U.S. patent application number 17/004690 was filed with the patent office on 2022-03-03 for multiferroic-assisted voltage controlled magnetic anisotropy memory device and methods of manufacturing the same.
The applicant listed for this patent is WESTERN DIGITAL TECHNOLOGIES, INC.. Invention is credited to Alan KALITSOV, Bhagwati PRASAD, Neil SMITH.
Application Number | 20220069200 17/004690 |
Document ID | / |
Family ID | 1000006150098 |
Filed Date | 2022-03-03 |
United States Patent
Application |
20220069200 |
Kind Code |
A1 |
PRASAD; Bhagwati ; et
al. |
March 3, 2022 |
MULTIFERROIC-ASSISTED VOLTAGE CONTROLLED MAGNETIC ANISOTROPY MEMORY
DEVICE AND METHODS OF MANUFACTURING THE SAME
Abstract
A magnetic memory device includes a first electrode, a second
electrode, and a layer stack located between the first electrode
and the second electrode. The layer stack includes a reference
layer, a tunnel barrier layer, a free layer, and a magnetoelectric
multiferroic layer including at least one crystalline grain. The
magnetization of the magnetoelectric multiferroic layer may be
axial, canted, or in-plane. For axial or canted magnetization of
the magnetoelectric multiferroic layer, a deterministic switching
of the free layer may be achieved through coupling with the axial
component of magnetization of the magnetoelectric multiferroic
layer. Alternatively, the in-plane magnetization of the
magnetoelectric multiferroic layer may be employed to induce
precession of the magnetization angle of the free layer.
Inventors: |
PRASAD; Bhagwati; (San Jose,
CA) ; KALITSOV; Alan; (San Jose, CA) ; SMITH;
Neil; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
WESTERN DIGITAL TECHNOLOGIES, INC., |
San Jose |
CA |
US |
|
|
Family ID: |
1000006150098 |
Appl. No.: |
17/004690 |
Filed: |
August 27, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 43/10 20130101;
G11C 11/1673 20130101; G11C 11/161 20130101; H01L 43/02 20130101;
H01L 27/222 20130101; G11C 11/1675 20130101 |
International
Class: |
H01L 43/02 20060101
H01L043/02; H01L 27/22 20060101 H01L027/22; H01L 43/10 20060101
H01L043/10; G11C 11/16 20060101 G11C011/16 |
Claims
1. A magnetic memory device, comprising: a first electrode; a
second electrode; and a layer stack located between the first
electrode and the second electrode and comprising, from one side to
another, a reference layer, a tunnel barrier layer, a free layer,
and a voltage controlled magnetic anisotropy (VCMA) assist
structure having an in-plane magnetization direction.
2. The magnetic memory device of claim 1, wherein the VCMA assist
structure comprises a magnetoelectric multiferroic layer including
at least one crystalline grain having an easy axis of magnetization
that is parallel to an interface between the free layer and the
tunnel barrier layer, and a net in-plane magnetization of the
magnetoelectric multiferroic layer is non-zero.
3. The magnetic memory device of claim 2, wherein the
magnetoelectric multiferroic layer has a net ferroelectric
polarization that is parallel to the interface between the free
layer and the tunnel barrier layer.
4. The magnetic memory device of claim 2, wherein the
magnetoelectric multiferroic layer is polycrystalline, wherein a
predominant subset in volume of crystalline grains within the
magnetoelectric multiferroic layer has a respective magnetization
direction that is aligned to, or has an angle less than 145 degrees
from a direction of the net in-plane magnetization of the
magnetoelectric multiferroic layer.
5. The magnetic memory device of claim 4, wherein the net in-plane
magnetization of the magnetoelectric multiferroic layer is parallel
to or is antiparallel to the direction of the net in-plane
magnetization of the magnetoelectric multiferroic layer.
6. The magnetic memory device of claim 1, wherein the
magnetoelectric multiferroic layer comprises a material selected
from BiFeO.sub.3, h-YMnO.sub.3, BaNiF.sub.4, PbVO.sub.3,
BiMnO.sub.3, LuFe.sub.2O.sub.4, HoMn.sub.2O.sub.5, h-HoMnO.sub.3,
h-ScMnO.sub.3, h-ErMnO.sub.3, h-TmMnO.sub.3, h-YbMnO.sub.3,
h-LuMnO.sub.3, K.sub.2SeO.sub.4, Cs.sub.2CdI.sub.4, TbMnO.sub.3,
Ni.sub.3V.sub.2O.sub.8, MnWO.sub.4, CuO, ZnCr.sub.2Se.sub.4,
LiCu.sub.2O.sub.2, and Ni.sub.3B.sub.7O.sub.13I.
7. The magnetic memory device of claim 6, wherein the
magnetoelectric multiferroic layer comprises a BiFeO.sub.3 layer
having a (111) preferred grain orientation and a thickness of 1 nm
or less.
8. The magnetic memory device of claim 1, wherein the VCMA assist
structure comprises a ferromagnetic layer having an in-plane
magnetization direction and an electrically conductive nonmagnetic
spacer layer located between the free layer and the ferromagnetic
layer.
9. The magnetic memory device of claim 1, wherein the VCMA assist
structure comprises a ferrimagnetic layer having an in-plane
magnetization direction and an electrically conductive nonmagnetic
spacer layer located between the free layer and the ferrimagnetic
layer.
10. The magnetic memory device of claim 1, wherein the layer stack
further comprises a nonmagnetic capping layer including a
nonmagnetic metal contacting the magnetoelectric multiferroic layer
and one of the electrode and the second electrode.
11. The magnetic memory device of claim 1, wherein the layer stack
further comprises a SAF structure contacting the reference
layer.
12. The magnetic memory device of claim 1, wherein the tunnel
barrier layer comprises a dielectric layer having a thickness of 1
nm to 2 nm.
13. The magnetic memory device of claim 1, further comprising a
sensing circuit configured to determine tunneling magnetoresistance
of the layer stack between the first electrode and the second
electrode.
14. The magnetic memory device of claim 13, further comprising
programming circuit connected to the first electrode and the second
electrode and configured to non-deterministically program the
magnetic memory device using non-deterministic VCMA programming
15. The magnetic memory device of claim 14, wherein the programming
circuit is configured to: to determine whether a memory state of
the magnetic memory device is in a target state or not by comparing
a measured value of the tunneling magnetoresistance of the layer
stack to a target value; and to apply a programming voltage pulse
that induces precession of an angle between a magnetization
direction of the free layer and an axial direction that is
perpendicular to the interface between the free layer and the
tunnel barrier layer if the magnetic memory device is not in the
target state, and not to apply any programming voltage pulse if the
magnetic memory device is in the target state.
16. The magnetic memory device of claim 15, wherein the in-plane
magnetization of the VCMA assist structure induces precession of
the angle between the magnetization direction of the free layer and
the axial direction between 0 degree and 180 degrees upon
application of the programming voltage pulse without applying an
external magnetic field.
17. A method of operating the magnetic memory device of claim 1,
comprising determining tunneling magnetoresistance of the layer
stack.
18. The method of claim 17, further comprising
non-deterministically programming the magnetic memory device
without applying an external magnetic field by applying a VCMA
programming voltage pulse between the first electrode and the
second electrode.
19. The method of claim 18, further comprising: determining whether
a memory state of the magnetic memory device is in a target state
or not by comparing a measured value of the tunneling
magnetoresistance of the layer stack to a target value; and
applying the VCMA programming voltage pulse that induces a
precession of an angle between a magnetization direction of the
free layer and an axial direction that is perpendicular to the
interface between the free layer and the tunnel barrier layer if
the magnetic memory device is not in the target state, and not
applying any programming voltage pulse if the magnetic memory
device is in the target state.
