Title: High speed low power magnetic devices based on current induced spin-momentum transfer
Abstract: The present invention generally relates to the field of magnetic devices for memory cells that can serve as non-volatile memory. More specifically, the present invention describes a high speed and low power method by which a spin polarized electrical current can be used to control and switch the magnetization direction of a magnetic region in such a device. The magnetic device comprises a pinned magnetic layer with a fixed magnetization direction, a free magnetic layer with a free magnetization direction, and a read-out magnetic layer with a fixed magnetization direction. The pinned magnetic layer and the free magnetic layer are separated by a non-magnetic layer, and the free magnetic layer and the read-out magnetic layer are separated by another non-magnetic layer. The magnetization directions of the pinned and free layers generally do not point along the same axis. The non-magnetic layers minimize the magnetic interaction between the magnetic layers. A current is applied to the device to induce a torque that alters the magnetic state of the device so that it can act as a magnetic memory for writing information. The resistance, which depends on the magnetic state of the device, is measured to thereby read out the information stored in the device.
Patent Number: 6,980,469 Issued on 12/27/2005 to Kent, et al.
| Inventors:
|
Kent; Andrew (New York, NY);
Gonzalez Garcia; Enrique (New York, NY);
Özyilmaz; Barbaros (Brooklyn, NY)
|
| Assignee:
|
New York University (New York, NY)
|
| Appl. No.:
|
643762 |
| Filed:
|
August 19, 2003 |
| Current U.S. Class: |
365/171; 365/158; 365/173; 365/225.5; 365/97; 365/66; 257/421 |
| Intern'l Class: |
G11C 011/14 |
| Field of Search: |
365/171,173,158,97,225.5,66
257/421
|
References Cited [Referenced By]
U.S. Patent Documents
| 5695864 | Dec., 1997 | Slonczewski.
| |
| 5856897 | Jan., 1999 | Mauri.
| |
| 5896252 | Apr., 1999 | Kanai.
| |
| 5966323 | Oct., 1999 | Chen et al.
| |
| 6134138 | Oct., 2000 | Lu et al.
| |
| 6154349 | Nov., 2000 | Kanai et al.
| |
| 6256223 | Jul., 2001 | Sun.
| |
| 6522137 | Feb., 2003 | Sun et al.
| |
| 6532164 | Mar., 2003 | Redon et al.
| |
| 6538918 | Mar., 2003 | Swanson et al.
| |
| 6603677 | Aug., 2003 | Redon et al.
| |
| 6654278 | Nov., 2003 | Engel et al.
| |
| 6744086 | Jun., 2004 | Daughton et al.
| |
| 6750491 | Jun., 2004 | Sharma et al.
| |
| 6812537 | Nov., 2004 | Okazawa et al.
| |
| 2003/0218903 | Nov., 2003 | Luo.
| |
| 2004/0130936 | Jul., 2004 | Nguyen et al.
| |
| Foreign Patent Documents |
| 0200/1195878 | Jul., 2001 | JP.
| |
Other References
R.H. Koch, et al. Physical Review Letters, vol. 84, No. 23, Jun. 2000 Thermally
Assisted Magnetization Reversal in Submicron-Sized Magnetic Thin Films.
Lee et al., Analytical investigation of spin-transfer dynamics using a perpendicular-to-plane
polarizer,, Applied Physics Letters 86, pp. 022505-022505-3 2005.
|
Primary Examiner: Nguyen; Viet Q.
Attorney, Agent or Firm: Darby & Darby
Claims
1. A magnetic device comprising:
a pinned magnetic layer with a magnetization vector with a fixed magnetization direction;
a free magnetic layer with at least one magnetization vector with a changeable
magnetization direction;
a first non-magnetic layer spatially separating said free magnetic layer and
said pinned magnetic layer;
a read-out magnetic layer with a magnetization vector with a fixed magnetization
direction; and
a second non-magnetic layer that spatially separates said free magnetic layer
and said read-out magnetic layer such that the mutual magnetic interaction between
said free magnetic layer and said read-out magnetic layer is minimized.
2. The magnetic device according to claim 1, wherein one of said magnetization
directions of said pinned magnetic layer, said free magnetic layer, and said read-out
magnetic layer lies along an axis which is different than at least one of axes
along which said other magnetization directions lie.
3. The magnetic device according to claim 1, wherein:
said fixed magnetization direction of said pinned magnetic layer is perpendicular
to a plane of said free magnetic layer; and
said changeable magnetization direction of said free magnetic layer is perpendicular
to an axis extending longitudinally through said magnetic device.
4. The magnetic device according to claim 1, wherein said changeable magnetization
direction of said free magnetic layer and said fixed magnetization direction of
said read-out layer switch between being in anti-parallel alignment and parallel alignment.
5. The magnetic device according to claim 1, wherein said free magnetic layer
has a single magnetization vector with a changeable magnetization vector.
6. The magnetic device according to claim 1, wherein said magnetization direction
of said magnetization vector of said free magnetic layer represents a bit of information.
7. The magnetic device according to claim 1, wherein:
said magnetic device is pillar-shaped; and
said pinned magnetic layer, said first non-magnetic layer, said free magnetic
layer, said second magnetic layer, and said read-out magnetic layer are less than
approximately 200 nm laterally and approximately 1 nm to 50 nm thick.
8. The magnetic device according to claim 1, wherein said pinned magnetic layer,
said free magnetic layer, and said read-out magnetic layer are comprised of a member
of the group consisting of Co, Ni, Fe, an alloy of Co and Ni, an alloy of Co and
Fe, an alloy of Ni and Fe, an alloy of Co, Ni, and Fe, and permalloy Ni1-xFex.
