U.S. patent application number 10/899057 was filed with the patent office on 2005-11-17 for magnetic random access memory and method of writing data in magnetic random access memory.
Invention is credited to Fukuzumi, Yoshiaki.
Application Number | 20050254288 10/899057 |
Document ID | / |
Family ID | 35309236 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050254288 |
Kind Code |
A1 |
Fukuzumi, Yoshiaki |
November 17, 2005 |
MAGNETIC RANDOM ACCESS MEMORY AND METHOD OF WRITING DATA IN
MAGNETIC RANDOM ACCESS MEMORY
Abstract
A magnetic random access memory includes first and second write
wirings extended in first and second directions, a
magneto-resistance element located between the first and second
write wirings, a first yoke layer provided on a first outer surface
and both sides of the first write wiring and being formed of a
magnetic layer, and a second yoke layer provided on a second outer
surface and both sides of the second write wiring and being formed
of a magnetic layer, wherein the magneto-resistance element has a
recording layer formed of a ferromagnetic substance and comprising
a first surface and a second surface, a first ferromagnetic layer
provided on the first surface, a second ferromagnetic layer
provided on the second surface, a first nonmagnetic layer provided
between the recording layer and the first ferromagnetic layer, and
a second nonmagnetic layer provided between the recording layer and
the second ferromagnetic layer.
Inventors: |
Fukuzumi, Yoshiaki;
(Yokohama-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
35309236 |
Appl. No.: |
10/899057 |
Filed: |
July 27, 2004 |
Current U.S.
Class: |
365/158 ;
257/E21.665; 257/E27.005 |
Current CPC
Class: |
B82Y 25/00 20130101;
H01L 27/228 20130101; B82Y 10/00 20130101; G11C 11/16 20130101;
H01F 10/3254 20130101 |
Class at
Publication: |
365/158 |
International
Class: |
G11B 011/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2004 |
JP |
2004-146441 |
Claims
What is claimed is:
1. A magnetic random access memory comprising: a first write wiring
extended in a first direction; a second write wiring extended in a
second direction different from the first direction; a
magneto-resistance element provided at a point of intersection of
the first and second write wirings and located between the first
and second write wirings; a first yoke layer provided on a first
outer surface and both sides of the first write wiring and being
formed of a magnetic layer, the first outer surface being opposite
a first inner surface of the first write wiring facing the
magneto-resistance element; and a second yoke layer provided on a
second outer surface and both sides of the second write wiring and
being formed of a magnetic layer, the second outer surface being
opposite a second inner surface of the second write wiring facing
the magneto-resistance element, wherein the magneto-resistance
element has: a recording layer formed of a ferromagnetic substance
and comprising a first surface and a second surface; a first
ferromagnetic layer provided on the first surface of the recording
layer; a second ferromagnetic layer provided on the second surface
of the recording layer; a first nonmagnetic layer provided between
the recording layer and the first ferromagnetic layer; and a second
nonmagnetic layer provided between the recording layer and the
second ferromagnetic layer.
2. The magnetic random access memory according to claim 1, wherein
each of the recording layer and first and second ferromagnetic
layers has a magnetostriction constant with an absolute value of at
least 10.sup.-6.
3. The magnetic random access memory according to claim 1, wherein
the recording layer has an easy axis of magnetization oriented in
the first or second direction during no-current flow time.
4. The magnetic random access memory according to claim 1, wherein
the first ferromagnetic layer is formed of a ferromagnetic
substance having magnetization oriented parallel to the first
direction during no-current flow time and applies a first stress
resulting from magnetostriction, to the recording layer, and the
second ferromagnetic layer is formed of a ferromagnetic substance
having magnetization oriented parallel to the second direction
during no-current flow time and applies a second stress resulting
from magnetostriction, to the recording layer.
5. The magnetic random access memory according to claim 1, wherein
each of the first and second ferromagnetic layers has a larger film
thickness than the recording layer.
6. The magnetic random access memory according to claim 1, wherein
each of the first and second ferromagnetic layers have a width
equal to or larger than that of each of the first and second write
wirings.
7. The magnetic random access memory according to claim 1, wherein
the first or second nonmagnetic layer is a tunnel barrier
layer.
8. The magnetic random access memory according to claim 1, wherein
the magneto-resistance element further has: a first fixation layer
provided between the second nonmagnetic layer and the second
ferromagnetic layer; a second fixation layer provided between the
second nonmagnetic layer and the second ferromagnetic layer and
magnetically coupled to second ferromagnetic layer; a third
nonmagnetic layer provided between the second ferroelectric layer
and the second fixation layer; and an antiferromagnetic layer
provided between the first and second fixation layers.
9. The magnetic random access memory according to claim 1, wherein
the magneto-resistance element further has: a fixation layer
provided between the second nonmagnetic layer and second write
wiring and magnetically coupled to second ferromagnetic layer; a
third nonmagnetic layer provided between the second ferroelectric
layer and the fixation layer; and an antiferromagnetic layer
provided between the fixation layer and the second write
wiring.
10. The magnetic random access memory according to claim 1, further
comprising an interlayer insulating film provided around the
magneto-resistance element and having gap.
11. The magnetic random access memory according to claim 1, further
comprising a metal layer provided between first write wiring and
the first ferromagnetic layer.