20. The method of claim 19, wherein a duration of the programming
voltage pulse is selected such that the angle between the
magnetization direction and the axial direction changes by more
than 135 degrees at an end of the programming voltage pulse.
Description
FIELD
[0001] The present disclosure relates generally to the field of
magnetic (e.g., spintronic) memory devices, and specifically to a
magnetoresistive random access memory ("MRAM") device including a
multiferroic layer and methods of manufacturing the same.
BACKGROUND
[0002] A magnetoresistive memory device can store information
employing the difference in electrical resistance of a first
configuration in which a ferromagnetic free layer has a
magnetization direction that is parallel to the magnetization of a
ferromagnetic reference layer and a second configuration in which
the free layer has a magnetization direction that is antiparallel
to the magnetization of the reference layer. Programming of the
magnetoresistive memory device requires flipping of the direction
of the magnetization of the free layer employing various external
power sources, which may be magnetic in nature or may employ a spin
transfer mechanism.
[0003] Voltage controlled magnetic anisotropy (VCMA) refers to an
effect in which the perpendicular magnetic anisotropy of the free
layer has a first-order dependence on the externally applied
voltage across the free layer located in a magnetic tunnel junction
which includes a dielectric tunnel barrier layer between the free
layer and a ferromagnetic reference layer. The thickness of the
dielectric tunnel barrier layer may be at least 1 nm, which reduces
tunneling current flow through the dielectric tunnel barrier layer
below a critical current required to switch the magnetization
direction of the free layer during programming. Therefore, an
applied voltage is used to switch the magnetization direction of
the free layer. The applied voltage lowers the energy perpendicular
magnetic anisotropy in one bias direction and raises it for the
other bias direction. However, the VCMA programming is non
deterministic and requires precise control of the timing of the
applied voltage pulses to obtain a desired magnetization direction
of the free layer.
SUMMARY
[0004] According to an aspect of the present disclosure, a magnetic
memory device is provided, which comprises: a first electrode; a
second electrode; and a layer stack located between the first
electrode and the second electrode and comprising, from one side to
another, a reference layer, a tunnel barrier layer, a free layer,
and a magnetoelectric multiferroic layer including at least one
crystalline grain having an easy axis of magnetization along an
axial direction or along a first tilted direction having a first
tilt angle less than 90 degrees relative to the axial direction,
the axial direction being perpendicular to an interface between the
free layer and the tunnel barrier layer.
[0005] According to another aspect of the present disclosure a
method of programming the magnetic memory device includes applying
a first programming voltage pattern comprising a first positive
voltage having a magnitude greater than a coercive voltage
sufficient to switch a magnetization direction of the
magnetoelectric multiferroic layer, and applying a second
programming voltage pattern comprising a negative voltage having a
magnitude greater than the coercive voltage followed by a second
positive voltage having a magnitude less than the coercive
voltage.
[0006] In one embodiment, the first positive voltage
deterministically programs the magnetization direction of the
magnetoelectric multiferroic layer to be parallel to a
magnetization direction of the reference layer, and the first
positive voltage has a sufficient magnitude to lower a
perpendicular magnetic anisotropy of the free layer that allows the
free layer to switch its magnetization direction to be parallel to
the magnetization direction of the reference layer. The negative
voltage deterministically programs the magnetization direction of
the magnetoelectric multiferroic layer to be antiparallel to the
magnetization direction of the reference layer, and the negative
voltage increases the perpendicular magnetic anisotropy of the free
layer such that the free layer cannot switch its magnetization
direction. The second positive voltage does not have a sufficient
magnitude to switch the magnetization direction of the
magnetoelectric multiferroic layer, and the second positive voltage
has a sufficient magnitude to lower the perpendicular magnetic
anisotropy of free layer that allows the free layer to
deterministically switch its magnetization direction to be
antiparallel to magnetization direction of the reference layer.
[0007] According to yet another aspect of the present disclosure, a
magnetic memory device comprises a first electrode, a second
electrode, and a layer stack located between the first electrode
and the second electrode and comprising, from one side to another,
a reference layer, a tunnel barrier layer, a free layer, and a
voltage controlled magnetic anisotropy (VCMA) assist structure
having an in-plane magnetization direction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic diagram of a random access array of
magnetic tunnel junction devices according to an embodiment of the
present disclosure.
[0009] FIG. 2A is a vertical cross-sectional view of a first
exemplary structure after formation according to a first embodiment
of the present disclosure.
[0010] FIG. 2B is a vertical cross-sectional view of an alternative
embodiment of the first exemplary structure according to the first
embodiment of the present disclosure.
[0011] FIG. 3A is a perspective view of a unit cell of BiFeO.sub.3
with ferroelectric polarization and magnetic moment
[0012] FIG. 3B illustrates the relative spatial orientation between
the ferroelectric polarization and the magnetic moment directions
of the unit cell of BiFeO.sub.3 of FIG. 3A.
[0013] FIG. 4A is a vertical cross-sectional view of the first
exemplary structure in a first programmed state according to the
first embodiment of the present disclosure.
[0014] FIG. 4B is a vertical cross-sectional view of the first
exemplary structure in a second programmed state according to the
first embodiment of the present disclosure.
[0015] FIG. 5 is a diagram illustrating changes in the axial
ferroelectric polarization of the magnetoelectric multiferroic
layer according to the first embodiment of the present
disclosure.
[0016] FIG. 6A is a diagram illustrating a first voltage pulse
pattern for programming the magnetic memory cell of the first
exemplary structure into a first magnetic state according to an
aspect of the present disclosure.
[0017] FIG. 6B is a diagram illustrating a second voltage pulse
pattern for programming the magnetic memory cell of the first
exemplary structure into a second magnetic state according to an
aspect of the present disclosure.
[0018] FIG. 7A is a vertical cross-sectional view of a second
exemplary structure after formation according to a first embodiment
of the present disclosure.
[0019] FIG. 7B is a vertical cross-sectional view of an alternative
embodiment of the second exemplary structure according to the first
embodiment of the present disclosure.
[0020] FIG. 8 is a vertical cross-sectional view of another
alternative embodiment of the second exemplary structure according
to the second embodiment of the present disclosure.
[0021] FIG. 9A is a vertical cross-sectional view of a second
exemplary structure in a first magnetic state according to a second
embodiment of the present disclosure.
[0022] FIG. 9B is a vertical cross-sectional view of the second
exemplary structure in a second magnetic state according to the
second embodiment of the present disclosure.
[0023] FIG. 10A is a diagram of a sensing voltage pattern for
sensing a magnetic state of the magnetic memory device of the
second exemplary structure according to the second embodiment of
the present disclosure.
[0024] FIG. 10B is a diagram of a programming voltage pattern for
programming a magnetic state of the magnetic memory device of the
second exemplary structure according to the second embodiment of
the present disclosure.
DETAILED DESCRIPTION
[0025] Embodiments of the present disclosure are directed to a
voltage controlled magnetic anisotropy memory device containing a
magnetoelectric multiferroic layer and methods of operating the
same, the various aspects of which are described in detail
herebelow. The magnetoelectric multiferroic layer provides a
deterministic VCMA programming mechanism (e.g., where the
magnetization direction of the free layer is not dependent on the
duration of the programming voltage pulse), and/or provides a
precisely controlled, in-plane assist magnetic field that is
determined by the crystalline characteristics of the
magnetoelectric multiferroic layer.
