U.S. patent application number 10/093344 was filed with the patent office on 2003-09-11 for synthetic ferrimagnet reference layer for a magnetic storage device.
Invention is credited to Sharma, Manish, Tran, Lung The.
Application Number | 20030169620 10/093344 |
Document ID | / |
Family ID | 27754047 |
Filed Date | 2003-09-11 |
United States Patent
Application |
20030169620 |
Kind Code |
A1 |
Sharma, Manish ; et
al. |
September 11, 2003 |
SYNTHETIC FERRIMAGNET REFERENCE LAYER FOR A MAGNETIC STORAGE
DEVICE
Abstract
A synthetic ferrimagnet reference layer for a magnetic storage
device. The reference layer has first and second layers of magnetic
material operable to be magnetized in first and second magnetic
orientations. A spacer layer between the layers of magnetic
material is of suitable dimensions to magnetically couple the
magnetic layers in opposite directions. The layers of magnetic
material have substantially the same coercivities.
Inventors: |
Sharma, Manish; (Sunnyvale,
CA) ; Tran, Lung The; (Saratoga, CA) |
Correspondence
Address: |
HEWLETT-PACKARD COMPANY
Intellectual Property Administration
P.O. Box 272400
Fort Collins
CO
80527-2400
US
|
Family ID: |
27754047 |
Appl. No.: |
10/093344 |
Filed: |
March 6, 2002 |
Current U.S.
Class: |
365/173 |
Current CPC
Class: |
G11C 11/161 20130101;
H01F 10/324 20130101; G11C 11/1673 20130101; H01F 10/3277 20130101;
B82Y 25/00 20130101; H01F 10/3254 20130101 |
Class at
Publication: |
365/173 |
International
Class: |
G11C 011/15 |
Claims
What is claimed is:
1. A reference layer for a magnetic storage device, said reference
layer comprising: a first layer of magnetic material operable to be
magnetized in first and second magnetic orientations; a second
layer of magnetic material operable to be magnetized in first and
second magnetic orientations; said first and second layers having
substantially the same coercivities; and a spacer layer between
said first and second layers, said spacer layer of suitable
dimensions to magnetically couple said first and second layers in
opposite directions.
2. The reference layer of claim 1, wherein said first and second
layers are constructed such that their magnetic orientations will
tend to be substantially orthogonal to the direction of an applied
magnetic field.
3. The reference layer of claim 2, wherein the intensity of said
applied magnetic field is within the range of typical magnetic
fields for reading and writing a magneto-resistive memory
device.
4. The reference layer of claim 2, wherein the intensity of said
applied magnetic field is on the order of less than a few thousand
Amperes/Meter.
5. The reference layer of claim 1, wherein said first and second
layers are ferromagnetic layers.
6. The reference layer of claim 1, wherein said spacer layer is
electrically conductive and magnetically non-conductive.
7. The reference layer of claim 1, wherein said first and second
layers are substantially the same thickness.
8. A magnetic storage device comprising: a synthetic ferrimagnet
reference layer; a data layer having a first and a second magnetic
orientations; a tunnel layer between said reference layer and said
data layer; said reference layer comprising: a first layer of
magnetic material; a second layer of magnetic material; said first
and said second layers operable to be magnetized to cause the net
magnetic moment of the reference layer to be substantially zero;
and a spacer layer between said first layer and said second layer,
said spacer layer with dimensions to magnetically couple said first
layer and said second layer in opposite directions; said reference
layer having two stable magnetic orientations, a first in which
said first layer is in a first direction and said second layer is
in an opposite direction and a second stable state in which said
first layer is in said opposite direction and said second layer is
in said first direction, said directions being substantially
orthogonal to an applied magnetic field.
9. The device of claim 8, further comprising means for creating a
magnetic orientation in said second layer that is either
substantially parallel or anti-parallel to said magnetic
orientation of said data layer.
10. The device of claim 8, wherein said second layer is positioned
relative to said data layer such that said two stable magnetic
orientations comprise a first range of angles and a second range of
angles relative to said magnetic orientation in said data layer,
wherein the electrical resistance across said spacer layer has
first and second resistances corresponding to said two stable
orientations.
