
component, the ferromagnetic layer 302, the ferromagnetic layer 304
and the ferromagnetic hard layer 306 arranged one over the other.
The ferromagnetic layer 302, the ferromagnetic layer 304 and the
ferromagnetic hard layer 306 may be as described in the context of
the embodiments of FIG. 3A.
[0187] The oscillating ferromagnetic structure 320 further includes
a separating layer 308 arranged in between the ferromagnetic layer
302 and the ferromagnetic layer 304. The oscillating ferromagnetic
layer 300 further includes a separating layer 310 arranged in
between the ferromagnetic layer 304 and the ferromagnetic hard
layer 306. The separating layers 308, 310 may be as described in
the context of the embodiments of FIG. 3A.
[0188] The oscillating ferromagnetic structure 320 may further
include an antiferromagnetic (AFM) layer 322, where the
ferromagnetic hard layer 306 may be arranged over or stacked on top
of the antiferromagnetic layer 322, which pins the magnetization of
the ferromagnetic hard layer 306.
[0189] In various embodiments, the antiferromagnetic layer 322 may
include a material of X-manganese or X-Y-manganese, wherein each of
X and Y may be platinum (Pt), palladium (Pd), iridium (Ir), rhodium
(Rh) or ruthenium (Ru).
[0190] In various embodiments, the antiferromagnetic layer 322 may
have a thickness in a range of between about 5 nm and about 20 nm,
for example between about 5 nm and about 15 nm, between about 5 nm
and about 10 nm or between about 10 nm and about 20 nm.
[0191] FIG. 3C shows a schematic cross-sectional view of an
oscillating ferromagnetic structure 330, according to various
embodiments, during operation, or in-motion or oscillating mode
(e.g. when a current or a voltage is applied across the oscillating
ferromagnetic structure 330). The oscillating ferromagnetic
structure 330 includes a ferromagnetic layer 332 with a
perpendicular anisotropy component and a ferromagnetic hard layer
334 with an in-plane anisotropy component, the ferromagnetic layer
332 and the ferromagnetic hard layer 334 arranged one over the
other. In other words, the ferromagnetic layer 332 has a
magnetization orientation configured to orient in a direction at
least substantially perpendicular to a plane defined by an
interface, for example an interface between the ferromagnetic layer
332 and the ferromagnetic hard layer 334. The ferromagnetic hard
layer 334 has a magnetization orientation configured to orient in a
direction at least substantially parallel to a plane defined by an
interface, for example an interface between the ferromagnetic layer
332 and the ferromagnetic hard layer 334. As a non-limiting
example, FIG. 3C illustrates that the ferromagnetic layer 332 is
arranged or stacked on top of the ferromagnetic hard layer 334.
[0192] As shown in FIG. 3C, during operation or during in-motion or
oscillating mode, the magnetization orientation of the
ferromagnetic layer 332 may oscillate or precess in a direction,
such as a cone direction or a direction resembling a cone, in
response to a current or a voltage applied across the oscillating
ferromagnetic structure 330 so as to change the magnetization
orientation of a corresponding ferromagnetic soft layer, for
example due to spin transfer torque. As a non-limiting example,
with reference to FIGS. 2A and 3C, the structure of the first
oscillating ferromagnetic structure 206a may be that of the
oscillating ferromagnetic structure 330. Where the first
oscillating ferromagnetic structure 206a is configured such that
the magnetization orientation of the first oscillating
ferromagnetic structure 206a oscillates in a direction, e.g. the
cone direction as represented by the arrow 207a (e.g. clockwise
direction), the magnetization orientation of the ferromagnetic
layer 332 oscillates in a similar clockwise direction, as
represented by the arrow 333a. Where the first oscillating
ferromagnetic structure 206a is configured such that the
magnetization orientation of the first oscillating ferromagnetic
structure 206a oscillates in a direction, e.g. the cone direction
as represented by the arrow 207b (e.g. anti-clockwise direction),
the magnetization orientation of the ferromagnetic layer 332
oscillates in a similar anti-clockwise direction, as represented by
the arrow 333b. In various embodiments, the direction of the
in-plane anisotropy (e.g. in the left or right direction) of the
ferromagnetic hard layer 334 may affect the direction of the
oscillation of the ferromagnetic layer 332 (e.g. clockwise
direction 333a or anti-clockwise direction 333b).
[0193] In various embodiments, the ferromagnetic layer 332 may be
`softer` (i.e. ferromagnetically softer) than the ferromagnetic
hard layer 334. In various embodiments, the ferromagnetic layer 332
may or may not be `softer` (i.e. ferromagnetically softer) than the
ferromagnetic soft layers (e.g. 204a, 204b, 224).
[0194] The oscillating ferromagnetic structure 330 further includes
a separating layer 336 arranged in between the ferromagnetic layer
332 and the ferromagnetic hard layer 334. In various embodiments,
the separating layer 336 may be of a conductive and non-magnetic
material (e.g. Cu, Ag or Au). In various embodiments, the
separating layer 336 may have a thickness of at least 1.5 nm, for
example a thickness of between about 1.5 nm and 20 nm, e.g. between
about 1.5 nm and 10 nm, between about 1.5 nm and 5 nm, between
about 5 nm and 20 nm or between about 5 nm and 10 nm.