[0026] The drawings are not drawn to scale. Multiple instances of
an element may be duplicated where a single instance of the element
is illustrated, unless absence of duplication of elements is
expressly described or clearly indicated otherwise. Ordinals such
as "first," "second," and "third" are employed merely to identify
similar elements, and different ordinals may be employed across the
specification and the claims of the instant disclosure. The term
"at least one" element refers to all possibilities including the
possibility of a single element and the possibility of multiple
elements. The same reference numerals refer to the same element or
similar element. Unless otherwise indicated, elements having the
same reference numerals are presumed to have the same composition
and the same function. Unless otherwise indicated, a "contact"
between elements refers to a direct contact between elements that
provides an edge or a surface shared by the elements. If two or
more elements are not in direct contact with each other or among
one another, the two elements are "disjoined from" each other or
"disjoined among" one another. As used herein, a first element
located "on" a second element can be located on the exterior side
of a surface of the second element or on the interior side of the
second element. As used herein, a first element is located
"directly on" a second element if there exist a physical contact
between a surface of the first element and a surface of the second
element. As used herein, a first element is "electrically connected
to" a second element if there exists a conductive path consisting
of at least one conductive material between the first element and
the second element. As used herein, a "prototype" structure or an
"in-process" structure refers to a transient structure that is
subsequently modified in the shape or composition of at least one
component therein.
[0027] As used herein, a "layer" refers to a material portion
including a region having a thickness. A layer may extend over the
entirety of an underlying or overlying structure, or may have an
extent less than the extent of an underlying or overlying
structure. Further, a layer may be a region of a homogeneous or
inhomogeneous continuous structure that has a thickness less than
the thickness of the continuous structure. For example, a layer may
be located between any pair of horizontal planes between, or at, a
top surface and a bottom surface of the continuous structure. A
layer may extend horizontally, vertically, and/or along a tapered
surface. A substrate may be a layer, may include one or more layers
therein, or may have one or more layer thereupon, thereabove,
and/or therebelow. As used herein, a "layer stack" refers to a
stack of layers. As used herein, a "line" or a "line structure"
refers to a layer that has a predominant direction of extension,
i.e., having a direction along which the layer extends the most. As
used herein, a "ferroelectric material" refers to any material that
exhibits spontaneous ferroelectric polarization that can be
reversed by the application of an external electric field (e.g.,
exhibits ferroelectricity). As used herein, a "multiferroic"
material refers to a material that exhibits at least two ferroic
orders, such as magnetic (ferromagnetism, antiferromagnetism, or
ferrimagnetism) and ferroelectric. As used herein, a
"magnetoelectric multiferroic" refers to a material that exhibits a
ferromagnetic-type order and ferroelectricity. A change in total
magnetization is coupled to a change in total ferroelectric
polarization in a magnetoelectric multiferroic material, and thus,
a change in a magnetic moment direction of the material can be
coupled to a change in the ferroelectric polarization direction and
vice versa.
[0028] FIG. 1 is a schematic diagram of a random access memory
device 501 of magnetic tunnel junction devices 180 according to an
embodiment of the present disclosure. As used herein, a "random
access memory device" refers to a memory device including memory
cells that allow random access, i.e., access to any selected memory
cell upon a command for reading the contents of the selected memory
cell.
[0029] The random access memory device 501 of the embodiments of
the present disclosure may comprise an MRAM device, such as a
multistate STT-type MRAM device containing multiferroic portions.
The device 501 includes a memory array region 550 containing an
array of the respective magnetic devices, such as magnetic tunnel
junction devices (e.g., magnetoresistive memory cells) 180 located
at intersections of word lines (which may comprise first
electrically conductive lines 30 as illustrated or as second
electrically conductive lines 90 in an alternate configuration) and
bit lines (which may comprise second electrically conductive lines
90 as illustrated or as first electrically conductive lines 30 in
an alternate configuration). For example, the first electrically
conductive lines 30 may be electrically connected to, and/or may
comprise, bottom electrodes of a respective row of magnetic tunnel
junction devices 180 in the memory array region 550, while the
second electrically conductive lines 90 may be electrically
connected to, and/or may comprise, top electrodes of a respective
column of magnetic tunnel junction devices 180 in the memory array
region 550.
[0030] The random access memory device 501 may also contain a row
decoder 560 connected to the word lines, a sensing and programming
circuit 570 (which may include sense amplifiers, programming
transistors, and control circuits) connected to the bit lines, a
column decoder 580 connected to the bit lines, and a data buffer
590 connected to the sense circuitry. The magnetic tunnel junction
devices 180 are provided in an array configuration that forms the
random access memory device 501. In one embodiment, the magnetic
tunnel junction devices 180 may be provided as a rectangular array.
As such, each of the magnetic tunnel junction devices 180 can be a
two-terminal device including a respective first electrode and a
respective second electrode. It should be noted that the location
and interconnection of elements are schematic, and the elements may
be arranged in a different configuration. Further, a magnetic
tunnel junction device 180 may be manufactured as a discrete
device, i.e., a single isolated device.
[0031] The random access configuration illustrated in the random
access memory device 501 is only exemplary, and the magnetic tunnel
junction devices 180 of the embodiments of the present disclosure
can be connected in different interconnection configurations.
[0032] Referring to FIGS. 2A and 2B, exemplary configurations of a
magnetic tunnel junction device 180 are illustrated in first
exemplary structures. The configuration of FIG. 2A and the
configuration of FIG. 2B are alternative configurations that may be
derived from each other by reversing the order of material
deposition during formation of a magnetic tunnel junction device
180, which is a magnetic memory device.
[0033] The first exemplary structure includes an optional
insulating material layer 110 that includes an insulating material
such as undoped silicate glass, a doped silicate glass,
organosilicate glass, silicon nitride, a dielectric metal oxide, or
a combination thereof. In one embodiment, the insulating material
layer 110 comprises an insulating substrate, such as a ceramic or a
glass substrate. In another embodiment, the insulating material
layer 110 can be provided over a semiconductor substrate (not
shown) with semiconductor devices (not shown) such as field effect
transistors thereupon. In this case, the insulating material layer
110 may include a plurality of interconnect-level dielectric
material layers embedding metal interconnect structures therein.
The metal interconnect structure can provide electrical connection
between the semiconductor devices and to the first electrically
conductive lines 30 and the second electrically conductive lines 90
that are formed above the insulating material layer 110. In this
case, structural elements that are formed above the insulating
material layer 110 may be embedded within a dielectric matrix (not
expressly shown) that embeds the first electrically conductive
lines 30 and the second electrically conductive lines.
[0034] First electrically conductive lines 30 can be formed over
the insulating material layer 110. The first electrically
conductive lines 30 may be formed over the top surface of the
insulating material layer 110 by depositing and patterning at least
one metallic layer and patterning the at least one metallic layer
into line-shaped structures that laterally extend along a first
horizontal direction. In one embodiment, the at least one metallic
layer may include a stack of a metallic barrier layer including a
conductive metallic nitride and a high-electrical-conductivity
metal layer such a copper layer or a tungsten layer. Alternatively,
line cavities may be formed in an upper portion of the insulating
material layer 110, and the first electrically conductive lines 30
may be formed by a damascene method in which at least one metallic
material is deposited in the line cavities, and excess portions of
the at least one metallic material is removed from above the
horizontal plane including the top surface of the insulating
material layer 110. Each portion of a first electrically conductive
line 30 that contacts an overlying magnetic tunnel junction device
180 comprises a first electrode of the magnetic tunnel junction
device 180, which is a magnetic memory device.
[0035] A continuous layer stack can be deposited over the first
electrically conductive lines 30. The continuous layer stack
includes, from bottom to top or from top to bottom, an optional
continuous synthetic antiferromagnetic (SAF) structure, a
continuous ferromagnetic reference layer, a continuous tunnel
barrier layer, a continuous ferromagnetic free layer, a continuous
magnetoelectric multiferroic layer, and an optional continuous
nonmagnetic capping layer.