12. A method of writing data in a magnetic random access memory
comprising: a first write wiring extended in a first direction; a
second write wiring extended in a second direction different from
the first direction; a magneto-resistance element provided at a
point of intersection of the first and second write wirings and
located between the first and second write wirings; a first yoke
layer provided on a first outer surface and both sides of the first
write wiring and being formed of a magnetic layer, the first outer
surface being opposite a first inner surface of the first write
wiring facing the magneto-resistance element; and a second yoke
layer provided on a second outer surface and both sides of the
second write wiring and being formed of a magnetic layer, the
second outer surface being opposite a second inner surface of the
second write wiring facing the magneto-resistance element, the
magneto-resistance element having: a recording layer formed of a
ferromagnetic substance and comprising a first surface and a second
surface; a first ferromagnetic layer provided on the first surface
of the recording layer; a second ferromagnetic layer provided on
the second surface of the recording layer; a first nonmagnetic
layer provided between the recording layer and the first
ferromagnetic layer; and a second nonmagnetic layer provided
between the recording layer and the second ferromagnetic layer
wherein when data is written in the magneto-resistance element,
first and second write currents flow through the first and second
write wirings, respectively, to generate first and second magnetic
fields, respectively, the first and second magnetic fields are
applied to the first and second ferromagnetic layers, respectively,
to rotate the magnetizations in the first and second ferromagnetic
layers, and a rotation of the magnetizations in the first and
second ferromagnetic layers causes magnetostriction resulting in
the first and second stresses, the first and second stresses being
applied to the recording layer to rotate magnetization in the
recording layer.
13. The method of writing data according to claim 12, wherein the
first and second write wirings are sequentially turned on, and the
first and second write wirings are then sequentially turned off,
and one of the first and second write wirings turned on first is
turned off first.
14. The method of writing data according to claim 12, wherein the
first yoke layer guides the first magnetic field to the first
ferromagnetic layer, and the second yoke layer guides the second
magnetic field to the second ferromagnetic layer.
15. The method of writing data according to claim 12, wherein
before data is written in the magneto-resistance element, data
already present in the magneto-resistance element is read.
16. The method of writing data according to claim 12, wherein the
first and second write currents flow in the same direction
regardless of whether "0" data or "1" data is written.
17. The method of writing data according to claim 12, wherein each
of the recording layer and first and second ferromagnetic layers
has a magnetostriction constant with an absolute value of at least
10.sup.-6.
18. The method of writing data according to claim 12, wherein the
recording layer has an easy axis of magnetization oriented in the
first or second direction during no-current flow time.
19. The method of writing data according to claim 12, wherein the
first ferromagnetic layer is formed of a ferromagnetic substance
having magnetization oriented parallel to the first direction
during no-current flow time and applies a first stress resulting
from magnetostriction, to the recording layer, and the second
ferromagnetic layer is formed of a ferromagnetic substance having
magnetization oriented parallel to the second direction during
no-current flow time and applies a second stress resulting from
magnetostriction, to the recording layer.
20. The method of writing data according to claim 12, wherein the
first or second nonmagnetic layer is a tunnel barrier layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2004-146441,
filed May 17, 2004, the entire contents of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a magnetic random access
memory (MRAM) comprising a magneto-resistance element and a method
of writing data in the magnetic random access memory.
[0004] 2. Description of the Related Art
[0005] In recent years, a magnetic random access memory (MRAM)
utilizing the tunnel magneto-resistance (MRAM) effect has been
proposed as a semiconductor memory.
[0006] In a memory cell of the MRAM, an MTJ (Magnetic Tunneling
Junction) element is provided at each point of intersection between
a bit line and a word line as an information storage element. If a
data write is carried out, currents are allowed to flow through a
selected bit line and a selected word line, respectively, to
generate a composite magnetic field. Then, the composite magnetic
field is used to write data in an MTJ element in the selected cell
located at the point of intersection between the selected bit line
and the selected word line. On the other hand, to read data from a
memory cell, a read current is allowed to flow through the MTJ
element in the selected cell. Then, "1" or "0" data is read on the
basis of the change in the resistance of the magnetized state of
the MTJ element.
[0007] In such an MRAM, when a data write is carried out, the write
current magnetic field may affect semi-selected cells selected by
one of the selected bit line and the selected bit line. Then, data
may be erroneously written in the semi-selected cells. This is
called a disturbance. The avoidance of disturbance is considered to
be one of the most important objects in the development of MRAMs.
However, an asteroid characteristic is sensitive to the shape of
the MTJ element or the like. Accordingly, the fine-grained
structure of the element may further significantly affect the
asteroid characteristic. Thus, the number of miswrites in
semi-selected cells increases with decreasing size of the element.
Then, the disturbance becomes more serious. To avoid this problem,
the write current must be increased in order to prevent miswrites
in semi-selected cells. There are MRAMs based on a toggle system
using a weakly coupled stacked recording layer. However, this
system must also increase the write current.
BRIEF SUMMARY OF THE INVENTION
[0008] According to a first aspect of the present invention, there
is provided a magnetic random access memory comprising a first
write wiring extended in a first direction; a second write wiring
extended in a second direction different from the first direction;
a magneto-resistance element provided at a point of intersection of
the first and second write wirings and located between the first
and second write wirings; a first yoke layer provided on a first
outer surface and both sides of the first write wiring and being
formed of a magnetic layer, the first outer surface being opposite
a first inner surface of the first write wiring facing the
magneto-resistance element; and a second yoke layer provided on a
second outer surface and both sides of the second write wiring and
being formed of a magnetic layer, the second outer surface being
opposite a second inner surface of the second write wiring facing
the magneto-resistance element, wherein the magneto-resistance
element has a recording layer formed of a ferromagnetic substance
and comprising a first surface and a second surface; a first
ferromagnetic layer provided on the first surface of the recording
layer; a second ferromagnetic layer provided on the second surface
of the recording layer; a first nonmagnetic layer provided between
the recording layer and the first ferromagnetic layer; and a second
nonmagnetic layer provided between the recording layer and the
second ferromagnetic layer.