9. The magnetic device according to claim 1, wherein said pinned magnetic layer,
said free magnetic layer, and said read-out magnetic layer are comprised of a non-magnetic
metal and a member of the group consisting of an alloy of Co and Ni, an alloy of
Co and Fe, an alloy of Ni and Fe, an alloy of Co, Ni, and Fe, such that said non-magnetic
metal and said member are ferromagnetically ordered at room temperature.
10. The magnetic device according to claim 9, wherein said non-magnetic metal
is a member of the group consisting of Cu, Pd, and Pt.
11. The magnetic device according to claim 1, wherein said pinned magnetic layer,
said free magnetic layer, and said read-out magnetic layer are comprised of a member
of the group consisting of NiMnSb and a conducting magnetic oxide.
12. The magnetic device according to claim 11, wherein said conducting magnetic
oxide is either CrO2 or Fe3O4.
13. The magnetic device according to claim 1, wherein said non-magnetic layers
are comprised of at least one member of the group consisting of Cu, Cr, Au, Ag,
and Al.
14. A memory system comprising:
a memory cell comprising:
a pinned magnetic layer with a magnetization vector with a fixed magnetization direction;
a free magnetic layer with at least one magnetization vector with a changeable
magnetization direction;
a first non-magnetic layer spatially separating said free magnetic layer and
said pinned magnetic layer;
a read-out magnetic layer with a magnetization vector with a fixed magnetization
direction; and
a second non-magnetic layer that spatially separates said free magnetic layer
and said read-out magnetic layer such that the mutual magnetic interaction between
said free magnetic layer and said read-out magnetic layer is minimized; and
an electric current source connected to said pinned magnetic layer and said read-out
magnetic layer so that an electric current can traverse said memory cell.
15. The memory system according to claim 14, wherein one of said magnetization
directions of said pinned magnetic layer, said free magnetic layer, and said read-out
magnetic layer lies along an axis which is different than at least one of axes
along which said other magnetization directions lie.
16. The memory system according to claim 14, further comprising a means for measuring
the resistance between said pinned magnetic layer and said read-out magnetic layer.
17. The memory system according to claim 16, wherein said resistance measuring
means comprises a voltmeter connected to said pinned magnetic layer and said read-out
magnetic layer.
18. The memory system according to claim 14, wherein said electric current comprises
a single current pulse.
19. The memory system according to claim 14, wherein said electric current comprises
two current pulses wherein one of said two current pulses is a negative current
pulse and the other of said two current pulses is a positive current pulse.
20. The memory system according to claim 14, wherein said electric current is
applied in a sub-nanosecond period of time.
21. A method of magnetic switching using current-induced spin-momentum transfer,
said method comprising the steps of:
applying an electric current to a magnetic device, wherein said electric current
comprises two current pulses wherein one of said two current pulses is a negative
current pulse and the other of said two current pulses is a positive current pulse,
wherein said current applying step occurs in a sub-nanosecond period of time; and
stopping said electric current when a magnetization vector of said magnetic device
has rotated 180° while said electric current is applied.
22. A method of making a memory cell, said method comprising the steps of:
forming a first non-magnetic layer on a pinned magnetic layer, said pinned magnetic
layer having a magnetization vector with a fixed magnetization direction;
forming a free magnetic layer with at least one magnetization vector with a changeable
magnetization direction on said first non-magnetic layer;
forming a second non-magnetic layer on said free magnetic layer; and
forming a read-out magnetic layer with a magnetization vector with a fixed magnetization
direction on said second non-magnetic layer.
23. The method of making a memory cell according to claim 22, wherein one of
said magnetization directions of said pinned magnetic layer, said free magnetic
layer, and said read-out magnetic layer lies along an axis which is different than
at least one of axes along which said other magnetization directions lie.
24. The method of making a memory cell according to claim 22, further comprising
the step of:
connecting an electric current source to said pinned magnetic layer and said
read-out magnetic layer so that an electric current can traverse said memory cell.
25. The method of making a memory cell according to claim 22, further comprising
the step of:
measuring the resistance between said pinned magnetic layer and said read-out
magnetic layer.
26. The method of making a memory cell according to claim 25, wherein said resistance
measuring step comprises the step of connecting a voltmeter to said pinned magnetic
layer and said read-out magnetic layer.
27. The method of making a memory cell according to claim 22, wherein said pinned
magnetic layer, said free magnetic layer, and said read-out magnetic layer are
comprised of a member of the group consisting of Co, Ni, Fe, an alloy of Co and
Ni, an alloy of Co and Fe, an alloy of Ni and Fe, an alloy of Co, Ni, and Fe, and
a permalloy Ni1-xFex.
28. The method of making a memory cell according to claim 22, wherein said pinned
magnetic layer, said free magnetic layer, and said read-out magnetic layer are
comprised of a non-magnetic metal and a member of the group consisting of an alloy
of Co and Ni, an alloy of Co and Fe, an alloy of Ni and Fe, an alloy of Co, Ni,
and Fe, such that said non-magnetic metal and said member are ferromagnetically
ordered at room temperature.
29. The method of making a memory cell according to claim 28, wherein said non-magnetic
metal is a member of the group consisting of Cu, Pd, and Pt.
30. The method of making a memory cell according to claim 22, wherein said pinned
magnetic layer, said free magnetic layer, and said read-out magnetic layer are
comprised of a member of the group consisting of NiMnSb and a conducting magnetic oxide.
31. The method of making a memory cell according to claim 30, wherein said conducting
magnetic oxide is either CrO2 or Fe3O4.