[0036] The optional SAF structure may include a superlattice
structure including an alternating sequence of ferromagnetic
material layers and conductive nonmagnetic material layers. In an
illustrative example, the superlattice structure may include
[X/Q].sub.n, in which X represents a ferromagnetic material layer,
such as Co, CoFe, Fe, or CoFeB layer, Q represents a nonmagnetic
material layer, such as Pt or Pd layer, and n represents the total
number of repetitions of a bilayer stack of the ferromagnetic
material layer and the nonmagnetic material layer. The total number
n of repetitions may be in a range from 2 to 20, such as from 3 to
8, although a greater number may also be employed.
[0037] The continuous reference layer includes a ferromagnetic
material having perpendicular magnetic anisotropy. The continuous
reference layer includes a ferromagnetic material such as CoFe or
CoFeB. The continuous reference layer may be deposited, for
example, by physical vapor deposition, and may have a thickness in
a range from 1 nm to 3 nm, although lesser and greater thicknesses
may also be employed.
[0038] The continuous tunnel barrier layer includes a tunneling
dielectric material such as MgO. The continuous tunnel barrier
layer may be deposited, for example, by physical vapor deposition.
The thickness of the continuous tunnel barrier layer may be in a
range from 1 nm to 2 nm, such as 1.5 nm to 2 nm, although lesser
and greater thicknesses may also be employed.
[0039] The continuous free layer includes a ferromagnetic material
having perpendicular magnetic anisotropy. The continuous free layer
includes a ferromagnetic material such as CoFe or CoFeB. The
continuous free layer may be deposited, for example, by physical
vapor deposition, and may have a thickness in a range from 0.6 nm
to 2 nm, although lesser and greater thicknesses may also be
employed.
[0040] The continuous magnetoelectric multiferroic layer comprises
a magnetoelectric multiferroic material. As used herein, a
"multiferroic" refers to a material that exhibits at least two
ferroic orders (such as ferromagnetism and ferroelectricity) As
used herein, a "magnetoelectric multiferroic" refers to a material
that exhibits a ferromagnetic-type order and ferroelectricity. A
change in total magnetization is coupled to a change in total
electric polarization in a magnetoelectric multiferroic, and thus,
a magnetic transition can be coupled to a change in the
ferroelectric polarization and vice versa.
[0041] The continuous magnetoelectric multiferroic layer may
comprise any polycrystalline multiferroic material or a single
crystalline multiferroic material that can generate a non-zero net
magnetization either as deposited, or upon application of an
initializing magnetic field or an initializing electric field. In
an illustrative example, the continuous magnetoelectric
multiferroic layer may comprise a material selected from
BiFeO.sub.3, h-YMnO.sub.3, BaNiF.sub.4, PbVO.sub.3, BiMnO.sub.3,
LuFe.sub.2O.sub.4, HoMn.sub.2O.sub.5, h-HoMnO.sub.3, h-ScMnO.sub.3,
h-ErMnO.sub.3, h-TmMnO.sub.3, h-YbMnO.sub.3, h-LuMnO.sub.3,
K.sub.2SeO.sub.4, Cs.sub.2CdI.sub.4, TbMnO.sub.3,
Ni.sub.3V.sub.2O.sub.8, MnWO.sub.4, CuO, ZnCr.sub.2Se.sub.4,
LiCu.sub.2O.sub.2, and Ni.sub.3B.sub.7O.sub.13I. The continuous
magnetoelectric multiferroic layer may be deposited by a suitable
deposition method, such as physical vapor deposition. The thickness
of the continuous magnetoelectric multiferroic layer can be 1 nm or
less, such as in a range from 0.3 nm to 1 nm, such as from 0.5 nm
to 0.8 nm, although lesser and greater thicknesses can also be
employed.
[0042] In one embodiment, the polarization and magnetization
directions of the continuous magnetoelectric multiferroic layer are
orthogonal to each other. The continuous magnetoelectric
multiferroic layer includes at least one crystalline grain having
an easy axis of magnetization along an axial direction or along a
first tilted (i.e., canted) direction having a first tilt angle
less than 90 degrees relative to the axial direction. The axial
direction is perpendicular to the interface between the continuous
free layer and the continuous tunnel barrier layer. In one
embodiment, the continuous magnetoelectric multiferroic layer
includes a plurality of crystalline grains having an easy axis of
magnetization along the axial direction or along a respective first
tilted direction having a respective first tilt angle less than 90
degrees relative to the axial direction.
[0043] Generally, the magnetization direction in the crystalline
grains of the continuous magnetoelectric multiferroic layer may
depend on the preferred grain orientation and/or impurity doping
within the magnetoelectric multiferroic layer. If the
magnetoelectric multiferroic material comprises BiFeO.sub.3, then a
polycrystalline BiFeO.sub.3 layer having a preferred grain
orientation along the (110) plane can provide a predominantly axial
magnetization direction, i.e., more than 50% all crystalline grains
can have a respective magnetization direction (e.g., spin
orientation) that is perpendicular to the interface between the
continuous tunnel barrier layer and the continuous free layer. A
polycrystalline BiFeO.sub.3 layer having a preferred grain
orientation along the (100) plane can provide a predominantly
canted magnetization direction along the <111> direction,
i.e., more than 50% all crystalline grains can have a respective
magnetization direction (e.g., spin orientation) that is not
perpendicular to, and not parallel to, the interface between the
continuous tunnel barrier layer and the continuous free layer. In
contrast, a polycrystalline BiFeO3 layer having the preferred grain
orientation along the (100) plane but which is doped with La on the
Bi lattice sites can provide a predominantly axial magnetization
direction, i.e., more than 50% all crystalline grains can have a
respective magnetization direction (e.g., spin orientation) that is
perpendicular to the interface between the continuous tunnel
barrier layer and the continuous free layer. A polycrystalline
BiFeO3 layer having a preferred grain orientation along the (111)
plane can provide predominantly in-plane magnetization direction,
i.e., more than 50% all crystalline grains can have a respective
magnetization direction (e.g., spin orientation) that is parallel
to the interface between the continuous tunnel barrier layer and
the continuous free layer.
[0044] The optional continuous nonmagnetic capping layer includes a
nonmagnetic metal that is resistant to oxidation and/or diffusion.
In one embodiment, the continuous nonmagnetic capping layer may
include a metal having a melting point higher than 1,500 degrees
Celsius. For example, the metal of the continuous nonmagnetic
capping layer may include ruthenium, tantalum, platinum or gold.
The continuous nonmagnetic capping layer may be deposited by a
physical vapor deposition. The thickness of the continuous
nonmagnetic capping layer may be in a range from 0.5 nm to 2 nm,
although lesser and greater thicknesses may also be employed.
[0045] The continuous layer stack can be patterned into a
two-dimensional array of pillar structures. The patterning can be
performed using ion beam milling and/or photolithography and
etching. Each of the pillar structures may include a magnetic
tunnel junction device 180, which is a magnetic memory device
(e.g., an MRAM cell).
[0046] In each pillar structure (e.g., in each magnetic tunnel
junction device 180 which comprises an MRAM cell), each patterned
portion of the continuous SAF structure comprises a SAF structure
120. Each patterned portion of the continuous reference layer
comprises a reference layer 132. If the SAF structure 120 includes
the superlattice, then the reference layer 132 may contact the
nonmagnetic layer of the SAF structure superlattice. Each patterned
portion of the continuous tunnel barrier layer comprises a tunnel
barrier layer (i.e., tunneling dielectric layer) 134. Each
patterned portion of the continuous free layer comprises a free
layer 136. Each patterned portion of the continuous magnetoelectric
multiferroic layer comprises a magnetoelectric multiferroic layer
140.
