U.S. patent application number 11/337667 was filed with the patent office on 2006-07-06 for hybrid memory cell for spin-polarized electron current induced switching and writing/reading process using such memory cell.
Invention is credited to Jacques Miltat, Yoshinobu Nakatani.
Application Number | 20060146598 11/337667 |
Document ID | / |
Family ID | 36576529 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060146598 |
Kind Code |
A1 |
Miltat; Jacques ; et
al. |
July 6, 2006 |
Hybrid memory cell for spin-polarized electron current induced
switching and writing/reading process using such memory cell
Abstract
The present invention relates to a magnetoresistive hybrid
memory cell comprising a first stacked structure comprising a
magnetic tunnel junction including first and second magnetic
regions stacked in a parallel, overlying relationship separated by
a layer of non-magnetic material, wherein said first magnetic
region being provided with a fixed first magnetic moment vector and
said second magnetic region being provided with a free second
magnetic moment vector which is free to be switched between the
same and opposite directions with respect to said fixed first
magnetic moment vector of said first magnetic region, a second
stacked structure being at least partly arranged in a lateral
relationship as to said first stacked structure and comprising a
third magnetic region being provided with a fixed third magnetic
moment vector and said second magnetic region; wherein said first
and second structures being arranged in between at least two
electrodes in electrical contact therewith. It further relates to a
method of writing to and reading of a magnetoresistive hybrid
memory cell, wherein a writing voltage pulse is applied to
electrodes on both sides of only said second structure, and wherein
a reading voltage pulse is applied to electrodes on both sides of
only said first structure.
Inventors: |
Miltat; Jacques; (Paris,
FR) ; Nakatani; Yoshinobu; (Tokyo, JP) |
Correspondence
Address: |
EDELL, SHAPIRO & FINNAN, LLC
1901 RESEARCH BOULEVARD
SUITE 400
ROCKVILLE
MD
20850
US
|
Family ID: |
36576529 |
Appl. No.: |
11/337667 |
Filed: |
January 24, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11024945 |
Dec 30, 2004 |
|
|
|
11337667 |
Jan 24, 2006 |
|
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Current U.S.
Class: |
365/158 ;
365/171; 365/173 |
Current CPC
Class: |
G11C 11/16 20130101 |
Class at
Publication: |
365/158 ;
365/171; 365/173 |
International
Class: |
G11C 11/00 20060101
G11C011/00; G11C 11/14 20060101 G11C011/14; G11C 11/15 20060101
G11C011/15 |
Claims
1. A method of writing to and reading of a magnetoresistive hybrid
memory cell, comprising the following steps: providing of a
magnetoresistive hybrid memory cell comprising: a first stacked
structure comprising a magnetic tunnel junction including first and
second magnetic regions stacked in a parallel, overlying
relationship separated by a layer of non-magnetic material, wherein
said first magnetic region being provided with a fixed first
magnetic moment vector and said second magnetic region being
provided with a second free magnetic moment vector which is free to
be switched between the same and opposite directions with respect
to said fixed first magnetic moment vector of said first magnetic
region; a second stacked structure being arranged in a lateral
relationship to said first stacked structure and comprising a third
magnetic region being provided with a fixed third magnetic moment
vector and said second magnetic region; wherein said first and
second structures being arranged in between at least two electrodes
in electrical contact therewith; applying of a writing voltage
pulse resulting in a writing current pulse flowing trough said
second magnetic region for switching of its free magnetic moment
vector to electrodes on both sides of only said second structure;
applying of a reading voltage pulse resulting in a reading current
pulse flowing through said magnetic tunnel junction to electrodes
on both sides of only said first structure.
2. The method of switching a magnetoresistive hybrid memory cell as
claimed in claim 7, wherein said writing voltage pulse is applied
adapted to result in a coherent rotation over half a full turn in
total.