[0009] According to a second aspect of the present invention, there
is provided a method of writing data in a magnetic random access
memory comprising a first write wiring extended in a first
direction; a second write wiring extended in a second direction
different from the first direction; a magneto-resistance element
provided at a point of intersection of the first and second write
wirings and located between the first and second write wirings; a
first yoke layer provided on a first outer surface and both sides
of the first write wiring and being formed of a magnetic layer, the
first outer surface being opposite a first inner surface of the
first write wiring facing the magneto-resistance element; and a
second yoke layer provided on a second outer surface and both sides
of the second write wiring and being formed of a magnetic layer,
the second outer surface being opposite a second inner surface of
the second write wiring facing the magneto-resistance element, the
magneto-resistance element having a recording layer formed of a
ferromagnetic substance and comprising a first surface and a second
surface; a first ferromagnetic layer provided on the first surface
of the recording layer; a second ferromagnetic layer provided on
the second surface of the recording layer; a first nonmagnetic
layer provided between the recording layer and the first
ferromagnetic layer; and a second nonmagnetic layer provided
between the recording layer and the second ferromagnetic layer
wherein when data is written in the magneto-resistance element,
first and second write currents flow through the first and second
write wirings, respectively, to generate first and second magnetic
fields, respectively, the first and second magnetic fields are
applied to the first and second ferro-magnetic layers,
respectively, to rotate the magnetizations in the first and second
ferromagnetic layers, and a rotation of the magnetizations in the
first and second ferromagnetic layers causes magnetostriction
resulting in the first and second stresses, the first and second
stresses being applied to the recording layer to rotate
magnetization in the recording layer.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0010] FIG. 1 is a schematic plan view showing a magnetic random
access memory according to a first embodiment of the present
invention;
[0011] FIG. 2 is a sectional view of the magnetic random access
memory taken along a line II-II in FIG. 1;
[0012] FIG. 3 is a diagram showing the magnetization direction of
each layer in the magnetic random access memory according to the
first embodiment of the present invention;
[0013] FIG. 4 is a diagram showing a data write to the magnetic
random access memory according to the first embodiment of the
present invention;
[0014] FIG. 5 is a diagram showing an initial state of the data
write to the magnetic random access memory according to the first
embodiment of the present invention;
[0015] FIG. 6 is a diagram showing a first cycle of the data write
to the magnetic random access memory according to the first
embodiment of the present invention;
[0016] FIG. 7 is a diagram showing a second cycle of the data write
to the magnetic random access memory according to the first
embodiment of the present invention;
[0017] FIG. 8 is a diagram showing a third cycle of the data write
to the magnetic random access memory according to the first
embodiment of the present invention;
[0018] FIG. 9 is a diagram showing a fourth cycle of the data write
to the magnetic random access memory according to the first
embodiment of the present invention;
[0019] FIG. 10 is a schematic diagram showing a read of "0" data
from the magnetic random access memory according to the first
embodiment of the present invention;
[0020] FIG. 11 is a schematic diagram showing a read of "1" data
from the magnetic random access memory according to the first
embodiment of the present invention;
[0021] FIG. 12 is a schematic plan view showing a magnetic random
access memory according to a second embodiment of the present
invention;
[0022] FIG. 13 is a schematic plan view showing a magnetic random
access memory according to a third embodiment of the present
invention; and
[0023] FIG. 14 is a schematic plan view showing a magnetic random
access memory according to a fourth embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Embodiments of the present invention will be described with
reference to the drawings. In the description, common components
are denoted by common reference numerals throughout the
drawings.
First Embodiment
[0025] In a first embodiment, in an MTJ (Magnetic Tunnel Junction)
element, ferromagnetic layers are provided over and under a
recording layer, respectively, via nonmagnetic layers so that
magnetostriction in the ferromagnetic layers can be transmitted to
the recording layer. That is, toggle writes are carried out
utilizing a magnetostriction interaction.
[0026] (1) Structure
[0027] FIG. 1 is a schematic plan view showing a magnetic random
access memory according to a first embodiment of the present
invention. FIG. 2 is a sectional view of the magnetic random access
memory taken along a line II-II in FIG. 1. FIG. 3 is a diagram
showing the magnetization direction of each layer in the magnetic
random access memory according to the first embodiment of the
present invention. Description will be given of the structure of
the magnetic random access memory according to the first
embodiment.
[0028] As shown in FIGS. 1 and 2, a memory cell is shaped like a
matrix in which bit lines (BL) 11 functioning as write and read
wires are extended in a Y direction and word lines (WL) 12
functioning as write wires are extended in an X direction. At least
a part 10b of an MTJ element 10 that is a magneto-resistance
element is provided in a P area that is a point of intersection of
one bit line 11 and one word line which point is located between
the bit line 11 and the word line 12. One end of the MTJ element 10
is electrically connected to the corresponding bit line 11. The
other end of the MTJ element 10 is electrically connected to a
lower electrode layer 13. The lower electrode layer 13 is
electrically connected via a contact 14 to a MOSFET 15 that is a
reading-switching element. A gate electrode of the MOSFET 15
functions as a read word line.
[0029] The MTJ element 10 is formed of an upper ferromagnetic layer
30, a nonmagnetic layer 31, a recording layer (free layer) 32, a
tunnel barrier layer (nonmagnetic layer) 33, a first fixation layer
(pin layer) 34, an antiferromagnetic layer 35, a second fixation
layer 36, a nonmagnetic layer 37, and a lower ferromagnetic layer
38. The MTJ element 10 is composed of a first portion 10a, a second
portion 10b, and a third portion 10c. The first portion 10a is
composed of the upper ferromagnetic layer 30. The second portion
10b is composed of the nonmagnetic layer 31 and the recording layer
32. The third portion 10c is composed of the tunnel barrier layer
33, the first fixation layer 34, the antiferromagnetic layer 35,
the second fixation layer 36, the nonmagnetic layer 37, and the
lower ferromagnetic layer 38. In this MTJ element 10, the
nonmagnetic layer 31 is provided between the recording layer 32 and
the upper ferromagnetic layer 30. The nonmagnetic layers 33 and 37
are provided between the recording layer 32 and the lower
ferromagnetic layer 38.
[0030] At least parts of the peripheries of the bit line 11 and
word line 12 are surrounded by first and second yoke layers 21 and
22, respectively, each composed of a magnetic layer. For example,
the first yoke layer 21 is formed on a top surface of the bit line
11 (the surface of the bit line 11 which is opposite its surface
facing the MTJ element 10) and on both sides of the bit line 11.