[0047] A dielectric matrix layer 190 may be deposited over and
around the array of magnetic tunnel junction devices 180 (i.e.,
around the pillar structures). The dielectric matrix layer 190
includes a dielectric material such as silicon nitride, silicon
oxide, organosilicate glass, and/or a dielectric metal oxide. In
one embodiment, the dielectric matrix layer 190 may include a
dielectric diffusion liner (such as a silicon nitride liner) and a
dielectric fill material (such as silicon oxide). The dielectric
matrix layer 190 may be planarized to provide a horizontal top
surface above a horizontal plane including the top surfaces of the
magnetic tunnel junction devices 180. For example, chemical
mechanical planarization (CMP) may be employed to planarize the top
surface of the dielectric matrix layer 190.
[0048] Line cavities that laterally extend along a second
horizontal direction can be formed in an upper portion of the
dielectric matrix layer 190. The second horizontal direction is
different from the first horizontal direction, and may be
perpendicular to the first horizontal direction. At least one
conductive material can be deposited in the line cavities, and
excess portions of the at least one conductive material can be
removed from above the horizontal plane including the top surface
of the dielectric matrix layer 190. Each remaining portion of the
at least one conductive material filling the line cavities
comprises a second electrically conductive line 90. Each second
electrically conductive line 90 can comprise at least one second
electrode of the two-dimensional array of magnetic tunnel junction
devices 180. Alternatively, the second electrically conductive
lines 90 may be formed on each of the pillar structures followed by
forming the dielectric matrix layer 190 over the second
electrically conductive lines 90.
[0049] In one embodiment, each first electrically conductive line
30 can contact a respective row of magnetic tunnel junction devices
180, and each second electrically conductive line 90 can contact a
respective column of magnetic tunnel junction devices 180. In this
case, each first electrically conductive line 30 can comprise a row
of first electrodes, and each second electrically conductive line
90 can comprise a column of second electrodes.
[0050] In one embodiment, a predominant subset in volume of the
crystalline grains (i.e., a set of crystalline grains that occupy
more than 50% of the entire volume of each magnetoelectric
multiferroic layer 140) within the magnetoelectric multiferroic
layers 140 can have an easy axis of magnetization along an axial
direction or along a first tilted direction having a first tilt
angle less than 90 degrees relative to the axial direction (such as
the <111> direction). The axial direction is perpendicular to
each interface between the free layer 136 and the tunnel barrier
layer 134 within each magnetic tunnel junction device 180. In one
embodiment, each magnetoelectric multiferroic layer 140 can have a
respective easy axis of ferroelectric polarization along the axial
direction or along a second tilted direction having a second tilt
angle less than 90 degrees relative to the axial direction. The
second direction of each crystalline grain may, or may not, be
parallel or antiparallel to the first direction of the respective
crystalline grain. Each magnetoelectric multiferroic layer 140 may
be either single crystalline or polycrystalline, i.e., may contain
a plurality of crystalline grains that are adjoined at grain
boundaries at which crystal structures of the magnetoelectric
multiferroic material are discontinuous.
[0051] According to an embodiment of the present disclosure, a net
axial magnetization can be induced within each magnetoelectric
multiferroic layer 140 within the array of magnetic tunnel junction
devices 180 by application of an axial initialization magnetic
field or electric field. The electric field can switch the
ferroelectric polarization and corresponding magnetization due to
the magnetoelectric coupling. The axial initialization magnetic
field or electric field can be applied along a vertical direction
that is perpendicular to the interface between each free layer 136
and the tunnel barrier layer 134. The axial initialization magnetic
field or electric field can align the vertical component of the
magnetization of each crystalline grain in each magnetoelectric
multiferroic layer 140 along the direction of the axial
initialization magnetic field or electric field. Thus, each
magnetoelectric multiferroic layer 140 can have a net non-zero
axial magnetization. In this case, each magnetoelectric
multiferroic layer 140 can have a net non-zero ferroelectric
polarization that is associated with the net non-zero axial
magnetization.
[0052] According to another embodiment of the present disclosure, a
net axial ferroelectric polarization can be induced within each
magnetoelectric multiferroic layer 140 within the array of magnetic
tunnel junction devices 180 by application of an axial
initialization electric field. The axial initialization electric
field can be applied along a vertical direction that is
perpendicular to the interface between each free layer 136 and the
tunnel barrier layer 134. The axial initialization electric field
can align the vertical component of the ferroelectric polarization
of each crystalline grain in each magnetoelectric multiferroic
layer 140 along the direction of the axial initialization electric
field. Thus, each magnetoelectric multiferroic layer 140 can have a
net non-zero axial ferroelectric polarization. In this case, each
magnetoelectric multiferroic layer 140 can have a net non-zero
magnetization that is associated with the net non-zero axial
ferroelectric polarization.
[0053] According to an aspect of the present disclosure, a magnetic
memory device is provided, which comprises: a first electrode
(comprising a portion of a first electrically conductive line 30);
a second electrode (comprising a portion of a second electrically
conductive line 90); and a layer stack (120, 132, 134, 136, 140,
170) located between the first electrode and the second electrode
and comprising, from one side to another, a reference layer 132, a
tunnel barrier layer 134, a free layer 136, and a magnetoelectric
multiferroic layer 140 including at least one crystalline grain
having an easy axis of magnetization along an axial direction or
along a first tilted direction having a first tilt angle less than
90 degrees relative to the axial direction, the axial direction
being perpendicular to an interface between the free layer 136 and
the tunnel barrier layer 134.
[0054] In one embodiment, the layer stack (120, 132, 134, 136, 140,
170) also comprises a nonmagnetic capping layer 170 including a
nonmagnetic metal and contacting the magnetoelectric multiferroic
layer 140 and one of the first electrode and the second electrode.
In one embodiment, the magnetoelectric multiferroic layer 140
comprises, and/or consist essentially of, at least one material
selected from BiFeO.sub.3, h-YMnO.sub.3, BaNiF.sub.4, PbVO.sub.3,
BiMnO.sub.3, LuFe.sub.2O.sub.4, HoMn.sub.2O.sub.5, h-HoMnO.sub.3,
h-ScMnO.sub.3, h-ErMnO.sub.3, h-TmMnO.sub.3, h-YbMnO.sub.3,
h-LuMnO.sub.3, K2SeO.sub.4, Cs.sub.2CdI.sub.4, TbMnO.sub.3,
Ni.sub.3V.sub.2O.sub.8, MnWO.sub.4, CuO, ZnCr.sub.2Se.sub.4,
LiCu.sub.2O.sub.2, or Ni.sub.3B.sub.7O.sub.13I.
[0055] In one embodiment, a two-dimensional array of instances of
the magnetic memory device of FIG. 2A or FIG. 2B can be provided.
The magnetic memory array can comprise: first electrically
conductive lines 30 that are parallel to each other and extend
along a first direction, and second electrically conductive lines
90 that are parallel to each other and extend along a second
direction that is perpendicular to the first direction. Each of the
first electrically conductive lines 30 comprises first electrodes
of a respective row of the instances of the magnetic memory device
in the magnetic memory array. Each of the second electrically
conductive lines 90 comprises second electrodes of a respective
column of the instances of the magnetic memory device in the
magnetic memory array.
[0056] In one embodiment, the magnetoelectric multiferroic layer
140 can have a net non-zero axial magnetization. The net in-plane
ferromagnetic moment of the magnetoelectric multiferroic layer 140
may be zero (for axial multiferroic material) or not zero (for
canted multiferroic material). Thus, the net in-plane magnetization
of the magnetoelectric multiferroic layer 140 may be zero (for
axial multiferroic material) or not zero (for canted multiferroic
material).
