U.S. patent application number 09/732030 was filed with the patent office on 2001-07-12 for magnetic element and magnetic memory device.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Kikuchi, Hideyuki, Kobayashi, Kazuo, Sato, Masashige.
Application Number | 20010007532 09/732030 |
Document ID | / |
Family ID | 18531043 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007532 |
Kind Code |
A1 |
Sato, Masashige ; et
al. |
July 12, 2001 |
Magnetic element and magnetic memory device
Abstract
A magnetic element comprises a first ferromagnetic layer, an
insulating layer, and a second ferromagnetic layer laminated in
this order. At least one of the first and second ferromagnetic
layers comprises a lower ferromagnetic layer, a nonmagnetic
conductive layer, and an upper ferromagnetic layer laminated in
this order. By changing kind or composition of material of the
upper and lower ferromagnetic layers, the amount of magnetization
of each layer can be controlled to reduce affection by
magnetostatic coupling. Changeability of magnetized direction of
the first or second ferromagnetic layer can be regulated thereby.
This realizes an improvement of sensitivity.
Inventors: |
Sato, Masashige; (Kawasaki,
JP) ; Kikuchi, Hideyuki; (Kawasaki, JP) ;
Kobayashi, Kazuo; (Kawasaki, JP) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR
25TH FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
FUJITSU LIMITED
|
Family ID: |
18531043 |
Appl. No.: |
09/732030 |
Filed: |
December 7, 2000 |
Current U.S.
Class: |
365/173 |
Current CPC
Class: |
G11C 11/161 20130101;
H01F 10/3254 20130101; B82Y 25/00 20130101; H01F 10/3272
20130101 |
Class at
Publication: |
365/173 |
International
Class: |
G11C 011/15 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 7, 2000 |
JP |
2000-001861 |
Claims
What is claimed is:
1. A magnetic element comprising a first ferromagnetic layer, an
insulating layer, and a second ferromagnetic layer laminated in
this order, at least one of said first and second ferromagnetic
layers comprising a lower ferromagnetic layer, a nonmagnetic
conductive layer, and an upper ferromagnetic layer laminated in
this order.
2. A magnetic element according to claim 1, wherein said upper and
lower ferromagnetic layers sandwiching said non-magnetic conductive
layer are antiferromagnetically coupled with each other.
3. A magnetic element according to claim 2, wherein the magnetized
direction of at least one of said first and second ferromagnetic
layers is fixed by an adjacent antiferromagnetic layer.
4. A magnetic element according to claim 1, wherein said
non-magnetic conductive layer is made of one of Ru and Cu.
5. A magnetic element according to claim 1, wherein the amount of
magnetization of said upper ferromagnetic layer differs from that
of said lower ferromagnetic layer.
6. A magnetic element according to claim 1, wherein the thickness
of said upper ferromagnetic layer differs from that of said lower
ferromagnetic layer.
7. A magnetic memory device comprising magnetic elements each of
which comprises a first ferromagnetic layer, an insulating layer,
and a second ferromagnetic layer laminated in this order, the
magnetized direction of one of said first and second ferromagnetic
layers being changeable in accordance with data to be stored, at
least one of said first and second ferromagnetic layers comprising
a lower ferromagnetic layer, a non-magnetic conductive layer, and
an upper ferromagnetic layer laminated in this order.
8. A magnetic memory device according to claim 7, wherein said
upper and lower ferromagnetic layers sandwiching said non-magnetic
conductive layer are antiferromagnetically coupled with each other
to form an antiferromagnetic layer.
9. A magnetic memory device according to claim 7, wherein the
magnetized direction of at least one of said first and second
ferromagnetic layers is fixed by an adjacent antiferromagnetic
layer.
10. A magnetic memory device according to claim 7, wherein said
non-magnetic conductive layer is made of one of Ru and Cu.
11. A magnetic memory device according to claim 7, wherein the
amount of magnetization of said upper ferromagnetic layer differs
from that of said lower ferromagnetic layer.
12. A magnetic memory device according to claim 7, wherein the
thickness of said upper ferromagnetic layer differs from that of
said lower ferromagnetic layer.