3. A method of writing to and reading of a magnetoresistive hybrid
memory cell, comprising the following steps: providing of a
magnetoresistive hybrid memory cell comprising: a first stacked
structure comprising a magnetic tunnel junction including first and
second magnetic regions stacked in a parallel, overlying
relationship separated by a layer of non-magnetic material, wherein
said first magnetic region being provided with a fixed first
magnetic moment vector and said second magnetic region being
provided with a second free magnetic moment vector which is free to
be switched between the same and opposite directions with respect
to said fixed first magnetic moment vector of said first magnetic
region; a second stacked structure being at least partly arranged
in a lateral relationship to said first stacked structure and
comprising a third magnetic region being provided with a fixed
third magnetic moment vector and said second magnetic region;
wherein said first and second structures being arranged in between
at least two electrodes in electrical contact therewith; applying
of a writing voltage pulse resulting in a writing current pulse
flowing trough said second magnetic region for switching of its
free magnetic moment vector to electrodes on both sides of only
said second structure, wherein said switching voltage pulse is
applied adapted to result in a coherent rotation over half a full
turn in total having a slow rise time and a fast fall time;
applying of a reading voltage pulse resulting in a reading current
pulse flowing through said magnetic tunnel junction to electrodes
on both sides of only said first structure.
4. The method as claimed in claim 7, wherein said fixed magnetic
moment vector of said third magnetic region being perpendicularly
aligned to said free magnetic moment vector of said second magnetic
region.
5. The method as claimed in claim 7, wherein one of said electrodes
arranged on one side of said first and second structures is a
common electrode connecting said first and second structures.
6. The method as claimed in claim 11, wherein said common electrode
is positioned adjacent said second magnetic region.
7. The method as claimed in claim 12, wherein separate electrodes
for each one of said first and second structures are provided on
the other side of said first and second structures.
8. The magnetoresistive hybrid memory cell as claimed in claim 7,
wherein separate electrodes for each one of said first and second
structures are provided on both sides of said first and second
structures.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/024,945, filed Dec. 30, 2004, entitled "Hybrid Memory
Cell for Spin-Polarized Electron Current Induced Switching and
Writing/Reading Process Using Such Memory Cell" the entire contents
of which are hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to non-volatile semiconductor
memory chips and more particularly to magnetoresistive memory cells
adapted for spin-polarized electron current induced switching.
BACKGROUND
[0003] Magnetic (or magnetoresistive) random access memory (MRAM)
is a non-volatile memory technology considered to be of great
future importance as the standard memory technology for computing
devices.
[0004] A schematic representation of a typical magnetoresistive
memory cell is shown in FIG. 1. A magnetoresistive memory cell
(also referred to as a tunneling magneto-resistive or TMR-device)
includes a structure having ferromagnetic layers 2, 4 respectively
having a resultant magnetic moment vector 5, 6 separated by a
non-magnetic layer (tunnel barrier) 3 and arranged into a magnetic
tunnel junction (MTJ) 1. Digital information is stored and
represented in the magnetic memory cell as directions of magnetic
moment vectors in the ferromagnetic layers. More specifically, the
resultant magnetic moment vector 6 of one ferromagnetic layer 4 is
magnetically fixed or pinned (typically also referred to as the
"reference layer", "pinned layer" or "fixed layer"), while the
resultant magnetic moment vector 5 of the other ferromagnetic layer
2 (typically also referred to as the "free layer") is free to be
switched between two preferred directions, i.e., the same and
opposite directions with respect to the fixed magnetization 6 of
the reference layer 4. The orientations of the magnetic moment
vector 5 of the free layer 2 are also known as "parallel" and
"antiparallel" states, respectively, wherein a parallel state
refers to the same magnetic alignment of the free and reference
layers (upper diagram of FIG. 1), while an antiparallel state
refers to opposing alignments therebetween (lower diagram of FIG.
1). Accordingly, a logic state of a magnetoresistive memory cell is
not maintained by power as in DRAMs, but rather by the direction of
the magnetic moment vector of the free layer with respect to the
direction of the magnetic moment vector of the reference layer (for
instance, a logic "0" in the case of a parallel alignment of
magnetic moment vectors and a logic "1" in the case of an
antiparallel alignment therebetween).