[0195] In various embodiments, the ferromagnetic layer 332 may
include a material having at least one of iron (Fe), cobalt (Co)
and nickel (Ni) elements, for example cobalt-iron-boron (CoFeB),
(Co/Ni) multilayer, (Co/X) multilayer, (CoFe/X) multilayer,
(CoFeB/X) multilayer where X is palladium (Pd) and/or platinum
(Pt). Any combination of cobalt-iron-boron (CoFeB), (Co/X)
multilayer, (Co/Ni) multilayer, (CoFe/X) multilayer and (CoFeB/X)
multilayer may also be provided. The (Co/X) multilayer, the
(CoFe/X) multilayer and the (CoFeB/X) multilayer may include a
plurality of a bilayer structure having a first layer of material
(Co, CoFe and CoFeB respectively) and a second layer of Pd and/or
Pt. As a non-limiting example, the ferromagnetic layer 332 may
include (CoFe/Pd).sub.5, of 5 layers of CoFe arranged alternately
with 5 layers of Pd, i.e.
(CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd/CoFe/Pd).
[0196] In various embodiments, the ferromagnetic layer 332 may have
a thickness in a range of between about 0.3 nm and about 4 nm, for
example between about 0.3 nm and about 2 nm, between about 0.3 nm
and about 1 nm or between about 1 nm and about 4 nm.
[0197] In various embodiments, the ferromagnetic hard layer 334 may
be a single ferromagnetic layer or a synthetic antiferromagnetic
layer (SAF). In various embodiments, the ferromagnetic hard layer
334 may include a material, including but not limited to cobalt
(Co), cobalt-iron (CoFe), cobalt-iron-boron (CoFeB) or any
combinations thereof.
[0198] In embodiments where the ferromagnetic hard layer 334 may be
a synthetic antifferomagnetic layer (SAF), the SAF may include two
antiferromagnetically coupled ferromagnetic layers, pinned by an
antiferromagnetic layer. A metal separating layer, for example a
conductive and non-magnetic separating layer (e.g. ruthenium (Ru))
may be sandwiched in between the two antiferromagnetically coupled
ferromagnetic layers. As a non-limiting example, the synthetic
antiferromagnetic layer may have a structure or arrangement having
two ferromagnetic layers, with a metal separating layer having
Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, such as but not
limited to ruthenium (Ru), sandwiched in between the two
ferromagnetic layers. One of the ferromagnetic layers may be a
reference layer while the other ferromagnetic layer is a pinned
layer, and in contact with an antiferromagnetic layer (e.g.
including a material including X-manganese or X-Y-manganese,
wherein each of X and Y is selected from the group consisting of
platinum (Pt), palladium (Pd), iridium (Ir), rhodium (Rh) and
ruthenium (Ru)). By employing the metal separating layer with an
appropriate thickness, the magnetization orientations of the two
ferromagnetic layers, may be anti-aligned (i.e. the layers may be
antiferromagnetically coupled). In various embodiments, the SAF may
pin the reference layer and may reduce stray magnetic field that
may act on the ferromagnetic soft layer (e.g. 204a, 204b, 224).
[0199] In various embodiments, the ferromagnetic hard layer 334 may
have a thickness in a range of between about 0.5 nm and about 4 nm,
for example between about 0.5 nm and about 2 nm, between about 0.5
nm and about 1 nm or between about 1 nm and about 4 nm.
[0200] FIG. 3D shows a schematic cross-sectional view of an
oscillating ferromagnetic structure 340, according to various
embodiments. The oscillating ferromagnetic structure 340 includes a
ferromagnetic layer 332 with a perpendicular anisotropy component
and a ferromagnetic hard layer 334 with an in-plane anisotropy
component, the ferromagnetic layer 332 and the ferromagnetic hard
layer 334 arranged one over the other. The ferromagnetic layer 332
and the ferromagnetic hard layer 334 may be as described in the
context of the embodiments of FIG. 3C.
[0201] The oscillating ferromagnetic structure 340 further includes
a separating layer 336 arranged in between the ferromagnetic layer
332 and the ferromagnetic hard layer 334. The separating layer 336
may be as described in the context of the embodiments of FIG.
3C.
[0202] The oscillating ferromagnetic structure 340 may further
include an antiferromagnetic layer 342, where the ferromagnetic
hard layer 334 may be arranged over or stacked on top of the
antiferromagnetic layer 342, which pins the magnetization of the
ferromagnetic hard layer 334.
[0203] In various embodiments, the antiferromagnetic layer 342 may
include a material of X-manganese or X-Y-manganese, wherein each of
X and Y may be platinum (Pt), palladium (Pd), iridium (Ir), rhodium
(Rh) or ruthenium (Ru).
[0204] In various embodiments, the antiferromagnetic layer 342 may
have a thickness in a range of between about 5 nm and about 20 nm,
for example between about 5 nm and about 15 nm, between about 5 nm
and about 10 nm or between about 10 nm and about 20 nm.