13. A magnetic memory device according to claim 7, wherein each of
said magnetic elements forms a memory cell, with one of said first
and second ferromagnetic layers being connected to a word line
while the other being connected to a bit line.
14. A magnetic memory device according to claim 7, wherein each of
said magnetic elements forms a memory cell interposed between a
word line and a bit line at the intersection of said word and bit
lines, with one of said first and second ferromagnetic layers being
connected to said word line while the other being connected to said
bit line.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims priority of
Japanese Patent Application No. 2000-001861, filed on Jan. 7, 2000,
the contents being incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to magnetic elements using
ferromagnetic tunnel junction, and magnetic memory devices
comprising such elements.
[0004] 2. Description of the Related Art
[0005] MRAM (Magnetic Random Access Memory) is one of memory
devices such as DRAM (Dynamic Random Access Memory) and SRAM
(Static Random Access Memory) using semiconductor substances. While
DRAM and SRAM store data in accordance with presence/absence of
electric charges, MRAM stores data in accordance with magnetized
directions of magnetic substances. MRAM has many merits, e.g., it
is suitable for high speed operation, it shows high radiation
resistance, and it shows little deterioration due to repetition of
data write operations. For this reason, the study of MRAM has been
prosecuted earnestly in recent years.
[0006] MRAM has its basic structure in which magnetic memory
elements are arranged into a matrix, word and bit lines are
disposed in columns and rows for generating magnetic field in a
selected element, and terminals are provided for reading out data
stored in the selected element. When one of the word lines and one
of the bit lines are selected, and electric currents are applied to
the selected word and bit lines, magnetic field is generated in the
magnetic element at the intersection of the selected word and bit
lines. The magnetized direction of the magnetic element can be
reversed by the magnetic field. In this manner, two different
magnetized states of each magnetic element can be realized. The two
magnetized states correspond to bit data of "0" and "1",
respectively.
[0007] For the memory structure of each magnetic element of MRAM,
usable are so-called MR (Magneto-Resistive) element, GMR (Giant
Magneto-Resistive) element, and ferromagnetic tunnel element, any
of which changes in its electric resistance in accordance with
magnetized directions. To read out data from a magnetic element, an
electric current is applied to the element and the electric
resistance thereof is measured.
[0008] MRAM using MR or GMR element for the memory structure of
each magnetic element has been realized. In this type of MRAM,
however, the sheet resistance of each magnetic element is measured.
Therefore, if the element size is reduced, the resistance to be
measured is reduced accordingly, so that the output is reduced.
Although reduction in size of such an element is a recent general
demand, this type of MRAM has its limit.
[0009] Ferromagnetic tunnel element has a tunnel junction structure
generally comprising a ferromagnetic layer, an insulating layer,
and another ferromagnetic layer laminated in this order. In this
structure, the tunnel resistance in case of both the magnetic
layers being magnetized in the same direction, differs from that in
case of those being magnetized in reverse directions. The amount of
change in resistance depends on the polarizability of each magnetic
layer. A change in resistance by more than 40-50% is expected if a
suitable material is chosen. Besides, the smaller the junction area
is, the higher the tunnel resistance is. Thus the element size can
be reduced without reducing the output.
[0010] For these reasons, use of such a ferromagnetic tunnel
junction structure for the memory structure of each magnetic memory
element of MRAM is expected to realize a higher packing density in
MRAM.
[0011] This idea for improving the packing density in MRAM,
however, includes the following vital problems.
[0012] First, each of the word and bit lines of MRAM must receive
an electric current for generating sufficient magnetic field that
can reverse the magnetized direction of the target magnetic layer
of each memory structure. For this reason, each of the word and bit
lines requires a certain degree of size in its cross section. This
requirement in size limits the packing density. To avoid this
problem, the electric current for generating magnetic field must be
made small. This requires selection of a suitable magnetic material
whose magnetized direction can be reversed with weaker magnetic
field.
[0013] Second, if the size of such a memory structure using
ferromagnetic tunnel junction is reduced, both the ferromagnetic
layers sandwiching the insulating layer may be magnetostatically
coupled through leakage fluxes out of the ferromagnetic layers.