11. The device of claim 8, wherein said second layer is positioned
relative to said data layer such that said two stable magnetic
orientations comprise a substantially parallel and a substantially
anti-parallel orientation relative to said magnetic orientation in
said data layer.
12. The device of claim 8, wherein easy axes of said first layer
and said second layer are substantially orthogonal to an easy axis
of said data layer.
13. The device of claim 8, wherein said applied magnetic field is
stronger in one layer of said first layer and said second layer,
wherein which of said two stable magnetic orientations that said
reference layer is known.
14. The device of claim 8, wherein said applied magnetic field is
less than 1000 Amperes/Meter.
15. A method of operating a magnetic storage device, said method
comprising: a) storing a bit in said device by applying a magnetic
field to a data layer in said device to create a magnetic
orientation in said data layer; b) applying a magnetic field to a
reference layer having first and second layers with substantially
the same coercivity and that are magnetically coupled in opposite
directions, said applied magnetic field establishing a magnetic
orientation in said first layer of said reference layer that is
substantially orthogonal to said applied magnetic field; and c)
measuring the resistance between said first layer and said data
layer to determine the magnetic orientation of said data layer,
wherein the bit stored in said storage device is determined.
16. The method of claim 15, wherein b) comprises applying said
magnetic field to cause said magnetic orientation in said first
layer of said reference layer to be either substantially parallel
or substantially anti-parallel to said magnetic orientation in said
data layer.
17. The method of claim 15, wherein b) comprises applying said
magnetic field in a direction that is substantially parallel to the
easy axis of said first layer of said reference layer.
18. The method of claim 15, wherein b) comprises applying said
magnetic field asymmetrically in a region of said first and second
layers, wherein said magnetic orientation of said first layer is
forced to a known position.
19. The method of claim 15, wherein b) comprises applying a current
near said reference layer.
20. The method of claim 15, wherein c) comprises applying a voltage
between said first layer and said data layer.
21. A magnetic storage device comprising: an array of storage
cells, said cells comprising: a first layer of magnetic material
having first and second magnetic orientations; a second layer of
magnetic material having first and second magnetic orientations;
said first and said second layers having substantially the same
coercivities; a spacer layer between said first and said second
layers, said spacer layer for magnetically coupling said first and
second layers in opposite directions, wherein said magnetic
orientations of said first and said second layers tend to be
substantially orthogonal to the direction of an applied magnetic
field.
22. The storage device of claim 21, further comprising: a plurality
of first electrically conductive elements extending along said
array in a first direction; a plurality of second electrically
conductive elements extending along said array in a second
direction; a plurality of third electrically conductive elements
extending in said second direction, said second and third
electrically conductive elements being separated by an electrical
insulator.
23. The storage device of claim 21, wherein the easy axis of at
least one of said data layers is substantially parallel to said
second electrically conductive elements and the easy axis of at
least one of said first layers is substantially orthogonal to said
second electrically conductive elements.
24. The storage device of claim 21, further comprising a circuit
for applying a current to a first selected element of said third
electrically conductive elements to set said magnetic orientation
of said first layer to a known position.
25. The storage device of claim 24, further comprising a circuit
for applying a voltage between a first selected element of said
first electrically conductive elements and a first selected element
of said second electrically conductive elements.
26. The storage device of claim 25, further comprising a circuit
for measuring current through said first selected element of said
second electrically conductive elements to measure resistance
between said first layer and said data layer.
27. A synthetic ferrimagnet reference layer for a magnetic storage
device, said reference layer comprising: a plurality of layers of
magnetic material having first and second magnetic orientations; at
least one spacer layer, said at least one spacer layer between
adjacent layers of said plurality of layers, said at least one
spacer layer of suitable dimensions to magnetically couple said
adjacent layers in opposite directions; and said plurality of
layers of magnetic material operable to be magnetized to cause the
net magnetic moment of the reference layer to be substantially
zero.
28. The reference layer of claim 27, wherein said plurality of
layers of magnetic material are constructed such that their
magnetic orientations will tend to be substantially orthogonal to
the direction of an applied magnetic field.