[0205] In various embodiments, the physics of the switching process
lies in the spin transfer from a polarized electric current to a
ferromagnetic layer, and the spin from the polarized current may
exert a "torque" which acts to reverse the magnetization of the
ferromagnetic layer. The mechanism of the spin torque transfer may
be governed by the Slonczewski spin torque term or other
symmetric/asymmetric factor torque terms.
[0206] As the switching time may be strongly correlated to the
magnitude of the spin torque, it would be desirable to increase the
average spin torque magnitude during the magnetization reversal
process, and the magnetoresistive device of various embodiments may
achieve this effect in two ways. Firstly, the incorporation of one
or more oscillating ferromagnetic structures (e.g. 206a, 206b, 226)
act as another source of spin torque, in addition to the
ferromagnetic hard layer (e.g. 202, 222). Secondly, the oscillating
nature of the oscillating ferromagnetic structures may increase the
average angle between the magnetization vectors or orientations of
the oscillating ferromagnetic structures and the ferromagnetic soft
layers (e.g. 204a, 204b, 224), as compared to the use of a static
polarizer. The oscillating ferromagnetic structures may provide the
required oscillating polarizer dynamics which may assist in the
spin torque switching, for example in perpendicular anisotropic
magnetoresistive devices.
[0207] FIG. 4 shows different configurations of magnetization and
polarization during the switching process. As shown by the
configurations of FIG. 4 of magnetization vector {right arrow over
(m)}, as represented by the arrow 400, of a ferromagnetic soft
layer (free layer) (e.g. layer 206a, 206b, 226) with respect to the
vector {right arrow over (P)}, as represented by the arrow 402, of
a polarizer (e.g. layers 202, 222), the polarization {right arrow
over (P)} needs to oscillate in order to increase the spin torque
magnitude. The arrow as represented by 404 shows the direction the
polarizer should move in order to increase the spin torque
magnitude (i.e. the arrow 404 indicates the oscillation of the
polarization vector {right arrow over (P)} 402. The circle as
represented by 406 shows the result of the vector which is the
wedge product of the magnetization vector {right arrow over (m)}
400 and the polarization vector {right arrow over (P)} 402 (i.e.
{right arrow over (m)} {right arrow over (P)}). The vector 406 is
perpendicular to both the magnetization vector {right arrow over
(m)} 400 and the polarization vector {right arrow over (P)} 402.
The circle 406 including a dot in the centre represents a direction
pointing out of the page, while the circle 406 including a cross
within the circle 406 represents a direction pointing into the
page. The arrow as represented by 408 shows the result of the
operation ({right arrow over (m)} ({right arrow over (m)} {right
arrow over (P)})). The symbol " " represents the wedge product of
vectors.
[0208] The polarizer or polarization direction 402 denotes the spin
polarization of the electrons acquired from passing through a
preceding layer of a free layer or a ferromagnetic soft layer, or
from back scattering from a succeeding layer of a free layer or a
ferromagnetic soft layer, when considered from the perspective of
the path of flow of electrons. In the scheme of Frequency-Induced
Spin Torque Switching (FISTS) as described below, the polarization
may be due to, but not limited to, the oscillating ferromagnetic
structure (e.g. 206a, 206b, 226).
[0209] In various embodiments, by applying the appropriate
oscillating frequency to the polarizer so that the angle between
{right arrow over (m)} 400 and {right arrow over (P)} 402 may be
optimized in the switching process, increased switching speed may
be achieved, while reducing the current asymmetry of the
P.fwdarw.AP and AP.fwdarw.P processes. This scheme may be referred
to as Frequency-Induced Spin Torque Switching (FISTS), and which
forms the basis for the magnetoresistive devices of various
embodiments, for example as that illustrated in FIG. 2A, where the
first oscillating ferromagnetic structure 206a and the second
oscillating ferromagnetic structure 206b may provide the required
oscillating polarizer dynamics which may assist in the spin torque
switching, for example in perpendicular anisotropic
magnetoresistive devices. For implementing FISTS, it is desirable
that the first oscillating ferromagnetic structure 206a and the
second oscillating ferromagnetic structure 206b oscillate or
precess with a fixed direction (e.g. a fixed cone direction) for
both positive and negative writing currents, so that the spin
torque induced, generated or provided by the first oscillating
ferromagnetic structure 206a and the second oscillating
ferromagnetic structure 206b may aid in the switching of both
P.fwdarw.AP and AP.fwdarw.P for the ferromagnetic soft layers.
[0210] The switching performance of the magnetoresistive devices of
various embodiments may be investigated using macrospin
simulations, based on the Frequency-Induced Spin Torque Switching
(FISTS) scheme. One or more of the following parameters may be used
for the macrospin simulation: [0211] .gamma.:gyromagnetic
ratio=1.76.times.10.sup.11 T.sup.-1 s.sup.-1 (1.76.times.10.sup.7
Oe.sup.-1 s.sup.-1) [0212] M.sub.s: saturation
magnetization=0.7.times.10.sup.6 A/m (700 emu/cm.sup.3) [0213]
.alpha.: Gilbert damping constant=0.01 [0214] t: thickness=2 nm
[0215] P.sub.ref: Polarizer (or reference layer) polarization
factor=0.4 [0216] P.sub.osc: Oscillator (or oscillating
ferromagnetic structure) polarization factor=0.4 [0217] H.sub.k:
Free layer (or ferromagnetic soft layer) perpendicular anisotropic
field=1 T
[0218] It should be appreciated that in some measurements, one or
more of the above-mentioned parameters may be varied.