The second yoke layer 22 is formed on a top surface of the word
line 12 (the surface of the word line 12 which is opposite its
surface facing the MTJ element 10) and on both sides of the word
line 12. Each of the first and second yoke layers 21 and 22 has an
easy axis of magnetization extending in a longitudinal direction (Y
direction). The easy axis of magnetization has uniaxial
anisotropy.
[0031] In this memory cell, the recording layer 32, the upper
ferromagnetic layer 30, and the lower ferromagnetic layer 38 has a
magnetostriction constant with a large absolute value. Accordingly,
data writes are carried out using a magnetostriction interaction.
Here, the magnetostriction constant has an absolute value of, for
example, at least 10.sup.-6. Further, the magnetostriction constant
may be either positive or negative. If the magnetostriction
constant is positive, magnetization is likely to be oriented in the
direction of tensile stress. If the magnetostriction constant is
negative, the magnetization is likely to be oriented in a direction
perpendicular to the direction of tensile stress.
[0032] In a data write based on the magnetostriction interaction, a
first stress resulting from magnetostriction induced in the upper
ferromagnetic layer 30 is transmitted to the recording layer 32.
Moreover, a second stress resulting from magnetostriction induced
in the lower ferromagnetic layer 38 is transmitted to the recording
layer 32. In this case, the first and second stresses desirably
have similar magnitudes.
[0033] To allow the first and second stresses to easily rotate the
magnetization in the recording layer 32, the film thickness T1 of
the upper ferromagnetic layer 30, the film thickness T2 of the
lower ferromagnetic layer 38, and the film thickness T3 of the
recording layer 32 desirably satisfy the following
relationship:
T1, T2>T3 (1)
[0034] Further, the ratio of energy A generated as a result of the
uniaxial anisotropy of the recording layer 32 to composite energy B
of the first and second stresses desirably satisfies the
relationship in Equation (2). The magnitude of the energy can be
adjusted by varying the materials and film thickness of the
recording layer 32 and nonmagnetic layers 31, 33, and 37.
energy A:energy B=1:2 (2)
[0035] The width of the upper ferromagnetic layer 30 and lower
ferromagnetic layer 38 is desirably equal to or larger than the
width of each of the bit line 11 and word line 12.
[0036] The first yoke layer 21 and the bit line 11 are in contact
with the upper ferromagnetic layer 30. The second yoke layer 22 and
the word line 12 are not in contact with the lower ferromagnetic
layer 38. However, the distance D between the lower electrode layer
13 and both second yoke layer 22 and word line 12 is small.
[0037] As shown in FIG. 3, the recording layer 32 has an easy axis
of magnetization oriented in the longitudinal direction of the
planar shape of the second portion 10b of the MTJ element 10. This
easy axis of magnetization has uniaxial anisotropy.
[0038] The first portion 10a is extended in a Y direction similarly
to the bit line 11. The second portion 10b is shaped like an
island-like rectangle. The third portion 10c is larger than the
second portion 10b and has the same planar shape as that of the
lower electrode layer 13. Accordingly, the first to third portions
10a, 10b, and 10c have different planar shapes.
[0039] In this MTJ element 10, the magnetization direction of each
layer is defined as described below. The magnetizations in the
first and second fixation layers 34 and 36 are fixed to an
orientation almost parallel to the direction (X direction) in which
the word line 12 is extended. During no-current flow time, the
magnetization in the recording layer 32 is oriented almost parallel
to the direction (X direction) in which the word line 12 is
extended. The magnetization in the upper ferromagnetic layer 30 is
oriented almost parallel to the direction (Y direction) in which
the bit line 11 is extended. The magnetization in the lower
ferromagnetic layer 38 is oriented almost parallel to the direction
(X direction) in which the word line 12 is extended.
[0040] In other words, during no-current flow time, the
magnetization directions of the upper ferromagnetic layer 30 and
lower ferromagnetic layer 38 are at 90.degree. to each other so
that the magnetization in the upper ferromagnetic layer 30 is
perpendicular to the magnetization in the lower ferromagnetic layer
38. The magnetization in the recording layer 32 is parallel or
antiparallel to the magnetization in one of the upper ferromagnetic
layer 30 and lower ferromagnetic layer 38. In the present
embodiment, during no-current flow time, the magnetization in the
recording layer 32 is set to be parallel or antiparallel to the
magnetization in the lower ferromagnetic layer 38. During
no-current flow time, it is possible to orient the magnetization in
the recording layer 32 almost parallel to the extending direction
(Y direction) of the bit line 11.
[0041] The longitudinal direction of the planar shape of the lower
ferromagnetic layer 38 corresponds to the extending direction (Y
direction) of the bit line 11. Accordingly, the magnetization in
the lower ferromagnetic layer 38 caused by the planar shape during
no-current flow time may be considered to be in the Y direction.
However, in the present embodiment, the lower ferromagnetic layer
38 is placed opposite the second fixation layer 36 via the
nonmagnetic layer 37. Accordingly, by adjusting the material or
film thickness of the nonmagnetic layer 37, a weakly magnetically
coupled state can be created between the lower ferromagnetic layer
38 and the second fixation layer 36. Thus, during no-current flow
time, the magnetization direction of the lower ferromagnetic layer
38 can be set to be parallel or antiparallel (X direction) to the
magnetization direction of the second fixation layer 36 regardless
of the planar shape of the lower ferromagnetic layer 38.
[0042] (2) Materials
[0043] The materials mentioned below are desirably used to form the
layers constituting the MTJ element 10 as well as the first and
second yoke layers 21 and 22.
[0044] The recording layer 32 and the fixation layers 34 and 36 are
formed of, for example, Fe, Co, Ni, or their alloys, a magnetite
having a large spin polarizability, an oxide such as CrO.sub.2 or
RXMnO.sub.3-y (R: rare earth, X: Ca, Ba, or Sr), or a Heusler alloy
such as MiMnSb or PtMnSb. These magnetic substances may contain a
small amount of a nonmagnetic element such as Ag, Cu, Au, Al, Mg,
Si, Bi, Ta, B, C, O, N, Pd, Pt, Zr, Ir, W, Mo, or Nb unless they
lose ferromagnetism.