29. The reference layer of claim 28, wherein the intensity of said
applied magnetic field is within the range of typical magnetic
fields for reading and writing a magneto-resistive memory
device.
30. The reference layer of claim 28, wherein the intensity of said
applied magnetic field is on the order of less than a few thousand
Amperes/Meter.
31. The reference layer of claim 27, wherein said plurality of
layers of magnetic material are ferromagnetic layers.
32. The reference layer of claim 27, wherein said at least one
spacer layer is electrically conductive and is not a magnetic
material.
33. The reference layer of claim 27, wherein said plurality of
layers of magnetic material are substantially the same thickness.
Description
TECHNICAL FIELD
[0001] The present invention relates to the field of magnetic
storage devices. More particularly, the present invention relates
to a synthetic ferrimagnet reference layer for a magnetic storage
device.
BACKGROUND ART
[0002] Magnetic Random Access Memory (MRAM) is a non-volatile
memory that has lower power consumption than short-term memory such
as DRAM, SRAM and Flash memory. MRAM can perform read and write
operations much faster (by orders of magnitude) than conventional
long-term storage devices such as hard drives. In addition, MRAM is
more compact and consumes less power than hard drives.
[0003] A typical MRAM device includes an array of memory cells,
word lines extending along rows of the memory cells, and bit lines
extending along columns of the memory cells. Each memory cell is
located at a cross point of a word line and a bit line.
[0004] The memory cells may be based on tunneling magneto-resistive
(TMR) devices such as spin dependent tunneling junctions (SDT). A
typical SDT junction includes a pinned layer, a sense layer and an
insulating tunnel barrier sandwiched between the pinned and sense
layers. The pinned layer has a magnetization orientation that is
fixed so as not to rotate in the presence of an applied magnetic
field in a range of interest. The sense layer has a magnetization
that can be oriented in either of two directions: the same
direction as the pinned layer magnetization or the opposite
direction of the pinned layer magnetization. If the magnetizations
of the pinned and sense layers are in the same direction, the
orientation of the SDT junction is said to be "parallel." If the
magnetizations of the pinned and sense layers are in opposite
directions, the orientation of the SDT junction is said to be
"anti-parallel." These two stable orientations, parallel and
anti-parallel, may correspond to logic values of `0` and `1`.
[0005] The magnetization orientation of the pinned layer may be
fixed by an underlying antiferromagnetic (AF) pinning layer. The AF
pinning layer provides a large exchange field, which holds the
magnetization of the pinned layer in one direction. Underlying the
AF layer are usually first and second seed layers. The first seed
layer allows the second seed layer to be grown with a crystal
structure orientation. The second seed layer establishes a crystal
structure orientation for the AF pinning layer.
[0006] The pinned layer in some conventional magneto-resistive
memory devices may have a net magnetic moment, which leads to
undesirable effects. One such effect is that of a demagnetizing
field. For example, the magnetic layer of the pinned layer reaches
and interacts with the sense layer. As the sense layer stores
information by the orientation of its magnetization, clearly its
magnetic orientation must be preserved. Thus, the interaction of
the magnetic field from the pinned layer may lead to loss of data
if this magnetic field becomes too strong. A second problem is that
the presence of the magnetic field from the pinned layer requires
that an asymmetric magnetic field be used to switch the state of
the data layer, which adds to the complexity of the writing
process. A still further problem is that the tolerance for stray
magnetic fields during writing is lowered.
[0007] As it is desirable to fabricate high capacity memories, it
is desirable to fabricate an array of such memory cells as dense as
possible. Unfortunately, the cumulative demagnetizing effects of
all of the reference layers may constrain how densely the memory
cells may be packed.
[0008] Another disadvantage of pinned structures is that the
materials needed to achieve pinning (e.g., the AF pinning layer and
the seed layer) are both complicated and expensive to
fabricate.
[0009] Therefore, a need exists for an information storage device
using magneto-resistive memory cells. A further need exists for
such a device that minimizes a demagnetizing field that may be
present in conventional magnetic storage devices. A still further
need exists for a device that may be fabricated more economically
and with fewer and simpler materials than conventional magnetic
storage devices.