[0219] The results for magnetoresistive devices based on the
Frequency Selection Switching Scheme (FSSS) will now be described
by way of the following non-limiting examples.
[0220] FIG. 5 shows a plot 500 of macrospin simulation results of
the critical current for P.fwdarw.AP (denoted by 502) and
AP.fwdarw.P (denoted by 504) respectively when a 3 GHz oscillation
is applied, according to various embodiments. The ferromagnetic
soft layer is initially set to have a very small tilt of about
1.degree. from the perpendicular axis. As shown in FIG. 5, there is
a reduction in the critical current, J.sub.C, when a 3 GHz
oscillation is applied, for both J.sub.C.sup.P.sup..fwdarw..sup.AP
and |J.sub.C.sup.AP.sup..fwdarw..sup.P|. In addition, there is a
sharp reduction of switching current asymmetry when the additional
polarizer (e.g. oscillating ferromagnetic structure) is oscillating
close to resonance frequency. In the context of various
embodiments, the resonance frequency refers to the oscillating
frequency of the oscillating ferromagnetic structure at which the
corresponding ferromagnetic soft layer may switch with the smallest
critical current for a given pulse length and amplitude, or
equivalently may switch in the fastest time for a given current
pulse time width and amplitude. In various embodiments, the
resonance frequency may be but not limited to 3 GHz. The frequency
of 3 GHz may also represent the optimum magnetization switching
frequency of the corresponding ferromagnetic soft layer, which may
be similar to the resonance frequency of the ferromagnetic soft
layer.
[0221] As illustrated in FIG. 5, the oscillation frequency may be
tuned or selected so that the ferromagnetic soft layer may switch
optimally at its resonance frequency for both P.fwdarw.AP 502 and
AP.fwdarw.P 504 via FISTS. Therefore, through selection of the
frequency of the respective oscillation ferromagnetic structures,
the respective corresponding ferromagnetic soft layers may be
switched independently. Such a Frequency Selection Switching Scheme
(FSSS) may help to address any related overwriting issue that may
occur in multi-bit per cell STT-MRAM.
[0222] For the Frequency Selection Switching Scheme (FSSS) to be
employed for multi-bit per cell STT-MRAM, the frequency bandwidth
should be reasonably narrow so that the selection of the frequency
of the respective oscillation ferromagnetic structures for
switching the corresponding ferromagnetic soft layers may be
optimal. FIG. 6 shows a plot 600 of macrospin simulation results of
the relationship between the frequency bandwidth and the applied
current density for P.fwdarw.AP (denoted by 602) and AP.fwdarw.P
(denoted by 604) switchings, according to various embodiments, in a
single magnetoresistive (e.g. GMR or TMR) device. FIG. 6 shows that
by changing the applied current density, it may be possible to
control the frequency bandwidth for both P.fwdarw.AP 602 and
AP.fwdarw.P 604 switchings.
[0223] In various embodiments, besides controlling the frequency
bandwidth, it is also possible to adjust the resonance frequency of
the ferromagnetic soft layer over a wide range. This may be helpful
for magnetoresistive devices with a plurality of ferromagnetic soft
layers, where individual ferromagnetic soft layers may be switched
at different frequencies.
[0224] FIG. 7 shows a plot 700 of macrospin simulation results of
the relationship between the optimum magnetization switching
frequency, where switching occurs at the lowest current amplitude,
and the perpendicular anisotropy field, H.sub.k, for P.fwdarw.AP
(denoted by 702) and AP.fwdarw.P (denoted by 704) switchings,
according to various embodiments. H.sub.k has been varied from 0.95
T to 1.10 T, with an applied constant current density, J, of
approximately 1.0.times.10.sup.7 A/cm.sup.2, with a pulse width of
about 2 ns.
[0225] As shown in FIG. 7, the optimum magnetization switching
frequency has an at least substantially linear dependence on the
perpendicular magnetic anisotropy field, H.sub.k, for both
P.fwdarw.AP 702 and AP.fwdarw.P 704 switchings. The results
indicate that by having different values of H.sub.k for different
ferromagnetic soft layers, it may be possible to switch their
magnetizations at different frequencies.
[0226] In various embodiments, it may also be possible to adjust
the optimum magnetization switching frequency by changing the
saturation magnetization, M.sub.s, of the oscillating ferromagnetic
structure and/or thickness, t, of the oscillating ferromagnetic
structure.