This lowers the sensitivity of the memory element to external
magnetic field.
[0014] FIG. 1 shows such a magnetic memory structure. Referring to
FIG. 1, ferromagnetic layers 101 and 103 sandwiches an insulating
layer 102. In this structure, the ferromagnetic layers 101 and 103
may be magnetically coupled with each other, so that leakage fluxes
become very few. This causes a bad sensitivity to external magnetic
field.
[0015] FIG. 2 shows a more specific structure of a magnetic memory
element in this type of MRAM. In this example, a ferromagnetic
layer 101 (thickness: 2.0 nm) made of CoFe, an insulating layer 102
(thickness: 1.5 nm) made of Al.sub.2O.sub.3, and a ferromagnetic
layer 103 (thickness: 1.0 nm) made of CoFe are laminated in this
order on a magnetic underlayer 104 (thickness: 10 nm) made of IrMn.
In this structure, external fluxes may be made from an end of the
CoFe layer 103 whose magnetized direction is fixed (hereinafter
referred to as fixed layer), to an end of the CoFe layer 101 whose
magnetized direction is to be reversed (hereinafter referred to as
free layer). This causes very bad sensitivity of the CoFe layer 101
to external magnetic field.
[0016] Besides, magnetic domain structure is a factor of stability
of such a magnetic element. In general, a magnetic substance
comprises a number of magnetic domains having the same magnetized
direction. The boundary between such domains is called magnetic
domain wall. In case of a magnetic element of MRAM in which a
magnetized direction is changed, such magnetic domain walls may
move. This causes noise and deterioration of sensitivity.
SUMMARY OF THE INVENTION
[0017] It is an object of the present invention to provide magnetic
elements having a relatively simple construction, being less
affected by magnetostatic coupling, and capable of realizing
improvement of the sensitivity for reversing magnetized direction
by a single domain structure. It is another object of the present
invention to provide magnetic memory devices comprising such
magnetic elements for realizing less power consumption, high-speed
operation, and high packing density.
[0018] According to an aspect of the present invention, a magnetic
element comprises a first ferromagnetic layer, an insulating layer,
and a second ferromagnetic layer laminated in this order. At least
one of the first and second ferromagnetic layers comprises a lower
ferromagnetic layer, a nonmagnetic conductive layer, and an upper
ferromagnetic layer laminated in this order.
[0019] Preferably, the upper and lower ferromagnetic layers
sandwiching the non-magnetic conductive layer are
antiferromagnetically coupled with each other.
[0020] Preferably, the magnetized direction of at least one of the
first and second ferromagnetic layers is fixed by an adjacent
antiferromagnetic layer.
[0021] Preferably, the non-magnetic conductive layer is made of one
of Ru and Cu.
[0022] Preferably, the amount of magnetization of the upper
ferromagnetic layer differs from that of the lower ferromagnetic
layer.
[0023] Preferably, the thickness of the upper ferromagnetic layer
differs from that of the lower ferromagnetic layer.
[0024] According to another aspect of the present invention, a
magnetic memory device comprises magnetic elements each of which
comprises a first ferromagnetic layer, an insulating layer, and a
second ferromagnetic layer laminated in this order. The magnetized
direction of one of the first and second ferromagnetic layers is
changeable in accordance with data to be stored. At least one of
the first and second ferromagnetic layers comprises a lower
ferromagnetic layer, a non-magnetic conductive layer, and an upper
ferromagnetic layer laminated in this order.
[0025] According to the present invention, at least one of the
first and second ferromagnetic layers has the structure that upper
and lower ferromagnetic layers sandwich a non-magnetic conductive
layer. By properly selecting the kind or composition of the
material and the thickness of each of the upper and lower
ferromagnetic layers, the amount of magnetization of each of them
can be so regulated as to reduce the affection by magnetostatic
coupling. Changeability of magnetized direction of the first or
second ferromagnetic layer in response to external magnetic field
can thereby be adjusted into a suitable value. This affords an
improvement of sensitivity.