32. The method of making a memory cell according to claim 22, wherein said non-magnetic
layers are comprised of at least one member of the group consisting of Cu, Cr,
Au, Ag, and Al.
33. The magnetic device according to claim 1, wherein the mutual magnetic interaction
between said free magnetic layer and said pinned magnetic layer is minimized.
34. The memory system according to claim 14, wherein the mutual magnetic interaction
between said free magnetic layer and said pinned magnetic layer is minimized.
35. The method of making a memory cell according to claim 22, wherein said first
and said second non-magnetic layers minimize the mutual magnetic interaction between
said pinned magnetic layer, said free magnetic layer, and said read-out magnetic layer.
Description
FIELD OF THE INVENTION
The present invention generally relates to magnetic devices used in memory and
information processing applications, such as giant magnetoresistance (GMR) devices.
More specifically, the present invention describes a high speed and low power method
by which a spin polarized electrical current can be used to control and switch
the direction of magnetization of a magnetic region in such a device.
BACKGROUND OF THE INVENTION
Magnetic devices that use a flow of spin-polarized electrons are of interest
for magnetic memory and information processing applications. Such a device generally
includes at least two ferromagnetic electrodes that are separated by a non-magnetic
material, such as a metal or insulator. The thicknesses of the electrodes are typically
in the range of 1 nm to 50 nm. If the non-magnetic material is a metal, then this
type of device is known as a giant magnetoresistance or spin-valve device. The
resistance of the device depends on the relative magnetization orientation of the
magnetic electrodes, such as whether they are oriented parallel or anti-parallel
(i.e., the magnetizations lie on parallel lines but point in opposite directions).
One electrode typically has its magnetization pinned, i.e., it has a higher coercivity
than the other electrode and requires larger magnetic fields or spin-polarized
currents to change the orientation of its magnetization. The second layer is known
as the free electrode and its magnetization direction can be changed relative to
the former. Information can be stored in the orientation of this second layer.
For example, "1" or "0" can be represented by anti-parallel alignment of the layers
and "0" or "1" by parallel alignment. The device resistance will be different for
these two states and thus the device resistance can be used to distinguish "1"
from "0." An important feature of such a device is that it is a non-volatile memory,
since the device maintains the information even when the power is off, like a magnetic
hard drive. The magnet electrodes can be sub-micron in lateral size and the magnetization
direction can still be stable with respect to thermal fluctuations.
In conventional magnetic random access memory (MRAM) designs, magnetic fields
are used to switch the magnetization direction of the free electrode. These magnetic
fields are produced using current carrying wires near the magnetic electrodes.
The wires must be small in cross-section because memory devices consist of dense
arrays of MRAM cells. As the magnetic fields from the wires generate long-range
magnetic fields (magnetic fields decay only as the inverse of the distance from
the center of the wire) there will be cross-talk between elements of the arrays,
and one device will experience the magnetic fields from the other devices. This
cross-talk will limit the density of the memory and/or cause errors in memory operations.
Further, the magnetic fields generated by such wires are limited to about 0.1 Tesla
at the position of the electrodes, which leads to slow device operation. Importantly,
conventional memory designs also use stochastic (random) processes or fluctuating
fields to initiate the switching events, which is inherently slow and unreliable
(see, for example, R. H. Koch et al., Phys. Rev. Lett. 84, 5419 (2000)).
In U.S. Pat. No. 5,695,864 and several other publications (e.g., J. Slonckewski,
Journal of Magnetism and Magnetic Materials 159, L1 (1996)), John Slonckewski described
a mechanism by which a spin-polarized current can be used to directly change the
magnetic orientation of a magnetic electrode. In the proposed mechanism, the spin
angular momentum of the flowing electrons interacts directly with the background
magnetization of a magnetic region. The moving electrons transfer a portion of
their spin-angular momentum to the background magnetization and produce a torque
on the magnetization in this region. This torque can alter the direction of magnetization
of this region and switch its magnetization direction. Further, this interaction
is local, since it only acts on regions through which the current flows. However,
the proposed mechanism was purely theoretical.
Slonckewski's patent describes MRAM devices that use spin-momentum
transfer for magnetic switching. However, the proposed devices are slow and rely
on fluctuating magnetic fields and stochastic processes to initiate magnetization
switching. Further, large current densities are needed to switch the devices. In
describing the preferred embodiment of his "latch or logic gate," Slonckewski states
" . . . the preferred axes of the 3 magnets F1, F2, and F3
are all "vertical" (i.e., in the same direction or orientation) as discussed above.
Other orientations can serve as long as they are parallel to the same axis." As
we describe below, our device makes use of layer magnetizations that are not parallel
to the same axis, to great advantage in speed, reliability, and power consumption.
U.S. Pat. No. 6,256,223 to Jonathan Sun also describes devices that use current-induced
magnetic fields and demonstrates in experiment the operation of such devices. However,
the devices proposed were unreliable, as there was little consistency with regard
to device characteristics. Further, the estimated time scale for magnetic switching
was 50 nsec for operation at large current densities.
Devices are needed that exhibit high speed and reliable operation under the
action of a spin-polarized current. This includes devices that operate with lower
power and have lower threshold currents for switching the magnetization orientation.
SUMMARY OF THE INVENTION
In view of the limitations associated with conventional designs of devices that
use spin-momentum transfer, an object of the present invention is to provide a
structure that is optimal for a magnetic memory or magnetic information processing device.
It is another object of the present invention to produce a magnetic device that
has advantages in terms of speed of operation.
It is a further object of the present invention to produce a magnetic device
that
has advantages in terms of reliability.
It is a further object of the present invention to produce a magnetic device
that
requires lower power to operate.