[0057] FIG. 3A illustrates a cubic unit cell of BiFeO.sub.3 with a
direction of ferroelectric polarization P along one of the
<111> family directions (e.g., the [111] direction) and a
canted magnetic moment direction Mc within the (111) plane. FIG. 3B
illustrates the in plane (100) and out of plane (001) components of
the magnetic moment direction Mc in the (111) plane. The relative
spatial orientation (e.g., 90 degree angle) between the magnetic
moment direction Mc and the polarization direction P is the same
for each magnetoelectric multiferroic unit cell. The out of plane
(001) component of the canted magnetic moment direction Mc of each
multiferroic portion is magnetically coupled to the magnetization
direction of corresponding free layer via exchange bias or coupling
(e.g., ferromagnetic or antiferromagnetic coupling) at their
interface.
[0058] In one embodiment, the easy axis of magnetization of
polycrystalline grains of the magnetoelectric multiferroic layer
140 may be along the axial direction. In another embodiment, the
easy axis of magnetization of polycrystalline grains of the
magnetoelectric multiferroic layer 140 may be along a respective
first tilted (i.e., canted) direction, such as one of the
<111> directions.
[0059] Referring to FIGS. 4A and 4B, magnetic states (which
correspond to magnetoresistance states) of a magnetic tunnel
junction device 180 in the first exemplary structure are
illustrated. FIG. 4A illustrates a first magnetic state of the
magnetic tunnel junction device 180, and FIG. 4B illustrates a
second magnetic state of the magnetic tunnel junction device 180.
The magnetization direction Mm of the magnetoelectric multiferroic
layer 140 is magnetically coupled to the magnetization direction Mf
of the free layer 136. Thus, in each of the first magnetic state
and the second magnetic state of the magnetic tunnel junction
device 180, the magnetization direction Mm of the magnetoelectric
multiferroic layer 140 can be aligned to the magnetization
direction Mf of the free layer 136. In FIG. 4A, the magnetization
direction Mf of the free layer 136 is parallel to the magnetization
direction Mr of the reference layer 132. Thus, the magnetic tunnel
junction device 180 (e.g., MRAM cell) is in the lower resistance
state. In FIG. 4B, the magnetization direction Mf of the free layer
136 is antiparallel to the magnetization direction Mr of the
reference layer 132. Thus, the magnetic tunnel junction device 180
(e.g., MRAM cell) is in the higher resistance state.
[0060] Referring to FIG. 5, the programming mechanism for the
magnetic memory device illustrated in FIGS. 4A and 4B is
schematically illustrated. FIG. 5 illustrates the hysteresis curve
for the axial electrical polarization of the magnetoelectric
multiferroic layer 140 as a function of an external voltage applied
across the second electrode and the first electrode, i.e., the
external voltage as applied to the second electrode with respect to
the first electrode. The direction of magnetization Mm of the
magnetoelectric multiferroic layer 140 can be switched upon
application of an electric voltage across the magnetoelectric
multiferroic layer 140 that exceeds the magnitude of the coercive
voltage (which may be Vc+ or Vc- depending on the polarity) of the
magnetoelectric multiferroic material of the magnetoelectric
multiferroic layer 140. The positive coercive voltage Vc+ and the
negative coercive voltage Vc- may have equal magnitudes, and may
differ from each other by the polarity.
[0061] FIG. 6A illustrates a first programming voltage pattern P1
that can be employed to program the magnetic tunnel junction device
180 into the lower resistance state in the configuration shown in
FIG. 4A. FIG. 6B illustrates a second programming voltage pattern
P2 that can be employed to program magnetic tunnel junction device
180 into the higher resistance state shown in FIG. 4B.
[0062] Referring to FIGS. 5 and 6A, the first programming voltage
pattern P1 includes a first polarity voltage having a magnitude
greater than the coercive voltage which is applied to magnetic
tunnel junction device 180 between the first and second electrodes.
In the embodiment of FIGS. 5 and 6A, the first polarity voltage is
a positive voltage VP1 having a magnitude greater than the coercive
voltage (Vc+) which is applied to magnetic tunnel junction device
180 between the first and second electrodes. However, it should be
understood that in an alternative embodiment, the first polarity
voltage can be a negative voltage having a magnitude greater than
the coercive voltage. The positive voltage deterministically
programs the magnetization direction Mm of the magnetoelectric
multiferroic layer 140 to be parallel to magnetization direction Mr
of the reference layer 132. Furthermore, the positive voltage has a
sufficient magnitude to lower the perpendicular magnetic anisotropy
("PMA") and magnetic anisotropy barrier height at the interface
between the tunnel barrier layer 134 and the free layer 136 to
allow the free layer 136 to deterministically switch its
magnetization direction Mf. Since the magnetization direction Mf of
the free layer 136 is coupled to the magnetization direction Mm of
the magnetoelectric multiferroic layer 140, the magnetization
direction Mf of the free layer 136 is aligned to the direction of
the magnetization of the Mm of the magnetoelectric multiferroic
layer 140. Therefore, the magnetization direction Mf of the free
layer 136 is deterministically programmed to be parallel to
magnetization direction Mr of the reference layer 132, as shown in
FIG. 4A. Thus, the magnetic tunnel junction device 180 is
deterministically programmed into the lower resistance state.
[0063] Referring to FIGS. 5 and 6B, the second programming voltage
pattern P2 includes a second polarity voltage opposite to the first
polarity voltage having a magnitude greater than the coercive
voltage which is applied to magnetic tunnel junction device 180
between the first and second electrodes. In the embodiment of FIGS.
5 and 6B, the second polarity voltage is a negative voltage VP2
having a magnitude greater than the coercive voltage (Vc-) which is
applied to magnetic tunnel junction device 180 between the first
and second electrodes. However, it should be understood that in an
alternative embodiment, the second polarity voltage can be a
positive voltage having a magnitude greater than the coercive
voltage. The negative voltage deterministically programs the
magnetization direction Mm of the magnetoelectric multiferroic
layer 140 to be antiparallel to magnetization direction Mr of the
reference layer 132. However, the negative voltage increases the
perpendicular magnetic anisotropy ("PMA") and magnetic anisotropy
barrier height at the interface between the tunnel barrier layer
134 and the free layer 136 such that the free layer 136 cannot
switch its magnetization direction Mf.
[0064] Therefore, the second programming voltage pattern P2 also
includes a first polarity voltage having a magnitude less than the
coercive voltage which is applied to magnetic tunnel junction
device 180 between the first and second electrodes. In the
embodiment of FIGS. 5 and 6B, the first polarity voltage is a
positive voltage VP3 having a magnitude less than the coercive
voltage (Vc+) which is applied to magnetic tunnel junction device
180 between the first and second electrodes. However, it should be
understood that in an alternative embodiment, the first polarity
voltage can be a negative voltage having a magnitude less than the
coercive voltage. The positive voltage VP3 does not have a
sufficient magnitude to switch the magnetization direction Mm of
the magnetoelectric multiferroic layer 140. However, the positive
voltage VP3 does have a sufficient magnitude to lower the
perpendicular magnetic anisotropy ("PMA") and magnetic anisotropy
barrier height at the interface between the tunnel barrier layer
134 and the free layer 136 to allow the free layer 136 to
deterministically switch its magnetization direction Mf. Since the
magnetization direction Mf of the free layer 136 is coupled to the
magnetization direction Mm of the magnetoelectric multiferroic
layer 140, the magnetization direction Mf of the free layer 136 is
aligned to the direction of the magnetization of the Mm of the
magnetoelectric multiferroic layer 140. Therefore, the
magnetization direction Mf of the free layer 136 is
deterministically programmed to be antiparallel to magnetization
direction Mr of the reference layer 132, as shown in FIG. 4B. Thus,
the magnetic tunnel junction device 180 is deterministically
programmed into the higher resistance state when the negative
voltage pulse VP2 is followed by the smaller positive voltage pulse
VP3.