[0005] Depending upon the magnetic states of the free layer, the
magnetic memory cell exhibits two different resistance values in
response to a voltage applied across the magnetic tunnel junction
barrier, wherein the resistance is "low" when the magnetization is
parallel and "high" when the magnetization is antiparallel, so that
a detection of changes in resistance allows an MRAM-device to
provide logic information stored in the magnetic memory
element.
[0006] A magnetic memory cell typically is written to through the
application of magnetic fields from bi- or uni-directional
currents. For writing of magnetic memory cells different writing
(switching) scenarios are known depending on the actual
configuration of the magnetoresistive memory cell such as
Stoner-Wohlfahrt-switching or adiabatic rotational switching
(toggle-switching) which are well-known to those skilled in the art
and therefore need not be further detailed here.
[0007] To be useful in present day electronic devices, such as
digital cameras or the like, very high density arrays of magnetic
memory cells must be used, thus rendering a scaling-down of MRAM
cells one of the most important issues, which, however, requires
several problems to be solved.
[0008] Down-scaling of MRAM cells requires smaller and smaller
magnetic tunnel junctions, which proves problematic, since for a
given aspect ratio and free layer thickness, the activation energy,
being dependent on the free layer volume, scales down like w, where
w is the width of the magnetic cell. Otherwise, in down-scaling,
the switching fields increase roughly like 1/ {square root over
(w)}, so that magnetic field selected switching becomes ever
harder, but at the same time the magnetic cells loose their
information more and more rapidly due to thermal activation. A
major problem with having a small activation energy (energy
barrier) is that it becomes extremely difficult to selectively
switch one MRAM cell in an array, where selectability is seen to
allow switching without inadvertently switching other MRAM cells.
The memory cells therefore still need to retain a sizeable shape or
induced anisotropy in order to maintain thermal stability.
[0009] Reference is now made to FIG. 2 showing a diagram in which
the energy barrier height .DELTA.E for switching of magnetic moment
vector 5 of magnetic free layer 2 of rectangular MTJ 1 of FIG. 1
having lateral dimensions L for length and l for width (see insert)
and a low thickness of about 2 nm is plotted against its width l.
It is further assumed that magnetization of the magnetic free layer
2 is aligned along directions .+-.x. Considering a simple Arrhenius
law with a 0.1 nsec characteristic attempt time, requesting a ten
years stability is equivalent to setting the barrier height between
stable states (-x and -x) at about 45 k.sub.BT (T=300.degree. K.,
room temperature, k.sub.B is Boltzmann constant).
[0010] As can be seen from FIG. 2, an aspect ratio L/l=2 proves
sufficient for overcoming the energy barrier height lower limit
criterion if l remains greater than about 60 nm. A slight increase
of the aspect ratio pushes the limit further out. It also becomes
clear that as sizes shrink down, the superparamagnetic limit
becomes closer and closer.
[0011] Another problem in scaling down magnetoresistive memory
cells may be seen in that in the case of magnetic field selected
switching of memory cells the cell sizes need to be smaller than
sizes of the current lines for generating of magnetic fields in
order to ensure essentially homogeneous magnetic fields over the
whole memory cell area.
[0012] In an attempt to overcome the above problems, a new concept
of writing to magnetoresistive memory cells has been recently
proposed, where the reversal of the magnetic moment vector of the
magnetic free layer is generated not by external magnetic fields
but by spin-polarized electrons passing perpendicularly through the
stack of memory cell layers. For a detailed description of that
concept, see for instance seminal U.S. Pat. No. 5,695,864 to
Slonczewski and U.S. Pat. No. 6,532,164 to Redon et al., the
disclosures of which are incorporated herein by reference.
[0013] In the above new concept, by sending an electric current
through a magnetic layer having a particular magnetization, spins
of electrons are oriented by quantum-mechanical magnetic exchange
interaction with the result that the current electrons leave the
magnetic layer with a polarized spin. Alternatively, where
spin-polarized electrons are passed through a magnetic layer having
a particular magnetic moment vector in a preferred easy axis
direction, these spin-polarized electrons will cause a continuous
rotation of the magnetic moment vector which may result in a
reversal of the magnetic moment vector along its easy axis. Hence,
switching of the magnetic moment vector between its two preferred
directions along the easy axis may be effected by passing
spin-polarized electrons perpendicularly through the magnetic
layer.