[0227] In order to show the feasibility of using a selective
frequency to switch different ferromagnetic soft layers, H.sub.k
may be varied while fixing all other parameters. FIG. 8 shows a
plot 800 of macrospin simulation results of the relationship
between the switching time and the applied oscillation frequency
for P.fwdarw.AP and AP.fwdarw.P switchings for different
perpendicular anisotropy fields, H.sub.k, according to various
embodiments. The applied constant current density, J, is
approximately 1.0.times.10.sup.7 A/cm.sup.2, with a pulse width of
about 2 ns. The plot 800 shows the results for P.fwdarw.AP
switching for H.sub.k=0.95 T 802a, H.sub.k=1.01 T 804a and
H.sub.k=1.06 T 806a, and the results for AP.fwdarw.P switching for
H.sub.k=0.95 T 802b, H.sub.k=1.01 T 804b and H.sub.k=1.06 T 806b.
The results illustrate the separability of the ferromagnetic soft
layers' frequency bandwidths.
[0228] In various embodiments, independent switching of the
magnetizations of two ferromagnetic soft layers using two different
switching current amplitudes (e.g. J.sub.low and J.sub.high) may be
possible, where for example, the resonance frequency of one
ferromagnetic soft layer (e.g. free layer 1) is low, while the
resonance frequency of the other ferromagnetic soft layer (e.g.
free layer 2) is high, and that the resonance frequencies are
sufficiently far apart.
[0229] As a non-limiting example, when J.sub.low is applied, the
oscillating ferromagnetic structures have a low frequency, and
FISTS may be optimal for free layer 1, due to its low resonance
frequency, but not for free layer 2, and hence switching only
occurs for free layer 1. In a similar manner, when J.sub.high is
applied, the oscillating ferromagnetic structures have a high
frequency, and FISTS may be optimal for free layer 2, due to its
high resonance frequency, but not for free layer 1, and hence
switching only occurs for free layer 2. The frequencies need to be
sufficiently far apart so that the frequency bandwidths of the two
ferromagnetic soft layer s do not overlap at the operating current
amplitudes.
[0230] FIG. 9A shows plots of macrospin simulation results of the
respective perpendicular magnetization components, Mz, of two
ferromagnetic soft layers for a 2-bit STT-MRAM device, as a
function of time, according to various embodiments. The
perpendicular anisotropy field, H.sub.k, and the thickness, t, may
be varied for the two ferromagnetic soft layers, with all other
parameters kept constant, so as to change the optimum switching
frequencies. The parameters corresponding to one ferromagnetic soft
layer (e.g. free layer 1) are .alpha..sub.1=0.01, M.sub.S1=1000
emu/cm.sup.3, .gamma..sub.1=1.76.times.10.sup.7 Oe.sup.-1 s.sup.-1,
H.sub.k1=1.5 T, t.sub.1=1.2 nm, P.sub.ref=0.4, P.sub.osc1=0.4,
while the parameters corresponding to the other ferromagnetic soft
layer (e.g. free layer 2) are .alpha..sub.2=0.01, M.sub.S2=1000
emu/cm.sup.3, .gamma..sub.2=1.76.times.10.sup.7 Oe.sup.-1 s.sup.-1,
H.sub.k2=1.4 T, t.sub.2=1.5 nm, P.sub.ref=0.4, P.sub.osc2=0.4. The
applied constant current density, J.sub.low, is approximately
6.5.times.10.sup.6 A/cm.sup.2, with a pulse width of about 2
ns.
[0231] As shown in FIG. 9A, when J.sub.low, of approximately
6.5.times.10.sup.6 A/cm.sup.2 is applied for about 2 ns, the
switching of the magnetization of free layer 1 does not occur for
both P.fwdarw.AP 902a and AP.fwdarw.P 902b. On the other hand,
under the same conditions, magnetization switching for free layer 2
occurs for both P.fwdarw.AP 904a and AP.fwdarw.P 904b.
[0232] FIG. 9B shows plots of macrospin simulation results of the
respective perpendicular magnetization components, Mz, of two
ferromagnetic soft layers for a 2-bit STT-MRAM device, as a
function of time, according to various embodiments. The
perpendicular anisotropy field, H.sub.k, and the thickness, t, may
be varied for the two ferromagnetic soft layers, with all other
parameters kept constant, so as to change the optimum switching
frequencies. The parameters corresponding to one ferromagnetic soft
layer (e.g. free layer 1) are .alpha..sub.1=0.01, M.sub.S1=1000
emu/cm.sup.3, .gamma..sub.1=1.76.times.10.sup.7 Oe.sup.-1 s.sup.-1,
H.sub.k1=1.5 T, t.sub.1=1.2 nm, P.sub.ref=0.4, P.sub.osc1=0.4,
while the parameters corresponding to the other ferromagnetic soft
layer (e.g. free layer 2) are .alpha..sub.2=0.01, M.sub.S2=1000
emu/cm.sup.3, .gamma..sub.2=1.76.times.10.sup.7 Oe.sup.-1 s.sup.-1,
H.sub.k2=1.4 T, t.sub.2=1.5 nm, P.sub.ref=0.4, P.sub.osc2=0.4. The
applied constant current density, J.sub.high, is approximately
1.0.times.10.sup.7 A/cm.sup.2, with a pulse width of about 1.5
ns.
[0233] As shown in FIG. 9B, when J.sub.high, of approximately
1.0.times.10.sup.7 A/cm.sup.2 is applied for about 1.5 ns, the
switching of the magnetization of free layer 1 occurs for both
P.fwdarw.AP 912a and AP.fwdarw.P 912b. On the other hand, under the
same conditions, magnetization switching for free layer 2 does not
occur for both P.fwdarw.AP 914a and AP.fwdarw.P 914b.
[0234] The results of FIGS. 9A and 9B illustrate the possibility of
independent switching of magnetizations of two ferromagnetic soft
layers (or free layers) having different properties.
[0235] The results for magnetoresistive devices based on the
Oscillator Selection Switching Scheme (OSSS) will now be described
by way of the following non-limiting examples.
[0236] FIG. 10 shows a plot 1000 of an oscillator's frequency as a
function of applied current density. FIG. 10 illustrates that an
out-of-plane spin torque oscillator has two critical current
densities, and oscillations only occur within the range 1002 of
applied current densities between the two critical current
densities.
[0237] For the Oscillator Selection Switching Scheme (OSSS) to be
employed for multi-bit per cell STT-MRAM, the operating current
amplitudes may be chosen such that only one oscillator or
oscillating ferromagnetic structure is oscillating (hereinafter
referred as "ON"), while the remaining oscillators or oscillating
ferromagnetic structure are not oscillating (hereinafter referred
as "OFF"), e.g. the current amplitude applied is within the range
of operating current amplitudes of only one oscillator.
[0238] In various embodiments, the frequency and critical currents
of a spin torque oscillating ferromagnetic structure may be
controlled via intrinsic material parameters such as .alpha.
(Gilbert damping constant), M.sub.s (saturation magnetization),
H.sub.k (perpendicular anisotropy field) and t (thickness) of the
oscillating ferromagnetic structure. As a non-limiting example, in
embodiments of magnetoresistive devices having two oscillating
ferromagnetic structures, the two oscillating ferromagnetic
structures may be designed to have different critical currents, and
the resonance frequencies of the corresponding ferromagnetic soft
layers may also be designed and adjusted over a wide range. As
shown in FIG. 7, by having ferromagnetic soft layers with different
H.sub.k, the resonance frequencies of the ferromagnetic soft layers
may be tuned to match the frequencies of the corresponding
oscillating ferromagnetic structures.
[0239] FIGS. 11A and 11B show schematic cross-sectional views of
the magnetoresistive device 200 of FIG. 2A employing OSSS, in
operation, according to various embodiments, using the switching
current amplitudes, J.sub.low and J.sub.high, respectively. The
ferromagnetic soft layers are referred to as FLs (free layers) and
the oscillating ferromagnetic structures are referred to as OLs
(oscillating layers).
[0240] FIGS. 11A and 11B illustrate the independent switching of
the magnetizations of the two ferromagnetic soft layers using two
different switching current amplitudes (J.sub.low, and J.sub.high),
where one of the oscillating ferromagnetic structures is ON while
the other oscillating ferromagnetic structure is OFF.
[0241] As a non-limiting example, when J.sub.low is applied, OL1
206a may be ON and FISTS is in effect for FL1 204a, and the
magnetization orientation of FL1 204a switches within the duration
of the width of the current pulse of J.sub.low applied. At the same
time, OL2 206b is OFF as the applied current, J.sub.low, is too low
to induce oscillation in OL2 206b, and FISTS is not in effect for
FL2 204b, and its magnetization orientation therefore does not
switch.
[0242] When J.sub.high is applied, OL1 206a may be OFF as the
applied current, J.sub.high, is too high to induce oscillation in
OL1 206a and FISTS is not in effect for FL1 204a, and the
magnetization orientation of FL1 204a therefore does not switch
within the duration of the width of the current pulse of
J.sub.high, applied. At the same time, OL2 206b is ON and FISTS is
in effect for FL2 204b, and its magnetization orientation therefore
switches.
[0243] FIG. 12A shows plots of macrospin simulation results of the
respective perpendicular magnetization components, Mz, of two
ferromagnetic soft layers for a 2-bit STT-MRAM device, as a
function of time, according to various embodiments. The parameters
corresponding to both ferromagnetic soft layers (e.g. free layers 1
and 2) are .alpha.=0.01, M.sub.S=1100 emu/cm.sup.3,
.gamma.=1.76.times.10.sup.7 Oe.sup.-1 s.sup.-1, H.sub.k=1.6 T,
t=1.2 nm, P.sub.ref=0.4, P.sub.osc=0.4. The applied constant
current density, J.sub.low, is approximately 6.5.times.10.sup.6
A/cm.sup.2, with a pulse width of about 2 ns. The oscillating
ferromagnetic structures corresponding to free layer 1 and free
layer 2 are ON and OFF respectively, at this applied current
density, J.sub.low.