[0065] Referring to FIGS. 5, 6A and 6B, the positive voltage pulse
VP1 may have a magnitude between 0.5 V and 2 V, the negative
voltage pulse VP2 may have a magnitude between -0.5 V and -2 V, and
the third positive voltage pulse VP3 may have a magnitude between
0.3 V and 1 V, and which has an absolute value which is less than
an absolute value of the negative voltage pulse VP2.
[0066] Generally, the magnetic memory device of the embodiments
present disclosure can comprise a programming circuit, such as a
component of the sensing and programming circuit 570 of the random
access memory device 501 of FIG. 1 which applies the avoid
voltages. Alternatively, each magnetic tunnel junction device 180
may be individually connected to a respective programming circuit
configured to program a single magnetic tunnel junction device
180.
[0067] Referring to FIGS. 7A and 7B, configurations of a second
exemplary structure according to a second embodiment of the present
disclosure are illustrated. The configurations of the second
exemplary structure may be derived from the configurations of the
first exemplary structure illustrated in FIGS. 2A and 2B by
replacing the magnetoelectric multiferroic layer 140 with a VCMA
assist structure 200 having an in-plane magnetization direction. In
the configuration of FIGS. 7A and 7B, the VCMA assist structure 200
comprises a magnetoelectric multiferroic layer 140' having an
in-plane magnetization direction Mm. The in-plane magnetization
provides an in-plane magnetic dipolar magnetic field for VCMA
precessional switching. Layer 140' may comprise a polycrystalline
BiFeO.sub.3 layer having a preferred grain orientation along the
(111) plane can provide predominantly in-plane magnetization
direction, i.e., more than 50% all crystalline grains can have a
respective magnetization direction (e.g., spin orientation) that is
parallel to the interface between the tunnel barrier layer 134 and
the free layer 136.
[0068] A net in-plane magnetization of the magnetoelectric
multiferroic layer 140' can be induced by application of an
in-plane magnetic field or by application of an out-of-plane
external electric field. Thus, a predominant portion (i.e., more
than 50% in volume) of the crystalline grains of the
magnetoelectric multiferroic layer 140' can have a net non-zero
in-plane magnetization. In this case, each magnetoelectric
multiferroic layer 140' can have a net non-zero ferroelectric
polarization that is associated with the net non-zero in-plane
magnetization. In other words, the net in-plane magnetization of
each magnetoelectric multiferroic layer 140' can be induced by
applying external magnetic field along an in-plane direction that
is parallel to the interface between the free layer 136 and the
tunnel barrier layer 134. A net in-plane ferroelectric polarization
of the magnetoelectric multiferroic layer 140' and the net in-plane
magnetization of the magnetoelectric multiferroic layer 140' can be
simultaneously induced by the external magnetic field.
[0069] According to another embodiment of the present disclosure, a
net in-plane ferroelectric polarization can be induced within each
magnetoelectric multiferroic layer 140' within the array of
magnetic tunnel junction devices 180 by application of an in-plane
or out-of plane initialization electric field. The in-plane
initialization electric field can be applied along a horizontal
direction that is parallel to the interface between each free layer
136 and the tunnel barrier layer 134. The in-plane initialization
electric field can align the in-plane component of the
ferroelectric polarization of each crystalline grain in each
magnetoelectric multiferroic layer 140' along the direction of the
in-plane initialization electric field. Thus, each magnetoelectric
multiferroic layer 140' can have a net non-zero in-plane
ferroelectric polarization. In this case, each magnetoelectric
multiferroic layer 140' can have a net non-zero in-plane
magnetization that is associated with the net non-zero in-plane
ferroelectric polarization. In other words, the net in-plane
magnetization of each magnetoelectric multiferroic layer 140' can
be induced by applying external electric field along an in-plane
direction that is parallel to the interface between the free layer
136 and the tunnel barrier layer 134. A net in-plane ferroelectric
polarization of each magnetoelectric multiferroic layer 140' and
the net in-plane magnetization of each magnetoelectric multiferroic
layer 140' can be simultaneously induced by the external electric
field.
[0070] Generally, the magnitude of the net in-plane magnetization
of each magnetoelectric multiferroic layer 140' can be greater than
the magnitude of any net axial magnetization of the respective
magnetoelectric multiferroic layer 140' by at least one order of
magnitude, i.e., by a factor greater than 10 such as by a factor
greater than 30 or 100.
[0071] Referring to FIG. 8, an alternative configuration of the
second exemplary structure is illustrated, which can be derived
from the configurations of the second exemplary structure
illustrated in FIGS. 7A and 7B by using a VCMA assist structure 200
which comprises a ferromagnetic or ferrimagnetic layer 240 having
an in-plane magnetization direction and an electrically conductive
nonmagnetic spacer layer 138 located between the free layer 136 and
the ferromagnetic or ferrimagnetic layer 240. The ferromagnetic or
ferrimagnetic layer 240 may comprise a CoFe or CoFeB ferromagnetic
layer or a Fe.sub.3O.sub.4 ferrimagnetic layer. The electrically
conductive nonmagnetic spacer layer 138 may comprise any suitable
nonmagnetic metal layer, such as Pt, Ta, Ru, W, etc. The thickness
of each of layers 238 and 240 may be between 1 nm and 2 nm,
although lesser and greater thicknesses may also be employed
[0072] Referring collectively to FIGS. 7A, 7B, and 8, a magnetic
memory device is provided, which comprises: a first electrode
(comprising portion of a first electrically conductive line 30); a
second electrode (comprising portion of a second electrically
conductive line 90); and a layer stack (optional 120, 132, 134,
136, 200, optional 170) located between the first electrode and the
second electrode and comprising, from one side to another, a
reference layer 132 having a fixed axial magnetization direction, a
tunnel barrier layer 134, a free layer 136 having an easy axis of
magnetization that is parallel or antiparallel to the fixed axial
magnetization direction, and a VCMA assist structure 200 having an
in-plane magnetization direction.
[0073] In one embodiment, the VCMA assist structure comprises a
magnetoelectric multiferroic layer 140' including at least one
crystalline grain which has an easy axis of magnetization that is
parallel to an interface between the free layer 136 and the tunnel
barrier layer 134, and a net in-plane magnetization of the
magnetoelectric multiferroic layer 140' is non-zero.
[0074] In one embodiment, the magnetoelectric multiferroic layer
140' has a net ferroelectric polarization that is parallel to the
interface between the free layer 136 and the tunnel barrier layer
134. In one embodiment, the magnetoelectric multiferroic layer 140'
can be polycrystalline, and a predominant subset in volume of
crystalline grains i.e., a set of crystalline grains that occupy
more than 50% of the entire volume of the magnetoelectric
multiferroic layer 140) within the magnetoelectric multiferroic
layer 140' can have a respective magnetization direction that is
aligned, or has an angle less than 145 degrees from, a direction of
the net in-plane magnetization of the magnetoelectric multiferroic
layer 140'. In one embodiment, the net in-plane magnetization of
the magnetoelectric multiferroic layer 140' can be parallel to, or
is antiparallel to, the direction of the net in-plane magnetization
of the magnetoelectric multiferroic layer 140'.