These and additional objects of the invention are accomplished by a device
that employs magnetic layers in which the layer magnetization directions do not
lie along the same axis. For instance in one embodiment, two magnetic regions have
magnetizations that are orthogonal.
The invention is a magnetic device comprised of ferromagnetic and non-magnetic
layers through which current can flow. The magnetic device is comprised of a ferromagnetic
layer with a fixed magnetization direction and another ferromagnetic layer separated
from the first by a non-magnetic region that has a magnetization that is free to
rotate in response to applied currents. A third ferromagnetic layer, again, separated
from the others by a non-magnetic layer, has a fixed magnetization direction and
can be employed to readout the magnetization direction of the free ferromagnetic
layer. The magnetization directions of the ferromagnetic layers are not all along
the same axis. In one of the preferred embodiments, the first fixed ferromagnetic
layer's magnetization direction is perpendicular to the plane of the layer, while
the free ferromagnetic layer's magnetization is in the plane of the layer. As described
above, a current flow between the layers transfers spin-angular momentum from the
fixed magnetization layer to the free magnetization layer and produces a torque
on the magnetization of the free layer. The torque is proportional to the vector
triple product of the magnetization direction of the fixed and free layer, with
a factor of proportionality that depends on the current and the spin polarization
of the current. A large torque is produced when the magnetization directions of
the fixed and free layers are orthogonal.
This large torque acting on the magnetization direction of the free magnetic
layer causes the magnetization of the free magnetic layer to rotate out of the
plane of the layer. Since the thickness of the free magnetic layer is less than
the width and length dimensions, the rotation of the magnetization of the free
magnetic layer out of the plane of the layer generates a large magnetic field,
a 'demagnetizing' field, which is perpendicular to the plane of the layer.
This demagnetizing field forces the magnetization vector of the free magnetic
layer to precess, i.e., for the magnetization direction to rotate around the direction
of the demagnetization magnetic field. The demagnetizing field also determines
the rate of precession. A large demagnetizing field results in a high precession
rate, which is an optimal condition for fast magnetic switching. An advantage of
this magnetic device is that random fluctuating forces or fields are not necessary
to initiate or control the magnetic response of the layers.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features of the present invention will be more readily
apparent from the following detailed description and drawings of the illustrative
embodiments of the invention wherein like reference numbers refer to similar elements
throughout the views and in which:
FIG. 1 is an illustration of a magnetic device according to the present invention;
FIGS. 2A-2E are illustrations of the free magnetic layer showing the magnetization
vector and the demagnetizing field of the electronic device of FIG. 1 during the
application of pulses of current as illustrated in FIG. 3A;
FIG. 3A is an illustration of a current waveform that may be applied to the
magnetic device;
FIG. 3B is an illustration of an alternate current waveform that may be applied
to the magnetic device;
FIG. 4 is an illustration of a memory cell according to one embodiment of the
present invention;
FIGS. 5A-5E are illustrations of the free magnetic layer showing the magnetization
vector and the demagnetizing field of the memory cell of FIG. 4;
FIG. 6A is an illustration of a current waveform that may be applied to the
memory cell of FIG. 4 during a write operation;
FIG. 6B is an illustration of a resistance measured from the memory cell during
a read-out operation before and after the current pulse shown in FIG. 6A is applied;
FIG. 7 is an illustration of the free magnetic layer of a 4-state memory cell;
FIG. 8 is an illustration of an example of a current waveform applied to the
magnetic device;
FIG. 9 is an illustration of the magnetization components of the free magnetic
layer during and after the application of the current pulse shown in FIG. 8; and
FIG. 10 is an illustration of a memory cell according to one embodiment of the
present invention in which during writing operations no net current passes through
the free magnetic layer.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Structure of a Basic Magnetic Device
To illustrate the basic concept, FIG. 1 shows a multilayered, pillar-shaped magnetic
device comprising a pinned magnetic layer FM
1 with a fixed magnetization
direction and a free magnetic layer FM
2 with a free magnetization direction.
{right arrow over (m)}
1 is the magnetization vector of the pinned magnetic
layer FM
1, and {right arrow over (m)}
2 is the magnetization vector
of the free magnetic layer FM
2. The pinned magnetic layer FM
1 acts
as a source of spin angular momentum.
The pinned magnetic layer FM
1 and the free magnetic layer FM
2 are
separated by a first non-magnetic layer N
1 that spatially separates the
two layers FM
1 and FM
2 such that their mutual magnetic interaction
is minimized. The pillar-shaped magnetic device is typically sized in nanometers,
e.g., it may be less than approximately 200 nm laterally.
The free magnetic layer FM
2 is essentially a magnetic thin film element
imbedded in a pillar-shaped magnetic device with two additional layers—the
pinned magnetic layer FM
1 and the non-magnetic layer N
1. The layer
thicknesses are typically approximately 1 nm to 50 nm.
These pillar-shaped magnetic devices can be fabricated in a stacked sequence
of layers by many different means, including sputtering, thermal and electron-beam
evaporation through a sub-micron stencil mask. These magnetic devices can also
be fabricated in a stack sequence using sputtering, thermal and electron-beam evaporation
to form a multilayered film followed by a subtractive nanofabrication process that
removes materials to leave the pillar-shaped magnetic device on a substrate surface,
such as that of a silicon of other semiconducting or insulating wafer.
Materials for the ferromagnetic layers include (but are not limited to)
Fe, Co, Ni, and alloys of these elements, such as Ni
1-xFe
x;
alloys of these ferromagnetic metals with non-magnetic metals, such as Cu, Pd,
Pt, NiMnSb, at compositions in which the materials are ferromagnetically ordered
at room temperature; conducting materials; and conducting magnetic oxides such
as CrO
2 and Fe
3O
4. For the nonmagnetic layers,
materials include (but are not limited to) Cu, Cr, Au, Ag, and Al. The main requirement
for the non-magnetic layer is the absence of scattering of the electron spin-direction
on a short length scale, which is less than about the layer thickness.
An electric current source is connected to the pinned magnetic layer FM
1
and the free magnetic layer FM
2 so that an electric current I can traverse
the pillar device.
Method of Magnetic Switching
An electric current I is applied to the pillar-shaped magnetic device so that
the current I flows through the various layers of the device, from the pinned magnetic
layer FM
1 to the first non-magnetic layer N
1 to the free magnetic
layer FM
2. The applied current I results in a transfer of angular momentum
from the pinned magnetic layer FM
1 to the free magnetic layer FM
2.
As stated above, a transfer of angular momentum from one magnetic region to another
can produce a torque.
FIGS. 2A-2E show steps in the method of magnetic switching using the magnetic
device shown in FIG. 1 and for convenience, FIGS. 2A-2E only show the free magnetic
layer FM
2 and the magnetization vector {right arrow over (m)}
2
of the free magnetic layer FM
2. FIG. 2A shows the initial state of the free
magnetic layer FM
2 before the current I is applied.
As shown in FIGS. 2B-2D, applying a current I, which can be of a form as shown
in FIGS. 3A and 3B, results in the transfer of angular momentum from the pinned
magnetic layer FM
1 to the free magnetic layer FM
2. This transfer
of angular momentum from the pinned magnetic layer FM
1 to the free magnetic
layer FM
2 produces a torque {right arrow over (τ)}
s on
the magnetic moment of the free magnetic layer FM
2.
The torque {right arrow over (τ)}
s per unit magnetization of
the free layer is proportional to the vector triple product a
I{circumflex
over (m)}
2×({circumflex over (m)}
2×{circumflex
over (m)}
1), where {circumflex over (m)}
2 is a unit vector
in the direction of the magnetic moment of the free magnetic layer FM
2 and
{circumflex over (m)}
1 is a unit vector in the direction of the magnetic
moment of the pinned magnetic layer FM
1. The prefactor, a
I, depends
on the current I, the spin-polarization P of the current I, and the cosine of the
angle between the free and pinned magnetic layers, cos(θ), such that a
I=ℏIg(P,cos(θ))/(mMV).
ℏ is the reduced Planck's constant, g is a function of the spin-polarization
P and cos(θ), M is the magnetization density of the free layer, m is the
mass of the electron, and V is the volume of the free layer (see, J. Slonczewski,
Journal of Magnetism and Magnetic Materials 159, L1 (1996)). Thus, a large torque
{right arrow over (τ)}
s is produced when the magnetic moments
of the pinned magnetic layer FM
1 and the free magnetic layer FM
2
are perpendicular.
This torque {right arrow over (τ)}
s, which acts on the magnetic
moment of the free magnetic layer FM
2, causes the magnetization of the free
magnetic layer FM
2 to rotate out of the plane of the layer. Since the thickness
of the free magnetic layer FM
2 is less than the width and length dimensions
of the free magnetic layer FM
2, the rotation of the magnetization vector
{right arrow over (m)}
2 of the free magnetic layer FM
2 out of
the plane of the layer generates a large magnetic field, a 'demagnetizing' field,
which is perpendicular to the plane of the layer.
This demagnetizing field forces the magnetization vector {right arrow over (m)}
2
of the free magnetic layer FM
2 to precess, i.e., to move such that the magnetization
direction rotates about the magnetic field axis. The demagnetizing field also determines
the rate of precession. A large demagnetizing field results in an extremely high
precession rate, which is an optimal condition for fast magnetic switching.
Thus, in an optimal configuration of the magnetic memory device for fast magnetic
switching, the magnetic moment of the pinned magnetic layer FM
1 is perpendicular
to the plane of the free magnetic layer FM
2, and the magnetic moment of
the free magnetic layer FM
2 is perpendicular to the axis of the pillar of
thin layers and lies in the plane of the free magnetic layer FM
2.
FIG. 2E shows the free magnetic layer FM
2 after the magnetic switching
process is completed. As shown in FIGS. 2A and 2E, the magnetic switching process
causes the magnetization vector {right arrow over (m)}
2 of the free
magnetic layer FM
2 to switch by reversing direction by rotating 180°.
FIGS. 3A and 3B show two different forms of current input that may be applied
to the magnetic device. The current input shown in FIG. 3A is comprised of two
current pulses of short duration, a first positive current pulse followed by a
second negative current pulse. This form of current input results in writing a
'1' or a '0'. Alternatively, the first current pulse can be negative and the second
current pulse can be positive, as long as the two current pulses are of opposite
polarity. In both cases, the state of the magnetic bit will be changed from '1'
to '0' or '0' to '1' (i.e., the final state will be the complement of the initial
state of the bit). The current input shown in FIG. 3A is used in the method of
magnetic switching described above and shown in FIGS. 2A-2E. Using a current input
formed of two current pulses results in a faster magnetic switching process.
The first current pulse starts the precession of the magnetization vector {right
arrow over (m)}
2 of the free magnetic layer FM
2. After the completion
of the first current pulse, the second current pulse is applied to stop the precession
at a desired state.
The second current pulse is not essential to the operation of the device, but
it enables higher speed switching. For example, the current input shown in FIG.
3B is comprised of a single positive current pulse. Alternatively, a single negative
current pulse may also be applied to the magnetic device. Simulations show that
many different types of current pulses switch FM
2. Therefore device operation
is certainly not limited to the current pulses shown in FIG. 3.