[0014] Recent experimental data (see S. I. Kiselev et al., Nature
425 (2003), 380 and W. H. Rippard et al., Phys. Rev. Lett. 92
(2004) 027201) confirm the very essence of magnetic moment transfer
as a source of magnetic excitations and, subsequently, switching.
These experiments confirm theoretical predictions (see J. C.
Sloncezwski, J. Magn. Magn. Mater. 159 (1996) L1 and M. D. Stiles
& A. Zangwill, Phys. Rev. B66, (2002) 014407) stating that the
leading torque term acting on the magnetization under conditions of
spin-polarized DC current is proportional to: d m d t .varies. P
.function. [ m .times. ( m .times. p ) ] ##EQU1## where m, p and P
are the magnetization direction in space, the polarization
direction of the electron current (density per unit area J) and a
polarization function, respectively. A direct inspection of above
equation indicates that the torque will be maximum when p is
orthogonal to m.
[0015] Reference is now made to FIGS. 3A and 3B, where a schematic
representation of both a magnetic free layer 2 and a magnetic layer
7 for spin-polarizing of current electrons in a stacked arrangement
is shown. In that configuration, the magnetic free layer 2 is
provided with a magnetization easy axis where a magnetic moment
vector 5 is free to be switched between two preferred directions
thereof. Magnetic layer 7 is provided with a fixed magnetic moment
vector 8 being perpendicular to the magnetic moment vector 5 in the
configuration of FIGS. 3A and 3B. FIG. 3A illustrates a case where
a current density J of an electron current (not illustrated)
flowing perpendicularly through the layers is assumed to be nil,
while in FIG. 3B the current density J is assumed to be different
from zero. Accordingly, on the one hand, in FIG. 3A where no
current is passing through the layers, magnetic moment vector 5
remains unchanged, while, on the other hand, in FIG. 3B, electrons
passing through the layers are spin-polarized when flowing through
magnetic layer 7 by the effect of magnetic exchange interaction. If
a polarization direction p of the current electrons belongs to the
plane of the magnetic free layer 2, then rotation of the magnetic
moment vector 5 occurs in the plane of magnetic free layer 2 and
the torque becomes nil when m becomes parallel to p (that case is
not shown in FIGS. 1A, 1B). Alternatively, if p is perpendicular to
the plane of the magnetic free layer 2 (case shown in FIGS. 1A,
1B), then the initial torque pulls the magnetic moment vector 5 out
of its plane, thus creating a demagnetizing field H.sub.D
perpendicular to the magnetic free layer 2 plane, with the result
that a precession movement of the magnetic moment vector 5 around
the demagnetizing field H.sub.D may now take place.
[0016] In other words, in a magnetic element such as the soft
element of an MRAM cell, the magnetization direction though not far
from being uniform fails to be so as a result of demagnetizing
effects. Coherence during magnetic switching may nevertheless be
preserved if the field exerting a torque on the magnetization is
perpendicular to the soft layer. In order to achieve this, the best
strategy is to apply a magnetic field normal to the mean
magnetization direction within the soft element and in the plane of
the layer. The initial torque .gamma..sub.0[m.times.H.sub.a], where
.gamma..sub.0, m, H.sub.a are a gyromagnetic ratio, magnetization
vector and applied magnetic field, respectively, pulls the
magnetization out of the plane leading to the growth of a
demagnetizing field that remains essentially normal to the plane of
the layer. The magnetization may now precess around the
demagnetizing field under the torque
.gamma..sub.0[m.times.H.sub.D], where H.sub.D is the demagnetizing
field.