[0026] According to the present invention, realized are magnetic
elements having a relatively simple construction, being less
affected by magnetostatic coupling, and capable of improving the
sensitivity for reversing magnetized direction by a single domain
structure. Also realized are magnetic memory devices comprising
such magnetic elements for less power consumption, high-speed
operation, and high packing density.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a schematic sectional view of a conventional
ferromagnetic tunnel junction structure comprising a ferromagnetic
layer, an insulating layer, and another ferromagnetic layer
laminated in this order, for explaining a problem of the
conventional structure;
[0028] FIG. 2 is a schematic sectional view showing a specific
structure of a conventional magnetic memory element of MRAM;
[0029] FIG. 3 is a schematic sectional view showing a specific
structure of a magnetic element according to the present invention;
and
[0030] FIG. 4 is a schematic plan view of a principal part of MRAM
according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] Hereinafter, embodiments of the present invention will be
described with reference to drawings.
[0032] (First Embodiment)
[0033] The first embodiment of the present invention will be
described. In this first embodiment, a magnetic element will be
described. FIG. 3 is a schematic sectional view showing a specific
structure of a magnetic element according to this embodiment.
[0034] The magnetic element comprises a ferromagnetic layer 1 as
fixed layer, an insulating layer 2, and a ferromagnetic layer 3 as
free layer, which are formed in this order on a magnetic underlayer
4. The ferromagnetic layers 1 and 3 and the insulating layer 2
functions as ferromagnetic tunnel junction. In this embodiment, the
ferromagnetic layer 3 is made up from the lower ferromagnetic layer
11, a non-magnetic conductive layer 12, and the upper ferromagnetic
layer 13 laminated in this order. More specifically, the magnetic
underlayer 4 is made of IrMn to have a thickness of about 10 nm.
The ferromagnetic layer 1 is made of CoFe to have a thickness of
about 2.0 nm. The insulating layer 2 is made of Al.sub.2O.sub.3 to
have a thickness of about 1.5 nm. The lower ferromagnetic layer 11
is made of CoFe to have a thickness of about 1.0 nm. The
non-magnetic conductive layer 12 is made of Ru to have a thickness
of about 0.8 nm. The upper ferromagnetic layer 13 is made of CoFe
to have a thickness of about 2.0 nm. Any of these layers can -be
formed by sputtering process. The non-magnetic conductive layer 12
may be made of Cu in place of Ru.
[0035] Generally in such a structure of ferromagnetic
layer/non-magnetic conductive layer/ferromagnetic layer, it is
known that the upper and lower ferromagnetic layers are
magnetically coupled if the thickness of the non-magnetic
conductive layer is regulated properly. In case of the non-magnetic
conductive layer of Ru, the ferromagnetic layers are
antiferromagnetically coupled when the thickness of the
non-magnetic conductive layer is about 0.8 nm.
[0036] If the upper and lower ferromagnetic layers 13 and 11
sandwiching the non-magnetic conductive layer 12 has the same
thickness, the magnetization of both the ferromagnetic layers
cancels out. As a result, leakage fluxes become very few and the
sensitivity becomes bad. If a little difference in thickness is
made between the ferromagnetic layers, there appears the
magnetization corresponding to the difference in thickness. The
magnetized direction of this apparent magnetization can be changed
with external magnetic field. The amount of the apparent
magnetization is determined by the amount of the magnetization and
the thickness of each ferromagnetic layer. Thus the amount of the
apparent magnetization can be finely controlled by changing the
thickness, or kind or composition of material of the ferromagnetic
layers.
[0037] According to this embodiment, in the ferromagnetic layer 3
as free layer, the lower ferromagnetic layer 11 is magnetically
coupled with the upper ferromagnetic layer 13 disposed on the
nonmagnetic conductive layer 12. The lower ferromagnetic layer 11
is, therefore, not magnetically coupled with the ferromagnetic
layer 1 as fixed layer. Besides, since the upper and lower
ferromagnetic layers 13 and 11 sandwiching the nonmagnetic
conductive layer 12 differ in the amount of magnetization, there
appears an apparent magnetization whose magnetized direction can be
changed with external magnetic field. Further, since the upper and
lower ferromagnetic layers 13 and 11 are antiferromagnetically
coupled to be in a stable state, there is formed a single domain.