[0045] The tunnel barrier layer 33 is formed of one of various
dielectrics, for example, Al.sub.2O.sub.3, SiO.sub.2, MgO, AlN,
Bi.sub.2O.sub.3, MgF.sub.2, CaF.sub.2, SrTiO.sub.2, and
AlLaO.sub.3.
[0046] The upper ferromagnetic layer 30 and the lower ferromagnetic
layer 38 are formed of, for example, Fe, Co, Ni, or their alloy, a
magnetite having a large spin polarizability, an oxide such as
CrO.sub.2 or RXMnO.sub.3-y (R: rare earth, X: Ca, Ba, or Sr), or a
Heusler alloy such as MiMnSb or PtMnSb.
[0047] The nonmagnetic layers 31 and 37 are formed of one of
various dielectrics, for example, Al.sub.2O.sub.3, SiO.sub.2, MgO,
AlN, Bi.sub.2O.sub.3, MgF.sub.2, CaF.sub.2, SrTiO.sub.2, and
AlLaO.sub.3. Also the nonmagnetic layers 31 and 37 are formed of,
for example, Ru, Ag, Cu, Au, Al, Mg, Si, Bi, Ta, B, C, O, N, Pd,
Pt, Zr, Ir, W, Mo, or Nb.
[0048] The antiferromagnetic layer 35 is formed of, for example,
Fe--Mn, Pt--Mn, Pt--Cr--Mn, Ni--Mn, Ir--Mn, NiO, or
Fe.sub.2O.sub.3.
[0049] The first and second yoke layers 21 and 22 are formed of,
for example, NiFe, CoFe, amorphous-CoZrNb, FeNx, or FeAlSi.
[0050] (3) Write/Read Operation
[0051] Description will be given of a write/read operation
performed on the magnetic random access memory according to the
first embodiment. In this case, the easy axis of magnetization of
the recording layer 32 is assumed to be almost parallel to the
extending direction (X direction) of the word line 12. Further, for
simplification, all of the three layers including the recording
layer 32, the upper ferro-magnetic layer 30, and the lower
ferromagnetic layer 38 are assumed to have positive
magnetostriction constants. Of course, any of the three layers may
have a negative magnetostriction constant. In that case, rotation
through 90.degree. may be assumed in the description below.
[0052] (a) Write Operation
[0053] FIGS. 4 to 9 are diagrams illustrating a write operation
performed on the magnetic random access memory according to the
first embodiment of the present invention.
[0054] The present embodiment is based on what is called toggle
writes in which before data is written in a selected cell, data
already present in the selected cell is read from the cell.
Accordingly, if arbitrary data is to be written in a selected cell
and when data already present in the selected cell is read from the
cell to find that the arbitrary data has already been written in
the cell, no writes are carried out. If it is found out that data
different from the arbitrary data has been written in the cell, a
write is carried out to rewrite the data. If for example, "0" data
is to be written in the selected cell, when data already present in
the selected cell is read from the cell and shows that "0" data has
already been written in the selected cell, no writes are carried
out. A write is carried out only if "1" data has already been
written in the selected cell. Likewise, if "1" data is to be
written in the selected cell, when data already present in the
selected cell is read from the cell and shows that "1" data has
already been written in the selected cell, no writes are carried
out. A write is carried out only if "0" data has already been
written in the selected cell.
[0055] If the data must be written in the selected cell after the
above check cycle, corresponding two write wirings are sequentially
turned on. The first turned-on write wiring is first turned off.
Then, the second turned-on write wiring is turned off. For example,
a four-cycle procedure comprises turning on the word line 12 and
allowing a write current 12 to flow through the word line 12,
turning on the bit line 11 and allowing a write current 11 to flow
through the bit line 11, turning off the word line 12 to stop the
flow of the write current 12, and turning off the bit line 11 to
stop the flow of the write current 11.
[0056] This write operation will be specifically described
below.
[0057] (Check Cycle)
[0058] First, it is checked what data is written in the MTJ element
10 in the selected cell. Thus, as in the case of a normal MRAM, the
magnetizing resistance of the MTJ element 10 is read by turning on
the MOSFET 15 to allow a read current to flow to the MTJ element 10
through the bit line 11. Specifically, if "0" data has already been
written in the MTJ element 10, the magnetizations in the recording
layer 32 and fixation layers 34 and 36 are, for example, parallel
to each other. Accordingly, the resistance is low. On the other
hand, if "1" data has already been written in the MTJ element 10,
the magnetizations in the recording layer 32 and fixation layers 34
and 36 are, for example, antiparallel to each other. Accordingly,
the resistance is high. Therefore, the data written in the MTJ
element 10 is determined by reading the magnetic resistance, which
may vary depending on whether "1" or "0" data has been written.
[0059] As a result, if the arbitrary data has already been written
in the selected cell, no writes are carried out. If data different
from the arbitrary data has already been written in the selected
cell, a write is carried out to rewrite the data.
[0060] (Initial State)
[0061] As a result of the above check cycle, a write operation is
required if (1) "1" data has already been written in the selected
cell when "0" data is to be written in the selected cell or (2) "0"
data has already been written in the selected cell when "1" data is
to be written in the selected cell.
[0062] Accordingly, in the initial state, if "1" data has already
been written in the MTJ element 10, the magnetization in the
fixation layers 34 and 36 is oriented in the direction of
0.degree., whereas the magnetization in the recording layer 32 is
oriented in the direction of 180.degree.. The magnetization
directions of these layers are in an antiparallel state (see the
schematic diagram in FIG. 4A, showing magnetizations). On the other
hand, if "0" data has already been written in the MTJ element 10,
the magnetization in the fixation layers 34 and 36 is oriented in
the direction of 0.degree., whereas the magnetization in the
recording layer 32 is also oriented in the direction of 0.degree..
The magnetization directions of these layers are in a parallel
state (see the schematic diagram in FIG. 4A, showing
magnetizations).