[0017] In order to observe precessional switching, three conditions
have to be fulfilled, namely, both the rise and fall times of the
field pulse need to be "short" and the length of the pulse has to
be tailored very accurately, where "short" means a time small when
compared to time requested for the magnetization to make half a
turn. Let T and f be the period and precession frequency,
respectively. A half a turn rotation means a time equal to T/2. One
has T=1/f and f depends on the amplitude of the demagnetizing
field: .omega.=2.PI.f=.gamma..sub.0H.sub.d. On the other hand, the
demagnetizing field scales with the angle of the magnetization out
of the sample plane.
[0018] An example may illustrate this: suppose the magnetization
leaves its plane by an angle of =10.degree., then the demagnetizing
field amplitude will amount to about H.sub.d.apprxeq.M.sub.s
sin(10.degree.). For a typical soft material with saturation
induction .mu..sub.0M.sub.s=1 Tesla, this means a precession
frequency equal to f=(.omega./2.PI.)=.gamma..sub.0M.sub.s
sin(10.degree.).apprxeq.5 GHz. The period then amounts to 200
picoseconds, and the time necessary for a half turn rotation would
typically be T/2=100.times.10.sup.-12 sec (100 picoseconds (ps)).
In summary, owing to values chosen in the sample, the pulse length
should be close to 100 ps and the fall and rise times much shorter
than 100 ps. Laboratory realizations allow for pulse rise and fall
times of the order of 20 ps.
[0019] Precessional switching is a very robust and fundamental
effect. In a large scale memory, however, due to various sources of
impedance, it is expected that maintaining such an accuracy in the
definition of the field pulses might prove extremely
problematic.
[0020] In order to result in a desired reversal of the free
magnetic moment vector, precession movement has to be controlled
appropriately, which, however, has not been demonstrated in prior
art.
SUMMARY
[0021] In light of the above, the invention provides a
magnetoresistive memory cell allowing a further cell size
down-scale without causing severe problems as to an increase of
switching-fields and decrease of activation energy.
[0022] The invention further provides a method of writing to
(switching) and reading of resistance states of above
magnetoresistive memory cells.
[0023] According to a first aspect of the invention, a
magnetoresistive hybrid memory cell comprises a first stacked
structure being provided with a magnetic tunnel junction including
first and second magnetic regions which are stacked in a parallel,
overlying relationship and are separated by a layer of non-magnetic
material. The first magnetic region is provided with a fixed first
magnetic moment vector, while the second magnetic region is
provided with a free second magnetic moment vector which is free to
be switched between the same and opposite directions with respect
to above fixed first magnetic moment vector of the first magnetic
region. The magnetoresistive hybrid memory cell further comprises a
second stacked structure which at least partly is arranged in a
lateral relationship as to the first stacked structure and
comprises both a third magnetic region and the second magnetic
region, the latter one thus being a common magnetic region of both
first and second stacked structures. The third magnetic region is
provided with a fixed third magnetic moment vector, which typically
and preferably is aligned in an orthogonal direction as to the free
second magnetic moment vector of the second magnetic region.
Furthermore, the first and second structures are arranged in
between at least two electrodes in electrical contact
therewith.
[0024] Magnetic anisotropy of the second magnetic region may be due
to shape anisotropy and/or intrinsic anisotropy. In the former
case, the second magnetic region may, for instance, be elliptic in
shape.
[0025] In a particularly preferred embodiment of the first aspect
of the invention, one of above-cited electrodes for contacting the
first and second structures being arranged on one side of the first
and second structures is a common electrode in electrical contact
with both first and second stacked structure. Such common electrode
is preferably positioned adjacent the second magnetic region, in
particular in direct electrical contact therewith.
[0026] In another particularly preferred embodiment which
preferably may be combined with a common electrode connecting the
first and second structures on the one side, separate electrodes
for each one of the first and second structures are provided on the
other side of the first and second structures. Such particular
design allows for an advantageous decoupling of write and read
functions, which, hence, can be optimized independently.
[0027] Alternatively, it is also possible to envisage separate
electrodes for each one of the first and second structures which
are provided on both sides of them.
[0028] According to a second aspect of the invention, a method of
writing to and reading of a magnetoresistive hybrid memory cell is
given, which comprises the following steps: providing of a
magnetoresistive hybrid memory cell as above-described with regard
to the first aspect of the invention; applying of a writing voltage
pulse to electrodes on both sides of only the second structure (and
not the first structure) resulting in a current pulse flowing
through the second magnetic region for writing of the free second
magnetic moment vector; applying of a reading voltage pulse to
electrodes on both sides of only the first structure (and not the
second structure) resulting in a current pulse flowing through the
magnetic tunnel junction. Accordingly, applying writing and reading
voltage pulses to second and first stacked structures,
respectively, allows for an advantageous decoupling of writing and
reading functions.
[0029] In a mostly preferred embodiment of the second aspect of the
invention, a switching voltage pulse is applied which is adapted to
result in a coherent rotation over half a full turn of the free
second magnetic moment vector in total. Such coherent rotation over
half a full turn of the free second magnetic moment vector may
preferably be achieved in applying writing voltage pulses having a
slow rise time and a fast fall time. The terms "slow" and "fast",
here, have a meaning exactly analogous to the precessional
switching case described in the introductory portion, that is,
"fast" means times shorter than a half precession cycle, while
"slow" means times substantially larger than a full precession
cycle. Hence, precessional switching requires both "fast" field
pulse rise and fall times, whereas spin injection in the present
geometry requires "slow" current rise times. This is a result of
extended numerical simulation work done by the inventors. A "fast"
current rise time would lead to a lot of unwanted magnetization
"ringing" (response oscillations).
[0030] As also explained in the introductory portion with respect
to the precessional switching case, "coherent rotation" means that,
irrespective of the magnetization distribution (not a perfectly
uniform distribution), the torque acts in such a way that all
moments are subjected to a torque acting in the same direction,
thus maintaining coherence of the distribution. This is not at all
the case for the conventional spin injection cells for which by
different simulations markedly chaotic behaviors have been
predicted. Other and further objects, features and advantages of
the invention will appear more fully from the following
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description given
below, serve to explain the principles of the invention.
[0032] FIG. 1 is an exemplary schematic representation of a typical
magnetic tunnel junction included in an MRAM cell;
[0033] FIG. 2 shows a diagram illustrating energy barrier height
for switching of an MRAM cell versus width dimension 1;
[0034] FIGS. 3A and 3B illustrate a stacked structure comprised of
a magnetic layer having fixed magnetization and a magnetic free
layer having a free magnetization free to be rotated with respect
to the fixed magnetization due to spin-polarized electron current
flowing therethrough;
[0035] FIG. 4 is a schematic representation of an embodiment of a
hybrid magnetoresistive memory cell of the invention;
[0036] FIG. 5 shows a diagram illustrating writing current I and
procession angle .PHI. in the single spin limit versus time;
[0037] FIGS. 6A and 6B show a typical writing current pulse having
slow rise and fast decay times resulting in a typical sawtooth
profile (FIG. 6A) and curve illustrating writing current versus
pulse length resulting in single reversal events; and
[0038] FIGS. 7A and 7B show diagrams analogous to that one FIGS. 6A
and 6B in the case of a limitation to the half of the platelet
area.
DETAILED DESCRIPTION
[0039] Embodiments of the present invention will be described in
detail below with reference to the accompanying drawings. Referring
to FIG. 4, an embodiment of the hybrid memory cell of the invention
is explained. Based on a conventional magnetic memory cell, the
hybrid magnetic memory cell of the invention comprises a first
stacked structure 9 being comprised of a magnetic tunnel junction
(MTJ) which includes a fixed first magnetic region 10 and a free
second magnetic region 11 stacked in parallel, overlying
relationship and separated by a layer 12 tunneling barrier.