[0244] As shown in FIG. 12A, when J.sub.low, of approximately
6.5.times.10.sup.6 A/cm.sup.2 is applied for about 2 ns, the
switching of the magnetization orientation of free layer 1 occurs
for both P.fwdarw.AP 1202a and AP.fwdarw.P 1202b. On the other
hand, under the same conditions, magnetization switching for free
layer 2 does not occur for both P.fwdarw.AP 1204a and AP.fwdarw.P
1204b.
[0245] FIG. 12B shows plots of macrospin simulation results of the
respective perpendicular magnetization components, Mz, of two
ferromagnetic soft layers for a 2-bit STT-MRAM device, as a
function of time, according to various embodiments. The parameters
corresponding to both ferromagnetic soft layers (e.g. free layers 1
and 2) are .alpha.=0.01, M.sub.S=1100 emu/cm.sup.3,
.gamma.=1.76.times.10.sup.7 Oe.sup.-1 s.sup.-1, H.sub.k=1.6 T,
t=1.2 nm, P.sub.ref=0.4, P.sub.osc=0.4. The applied constant
current density, J.sub.high, is approximately 1.0.times.10.sup.7
A/cm.sup.2, with a pulse width of about 1.5 ns. The oscillating
ferromagnetic structures corresponding to free layer 1 and free
layer 2 are OFF and ON respectively, at this applied current
density, J.sub.high.
[0246] As shown in FIG. 12B, when J.sub.high, of approximately
1.0.times.10.sup.7 A/cm.sup.2 is applied for about 1.5 ns, the
switching of the magnetization orientation of free layer 1 does not
occur for both P.fwdarw.AP 1212a and AP.fwdarw.P 1212b. On the
other hand, under the same conditions, magnetization switching for
free layer 2 occurs for both P.fwdarw.AP 1214a and AP.fwdarw.P
1214b.
[0247] The results of FIGS. 12A and 12B illustrate the possibility
of independent switching of magnetizations of two ferromagnetic
soft layers (or free layers) where the corresponding oscillating
ferromagnetic structures have different properties. The two
ferromagnetic soft layers may be identical or at least
substantially similar, e.g. in terms of properties and
parameters.
[0248] Various embodiments may provide n-bit per cell STT-MRAMs
(e.g. n.gtoreq.1, e.g. 1, 2, 3, 4, 5, 6 or any higher number) based
on the Frequency Selection Switching Scheme (FSSS) or the
Oscillator Selection Switching Scheme (OSSS). The n-bit per cell
STT-MRAMs may be formed based on the magnetoresistive device 200
(FIG. 2A), being a 2-bit STT-MRAM (hereinafter referred as "S2"),
and/or the magnetoresistive device 220 (FIG. 2B), being a 1-bit
STT-MRAM (hereinafter referred as "S1"). Therefore, any n-bit per
cell STT-MRAM may be achieved by arranging or stacking an
appropriate number of S1 (e.g. magnetoresistive device 220) and/or
S2 (e.g. magnetoresistive device 200), one over the other in a
stack, so that there are n bits in the overall stack.
[0249] Adjacent stack arrangements of S1s and/or S2s (e.g. adjacent
S1/S1, S2/S2 and S1/S2) may be separated by a stack separating
layer of a conductive and non-magnetic material, including but not
limited to one or more of copper (Cu), silver (Ag), gold (Au),
tantalum (Ta), chromium (Cr), palladium (Pd), platinum (Pt) or
ruthenium (Ru). However, it should be appreciated that the stack
separating layer between adjacent stacks may alternatively be of a
non-conductive and non-magnetic material, including but not limited
to one or more of magnesium oxide (MgO), aluminium oxide
(AlO.sub.x), or titanium oxide (TiO.sub.x).
[0250] In various embodiments, the n-bit per cell STT-MRAM may be a
giant magnetoresistance (GMR) device, with all the separating
layers in the overall stack being of a conductive and non-magnetic
material.
[0251] FIG. 13A shows a schematic cross-sectional view of a
magnetoresistive device 1300, according to various embodiments,
illustrating a non-limiting example of a 3-bit spin transfer torque
magnetoresistive random access memory (STT-MRAM), for
implementation using the Frequency Selection Switching Scheme
(FSSS) or the Oscillator Selection Switching Scheme (OSSS).
[0252] The magnetoresistive device 1300 may be a giant
magnetoresistive (GMR) device or a tunneling magnetoresistive (TMR)
device, e.g. a spin transfer torque magnetic random access memory
(STT-MRAM) with perpendicular anisotropy. The magnetoresistive
device 1300 has a stack structure, having for example a plurality
of ferromagnetic layers.
[0253] As a non-limiting example, the magnetoresistive device 1300
may include the stack arrangement of the magnetoresistive device
200 (i.e. S2), which may be as described in the context of the
embodiments of FIG. 2A, to provide 2 bits per cell. An additional
stack arrangement of a ferromagnetic hard layer 222, a
ferromagnetic soft layer 224, and an oscillating ferromagnetic
structure 226, e.g. of the magnetoresistive device 220 (i.e. S1),
which may be as described in the context of the embodiments of FIG.
2B, may be arranged on top of the magnetoresistive device 200, to
provide an additional 1 bit (i.e. an S1 stacked on top of an S2).