[0063] Furthermore, in the initial state, both bit line 11 and word
line 12 are turned off as shown in FIG. 4. Thus, the write currents
I1 and I2 do not flow through the bit line 11 and the word line 12,
respectively. That is, the bit line 11 and the word line 11 are in
a de-energized state. In this initial state, the magnetization in
the upper ferromagnetic layer 30 is in the direction of 90.degree.,
whereas the magnetization in the lower ferromagnetic layer 38 is in
the direction of 0.degree..
[0064] In this initial state, as shown in FIG. 5, the magnetic
energies of the upper ferromagnetic layer 30 and lower
ferromagnetic layer 38 have the same amplitude but different phases
shifted from each other through 90.degree.. The magnetic energies
have maximum values at -180.degree., -90.degree., 0.degree.,
90.degree., and 180.degree. The uniaxial anisotropic magnetic
energy have a maximum value at -180.degree., -90.degree.,
0.degree., 90.degree., and 180.degree.. Further, in the initial
state, the first stress acting on the recording layer 32 owing to
the magnetostriction in the upper ferromagnetic layer 30 offsets
the second stress applied to the recording layer 32 owing to the
magnetostriction in the lower ferromagnetic layer 38. Consequently,
the composite stress of the first and second stress is zero. Thus,
the magnetic energy applied to the recording layer 32 has a maximum
value similar to that of the uniaxial anisotropic magnetic
energy.
[0065] (First Cycle)
[0066] Then, in a first cycle, as shown in FIG. 4, the bit line is
kept off to hinder the write current 11 from flowing through the
bit line 11. The word line 12 is turned on to allow the write
current to flow through the word line 12. In this state, the
magnetization in the upper ferromagnetic layer 30 remains in the
direction of 90.degree.. However, the magnetization in the lower
ferromagnetic layer 38 is rotated and oriented in the direction of
90.degree.. Consequently, the magnetizations in both layers 30 and
38 are oriented in the same direction.
[0067] Specifically, the second yoke layer 22 guides a magnetic
field generated by the write current 12 flowing through the word
line 12 to the lower ferromagnetic layer 38. As a result, the
magnetization in the lower ferromagnetic layer 38 is rotated
through 90.degree. to shift the phase of the magnetic energy of the
lower ferromagnetic layer 38 by 90.degree. (see FIG. 6). Thus, the
magnetic energy of the upper ferromagnetic layer 32 and the
magnetic energy of the lower ferromagnetic layer 38 draw the same
curve (the magnetizations in the upper and lower ferromagnetic
layers 30 and 38 are oriented in the same direction). Since the sum
of the first and second stresses acts on the recording layer 32,
the composite stress increased in the directions of 90.degree. and
-90.degree. acts on the recording layer 32 (see the schematic
diagram in FIG. 4B, showing magnetizations). Thus, the
magnetization in the recording layer 32 is rotated through
90.degree. in the directions of the composite stress (the
directions of 90.degree. and -90.degree.) (see the schematic
diagram in FIG. 4B, showing magnetizations).
[0068] (Second Cycle)
[0069] Then, in a second cycle, as shown in FIG. 4, with the write
current 12 kept flowing through the word line I2, the bit line 11
is turned on to allow the write current to flow through the bit
line 11. In this state, the magnetization in the lower
ferromagnetic layer 38 remains oriented in the direction of
90.degree. The magnetization in the upper ferromagnetic layer 30 is
rotated and oriented in the direction 180.degree. Specifically, the
first yoke layer 21 guides a magnetic field generated by the write
current 11 flowing through the bit line 11, to the upper
ferromagnetic layer 30. As a result, the magnetization in the upper
ferromagnetic layer 30 is rotated through 90.degree. to shift the
phase of the magnetic energy of the upper ferromagnetic layer 30 by
90.degree. (see FIG. 7). Thus, the magnetic energy of the upper
ferromagnetic layer 30 and the magnetic energy of the lower
ferromagnetic layer 38 have the same amplitude but different phases
shifted from each other through 90.degree. The first and second
stresses are balanced, so that the composite stress is zero. As a
result, the magnetization in the recording layer 32 is oriented in
the direction of 180.degree. to the initial state (see the
schematic diagram in FIG. 4C, showing magnetizations).
[0070] (Third Cycle)
[0071] Then, in a third cycle, as shown in FIG. 4, with the write
current 11 kept flowing through the bit line 11, the word line 12
is turned off to stop the flow of the write current 12. In this
state, the magnetization in the upper ferromagnetic layer 30
remains oriented in the direction of 180.degree. The magnetization
in the lower ferromagnetic layer 38 returns to the direction of
0.degree. which corresponds to the original stable state.
[0072] That is, the magnetic field generated by the write current
12 flowing through the word line 12 is eliminated to prevent the
application of a magnetic field to the magnetization in the lower
ferromagnetic layer 38. This causes the magnetization in the lower
ferromagnetic layer 38 to return to the direction of 0, which
corresponds to the original stable state. Consequently, the
magnetization in the lower ferro-magnetic layer 38 is rotated
through 90.degree. to shift the phase of the magnetic energy of the
lower ferromagnetic layer 38 by 90.degree. (see FIG. 8). Thus, the
magnetic energy of the upper ferromagnetic layer 32 and the
magnetic energy of the lower ferromagnetic layer 38 exhibit the
same curve (the magnetizations in the upper and lower ferromagnetic
layers 30 and 38 are oriented in the opposite directions). Since
the sum of the first and second stresses acts on the recording
layer 32, the composite stress increased in the directions of
0.degree. and 180.degree. acts on the recording layer 32 (see the
schematic diagram in FIG. 4D, showing magnetizations). Thus, the
magnetization in the recording layer 32 remains oriented in the
directions of the composite stress (the directions of 0.degree. and
180.degree.) (see the schematic diagram in FIG. 4D, showing
magnetizations).