[0075] In one embodiment, the magnetoelectric multiferroic layer
140' comprises a material selected from BiFeO.sub.3, h-YMnO.sub.3,
BaNiF.sub.4, PbVO.sub.3, BiMnO.sub.3, LuFe.sub.2O.sub.4,
HoMn.sub.2O.sub.5, h-HoMnO.sub.3, h-ScMnO.sub.3, h-ErMnO.sub.3,
h-TmMnO.sub.3, h-YbMnO.sub.3, h-LuMnO.sub.3, K.sub.2SeO.sub.4,
Cs.sub.2CdI.sub.4, TbMnO.sub.3, Ni.sub.3V.sub.2O.sub.8, MnWO.sub.4,
CuO, ZnCr.sub.2Se.sub.4, LiCu.sub.2O.sub.2, and
Ni.sub.3B.sub.7O.sub.13I. For example, the magnetoelectric
multiferroic layer 140' comprises a BiFeO.sub.3 layer having a
(111) preferred grain orientation and a thickness of 1 nm or
less.
[0076] In one embodiment, the layer stack also includes the SAF
structure 120. In one embodiment, the layer stack (optional 120,
132, 134, 136, 200, optional 170) comprises a nonmagnetic capping
layer 170 including a nonmagnetic metal contacting the
magnetoelectric multiferroic layer 140' and one of the electrode
and the second electrode.
[0077] In another embodiment illustrated in FIG. 8, the VCMA assist
structure 200 comprises a ferromagnetic or ferrimagnetic layer 240
having an in-plane magnetization direction and an electrically
conductive nonmagnetic spacer layer 138 located between the free
layer 136 and the ferromagnetic or ferrimagnetic layer 240.
[0078] Referring to FIGS. 9A and 9B, a first programmed state and a
second programmed state of a second exemplary structure is
illustrated. FIG. 9A illustrates the first programmed state, and
FIG. 9B illustrates the second programmed state. The first
programmed state (e.g., the lower resistance state) has a parallel
alignment between the magnetization Mf of the free layer 136 and
the magnetization Mr of the reference layer 132. The second
programmed state (e.g., the higher resistance state) has an
antiparallel alignment between the magnetization Mf of the free
layer 136 and the magnetization Mr of the reference layer 132. The
first programmed state of FIG. 9A and the second programmed state
of FIG. 9B have different tunneling magnetoresistance values. Thus,
the magnetic state of the magnetic tunnel junction device 180 in
the second exemplary structure can be determined by applying a low
voltage sensing bias voltage across the second electrode and the
first electrode, and measuring the magnitude of the tunneling
current through the magnetic tunnel junction device 180.
[0079] Referring to FIG. 10A, the second exemplary structure can
include a sensing circuit (which may be a component of the sensing
and programming circuit 570 illustrated in FIG. 1 or may be a
stand-alone sensing circuit) configured to determine tunneling
magnetoresistance of the layer stack (optional 120, 132, 134, 136,
200, optional 170) between the first electrode and the second
electrode. The sensing circuit can be configured to determine
whether the memory state of the magnetic memory device (i.e., the
magnetic tunnel junction device 180) is in a target state or not by
comparing a measured value of the tunneling magnetoresistance of
the layer stack to a target value, which may be the target value
for the parallel state illustrated in FIG. 9A or the target value
for the antiparallel state illustrated in FIG. 9B.
[0080] The sensing circuit can be configured to apply a sensing
pulse Vs of a first polarity. The first polarity can be selected
such that the sensing pulse Vs increases perpendicular magnetic
anisotropy of the free layer 136. In other words, the magnetization
of the free layer 136 is less likely to make a transition between a
parallel state and an antiparallel state. Thus, the sensing
operation does not disturb the magnetic state of the magnetic
tunnel junction device 180 of the second exemplary structure. For
example, the sensing pulse may comprise a negative voltage pulse.
In one embodiment, the absolute magnitude of the voltage of the
sensing pulse Vs may be in a range from 0.05 V to 0.5 V such as
from 0.1 V to 0.2 V, although lesser and greater voltages may also
be employed.
[0081] In case the sensing operation determines that the measured
magnetic state of the magnetic tunnel junction device 180 is the
same as the target magnetic state for the magnetic tunnel junction
device 180, no further action is necessary.
[0082] Referring to FIG. 10B, in case the sensing operation
determines that the measured magnetic state of the magnetic tunnel
junction device 180 is the opposite of the target magnetic state of
the magnetic tunnel junction device 180, a programming circuit may
be employed to program the magnetic tunnel junction device 180 into
the target magnetic state. The programming circuit may be a
component of the sensing and programming circuit 570 illustrated in
FIG. 1, or may be a discrete circuit configured to program a single
magnetic tunnel junction device 180. The programming circuit can be
configured to apply a VCMA programming voltage pulse Vp that
induces precession of an angle between the magnetization direction
of the free layer 136 and an axial direction that is perpendicular
to the interface between the free layer 136 and the tunnel barrier
layer 134 if, the magnetic memory device is not in the target
state, and can be configured not to apply any programming voltage
pulse if the magnetic memory device is in the target state.
[0083] The VCMA programming voltage pulse Vp non-deterministically
programs the magnetic memory device without applying an external
magnetic field. The VCMA programming voltage pulse Vp can have a
second polarity that is an opposite of the first polarity. The
programming voltage pulse Vp reduces the perpendicular magnetic
anisotropy of the free layer 136. Thus, the programming voltage
pulse Vp enables movement of the direction of the magnetization of
the free layer 136. For example, the programming voltage Vp can be
a 0.3 V to 1 V positive voltage pulse which has a greater absolute
magnitude than the sensing pulse Vs.
[0084] The various embodiments of the present disclosure can be
employed to provide a voltage controlled magnetic anisotropy (VCMA)
tunnel magnetoresistance (TMR) memory device. The VCMA TMR device
may have a deterministic switching mechanism as in the case of the
first exemplary structure, or may have a pulse duration-controlled
switching mechanism as in the case of the second exemplary
structure. The outcome of programming is independent of the
variations in the pulse magnitude or pulse duration in the case of
the first exemplary structure due to the deterministic nature of
the programming mechanism. The VCMA assist structure of the second
exemplary structure provides a predictable and well controlled
stray magnetic field (residual magnetic field) that determines the
precession frequency of the angle between the magnetization of the
free layer 136 and the vertical axis, thereby improving the
accuracy and reliability of VCMA programming that switches the
magnetic states. Furthermore, when the VCMA assist structure
includes the in-plane multiferroic layer 140', such multiferroic
layer can provide additional PMA to free layer due to the
hybridization of oxygen orbitals and metal 3d orbital.
[0085] Furthermore, the multiferroic layer, such as BiFeO.sub.3,
has a smaller resistivity than the typical tunnel barrier material
(e.g., MgO). Since the multiferroic layer has similar thickness as
the tunnel barrier layer, the multiferroic layer does not
significantly increase the resistance of the memory device (i.e.,
does not add a significant parasitic resistance). Therefore, a
relatively small sense voltage may be used to read the memory
device without disturbing the written magnetic state similar to
conventional VCMA MRAMs which lack a multiferroic layer.
[0086] Although the foregoing refers to particular preferred
embodiments, it will be understood that the disclosure is not so
limited. It will occur to those of ordinary skill in the art that
various modifications may be made to the disclosed embodiments and
that such modifications are intended to be within the scope of the
disclosure. Where an embodiment employing a particular structure
and/or configuration is illustrated in the present disclosure, it
is understood that the present disclosure may be practiced with any
other compatible structures and/or configurations that are
functionally equivalent provided that such substitutions are not
explicitly forbidden or otherwise known to be impossible to one of
ordinary skill in the art. All of the publications, patent
applications and patents cited herein are incorporated herein by
reference in their entirety.
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