The magnetoresistive device 1300 further includes a stack
separating layer 1302 of a conductive and non-magnetic material
(e.g. Cu) or a non-conductive and non-magnetic material (e.g. MgO).
In further embodiments, the magnetoresistive device 200 (i.e. S2)
may alternatively be arranged on top of the magnetoresistive device
220 (i.e. S1) (i.e. an S2 stacked on top of an S1).
[0254] FIG. 13B shows a schematic cross-sectional view of a
magnetoresistive device 1310, according to various embodiments,
illustrating a further non-limiting example of a 3-bit spin
transfer torque magnetoresistive random access memory (STT-MRAM),
for implementation using the Frequency Selection Switching Scheme
(FSSS) or the Oscillator Selection Switching Scheme (OSSS). The
magnetoresistive device 1310 is similar to the magnetoresistive
device 1300, except that the positions of the ferromagnetic hard
layer 222 and the oscillating ferromagnetic structure 226 are
interchanged. In further embodiments, the magnetoresistive device
200 (i.e. S2) may alternatively be arranged on top of the
magnetoresistive device 220 (i.e. S1).
[0255] In further embodiments, other stack configurations may be
provided to realise a
3-bit STT-MRAM, e.g. by arranging three magnetoresistive devices
220 (i.e. three S1s) one over the other.
[0256] FIG. 14 shows a schematic cross-sectional view of a
magnetoresistive device 1400, according to various embodiments,
illustrating a non-limiting example of a 4-bit spin transfer torque
magnetoresistive random access memory (STT-MRAM), for
implementation using the Frequency Selection Switching Scheme
(FSSS) or the Oscillator Selection Switching Scheme (OSSS).
[0257] The magnetoresistive device 1400 may be a giant
magnetoresistive (GMR) device or a tunneling magnetoresistive (TMR)
device, e.g. a spin transfer torque magnetic random access memory
(STT-MRAM) with perpendicular anisotropy. The magnetoresistive
device 1400 has a stack structure, having for example a plurality
of ferromagnetic layers.
[0258] As a non-limiting example, the magnetoresistive device 1400
may include the stack arrangement of the magnetoresistive device
200 (i.e. S2), which may be as described in the context of the
embodiments of FIG. 2A, to provide 2 bits per cell. An additional
stack arrangement of a magnetoresistive device 1401, may be
arranged on top of the magnetoresistive device 200, to provide
additional 2 bits (i.e. two S2 stacked over each other). The
magnetoresistive device 1401 includes a ferromagnetic hard layer
1402, two ferromagnetic soft layers 1404a, 1404b, arranged adjacent
to the ferromagnetic hard layer with respective separating layers
1408a, 1408b in between, and two oscillating ferromagnetic
structures 1406a, 1406b arranged adjacent to the corresponding
ferromagnetic soft layers 1404a, 1404b, with respective separating
layers 1410a, 1410b in between. The magnetoresistive device 1401
may be similar to the magnetoresistive device 200 as described in
the context of the embodiments of FIG. 2A. The magnetoresistive
device 1400 further includes a stack separating layer 1412 of a
conductive and non-magnetic material (e.g. Cu) or a non-conductive
and non-magnetic material (e.g. MgO). In further embodiments, the
magnetoresistive device 200 (i.e. S2) may alternatively be arranged
on top of the magnetoresistive device 1401 (i.e. S2).
[0259] In further embodiments, other stack configurations may be
provided to realise a
4-bit STT-MRAM, e.g. by arranging four magnetoresistive devices 220
(i.e. four S1s) one over the other, or arranging a magnetoresistive
device 200 or 1401 (i.e. one S2) and two magnetoresistive devices
220 (i.e. two S1s), one over the other.
[0260] In the context of various embodiments, the FSSS writing
scheme may help to switch the magnetization orientation of each
ferromagnetic soft layer, without at least substantially affecting
the remaining ferromagnetic soft layer(s). The FSSS writing scheme
is based on frequency selection for each ferromagnetic soft layer,
where the frequency may be close to or equal to a resonance
frequency of the ferromagnetic soft layer, which may be tailored
through the intrinsic properties of each ferromagnetic soft
layer.
[0261] In the context of various embodiments, the OSSS writing
scheme may help to switch the magnetization orientation of each
ferromagnetic soft layer, without at least substantially affecting
the remaining ferromagnetic soft layer(s). The OSSS writing scheme
is based on oscillator selection for each ferromagnetic soft layer,
for example using the property of critical currents to turn each
oscillator or oscillating ferromagnetic structure corresponding to
a ferromagnetic soft layer, ON or OFF independently.
[0262] While the invention has been particularly shown and
described with reference to specific embodiments, it should be
understood by those skilled in the art that various changes in form
and detail may be made therein without departing from the spirit
and scope of the invention as defined by the appended claims. The
elements of the various embodiments may be incorporated into each
of the other species to obtain the benefits of those elements in
combination with such other species, and the various beneficial
features may be employed in embodiments alone or in combination
with each other. The scope of the invention is thus indicated by
the appended claims and all changes which come within the meaning
and range of equivalency of the claims are therefore intended to be
embraced.
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