[0073] (Fourth Cycle)
[0074] Then, in a fourth cycle, as shown in FIG. 4, the bit line 11
is turned off to stop the flow of the write current 11, as in the
case of the word line 12. In this state, the magnetization in the
lower ferromagnetic layer 38 remains oriented in the direction of
0.degree.. The magnetization in the upper ferromagnetic layer 30
returns to the direction of 90.degree., which corresponds to the
original stable state.
[0075] That is, the magnetic field generated by the write current
11 flowing through the bit line 11 is eliminated to prevent the
application of a magnetic field to the magnetization in the upper
ferromagnetic layer 30. This causes the magnetization in the upper
ferromagnetic layer 30 to return to the direction of 90.degree.,
which corresponds to the original stable state. Consequently, the
magnetization in the upper ferro-magnetic layer 30 is rotated
through 90.degree. (see the schematic diagram in FIG. 4E, showing
magnetizations) to shift the phase of the magnetic energy of the
upper ferromagnetic layer 30 by 90.degree. (see FIG. 9). Thus, the
magnetic energy of the upper ferromagnetic layer 30 and the
magnetic energy of the lower ferromagnetic layer 38 have the same
amplitude but different phases shifted from each other through
90.degree.. The first and second stresses are balanced, so that the
composite stress is zero. As a result, the magnetization in the
recording layer 32 is not rotated but remains oriented in the
directions of 0.degree. and 180.degree. (see the schematic diagram
in FIG. 4E, showing magnetizations). As a result, "0" or "1" data
is written in the MTJ element 10. The fourth cycle in FIG. 9 has
the same magnetizing energy status as that in the initial state
shown in FIG. 5.
[0076] As described above, when the write currents I1 and I2 are
allowed to flow through the bit and word lines 11 and 12,
respectively, the orientations of the magnetizations in the upper
and lower ferromagnetic layers 30 and 38 are set to rotate through
at least 45.degree. (desirably about 90.degree.) from the initial
state. Then, the magnetostriction effect of the upper and lower
ferromagnetic layers 30 and 38 strains the recording layer 32. The
resultant adverse effect of the magnetostriction enables the
magnetization in the recording layer 32 to be rotated. The above
four cycles enable the magnetization in the recording layer 32 to
be rotated from 0.degree. to 180.degree. or from 180.degree. to
0.degree..
[0077] Regardless of whether "0" or "1" data is written, the write
currents I1 and I2 are allowed to flow in the same direction.
[0078] (b) Read Operation
[0079] FIGS. 10 and 11 schematically show that "1" and "0" data
have been written in the magnetic random access memory according to
the first embodiment of the present invention.
[0080] If "0" data has been written in the MTJ element 10, then the
magnetization in the recording layer 32 is oriented parallel to the
magnetization in the fixation layers 34 and 36, for example, as
shown in FIG. 10. On the other hand, if "1" data has been written
in the MTJ element 10, then the magnetization in the recording
layer 32 is oriented antiparallel to the magnetization in the
fixation layers 34 and 36, for example, as shown in FIG. 11.
[0081] In this state, to read the written data, the MOSFET 15 is
turned on to allow a read current to flow to the MTJ element 10
through the bit line 11 to read the magnetizing resistance of the
MTJ element 10, as in the case of normal MRAMs. Specifically, if
"0" data has been written, the magnetization in the recording layer
32 is parallel to the magnetization in the fixation layers 34 and
36. Accordingly, the resistance is low. On the other hand, if "1"
data has been written, the magnetization in the recording layer 32
is antiparallel to the magnetization in the fixation layers 34 and
36. Accordingly, the resistance is high. Therefore, the data
written in the MTJ element 10 is determined by reading the magnetic
resistance, which may vary depending on whether "1" or "0" data has
been written.
[0082] When the read current is allowed to flow through the MTJ
element 10, a self-reference read can be carried out by also
allowing a current to flow through the word line 12 to rotate the
magnetization in the lower ferromagnetic layer 38 to increase or
reduce the resistance of the MTJ element 10 and then sensing the
increased or reduced resistance.
[0083] According to the first embodiment, the upper ferromagnetic
layer 30 and the lower ferromagnetic layer 38 are provided over and
under the recording layer 32, respectively, via the nonmagnetic
layers 31, 33, and 37. The recording layer 32, the upper
ferromagnetic layer 30, and the lower ferromagnetic layer 38 have
large magnetostriction constants. Thus, when a data write is
carried out, the magnetization in the recording layer 32 is rotated
by transmitting, to the recording layer 32, the first and second
stresses resulting from the magnetostriction of the upper and lower
ferromagnetic layers 30 and 38, caused by the magnetic fields
generated by the write currents I1 and I2.
[0084] For such a data write, a closed magnetic circuit is formed
of the yoke layer 21, provided around the bit line 11, and the
upper ferromagnetic layer 30. A magnetic circuit with a small width
is formed of the yoke layer 22, provided around the word line 12,
and the lower ferromagnetic layer 38. Accordingly, these magnetic
circuits enable the magnetic fields generated by the write currents
I1 and I2 to be guided to the upper and lower ferromagnetic layers
30 and 38. Thus, the magnetizations in the upper and lower
ferromagnetic layers 30 and 38 can be rotated using relatively
small write currents I1 and I2. This makes it possible to reduce
the write currents I1 and I2.
[0085] Further, these magnetic circuits enable the magnetic fields
generated by the write currents I1 and I2 to be efficiently guided
to the upper and lower ferromagnetic layers 30 and 38.
Consequently, disturbance (erroneous writes to semi-selected cells)
can be suppressed.
[0086] Furthermore, when data is written in the recording layer 32
on the basis of the magnetostriction interaction between the
recording layer 32 and both upper and lower ferromagnetic layers 30
and 38, it is possible to reduce a possible variation in the
reversal of the magnetization in the recording layer 32 attributed
to the roughness of the end of the element compared to the direct
write of data in the recording later 32 utilizing current magnetic
fields.
Second Embodiment
[0087] A second embodiment is obtained by deforming the structure
of a third portion of the MTJ element according to the first
embodiment.