Magnetic free region 11 is made of a magnetic material such as
CoFe/NiFe and is provided with a free second magnetic moment vector
18 free to be switched between oppositely aligned orientations
along its magnetic easy axis. Magnetic reference region 10 is
comprised of two layers 13, 14 of ferromagnetic materials such as
CoFe with its magnetizations being antiferromagnetically coupled
resulting in a fixed first magnetic moment vector 17. Intermediate
layer 12 is made of a nonmagnetic material such as AlO.sub.x. The
hybrid memory cell of the invention further comprises a second
stacked structure 23 which is comprised of the free second magnetic
region 11, a third magnetic region 20 being provided with a fixed
third magnetic moment vector 21 which is perpendicularly directed
to the second magnetic moment vector 18, a conductive layer 24 for
instance made of Au and being arranged on top of third magnetic
region 21 in contact therewith, and a further conductive layer 19
for instance made of Cu and being arranged beneath third magnetic
region 21 in contact therewith. Above second magnetic region 11,
first and second stacked structures 9 and 23, respectively, are
arranged in a lateral relationship leaving an intermediate gap G
between them. Further, first and second stacked structures are
arranged between a common bottom electrode 16 connecting both first
and second structures and separate top electrodes 15, 16, that is
to say a separate top electrode for each one of stacked
structures.
[0040] Having separate top electrodes 15, 22 for each one of both
first and second stacked structures, hybrid magnetoresistive memory
cell of FIG. 4 enables a favorite decoupling of write and read
functions.
[0041] Further characteristics of the invention are now
explained:
[0042] Let call F the minimum feature size (smallest dimension) of
the technology used, e.g. 0,11 .mu.m, 90 nm, 65 nm following the
semiconductor roadmap. A magnetic memory cell today may hardly be
smaller than 2 F.sup.2 due to the necessity for maintaining some
kind of shape anisotropy (toggle switching, however, allows for
circular elements). As mentioned above within the context of field
addressing, the field necessary to commute cells grows with
decreasing cell size. On the contrary, the smaller the active
region of spin-injection, the smaller the detrimental effect due to
the field created by the requested current density (the so-called
Oersted field). It is common knowledge that, for usual 3d
ferromagnetic materials, spin-injection ceases to be relevant for
cell sizes exceeding some 100 nm.
[0043] In the proposed scheme, the minimal cell size is 3 F.sup.2.
It means that distance G in FIG. 4 may not be smaller than F due to
processing constraints. On the other hand, allowing for a 1 F.sup.2
area for the spin-injection region (the right part of FIG. 4) is
extremely favorable because it complies with the necessity to
decrease as much as possible the Oersted field. The present scheme
mimics through spin-injection a precessional type motion of the
magnetization in the spin-injection region. It is a fundamental
process due to the relative orientations of the magnetization 21 in
layer 20 and magnetization 18 in layer 11.
[0044] Now, once the magnetization 18 in layer 11 has been reversed
under layer 20, a wall is created, which has inertia, so that once
it is set into motion, it will keep-up moving for some time that is
mainly controlled by the damping in the material. As simulations by
the inventors have shown, this "wall launching" mechanism allows
for wall motion through out the extent of the cell layer 11.
Additionally, some current flowing from layer 22 into sublayer 16
will also flow along the full length of the cell layer 11. Because
flowing in a ferromagnetic material, such a current is
spin-polarized and exerts a pressure on the wall, thus assisting
wall motion. This last effect, is however, hard to quantify because
it depends crucially on the difference in electrical resistivity
between layers 11 and 16. This last effect has been neglected by
the simulations made by the inventors.
[0045] Using cell design for spin injection suffers from the
drawback of needing to simultaneously optimize both the writing
current and the read signal. Giant magnetoresistance structures
would exhibit weak read signals. Moreover, the signal decreases
with decreasing cell size. Tunnel junctions do not suffer from this
basic drawback, but the mechanisms that eventually allow cell
switching through very shallow tunnel junctions remain unclear.
Shallow tunnel junctions result in smaller read signals. From an
engineering point of view, the larger the read signal, the
better.
[0046] In the proposed scheme, thermal stability is improved
through the geometry: a 3 F.sup.2 cell size remains thermally
stable on the long term for the smallest F dimensions because of
the aspect ratio as shown in FIG. 2 (F is 1 in FIG. 2). It may be
added that the magnetostatic coupling between layer 20 with
magnetization 21 and layer 11 with magnetization 18 will contribute
to an increased thermal stability.