DISCLOSURE OF THE INVENTION
[0010] Accordingly, embodiments of the present invention provide a
synthetic ferrimagnet reference layer for a magnetic storage
device. Embodiments of the present invention provide for a device
that minimizes a demagnetizing field that may be present in
conventional magnetic storage devices. A synthetic ferrimagnet
reference layer for a magnetic storage device is disclosed. The
reference layer has first and second layers of magnetic material
operable to be magnetized in first and second magnetic
orientations. A spacer layer between the layers of magnetic
material is of suitable dimensions to magnetically couple the
magnetic layers in opposite directions. The layers of magnetic
material have substantially the same coercivities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are incorporated in and
form a part of this specification, illustrate embodiments of the
invention and, together with the description, serve to explain the
principles of the invention:
[0012] FIG. 1 is an illustration of a magnetic memory device
according to embodiments of the present invention.
[0013] FIG. 2A and FIG. 2B are illustrations of reference layers
according to embodiments of the present invention.
[0014] FIG. 3A is an illustration of a hysteresis loop for a
reference layer according to embodiments of the present
invention.
[0015] FIG. 3B is a magnified illustration of the hysteresis loop
of FIG. 3A for a reference layer according to embodiments of the
present invention.
[0016] FIG. 4A and FIG. 4B are illustrations of reference layers
according to embodiments of the present invention.
[0017] FIG. 5 is a flowchart of steps of a process of reading a bit
on a magnetic memory device according to embodiments of the present
invention.
[0018] FIG. 6 is an illustration of a magnetic storage device
according to embodiments of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0019] In the following detailed description of the present
invention, numerous specific details are set forth in order to
provide a thorough understanding of the present invention. However,
it will be obvious to one skilled in the art that the present
invention may be practiced without these specific details or by
using alternate elements or methods. In other instances well known
methods, procedures, components, and circuits have not been
described in detail as not to unnecessarily obscure aspects of the
present invention.
[0020] Referring to FIG. 1, a magnetic memory device 10 includes a
magnetic tunnel junction 11 having a data layer 12, a reference
layer 14, and a tunnel barrier 16 between the data and reference
layers 12 and 14. Both layers 12 and 14 may be made of a
ferromagnetic material. The data layer 12 has a magnetization
(represented by the vector M1) that can be orientated in either of
two directions, typically along the easy axis (EA1) of the data
layer 12.
[0021] The reference layer 14 may be constructed to have a net
magnetic moment that is substantially zero. Hence, it may be stated
that the net magnetization vector of the reference layer 14 is
substantially zero. However, the reference layer 14 may have
multiple magnetization vectors, such as, for example, magnetization
vector M2A and M2B. These two vectors may be equal and opposite and
hence may cancel each other at a distance. However, magnetization
vector M2A may be closer to the tunnel barrier 16 than is
magnetization vector M2B. The two magnetization vectors M2A and M2B
may be orientated in either of two directions, typically along the
y-axis. However, the magnetization vectors M2A and M2B are not
necessarily orthogonal to the easy axis (EA2) of the reference
layer 14.
[0022] If the magnetizations vectors of the data and reference
layers 12 and 14 (M1 and M2A) are pointing in the same direction,
the orientation of the magnetic tunnel junction 11 may be referred
to as being "parallel." If the magnetization vectors (M1 and M2A)
of the data and reference layers 12 and 14 are pointing in opposite
directions, the orientation of the magnetic tunnel junction 11 may
be referred to as being "anti-parallel." These two orientations,
parallel and anti-parallel, may correspond to logic values of `0`
and `1`. The logic value that is represented by which orientation
is arbitrary.
[0023] The insulating tunnel barrier 16 allows quantum mechanical
tunneling to occur between the data and reference layers 12 and 14.
This tunneling phenomenon is electron spin dependent, causing the
resistance of the magnetic tunnel junction 11 to be a function of
the relative orientations of the magnetization vectors (M1 and M2A)
of the data and reference layers 12 and 14. Thus, a bit may be
stored by establishing the magnetization orientation of the data 12
and reference layers 14 to be different. For example, the
resistance of the magnetic tunnel junction 11 is a first value (R)
if the magnetization orientation of the magnetic tunnel junction 11
is parallel and a second value (R+.DELTA.R) if the magnetization
orientation is anti-parallel. However, the present invention is not
limited to the magnetization orientation of the two layers relative
to each other being either parallel or anti-parallel. More
generally, the magnetic orientation of each layer may be selected
such that the measured resistance between the layers is different
for two different states. The two stable orientations may comprise
a first range of angles and a second range of angles between the
magnetic orientation magnetization vectors (M1 and M2A) of the data
and reference layers 12 and 14, wherein the electrical resistance
across the tunnel layer has first and second resistances
corresponding to the two stables orientations.
[0024] The insulating tunnel barrier 16 may be made of aluminum
oxide (Al.sub.2O.sub.3), silicon dioxide (SiO.sub.2), tantalum
oxide (Ta.sub.2O.sub.5), silicon nitride (SiN.sub.4), aluminum
nitride (AlN.sub.x), or magnesium oxide (MgO). Other dielectrics
and certain semiconductor materials may be used for the insulating
tunnel barrier 16. Thickness of the insulating tunnel barrier 16
may range from about 0.5 nanometers to about three nanometers.
However, the present invention is not limited to this range.
[0025] The data layer 12 may be made of a ferromagnetic material.
The reference layer 14 may be implemented as a synthetic
ferrimagnet (SF), also known as an artificial antiferromagnet.
[0026] A first conductor 18 extending along the y-axis is in
contact with the data layer 12. A second conductor 20 extending
along the x-axis is in contact with the reference layer 14. The
first and second conductors 18 and 20 are shown as being
orthogonal. However, the present invention is not limited to an
orthogonal orientation. Below the second conductor 20 is a third
conductor 22, which also extends along the x-axis. An electrical
insulator 24 (e.g., a layer of dielectric material) separates the
second and third conductors 20 and 22. The conductors 18, 20 and 22
may be made of an electrically conductive material such as
aluminum, copper, gold or silver.
[0027] Referring now to FIGS. 2A and 2B, the synthetic ferrimagnet
reference layer 14 may include first and second ferromagnetic
layers 50 and 52 separated by a spacer layer 54 (e.g., the spacer
layer 54 may be metallic). The ferromagnetic layers 50 and 52 may
be made of a material such as, for example, cobalt iron (CoFe),
nickel iron (NiFe), Cobalt (Co), etc., and the spacer layer 54 may
be made of an electrically conductive, magnetically non-conductive
material such as, for example, Ruthenium (Ru), Rhenium (Re),
Rhodium (Rh), Copper (Cu), Tellurium (Te), Chromium (Cr), etc.
[0028] The dimensions (e.g., thickness) of the spacer layer 54 may
be selected to cause the first and second ferromagnetic layers 50
and 52 to be coupled, such that their magnetic orientations are
anti-parallel, as seen in FIGS. 2A and 2B. The thickness may depend
on the material that the spacer layer 54 is formed from. In one
embodiment, the thickness may be between about 0.2 nm and 2 nm.
However, other thicknesses may be suitable to couple the two
ferromagnetic layers 50 and 52.
[0029] For example, the magnetization vector of the upper
ferromagnetic layer 52 (M2A) is shown along the negative y-axis.
The magnetization vector of the lower ferromagnetic layer 50 (M2B)
is shown along the positive y-axis, in FIG. 2A.
[0030] The ultra-low coercivity allows the magnetization vectors
(M2A and M2B) of the SF reference layer 14 to be switched easily
between the orientations shown in FIGS. 2A and 2B. In practice, the
orientations of the magnetic vectors M2A and M2B for the
ferromagnetic layers 50 and 52 may be in any direction just before
the magnetic field is applied. Once the magnetic field is applied,
their orientations are known. Only a very small magnetic field
needs to be applied to the ferromagnetic layers 50 and 52 to push
the magnetization vectors M2A and M2B to known positions shown in
FIG. 2A or 2B, (e.g., orthogonal to the applied magnetic
field).
[0031] In some embodiments of the present invention, the synthetic
ferrimagnet reference layer 14 comprises more than two
ferromagnetic layers. In these embodiments, adjacent ferromagnetic
layers may be joined by spacer layers 54 and may have their
magnetic orientations in opposite directions. The net magnetic
moment of the synthetic ferrimagnet reference layer 14 is
substantially zero. Embodiments provide for both even and odd
numbers of ferromagnetic layers in a synthetic ferrimagnet
reference layer 14.
[0032] FIG. 3A illustrates a typical hysteresis loop for a
synthetic ferrimagnet reference layer 14 with exemplary dimensions
and materials of CoFe 3 nm/Ru 0.75 nm/CoFe 3 nm. Individually each
ferromagnetic layer 50, 52 may, for example, have a coercivity of
about 10-100Oersted (e.g., about 800-8000 Amperes/Meter) and have
similar hysteresis loops. However, the present invention is not
limited to this range of coercivities. The coercivities of the two
ferromagnetic layers 50 and 52 may be substantially the same. This
may be accomplished by using the same thickness of identical
materials; however, the present invention is not limited to this
way of achieving substantially identical coercivities. Furthermore,
it is not required that each ferromagnetic layer has the same
coercivity. For embodiments with more than two ferromagnetic
layers, the combined coercivities of the ferromagnetic layers may
cause the net magnetic moment of the reference layer 14 to be
substantially zero.
[0033] As illustrated, the hysteresis loop for the combined
ferromagnetic layers 50 and 52 may pass though the origin where the
net magnetic moment is zero. For embodiments with more than two
ferromagnetic layers, the combined moments of the ferromagnetic
layers causes the net magnetic moment of the reference layer 14 to
be substantially zero. Thus, in some embodiments, the ferromagnetic
layers do not have identical moments.
[0034] Exchange coupling between the magnetization vectors M2A, M2B
of the two ferromagnetic layers 50 and 52 may be very strong.
Consequently, a very large magnetic field may be needed to saturate
the magnetization vectors M2A, M2B of the ferromagnetic layers 50
and 52. For example, a field of 4000 Oersted (e.g.,
3.2.times.10.sup.5 Amperes/Meter) may be needed to completely
rotate both the ferromagnetic layers 50 and 52 in the same
direction. By selecting suitable thickness and material for the
spacer layer 54, the exchange coupling may be made suitably strong
such that at normal operating conditions, very little rotation
occurs and the ferromagnetic layers 50 and 52 point 180 degrees
apart.
[0035] Still referring to FIG. 3A, when exposed to a relatively
high magnetic field, the ferromagnetic layers 50 and 52 may rotate
and orient themselves in the direction of the applied field. This
is illustrated by the parallel double arrows below the hysteresis
loop at the point when Hs is large (positive or negative). This is
also illustrated in FIGS. 4A and 4B, in which the magnetization
vectors M2A and M2B are shown as being parallel to the applied
magnetic field (H). The fields required to saturate both the
ferromagnetic layers 50 and 52 are substantially larger than
normally used to read a magneto-resistive device.
[0036] Under lower magnetic fields, there may be two stable
magnetization orientations and these may be orthogonal to the
applied field. Referring again to FIG. 3, when near the y-axis, the
arrows are shown as pointing up and down to indicate the
magnetization vectors of the ferromagnetic layers 50 and 52 are
orthogonal to the applied field.
[0037] FIG. 3B illustrates a detail of the hysteresis loop of FIG.
3A showing low applied magnetic fields. Under magnetic fields that
are typical for read and write operations in a magneto-resistive
device the amount of rotation away from 180 degrees between the
magnetic vectors M2A and M2B may be very small. For example, when
the applied magnetic field is approximately 100 Oersted (8000
Amperes/Meter), the angle theta may be approximately 2 degrees.
Theta is the angle by which the magnetic vectors M2A and M2B
deviate from being orthogonal to the applied field. Thus, the
magnetic vectors are nearly orthogonal to the applied field under
normal operating conditions. This state is also shown in FIGS. 2A
and 2B.
[0038] Referring now to the flowchart of FIG. 5, an embodiment
provides for a method of operating a magnetic storage device. Step
510 of the process is storing bit of information in the data layer
12. This may be achieved by applying one or more currents to
selected word lines 18 and/or bit lines 20 to set the magnetization
vector M1 in the data layer 12. The magnetic orientations M2A, M2B
of the ferromagnetic layers 50 and 52 are not critical at this
time, as they may be established later.
[0039] For example, in one embodiment, data may be written to the
magnetic tunnel junction 11 by supplying write currents to the
first and second conductors 18 and 20. The current supplied to the
first conductor 18 creates a magnetic field about the first
conductor 18, and the current supplied to the second conductor 20
creates a magnetic field about the second conductor 20. The two
magnetic fields, when combined, exceed the coercivity of the data
layer 12 and, therefore, cause the magnetization vector (M1) of the
data layer 12 to be set in a desired orientation (the orientation
will depend upon the directions of the currents supplied to the
first and second conductors 18 and 20). The magnetization will be
set to either the orientation that corresponds to a logic `1` or
the orientation that corresponds to a logic `0`.
[0040] After write currents are removed from the conductors 18 and
20, the magnetization vector (M1) of the data layer 12 retains its
orientation. The magnetization vectors (M2A and M2B) of the
ferromagnetic layers 50 and 52 may be affected by the write process
and may or may not retain that orientation. If the reference layer
14 is "ultra-soft," the magnetization vectors (M2A and M2B) may
lose their magnetization orientations when the write currents are
removed from the first and second conductors 18 and 20.
[0041] In one embodiment, the third conductor 22 may be used to
assist with write operations. By supplying a current to the third
conductor 22 during write operations, the resulting magnetic field
about the third conductor 22 may combine with the other two
magnetic fields to help set the magnetization vector (M1) of the
data layer 12 in the desired orientation.
[0042] In step 520, a magnetic field is applied to a reference
layer 14 to establish a magnetic orientation (e.g., M2A) in a layer
(e.g., ferromagnetic layer 52) of the reference layer 14 that is
substantially orthogonal to the magnetic field. The reference layer
14 itself has first and second layers that have substantially the
same coercivity and are magnetically coupled in opposite
directions. The magnetic orientation of the first ferromagnetic
layer 52 of the reference layer 14 is either substantially parallel
or substantially anti-parallel to the magnetic orientation M1 in
the data layer 12.
[0043] For example, a current may be supplied to the third
conductor 22, and the resulting magnetic field causes the
magnetization vectors M2A and M2B of the ferromagnetic layers 50
and 52 to assume a specific orientation. Because the third
conductor 22 is farther from one ferromagnetic layer 50 or 52 than
the other, the magnetic orientation may be known by its preference
to favor one direction in each ferromagnetic layer 50, 52, the
direction depending, in part, on the direction of the current
through the third conductor 22. The resulting magnetic field does
not affect the magnetization vector (M1) of the data layer 12.
Furthermore, since the coercivity of the reference layer 14 is
extremely low, the magnitude of the third conductor current may be
low. For example, the coercivity of the balanced synthetic
ferrimagnet reference layer 14 may be only a few Oersteds (e.g., a
few hundred Amperes/Meter).
[0044] In step 530, the resistance between the first ferromagnetic
layer 52 and the data layer 12 is measured to determine the
magnetic orientation M1 of the data layer 12. In this fashion, the
bit stored in the storage device is determined. This step may be
accomplished by applying a voltage across the magnetic tunnel
junction 11 as the current is supplied to the third conductor 22.
The first and second conductors 18 and 20 may be used to apply the
voltage across the magnetic tunnel junction 11. The voltage causes
a sense current to flow through the magnetic tunnel junction 11.
The sensed current (I.sub.s) is inversely proportional to the
resistance of the magnetic tunnel junction 11. Thus, I.sub.s=V/R or
I.sub.s=V/(R+.DELTA.R), where V is the applied Voltage, I.sub.s is
the sensed current, R is the nominal resistance of the device 10,
and .DELTA.R is the difference in resistance between the parallel
magnetization orientation and the anti-parallel magnetization
orientation.
[0045] Reference is now made to FIG. 6, which illustrates an MRAM
device 610 including an array 612 of magnetic tunnel junctions 11.
The magnetic tunnel junctions 11 are arranged in rows and columns,
with the rows extending along a y-direction and the columns
extending along an x-direction. Only a relatively small number of
the magnetic tunnel junctions 11 are shown to simplify the
illustration of the MRAM device 610. In practice, arrays of any
size may be used.
[0046] Electrically conductive elements functioning as word lines
18 extend along the x-direction in a plane on one side of the array
612. The word lines 18 are in contact with the data layers 12 of
the magnetic tunnel junctions 11. Electrically conductive elements
functioning as bit lines 20 extend along the y-direction in a plane
on an adjacent side of the array 612. The bit lines 20 are in
contact with the reference layers 14 of the magnetic tunnel
junctions 11. There may be one word line 18 for each row of the
array 612 and one bit line 20 for each column of the array 612.
Each magnetic memory tunnel junction 11 is located at a cross point
of a word line 18 and a bit line 20.
[0047] Electrically conductive elements functioning as read lines
22 also extend along the y-direction. The read lines 22 may be
further from the tunnel junctions than the bit lines 20, and may be
insulated from the bit lines 20. The MRAM device 610 also includes
first and second row decoders 614a and 614b, first and second
column decoders 616a and 616b, and a read/write circuit 618. The
read/write circuit 618 includes a sense amplifier 620, ground
connections 622, a row current source 624, a voltage source 626,
and a column current source 628.
[0048] During a write operation on a selected magnetic tunnel
junction 11, the first row decoder 614a connects one end of a
selected word line 18 to the row current source 624, the second row
decoder 614b connects an opposite end of the selected word line 18
to ground, the first column decoder 616a connects one end of a
selected bit line 20 to ground, and the second column decoder 616b
connects the opposite end of the selected bit line 20 to the column
current source 628. As a result, write currents flow through the
selected word and bit lines 18 and 20. The write currents create
magnetic fields, which cause the magnetic tunnel junction 11 to
switch. The column decoders 616a and 616b may also cause a write
current to flow through the read line 22 crossing the selected
magnetic tunnel junction 11. This third write current creates an
additional magnetic field that assists in switching the selected
magnetic tunnel junction 11.
[0049] The easy axis of the data layers 12 may be oriented in the
y-direction. Thus, the data layers 12 may have their magnetic
vectors in the y-direction.
[0050] During a read operation on a selected magnetic tunnel
junction 11, the first and second column decoders 616a and 616b may
cause a steady read (reference) current to flow through the read
line 22 crossing the selected magnetic tunnel junction 11. The read
current will generate a magnetic field that causes the
magnetization vectors M2A and M2B to be oriented parallel to the
word lines 18.
[0051] While the read current is still applied, the first row
decoder 614a connects the voltage source 626 to a selected word
line 18, and the first column decoder 616a connects a selected bit
line 20 to a virtual ground input of the sense amplifier 620. As a
result, a sense current flows through the selected magnetic tunnel
junction 11 to the input of the sense amplifier 620. In this
fashion, the resistance of the selected magnetic tunnel junction 11
may be determined. However, the present invention is not limited to
this method of determining the resistance of the magnetic tunnel
junctions 11.
[0052] The magnetic tunnel junctions 11 described thus far include
individual reference layers 14, with each reference layer 14 having
the same geometry as its corresponding data layer 12 and tunnel
barrier 16. However, the present invention is not limited to
reference layers 14 having the same geometry as the data layers and
tunnel barriers. Instead, the reference layers 14 may have the same
geometry as the word and bit lines 18 and 20, or otherwise.
[0053] While the present invention has been described in particular
embodiments, it should be appreciated that the present invention
should not be construed as limited by such embodiments, but rather
construed according to the below claims.
* * * * *