Structure of a Memory Cell
The magnetic device described above can be incorporated into a memory cell for
inclusion into arrays of memory cells to make up a magnetic memory. According to
one embodiment as shown in FIG. 4, the magnetic device of the present invention,
when implemented as a memory cell, is a multilayered, pillar-shaped device having
a pinned magnetic layer FM
1 with a fixed magnetization direction, a free
magnetic layer FM
2 with a free magnetization direction, and a read-out magnetic
layer FM
3 with a fixed magnetization direction. {right arrow over (m)}
1
is the magnetization vector of the pinned magnetic layer FM
1, {right arrow
over (m)}
2 is the magnetization vector of the free magnetic layer FM
2,
and {right arrow over (m)}
3 is the magnetization vector of the read-out
magnetic layer FM
3.
The pinned magnetic layer FM
1 and the free magnetic layer FM
2 are
separated by a first non-magnetic layer N
1 that spatially separates the
two layers FM
1 and FM
2 such that their mutual magnetic interaction
is minimized. The free magnetic layer FM
2 and the read-out magnetic layer
FM
3 are separated by a second non-magnetic layer N
2 that spatially
separates the two layers FM
2 and FM
3 such that their mutual magnetic
interaction is minimized. The pillar-shaped magnetic device is typically sized
in nanometers, e.g., it may be less than approximately 200 nm.
An electric current source is connected to the pinned magnetic layer FM
1
and the read-out magnetic layer FM
3 so that an electric current I can traverse
the pillar device. A voltmeter is connected to the pinned magnetic layer FM
1
and the read-out magnetic layer FM
3 so that the resistance of the magnetic
device can be measured to thereby read the logical contents of the memory cell.
Method For Writing Information
The magnetic switching process is used when information is written into a memory
cell. To store a logical bit of information in a memory cell, the magnetization
direction of the magnetization vector inside the memory cell is set in one of two
possible orientations to code the logical values of '0' and '1'. This magnetic
device, when implemented as a memory cell, uses the method of magnetic switching
described previously in order to store bits of information. Current pulses are
applied to change the logical value in the magnetic device. The magnetic memory
device described above and shown in FIG. 4 stores one bit of information since
the free magnetic layer FM
2 has a single magnetization vector {right arrow
over (m)}
2 with two stable magnetic states.
An electric current I is applied to the pillar-shaped magnetic memory device
so
that the current I flows through the various layers of the magnetic memory device,
from the pinned magnetic layer FM
1 to the read-out magnetic layer FM
3.
The applied current I results in a transfer of angular momentum from the pinned
magnetic layer FM
1 to the free magnetic layer FM
2.
FIGS. 5A-5E show steps in the method of writing information using the magnetic
memory device shown in FIG. 4 and for convenience, FIGS. 5A-5E only show the free
magnetic layer FM
2 and the magnetization vector {right arrow over (m)}
2
of the free magnetic layer FM
2. FIG. 5A shows the initial state of the free
magnetic layer FM
2 before the current I is applied.
As shown in FIGS. 5B-5D, applying a current I, which can be of a form as shown
in FIGS. 3A and 3B, results in the transfer of angular momentum from the pinned
magnetic layer FM
1 to the free magnetic layer FM
2. FIGS. 2A-2E and
5A-
5E show the change in the orientation of the magnetization vector
{right arrow over (m)}
2 of the free magnetic layer FM
2 as a result
of applying the current to the magnetic device.
FIG. 6A shows a form of the current input that is applied to the magnetic memory
device shown in FIG. 4. The current input of FIG. 6A includes two current pulses
of short duration, a first positive current pulse followed by a second negative
current pulse, which results in writing a '1' or a '0'. Alternatively, the first
current pulse can be negative and the second current pulse can be positive, as
long as the two current pulses are of opposite polarity. In both cases, the state
of the magnetic bit will be changed from '1' to '0' or '0' to '1' (i.e., the final
state will be the complement of the initial state of the bit).
The first current pulse starts the precession of the magnetization vector {right
arrow over (m)}
2 of the free magnetic layer FM
2. After the completion
of the first current pulse, the second current pulse is applied to stop the precession
at a desired state. For this embodiment of the magnetic memory device of the present
invention, the precession is stopped when 180° rotation of the magnetization
vector {right arrow over (m)}
2 of the free magnetic layer FM
2
is achieved.
FIG. 6B shows an example of the corresponding resistance of the device as measured
by the voltmeter connected to the magnetic memory device shown in FIG. 4 with a
small current applied, i.e., a current intensity much less than that used in the
current pulses. The resistance increases after the current pulses of FIG. 6A are
applied to the device. At the initial state shown in FIG. 5A (before the first
positive current pulse), the resistance is at a constant low value. At the final
state shown in FIG. 5E, the resistance is at a constant high value.
Thus, the states shown in FIGS. 5A and 5E correspond to a logical value of
"0" in the initial state and a logical value of "1" in the final state, respectively.
The magnetization vector {right arrow over (m)}
2 of the free magnetic
layer FM
2 in the final state shown in FIG. 5E is in the opposite direction
than the magnetization vector {right arrow over (m)}
2 of the free magnetic
layer FM
2 in the initial state shown in FIG. 5A.
The necessary amplitude of the current pulses can be estimated by numerical modeling
using the equations of micromagnetics, the Landau-Lifzshitz Gilbert equations including
the spin-transfer torque discussed earlier (see, for example, B. Oezyilmaz et al.,
Phys. Rev. Lett. 91, 067203 (2003)). For a free layer comprised of Co with a magnetization
density of M=1400 emu/cm
3, a Gilbert damping parameter α of 0.01,
a spin-polarization of the current P of 0.4, and an in-plane uniaxial anisotropy
field of 1000 kOe. (In this case, the in-plane uniaxial anisotropy constant K is
K=7×10
5erg/cm
3.) For the purposes of this estimation,
the Co free layer is 3 nm thick and has lateral dimensions of 60 nm by 60 nm. We
find that a current pulse of amplitude of 5 mA is more than sufficient to switch
the layer. The current necessary to switch the device is reduced by decreasing
the size of the Co free layer; increasing the spin-polarization of the current,
for example, by using a pinned layer with a higher degree of spin-polarization;
and decreasing the in-plane anisotropy or decreasing the Gilbert damping. For this
current amplitude, a 35 psec pulse is sufficient to switch the device.
With a device resistance of 5 Ohms, the energy dissipation is 5×10
-15
J. This energy dissipation value can be compared to the energy needed to
switch a magnetic device with a spin-polarized current when the pinned layer and
the free layer magnetizations are initially aligned along the same axis. Recent
experiments show that this requires a current of approximately 10 mA applied for
approximately 10 ns in a device with a resistance of 5 Ohms (see, R. Koch et al.,
preprint to be published in Phys. Rev. Lett.). The energy dissipated is thus 5×10
-12
J. Thus, in comparison, the power requirement for our device is quite small.
Further, because the pulse is on only very briefly, in spite of the large current
densities, 1 A/μm
2, no electromigration is expected. Further,
we have operated such devices at current densities 5 times greater than this value
for extended periods (approximately 1 minute) with no device damage (see, B. Oezyilmaz
et al., Phys. Rev. Lett. 91, 067203 (2003)).
Method For Reading Information
The read-out magnetic layer FM
3 is required in the simplest implementation
of the magnetic memory device. The read-out magnetic layer FM
3 has a magnetization
vector {right arrow over (m)}
3 with a fixed magnetization direction.
The magnetization vector {right arrow over (m)}
3 of the read-out magnetic
layer FM
3 can be fixed in a number of ways. For example, the read-out magnetic
layer FM
3 can be formed thicker or of a higher anisotropic magnetic material
or can be placed adjacent to an antiferromagnetic layer to use the phenomena of
exchange biasing. In the phenomena of exchange biasing, the coupling between the
antiferromagnetic layer and the ferromagnetic layer and the large magnetic anisotropy
of the antiferromagnetic layer results in a hardening of the ferromagnetic layer
so that larger magnetic fields and currents are required to change its magnetization direction.
The resistance of the magnetic memory device is very sensitive to the relative
orientation of the magnetization vector {right arrow over (m)}
2 of the
free magnetic layer FM
2 and the magnetization vector {right arrow over (m)}
3
of read-out magnetic layer FM
3. The resistance of the magnetic memory
device is highest when the magnetization vectors {right arrow over (m)}
2 and
{right arrow over (m)}
3 of the free magnetic layer FM
2 and the
read-out layer FM
3, respectively, are in anti-parallel alignment. The resistance
of the magnetic device is lowest when the magnetization vectors {right arrow over
(m)}
2 and {right arrow over (m)}
3 of the layers FM
2
and FM
3, respectively, are in parallel alignment. Thus, a simple resistance
measurement can determine the orientation of the magnetization vector {right arrow
over (m)}
2 of the free magnetic layer FM
2.
The fixed orientation of the magnetization vector {right arrow over (m)}
3
of the read-out magnetic layer FM
3 is set so that it is either in parallel
or anti-parallel alignment with the magnetization vector {right arrow over (m)}
2
of the free magnetic layer FM
2, depending on the orientation of the
magnetization vector {right arrow over (m)}
2 of the free magnetic layer
FM
2. Since the orientation of the magnetization vector {right arrow over
(m)}
2 of the free magnetic layer FM
2 switches so that it can
be rotated 180°, the magnetization vectors {right arrow over (m)}
2 and
{right arrow over (m)}
3 of the free magnetic layer FM
2 and the
read-out layer FM
3, respectively, must be in either anti-parallel or parallel alignment.
Storage of Multiple Bits of Information
The magnetic memory device described above and shown in FIG. 4 has two stable
magnetic states and is able to store one bit of information. According to another
embodiment of the present invention, a magnetic memory device can be constructed
to store multiple bits of information. FIG. 6 shows an example of a free magnetic
layer FM
2 with four stable magnetic states. A magnetic memory device comprising
a free magnetic layer FM
2 with four stable magnetic states is able to store
two bits of information. In this embodiment, current pulses are applied to switch
the magnetization between directions that differ by 90° instead of 180°.
This can be accomplished by current pulses of a different form. For example, the
current pulses can be smaller in amplitude and/or shorter in duration. The readout
layer (FM
3) is then aligned such that each of the four magnetization states
has a different resistance. This requires that the readout layer magnetization
not have an in-plane component that points parallel to any of the four states nor
at 45° to these states.
EXAMPLE
The operation of the magnetic device was simulated using Landau-Lifzshitz Gilbert
equations including a spin-transfer torque.
FIG. 8 shows the amplitude of the current input applied to the magnetic memory
device starting at an initial time t=0 and ending at t=30 picoseconds. This current
input comprises two current pulses similar to the current input shown in FIGS.
3A and 6A.