[0047] As above stated, in the proposed scheme, write and read
functions may independently optimized, where optimization means
here both optimization of the read signal (state of the art tunnel
junction 9 in the low current regime, and, best couple of materials
between layers 11 and 13, and optimization of the write current
(optimized spin-polarization through the choice of materials in
layers 11 and 20, and, optimized spin-accumulation through a proper
choice of the thicknesses of layers 21, 20 and 19.
[0048] Now referring to FIGS. 5 through 7, a numeric simulation
concerning the method of writing to a magnetoresistive hybrid
memory cell is explained.
[0049] As can be seen from FIG. 5, a numeric simulation in the
single spin limit reveals that controlled precession of the free
second magnetic moment vector may be achieved through the
application of current pulses with a slow rise time and a fast fall
time.
[0050] Further characteristics of FIG. 5 are given: FIG. 5 (single
spin type simulations) shows that, for asymmetrical current pulses,
a proper choice of the current density allows for a controlled
magnetization rotation. .PHI. (in .degree.) to the right of the
figure is seen to move in steps of 180.degree., meaning one half a
turn, a full turn, three half-a-turn etc. The figure applies to the
case of FIG. 3B, not to FIG. 4. It was a first step in order to
explain that control was solely possible if allowing for pulse
asymmetry.
[0051] An extension of such calculations to the micromagnetic
regime confirms this prediction as can be seen from FIGS. 6A and
6B. Current injection through half of the platelet area yields the
following results, which are given in FIGS. 7A and 7B.
[0052] FIGS. 6B and 7B are computed operational margins as
determined by full micromagnetic simulations (meaning that now the
detailed aspects of the magnetization distribution both in space
and time are taken into account) in a parameter space where the
horizontal scale is the pulse length as defined in FIGS. 6A and 7A,
respectively, and the vertical scale the current density at the end
of the pulse.
[0053] FIG. 6 concern current densities that are homogeneous
through the entire cell. FIG. 6 are therefore not directly usable
for the present invention. On the contrary, FIG. 7 apply to cells
where the current flows in only half of a 2 F.sup.2 cell, i.e. a
cell, where distance G in FIG. 4 would be ideally zero. FIG. 7 show
that a fairly sizeable operational margin may be expected with
pulse durations in the 0.15 to 0.45 ns (150 to 450 ps) and maximum
current densities in 0.4 to close to 0.475 A/.mu.m.sup.2.
Constraints on pulse durations are expected to be rather weak.
Current densities are more challenging as the margin does not
exceed some 15%, according to extended and state of the art
numerical simulations.
[0054] Obviously many modifications and variations of the present
invention are possible in light of the above description. It is
therefore to be understood, that within the scope of appended
claims, the invention may be practiced otherwise than as
specifically devised.
[0055] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope
thereof. Accordingly, it is intended that the present invention
covers the modifications and variations of this invention provided
they come within the scope of the appended claims and their
equivalents.
REFERENCE LIST
[0056] 1 Magnetic tunnel junction [0057] 2 Ferromagnetic layer
[0058] 3 Tunnel barrier layer [0059] 4 Ferromagnetic layer [0060] 5
Magnetic moment vector [0061] 6 Magnetic moment vector [0062] 7
Ferromagnetic layer [0063] 8 Magnetic moment vector [0064] 9 First
stacked structure [0065] 10 Fixed first magnetic region [0066] 11
Free second magnetic region [0067] 12 Tunnel barrier layer [0068]
13 Ferromagnetic layer [0069] 14 Ferromagnetic layer [0070] 15 Top
electrode [0071] 16 Common bottom electrode [0072] 17 Fixed first
magnetic moment vector [0073] 18 Free second magnetic moment vector
[0074] 19 Conductive layer [0075] 20 Third magnetic region [0076]
21 Fixed third magnetic moment vector [0077] 22 Top electrode
[0078] 23 Second stacked structure [0079] 24 Conductive layer
* * * * *