Therefore, the element becomes hard to be affected by noise or
instability due to movement of magnetic domain walls. This makes it
possible to suppress the affection by the magnetostatic coupling of
the ferromagnetic layers 1 and 3, and keep the sensitivity to
external magnetic field good.
[0038] In this embodiment, the ferromagnetic layer 3 is made into
the structure of ferromagnetic layer/nonmagnetic conductive
layer/ferromagnetic layer. But, the ferromagnetic layer 1 may be
made into such a three-layer structure, in place of the
ferromagnetic layer 3. Further, both the ferromagnetic layers 1 and
3 may be made into three-layer structure. In this manner, fine
control of the amount of magnetization in accordance with
conditions becomes possible.
[0039] (Second Embodiment)
[0040] The second embodiment of the present invention will be
described next. In this second embodiment, a magnetic memory device
(MRAM) comprising such magnetic elements as described in the first
embodiment will be described.
[0041] FIG. 4 is a schematic plan view of a principal part of MRAM
according to this embodiment.
[0042] In MRAM, magnetic elements 21 are arranged into a matrix.
The magnetic elements 21 in each column are connected through a
word line 23, and the magnetic elements 21 in each row are
connected through a bit line 22. Terminals (not shown) are provided
for reading out data from any target magnetic element. When one of
the word lines 23 and one of the bit lines 22 are selected, and
electric currents are applied to the selected word and bit lines,
magnetic field is generated in the magnetic element 21 at the
intersection of the selected word and bit lines. The magnetized
direction of the magnetic element 21 is reversed by the magnetic
field. In this manner, the magnetic element 21 is set at one of two
different magnetized states. Bit data of "0" or "1" is thereby
stored in the magnetic element 21.
[0043] In this embodiment, since such magnetic elements as shown in
FIG. 3 are used for MRAM, sure data write and read can be performed
even if each element is reduced in size. This makes it possible to
realize highly reliable MRAM with less power consumption,
high-speed operation, and high packing density.
[0044] In this embodiment, the magnetic element described in the
first embodiment is applied to MRAM. In the present invention,
however, the magnetic element of the first embodiment is not
limited to such an application. The magnetic element of the first
embodiment can sufficiently be applied also to various magnetic
sensors including magnetic heads for example.
[0045] A magnetic element according to the present invention can be
used not only for MRAM but also for a magnetic sensor, e.g., as a
sensor device for a magnetic disk. That is, the present invention
can apply to either of a magnetic memory device and a magnetic
storage device. Here, "magnetic storage device" means, e.g., a data
storage device in which data write and read operations in relation
to a very small bit area on a magnetic storage medium rotating at a
high speed, such as a magnetic disk, are performed through a head
that a magnetic sensor is incorporated in and that is mechanically
driven to be put close to the storage surface of the medium, while
"magnetic memory device" means an electronic device in which data
write and read operations are performed entirely in an electronic
manner. The magnetic memory device has its basic structure in which
a memory cell as a data storage unit is provided at each
intersection of bit and word lines, like usual DRAM. In the
magnetic memory device, however, usual memory cells as capacitor
cells are replaced by magnetic elements, so that data write and
read operations in relation to each memory cell are performed
electromagnetically. Such a magnetic memory device is currently
called MRAM (Magnetic Random Access Memory) or the like and thus it
is premised on random access. But, even in case of a device called
MRAM, it has fundamentally no need of data refresh operations at
short intervals. In spite of the present situation, a magnetic
memory device according to the present invention can be used as a
read-only memory in which only read operations for data stored in a
semipermanent form are possible. Besides, it can also be used like
a flash memory in which data stored therein is electrically
erasable only in a lump. Thus the present invention is not limited
to such devices as dynamic random access memories in which data
refresh operations are performed at regular intervals.
[0046] Although the invention has been described in its preferred
form with a certain degree of particularity, it is understood that
various further changes, modifications, and alternations can be
made in the invention without departing from the spirit and the
scope thereof.
* * * * *