A 16-picosecond positive current pulse is applied to the magnetic memory device
to start the precession of the magnetization vector {right arrow over (m)}
2
of the free magnetic layer FM2. After this 16-picosecond current pulse,
a 14-picosecond negative current pulse is applied to the magnetic memory device
to stop the precession of the magnetization vector {right arrow over (m)}
2
of the free magnetic layer FM2 to achieve a desired state of the magnetization
vector {right arrow over (m)}
2. For magnetic memory devices, the precession
is stopped after achieving a 180° rotation of the magnetization vector {right
arrow over (m)}
2 of the free magnetic layer FM2.
FIG. 9 shows the magnetization components m
x and m
y of
the magnetization vector {right arrow over (m)}
2 of the free magnetic
layer FM2 in the x- and y-directions shown in FIGS. 2B and 5B. The magnetization
components m
x and m
y are measured during and after the application
of the current input shown in FIG. 8. FIG. 9 shows that the magnetization vector
{right arrow over (m)}
2 of the free magnetic layer FM2 reverses
180° from the initial state, which corresponds to FIG. 5A, to the final state,
which corresponds to FIG. 5E. The magnetization components (m
x, m
y)
are able to switch between (-1,0) to (1,0) as shown by the present invention.
Advantages
The high speed, low power magnetic device of the present invention uses energy
only for read and write operations or logic operations. When not energized, the
information is stored without significant loss. Thus, the magnetic device of the
present invention, when implemented as a memory cell, can be used as a non-volatile memory.
The non-volatile memory provided by the magnetic device of the present invention
is suitable for many applications, such as in computers and portable electronic
devices. In particular, the high speed, low power magnetic device of the present
invention provides several advantages. The performance of the high speed, low power
magnetic device of the present invention compares favorably with flash memory and
other types of non-volatile random access memory (RAM), such as conventional magnetic
RAM (MRAM) and ferroelectric RAM (FRAM).
The current-induced torques act only on the magnetic device that is energized,
i.e., to which a current is applied. Therefore, when multiple magnetic devices
are arranged in an array, such as in magnetic memory, the current-induced spin
transfer does not produce parasitic interactions ("cross-talk") between the neighboring
elements in the array, unlike in conventional magnetic memories in which magnetic
switching is accomplished by using magnetic fields produced by small current-carrying
wires near the magnetic elements.
The method of magnetic switching by current induced torque provided by the present
invention is faster than current conventional methods that use magnetic fields
to switch the magnetization direction of layers. Read-out and write operations
of the present invention can be completed in sub-nanosecond time scales. Conventional
magnetic hard drives are very slow compared to the magnetic memory of the present
invention since the conventional hard drives have data access times of the order
of milliseconds.
The method of magnetic switching by current induced torque provided by the present
invention requires low power. This is especially advantageous for use in portable
electronic devices.
The method of magnetic switching by current induced torque provided by the present
invention is ideal for sub-micron scale devices since the lateral dimension of
the magnetic device of the present invention may be less than approximately 200
nm. Therefore, the present invention is scaled to allow the fabrication of ultra-high
density memory cells so that a vast amount of information can be stored in the
magnetic memory provided by the present invention.
The basic architecture of the high speed, low power magnetic device of the present
invention is straightforward, and read-out and write operations are reliable and
are less sensitive to changes in temperature. Unlike conventional magnetic memory
devices, the present invention does not rely on stochastic (random) processes or
fluctuating fields to initiate switching events.
According to one embodiment of the present invention, multiple bits of
information can be stored on each device so that even more information can be stored
in the magnetic memory.
The method of magnetic switching by current induced torque provided by the present
invention can be used for logic operations, as well as for magnetic memory devices.
Since there is a threshold, which is dependent on the shape, amplitude, and period
of the current pulse, for the current pulse to produce a change in magnetization,
current input can be combined to produce a logic function, such as an AND gate.
For example, two current pulses can be combined to produce a current pulse that
traverses the device which is the sum of the two current pulses. The pulse characteristics
(shape, amplitude, and period) can be chosen such that each pulse individually
does not switch the device, yet the combined pulse does switch the device. Thus,
this is an AND operation. A NOT operation requires simply switching the state of
the device. A NOT and an AND operation can be combined to produce a NAND function,
which is a universal digital logic gate (i.e., all digital logic functions can
be constructed from NAND gates.)
There are several possible geometries and layer configurations that are provided
by the present invention. For example, an embodiment of the magnetic device of
the present invention may be configured so that no net current passes through the
free magnetic layer FM2 during write operations. This is illustrated in
FIG. 10 which shows an embodiment of the present invention including current source
A, current source B, and layer I2, which is a thin insulating layer made
of Al
2O
3, for example. In this device, layer I2 is
0.5 to 3 nm thick and is thin enough so that electrons can traverse the layer by
quantum mechanical tunneling.
In the device shown in FIG. 10, current pulses are applied with current source
A to change the magnetization direction of the free magnetic layer FM2.
Using current source A, current flows from FM1 to the non-magnetic layer
N1 and electron spin angular momentum is transferred to the free magnetic
layer FM2 by reflection of electrons at the interface between the non-magnetic
layer N1 and the free magnetic layer FM2. The device readout is performed
using current source B. The voltage is measured when a small current from B passes
between the free magnetic layer FM2 and the readout layer FM3. This
voltage will depend on the relative magnetization directions of the layers FM2
and FM3 so that the magnetization direction of the free magnetic layer FM2
can be determined to read-out the device. This device has the advantage that the
readout signal is large since the tunnel junction resistance can be large (1 Ohm
to 100 kOhm). Readout signals can be in the range from 10 mV to 1 V.
While there has been described what are at present considered to be embodiments
of the present invention, it will be understood that various modifications may
be made thereto, and it is intended that the appended claims cover all such modifications
as fall within the true spirit and scope of the invention.
*