[0088] FIG. 12 is a schematic sectional view of a magnetic random
access memory according to a second embodiment of the present
invention. Description will be given below of the structure of the
magnetic random access memory according to the second
embodiment.
[0089] As shown in FIG. 12, the second embodiment differs from the
first embodiment in a third portion 10c of the MTJ element 10. The
third portion 10c is composed of the tunnel barrier layer 33, the
lower ferromagnetic layer 38, the nonmagnetic layer 37, the
fixation layer 34, and the antiferromagnetic layer 35, which are
arranged in this order in the vertical direction; the tunnel
barrier layer 33 is located under the recording layer 32.
Accordingly, the nonmagnetic layer 31 is provided between the
recording layer 32 and the upper ferromagnetic layer 30. The
nonmagnetic layer (tunnel barrier layer) 33 is provided between the
recording layer 32 and the lower ferromagnetic layer 38.
[0090] Also in this structure, as in the case of the first
embodiment, a data write based on magnetostriction interaction is
carried out by transmitting the first stress resulting from the
magnetostriction induced in the upper ferromagnetic layer 30 to the
recording layer 32 and transmitting the second stress resulting
from the magnetostriction induced in the lower ferromagnetic layer
38 to the recording layer 32.
[0091] The magnetization in the fixation layer 34 is fixed in an
orientation almost parallel to the direction (X direction) in which
the word line 12 is extended. During no-current flow time, the
magnetization in the recording layer 32 is oriented almost parallel
to the extending direction (X direction) of the word line 12. The
magnetization in the upper ferromagnetic layer 30 is oriented
almost parallel to the extending direction (Y direction) of the bit
line 11. The magnetization in the lower ferromagnetic layer 38 is
oriented almost parallel to the extending direction (X direction)
of the word line 12.
[0092] The longitudinal direction of the planar shape of the lower
ferromagnetic layer 38 corresponds to the extending direction (Y
direction) of the bit line 11. Accordingly, the magnetization
direction of the lower ferromagnetic layer 38 attributed to the
planar shape during no-current flow time is also considered to be
the Y direction. However, in the present embodiment, the lower
ferromagnetic layer 38 is placed opposite the fixation layer 34 via
the nonmagnetic layer 37. Accordingly, a weakly magnetically
coupled state can be created between the lower ferromagnetic layer
38 and the fixation layer 34 by adjusting the material or film
thickness of the nonmagnetic layer 37. Thus, the magnetization
direction of the lower ferromagnetic layer 38 during no-current
flow time can be set to be parallel or antiparallel (X direction)
to the magnetization direction of the fixation layer 34 regardless
of the planar shape of the lower ferromagnetic layer 38.
[0093] The second embodiment can produce effects similar to those
of the first embodiment. Moreover, in the second embodiment, the
lower ferromagnetic layer 38 and the recording barrier 32 are
separated from each other only via the tunnel barrier layer 33.
Consequently, the stress induced by the lower ferromagnetic layer
38 acts more directly on the recording layer than that in the first
embodiment. Thus, the stress to be generated by the lower
ferromagnetic layer 38 on the basis of the adverse effect of
magnetostriction is more reliably transmitted to the recording
layer 32. Therefore, a reliable write operation can be
accomplished.
Third Embodiment
[0094] In a third embodiment, gaps are formed around MTJ elements
in order to relax the impact, on the MTJ elements, of the
magnetostriction caused by an interlayer insulating film.
[0095] FIG. 13 is a schematic sectional view of a magnetic random
access memory according to a third embodiment of the present
invention. Description will be given below of the structure of the
magnetic random access memory according to the third
embodiment.
[0096] As shown in FIG. 13, the third embodiment differs from the
first embodiment in that gaps 41 are formed in an interlayer
insulating film 40 provided around the MTJ elements 10. The gaps 41
may be formed anywhere within the interlayer insulating film 40.
However, the gaps 41 are desirably formed near the MTJ elements,
notably their recording layers 32.
[0097] The third embodiment produces effects similar to those of
the first embodiment. Moreover, according to the third embodiment,
the gaps 41 are formed around the MTJ elements 10 in the interlayer
insulating film 40. This makes it possible to relax the impact, on
the MTJ elements 10, of the stress induced by the interlayer
insulating film.
Fourth Embodiment
[0098] According to a fourth embodiment, a metal film is provided
between a ferromagnetic layer and a write wiring in order to relax
the impact, on the MTJ element, of the stress induced by the write
wiring.
[0099] FIG. 14 is a schematic sectional view of a magnetic random
access memory according to a fourth embodiment of the present
invention. Description will be given below of the structure of the
magnetic random access memory according to the fourth
embodiment.
[0100] As shown in FIG. 14, the fourth embodiment differs from the
first embodiment in that a metal film 43 is provided between the
upper ferromagnetic layer 30 and the bit line 11. The metal film 43
is desirably formed of, for example, a soft metal material such as
aluminum
[0101] The fourth embodiment produces effects similar to those of
the first embodiment. Moreover, the metal film 43 is provided
between the upper ferromagnetic layer 30 and the bit line 11. This
serves to relax the impact, on the MTJ element 10, of the stress
induced by the bit line 11.
[0102] Additionally, the present invention is not limited to the
above embodiments. In implementation, the embodiments may be varied
as described below without departing from the spirit of the
invention.
[0103] (1) In the above embodiments, the memory cell has a 1 MTJ+1
Tr (transistor) structure. However, the present invention is not
limited to this aspect. For example, in place of the transistor, a
diode may be used as a reading-switching element. Alternatively, a
cross-point structure may be used which does not use a reading
switching element for each cell.
[0104] (2) In the above embodiments, the recording layer 32 and the
fixation layers 34 and 36 have a single layer structure but may
have a stacked structure. Further, in the above embodiments, the
fixation layer 36 has a single layer structure but may have a
weakly coupled stacked pin structure.
[0105] (3) In the above embodiments, a write operation is preformed
in four cycles. However, the cycles may be allowed to overlap one
another to reduce they number. For example, the first and second
cycles may be executed simultaneously.
[0106] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *