Title: Magnetic ring unit and magnetic memory device
Abstract: The present invention relates to a magnetic ring unit and a magnetic memory device; an object of the invention is to control the direction of rotation of the magnetic flux freely and with high reproducibility in a simple structure without using a thermal process such as pinning; and a magnetic ring unit is formed of a magnetic ring in eccentric ring form where the center of the inner diameter is located at a decentered position relative to the center of the outer diameter.
Patent Number: 7,002,839 Issued on 02/21/2006 to Kawabata, et al.
| Inventors:
|
Kawabata; Makoto (Kanagawa, JP);
Harii; Kazuya (Kanagawa, JP);
Saitoh; Eiji (Kanagawa, JP);
Miyajima; Hideki (Kanagawa, JP)
|
| Assignee:
|
Keio University (Tokyo, JP)
|
| Appl. No.:
|
827366 |
| Filed:
|
April 20, 2004 |
Foreign Application Priority Data
| Apr 23, 2003[JP] | 2003-118198 |
| Current U.S. Class: |
365/171; 365/173; 365/66; 365/51; 365/55 |
| Current Intern'l Class: |
G11C 11/00 (20060101) |
| Field of Search: |
365/171,173,66,51,55
|
References Cited [Referenced By]
U.S. Patent Documents
| 5541868 | Jul., 1996 | Prinz.
| |
| 6391483 | May., 2002 | Zhu et al.
| |
| 6906369 | Jun., 2005 | Ross et al.
| |
| 6927073 | Aug., 2005 | Huggins.
| |
| 6936479 | Aug., 2005 | Sharma.
| |
| 6950332 | Sep., 2005 | Yamamoto et al.
| |
| 2002/0182557 | Dec., 2002 | Kuriyama et al.
| |
| 2004/0136558 | Jul., 2004 | Usuki et al.
| |
| 2004/0165426 | Aug., 2004 | Yamamoto et al.
| |
| 2004/0166640 | Aug., 2004 | Yagami et al.
| |
| Foreign Patent Documents |
| 2001-84758 | Mar., 2001 | JP.
| |
| 2002/-299584 | Oct., 2002 | JP.
| |
| 2003-31776 | Jan., 2003 | JP.
| |
Other References
J.-G. Zhu, et al.; "Ultrahigh density vertical magnetoresistive random access
memory (invited)"; Journal of Applied Physics; vol. 87; No. 9; May 1, 2000;
pp. 6668-6673./Discussed in the specification.
M. Kläui et al.; Vortex circulation control in mesoscopic ring magnets;
Applied Physics Letters; vol. 78; No. 21; May 21, 2001; pp. 3268-3270./Discussed
in the specification.
M. Schneider, et al.; "Magnetic switching of single vortex permalloy elements"
Applied Physics Letters; vol. 79; No. 19; Nov. 5, 2001; pp. 3113-3115.
R. Nakatani, et al; "Magnetization Reversal with In-Plane Magnetic Field in Asymmetric
Ring Dots"; Japanese Journal of Applied Physics; vol. 42; No. 1; Jan. 2003;
pp. 100-101.
|
Primary Examiner: Nguyen; Viet Q.
Attorney, Agent or Firm: Armstrong, Kratz, Quintos, Hanson & Brooks, LLP.
Claims
What is claimed is:
1. A magnetic ring unit, characterized by having at least a magnetic ring in
eccentric ring form wherein the center of the inner diameter is located at a decentered
position relative to the center of the outer diameter.
2. The magnetic ring unit according to claim 1, characterized in that said magnetic
ring in eccentric ring form comprises a pair of magnetic rings having coercive
forces that are different from each other and in that a non-magnetic layer is intervened
between said pair of magnetic rings.
3. A magnetic memory device that comprises: magneto-resistive memory elements
on a semiconductor substrate, which are respectively placed in intersection regions
of word lines and bit lines placed in the directions crossing each other and where
first magnetic layers of which the direction of rotation of magnetization is variable
and second magnetic layers of which the direction of rotation of magnetization
is fixed are layered via non-magnetic intermediate layers; and access transistors
of which the gates are sense lines placed in the direction that crosses said bit
lines, wherein the magnetic memory device is characterized in that each of said
magneto-resistive memory element is formed at least of: a first magnetic ring in
eccentric ring form where the center of the inner diameter is located at a decentered
position relative to the center of the outer diameter; a second magnetic ring in
eccentric ring form having a coercive force greater than that of said first magnetic
ring; and a non-magnetic layer provided between said first and second magnetic rings.
Description
TECHNICAL FIELD
The present invention relates to a magnetic ring unit and a magnetic memory device,
and in particular, to a magnetic ring unit and a magnetic memory device characterized
by the configuration for controlling the direction of rotation of magnetization
of the magnetic ring unit with high reproducibility.
BACKGROUND ART
In recent years, a dramatic increase in the density and miniaturization of magnetic
recording medium and magnetic memory device have progressed as a result of the
development of a microscopic processing technology and the recording density has
approximately arrived at a theoretical limit.
The direction of local magnetic moment M of a magnetic body corresponds to digital
data of either "0" or "1" in such a magnetic recording medium or magnetic memory device.
A magnetic random access memory device (MRAM), which is an example of a magnetic
memory device, is a memory device utilizing a change in the resistance value depending
on the direction of the spin of electrons in a magnetic body as a result of a current
flow in the magnetic structure, wherein GMR (giant magnetoresistance) elements
or TMR (tunneling magnetoresistance) elements have been examined concerning the
magnetic structure for the formation of memory cells [see for example, Japanese
Unexamined Patent Publication 2003-031776 (Patent document 1) or Japanese Unexamined
Patent Publication 2002-299584 (Patent document 2)].
Here, a great resistance change has been required in such an MRAM and therefore,
the TMR element structure is primarily used in research and development.
When such a magnetic memory device or magnetic recording medium is formed by
integrating magnetic units with a high density, the magnetic units are aligned
in proximity to each other in the configuration of the magnetic memory device or
magnetic recording medium, wherein the magnetostatic energy becomes the minimum
when the opposite poles are alternately aligned in the case where the poles of
magnetic bodies, that is to say N poles and S poles, are placed in proximity to
each other.
The magnetic pole alignments other than the above gradually transit to the minimum
energy condition due to thermal disturbance or as a result of a tunnel phenomenon
and thereby, the recorded data disappears.
This disappearance of the recorded data in course of time is a critical defect
in the magnetic recording medium or magnetic memory device and therefore, it is
necessary to reduce as much as possible the magnetic interaction between magnetic
units that hold data in order to prevent the above described disappearance of the
recorded data due to the magnetic interaction.
As one effective method for the above, usage of nanoscale magnetic bodies in
ring
form, that is to say, usage of nanoring units as the recording units has been proposed
[see for example, Japanese Unexamined Patent Publication 2001-084758 (Patent document
3) or Journal of Applied Physics, Vol. 87, No. 9, pp. 6668-6673, May 1
st,
2000 (Non-patent document 1)].
See FIG. 10.
FIG. 10 is a diagram showing a conceptual configuration of a nanoring unit,
wherein the nanoring unit is a ring with a diameter of approximately 100 nm fabricated
from a ferromagnetic body such as permalloy (NiFe alloy) having a small magnetic
anisotropy where a magnetic vortex structure (magnetic flux closure domain) is
formed so as to contain magnetic flux indicated by arrows inside.
In such a magnetic vortex structure, the clockwise direction and the counterclockwise
direction of magnetic flux have equal energy and therefore, magnetic memory cells
are formed in a manner where the direction of rotation corresponds to digital data
of either "0" or "1."
This magnetic vortex structure does not have flux leakage where magnetic interaction
between nanoring units is extremely small and accordingly, the data written in
a nanoring unit is stably retained even in the case where nanoring units are aligned
with a high density allowing the achievement of a recording density of approximately
400 Gbit/in
2 (≈62 Gbit/cm
2) which is a recording density
ten times, or more, higher than the present recording density.
As described above, a ferromagnetic nanoring unit has excellent characteristics
as a magnetic recording medium or magnetic memory device while the clockwise direction
and counterclockwise direction of flux have equal energy and therefore, it is necessary
to control the direction of rotation of magnetic flux for practical usage.
See FIGS. 11(
a) to 11(
c).
This is because whether the counterclockwise direction shown in FIG. 11(
b)
or the clockwise direction shown in FIG. 11(
c) is gained cannot be controlled
by adjusting energy during the process of conversion of the opposed domain structure
formed by applying the external magnetic field shown in FIG. 11(
a) into
the magnetic vortex structure when the magnetic field is reduced to 0.
Thus, in the above described Patent document 3, a non-magnetic conductor is
provided in order to penetrate through the center of a ferromagnetic nanoring unit
and the direction of rotation is regulated by the direction of current that flows
through this non-magnetic conductor.
In addition, an antiferromagnetic pattern is locally provided at a position that
shifts from the rotational symmetry on the surface of the ferromagnetic nanoring
unit so that the direction of magnetization of the pinned layer is fixed due to
the direction of magnetization provided to this antiferromagnetic pattern.
In addition, another method has been proposed wherein a constriction, or the
like,
is provided in a nanoring so that the direction of rotation is controlled by pinning
magnetic domain walls [see for example, Applied Physics Letters, Vol. 78, No. 21,
pp. 3268-3270, May 21
st, 2001 (Non-patent document 2)].
In the above described Patent document 1, however, it is necessary to make the
insulation between a feed-through conductor and a nanoring complete, and to do
so, it is necessary for an insulating film without a pinhole to be formed so as
to have a sufficient thickness that can prevent a tunnel phenomenon and in addition
a problem arises where an antiferromagnetic pattern is required.
In addition, a problem arises in the above described Non-patent document 2 where
a unit is thermally agitated as a result of the utilization of effects of pinning
magnetic domain walls and therefore, a stable operation cannot be expected at room temperature.
Accordingly, an object of the present invention is to control the direction
of rotation of the magnetic flux freely and with high reproducibility in a simple
structure where a thermal process such as pinning is not used.
DISCLOSURE OF THE INVENTION
FIGS. 1(
a) and 1(
b) are diagrams describing the configuration
according to the principles of the present invention and the means for solving
the problem according to the present invention is described in reference to these
FIGS. 1(
a) and 1(
b).
See FIGS. 1(
a) and 1(
b).
(1) According to the present invention, a magnetic ring unit is characterized
by having at least a magnetic ring 1 in eccentric ring form wherein the
center of the inner diameter is located at a decentered position relative to the
center of the outer diameter.
In such a magnetic ring 1 in eccentric ring form, as shown in FIGS. 1(
a)
and 1(
b), the magnetic domain walls 2 and 3 shift in
the direction toward the portion with a narrow ring width during the process of
conversion from the opposed domain structure formed by applying an external magnetic
field into the magnetic vortex structure when the magnetic field is reduced to
0 and therefore, the magnetic vortex structure rotates in the direction of the
magnetic moment on the side with a broad ring width so that the direction of rotation
can be controlled with high reproducibility by the direction of application of
the external magnetic field.
(2) In addition, according to the present invention, the above described (1)
is characterized in that the magnetic ring 1 in eccentric ring form is made
up of a pair of magnetic rings having coercive forces that are different from each
other and in that a non-magnetic layer is intervened between the pair of magnetic rings.
In this manner, a layered structure is formed of magnetic ring/non-magnetic layer/magnetic
ring where the coercive forces of the pair of magnetic rings are different from
each other and thereby, a magnetic sensor or magnetic memory cell can be formed.
Here, in this case a GMR element can be gained by forming the non-magnetic
layer of a conductive layer such as Cu, Au, Cr, or the like; and a TMR element
can be gained by forming the non-magnetic layer of a tunnel insulating film such
as Al
2O
3, SiO
2, or the like.
(3) In addition, according to the present invention, a magnetic memory device
that is provided with: magneto-resistive memory elements on a semiconductor substrate,
which are respectively placed in intersection regions of word lines and bit lines
placed in the directions crossing each other and where first magnetic layers, of
which the direction of rotation of magnetization is variable, and second magnetic
layers, of which the direction of rotation of magnetization is fixed, are layered
via non-magnetic intermediate layers; and access transistors of which the gates
are sense lines placed in the direction that crosses the bit lines, is characterized
in that each of the magneto-resistive memory element is formed at least of: a first
magnetic ring 1 in eccentric ring form where the center of the inner diameter
is located at a decentered position relative to the center of the outer diameter;
a second magnetic ring in eccentric ring form having a coercive force greater than
that of first magnetic ring 1; and a non-magnetic layer provided between
the first and the second magnetic rings.
A magnetic ring unit formed of magnetic ring/non-magnetic layer/magnetic ring
is
used as a magneto-resistive memory element in the above described manner and thereby,
a magnetic memory device with a high recording density, which is highly reliable,
can be implemented without requiring a complicated configuration or a thermal process.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(
a) and 1(
b) are diagrams describing the configuration
according to the principles of the present invention;
FIGS. 2(
a) to 2(
c) are views describing the steps halfway
through the manufacturing of a magnetic ring unit according to the first embodiment
of the present invention;
FIGS. 3(
d) and 3(
e) are views describing the steps following
FIG. 2(
c) of the manufacturing of the magnetic ring unit according to the
first embodiment of the present invention;
FIGS. 4(
a) to 4(
d) are diagrams describing the principles
of control of the direction of rotation of the magnetic moment in the magnetic
ring unit according to the first embodiment of the present invention;
FIG. 5 is a graph describing the hysteresis characteristics of the magnetic
ring unit according to the first embodiment of the present invention;
FIG. 6 is a graph describing the hysteresis characteristics of a non-eccentric
magnetic ring unit according to the prior art;
FIG. 7 is a schematic cross-sectional view showing a main portion of an MRAM
according to the second embodiment of the present invention;
FIGS. 8(
a) and 8(
b) are diagrams describing the circuit
configuration of the MRAM according to the second embodiment of the present invention;
FIGS. 9(
a) and 9(
b) are diagrams describing write-in and
read-out operations in the MRAM according to the second embodiment of the present invention;
FIG. 10 is a diagram showing a conceptual configuration of magnetic moment distribution
in a nanoring unit; and
FIGS. 11(
a) to 11(
c) are diagrams describing the conversion
from the opposed domain structure to the magnetic vortex structure in the nanoring unit.
BEST MODE FOR CARRYING OUT THE INVENTION
In reference to FIGS. 2(
a) to
6 a magnetic ring unit according
to
the first embodiment of the present invention is herein described. First, in reference
to FIGS. 2(
a) to
3(
e), the manufacturing steps of the magnetic
ring unit are described in the following.
See FIGS. 2(
a) and
2(
b).
FIG. 2(
a) is a plan view and FIG. 2(
b) is a schematic cross-sectional
view along one-dotted chain line A-A′ of FIG. 2(
a).
First, a photoresist layer
12 is applied to a silicon substrate
11
so as to have a thickness of, for example, 100 nm which is then exposed to light
and developed so that a recess
13 in ring form is created.
In this case, the outer diameter of recess
13 in ring form is, for example,
500 nm while the plan form of an inner protrusion
14 is an ellipse having
a major axis of 350 nm and a minor axis of 250 nm so that the recess is of an eccentric
ring form wherein the center of the ellipse shifts from the center of the outer
diameter by 50 nm in the direction of the minor axis.
See FIG. 2(
c).
Subsequently, a NiFe layer
15 is deposited on the entire surface
by means of the sputtering method so as to have a thickness of, for example, 20 nm.
See FIG. 3(
d).
Subsequently, photoresist layer
12 is removed and thereby, NiFe
layer
15 that has been deposited in recess
13 in ring form becomes
a magnetic ring unit
16.
See FIG. 3(
e).
FIG. 3(
e) is a plan view showing a condition where magnetic ring units
16 that have been fabricated in the above manner are aligned, wherein magnetic
ring units
16 are aligned in a matrix form with pitches of approximately
2 μm.
Next, in reference to FIGS. 4(
a) to
6, the principles of control
of the direction of rotation of the magnetic moment in a magnetic ring unit is described.
See FIG. 4(
a).
First, a magnetic field is applied in the direction perpendicular to the direction
in which magnetic ring unit
16 is decentered and thereby, an opposed domain
structure is formed where a domain
19 with a broad ring width and a domain
20 with a narrow ring width are opposed to each other via magnetic domain
walls
17 and
18.
See FIG. 4(
b).
Subsequently, magnetic domain walls
17 and
18 gradually
shift to the side of domain
20 with a narrow ring width as the magnetic
field is reduced to 0.
This is because magnetic domain wall energy e has a gradient ∇e on the
circumference of the ring due to the eccentricity and therefore, the stress f (=-∇e)
is applied to magnetic domain walls
17 and
18.
See FIGS. 4(
c) and
4(
d).
Subsequently, magnetic domain walls
17 and
18 shift further
to the side of domain
20 with a narrow ring width and ultimately a magnetic
vortex structure is formed.
At this time, the direction of rotation of the magnetic vortex structure agrees
with the direction of the magnetic moment in domain
19 with a broad ring width.
When magnetic ring unit
16 in such a condition is observed by using an
MFM (magnetic force microscope), it is confirmed that all magnetic ring units
16
have a magnetic vortex structure with the same direction of rotation.
Here, when the direction of application of an external magnetic field is reversed,
the direction of the magnetic moment of domain
19 with a broad ring width
becomes of a direction opposite to the case of FIG. 4(
a) and thus the direction
of rotation also becomes opposite.
See FIG. 5.
FIG. 5 is a graph describing the hysteresis characteristics of a magnetic ring
unit wherein it is understood that a stable magnetic vortex structure is formed
by applying a magnetic field of 3 [kOe] and the magnetic vortex structure is maintained
under an external magnetic field H
ex of approximately 2 [kOe].
In the case where a current is made to flow in the vicinity of the magnetic ring
unit in order to generate this magnetic field of 3 [kOe], it is possible to make
the amount of this current 1 μA or less indicating that data can be magnetically
written in with a sufficiently small current.
See FIG. 6.
FIG. 6 is a graph describing the hysteresis characteristics of a non-eccentric
magnetic ring unit according to the prior art, which has been shown for reference,
wherein it is understood that remanent magnetization M
r in the case
where external magnetic field H
ex has been reduced to 0 becomes approximately
0 (M
r≈0).
A magnetic ring array where magnetic ring units that are the same as the above
are aligned can be utilized as a magnetic recording medium where a cantilever of
an MFM may be used for read-out.
Next, in reference to FIGS. 7 to 9, an MRAM according to the second embodiment
of the present invention where a magnetic ring unit is used as a magnetic memory
cell is described.
See FIG. 7.
FIG. 7 is a schematic cross-sectional view showing a main portion of an MRAM
according to the second embodiment of the present invention wherein first a p-type
well region
22 is formed in a predetermined region of an n-type silicon
substrate
21 and n-type silicon substrate
21 is selectively oxidized
so as to form an element isolation oxide film
23 and after that a gate electrode
that becomes a sense line
25 for read-out is formed of WSi in an element
formation region via a gate insulating film
24 so that this gate electrode
is used as a mask for implanting ions such as As, or the like, and thereby, an
n
--type LDD (lightly doped drain) region
26 is formed.
Subsequently, an SiO
2 film or the like, is deposited on
the entire surface and anisotropic etching is carried out so that sidewalls
27
are formed, and after that, ions such as As or the like, are again implanted so
that n
+-type drain region
28 and n
+-type source region
29 are formed. Then, after a thick first interlayer insulating film
30
has been formed of a TEOS (Tetra-Ethyl-Ortho-Silicate)-NSG film, contact holes
reaching n
+-type drain region
28 and n
+-type source
region
29 are created and then, W plugs
31 and
32 are formed
by filling in these contact holes with W via Ti/TiN.
Subsequently, after TiN/Al/TiN has been deposited on the entire surface,
for example, patterning is carried out so that a connecting conductor
33
and a ground line
34 that is connected to n
+-type source region
29 are formed, and after that, a thick second interlayer insulating film
35 made of a TEOS-NSG film is again formed. Then a contact hole reaching
connecting conductor
33 is created and W plug
36 is formed by filling
this contact hole with W via Ti/TiN.
Subsequently, after TiN/Al/TiN has been deposited on the entire surface
again, patterning is carried out so that a connecting conductor
37 and a
word line
38 for write-in are formed, and after that, a thick third interlayer
insulating film
39 made of a TEOS-NSG film is again formed. Then a contact
hole reaching connecting conductor
37 is created and W plug
40 is
formed by filling this contact hole with W via Ti/TiN.
Subsequently, after TiN/Al/TiN has been deposited on the entire surface
again, patterning is carried out so that a lower electrode
41 is formed,
and after that, a thick fourth interlayer insulating film
42 made of a TEOS-NSG
film is again deposited, which is then flattened by means of CMP (chemical mechanical
polishing) until lower electrode
41 is exposed.
Subsequently, in the same manner as in the above described first embodiment,
a photoresist is applied so as to have a thickness of, for example, 100 nm which
is then exposed to light and developed so that a recess in eccentric ring form
is created. Then, after a NiFe layer
44 has been deposited so as to have
a thickness of, for example, 20 nm, a tunnel insulating layer
45 made of
Al
2O
3 has been deposited so as to have a thickness of, for
example, 1 nm and a CoFe layer
46 has been deposited so as to have a thickness
of, for example, 20 nm in a sequential manner, the photoresist is removed and thereby,
a magnetic ring unit
43 having a NiFe/Al
2O
3/CoFe layered
structure is formed.
In this case, the direction of eccentricity of magnetic ring unit
43 is
made to agree with the direction approximately perpendicular to the direction of
a synthesized magnetic field which is formed in the case where currents are made
to flow through word line
38 for write-in and through the below described
bit line
48.
Subsequently, a thin fifth interlayer insulating film
47 made
of a TEOS-NSG film is again deposited, which is then flattened by means of CMP
until CoFe layer
46 is exposed.
Subsequently, a multi-layered conductive layer having a TiN/Al/TiN
structure is deposited on the entire surface which is then patterned to form a
bit line
48 extending in the direction perpendicular to word line
38
for write-in and thereby, the basic structure of an MRAM is completed.
A high external magnetic field in the same direction as the direction of a synthesized
magnetic field which is formed in the case where currents are made to flow through
word line
38 for write-in and through bit line
48 is applied to the
above described MRAM and thereby, the direction of magnetization of CoFe layer
46, which becomes a pinned layer, is provided.
See FIG. 8(
a).
FIG. 8(
a) is a diagram showing an equivalent circuit of the above described
MRAM, wherein magnetic ring units
43 are placed at intersections of word
lines
38 and bit lines
48 in the configuration with a structure where
sense amplifiers
50 are connected to the ends of bit lines
48.
See FIG. 8(
b).
FIG. 8(
b) is a diagram showing a conceptual configuration of a magnetic
memory cell wherein the upper portion of magnetic ring unit
43 is connected
to bit line
48 and the lower portion of magnetic ring unit
43 is
connected to n
+-type drain region
28 that forms access transistor
49 in the configuration.
See FIG. 9(
a).
FIG. 9(
a) is a diagram showing a conceptual configuration of a magnetic
memory cell at the time of write-in wherein write-in is carried out on magnetic
ring unit
43 by making currents, of which the values are lower than those
that break the magnetic vortex structure of CoFe layer
46, flow through
bit line
48 and through word line
38 for write-in in the condition
where sense line
25 is biased to 0 while access transistor
49 has
been turned off, so that the generated synthesized magnetic field determines the
direction of rotation of NiFe layer
44 and thus data of "1" or "0" is written
in depending on the direction of rotation of NiFe layer
44 being the same
direction with or the opposite direction of CoFe layer
46.
See FIG. 9(
b).
FIG. 9(
b) is a diagram showing a conceptual configuration of a magnetic
memory cell at the time of read-out, wherein read-out of data that has been written
in magnetic ring unit
43 is carried out on magnetic ring unit
43
by applying V
read to bit line
48 in the condition where V
select
is applied to sense line
25 and access transistor
49 has been
turned on, so that the current that flows through bit line
48 is detected
by sense amplifier
50.
In this case, when the direction of rotation of NiFe layer
44 is the same
direction as the direction of rotation of CoFe layer
46, a low resistance
is gained. When the direction of rotation of NiFe layer
44 is the opposite
direction of the direction of rotation of CoFe layer
46, a high resistance
which is greater than the low resistance by, for example, 10% to 100% is gained
and therefore, the record of one bit can be read out by determining the amount
of current.
As described above, according to the second embodiment of the present invention,
a magneto-resistive effect element is formed of magnetic ring unit
43 in
eccentric ring form and therefore, the direction of rotation of magnetization can
be controlled with high reproducibility only by applying an external magnetic field.
Thereby it becomes possible to perpetuate memory retention as well as to increase
the density of an MRAM.
Though the respective embodiments of the present invention are described above,
the present invention is not limited to the configuration or the condition described
in each embodiment, but rather, a variety of modifications are possible.
For example, though in each of the above described embodiment the inner form
of the eccentric ring is elliptical where the direction of eccentricity is in the
direction of the minor axis of the ellipse, the direction of the eccentricity may
be the direction of the major axis of the ellipse.
In addition, though in each of the above described embodiment the inner form
of
the eccentric ring is elliptical, it may be a completely round form and moreover
it may be a polygonal form, that is to say, any form is acceptable as long as the
center of the inner diameter is decentered from the center of the outer diameter.
In addition, though in each of the above described embodiment the outer form
of
the eccentric ring is completely round, it is not limited to a completely round
form but rather, may be an elliptical form or furthermore, may be a polygonal form.
In addition, though in the above described first embodiment the magnetic ring
is formed of NiFe, it is not limited to NiFe but rather, a magnetic body exhibiting
soft magnet such as Fe, FeSi, FeAlSi, Co, Ni, CoFe, CoFeB, La
1-xSr
xMnO
3,
La
1-xCa
xMnO
3 and GaAsMn may be used, and furthermore,
it may be formed of a multi-layered structure such as NiFe/Co and the like instead
of a single-layered structure.
In addition, though in the above described second embodiment the magnetic ring
unit is formed of a NiFe/Al
2O
3/CoFe structure, the combination
of the compositions or composition ratios of the pair of magnetic rings is arbitrary
in a manner where the magnetic ring formed of the material having a relatively
high coercive force is used as the pinned layer while the magnetic ring formed
of the material having a relatively low coercive force is used as the free layer.
In this case, magnetic bodies forming the pair of magnetic rings may be appropriately
combined selecting from magnetic bodies having a large magnetic moment such as
Fe, Co, Ni, NiFe, CoFe, CoFeB, CrO
2, La
1-xSr
xMnO
3
and La
1-xCa
xMnO
3.
In addition, though in the above described second embodiment the pair of magnetic
rings is formed of single-layered magnetic bodies, at least one of the magnetic
rings may be formed of a multi-layered structure such as NiFe/Co and the like.
In addition, though in the above described second embodiment the magnetic ring
unit is formed of a TMR element, it may be formed of a GMR element. In this case,
the tunnel insulating film made of Al
2O
3 may be replaced
with a non-magnetic conductive layer such as Cu and the like.
In addition, though in the above described second embodiment a uniform external
magnetic field is applied in a specific direction at the time when fixed magnetization
is provided to the pinned layer that forms the magnetic ring unit, fixed magnetization
may be provided to the pinned layer by making currents, of which the amounts are
greater than those for providing magnetization to the free layer, flow through
the word line and bit line.
In addition, though in the above described second embodiment the pinned layer
and the bit line are connected to each other in the configuration at the time when
the magnetic ring unit is formed of a NiFe/Al
2O
3/CoFe structure,
the free layer and the bit line may be connected to each other in the configuration
by reversing the layered structure.
In addition, though in the above described second embodiment the magnetic ring
unit is formed of a NiFe/Al
2O
3/CoFe structure, an antiferromagnetic
layer may be joined to the pinned layer side so that the direction of rotation
of the pinned layer is more stably pinned by the fixed magnetization that has been
provided to the antiferromagnetic layer.
In this case, a variety of antiferromagnets such as IrMn, PtMn, FeMn, Fe
2O
3,
CrMnPt, ThCo, CrAs, NiMn, RhMn, PdPtMn, FeRh and the like can be used as the antiferromagnetic
layer, which may be deposited in order, allowing the antiferromagnetic layer to
make contact with the pinned layer, at the time when the magnetic ring unit is
formed according to the lift-off method.
Here in this case, the film formation may be carried out in the condition where
a magnetic field is applied or heat treatment may be carried out in the condition
where a magnetic field is applied after the film formation in order to provide
magnetization to the antiferromagnetic layer.
Furthermore, an antiferromagnetic layer may be locally provided so as
to make contact with a portion of the pinned layer in a manner as described in
the above Patent document 3.
In addition, though in the above described second embodiment the access transistors,
the peripheral circuits and the like, are formed by use of a semiconductor integrated
circuit device, the present invention is not limited to a semiconductor integrated
circuit device but rather, switching elements, peripheral circuits and the like,
may be formed by a superconducting circuit device using Josephson junctions.
In addition, though in the above described second embodiment a magnetic memory
cell of the MRAM is formed of a magnetic ring unit made up of free layer/tunnel
insulating layer/pinned layer, the present invention is not limited to an MRAM
but rather, a magnetic sensor having data retention ability may be formed of a
similar magnetic ring unit including a GMR structure.
In addition, though in each of the above described embodiment the outer diameter
of the eccentric ring is 500 nm, 500 nm is merely an example but rather, the outer
diameter is miniaturized as the lithographic technology advances while fabrication
of an eccentric ring having an outer diameter of approximately 100 nm is possible
in a laboratory even at the present time and accordingly, it is possible to apply
the present invention to a nanoring unit.
According to the present invention, a magnetic ring unit is in an eccentric
ring form so that fabrication of a magnetic ring unit that can control the direction
of rotation with high reproducibility becomes possible only by using a lift-off
process and thereby, the present invention greatly contributes to the implementation
of a magnetic recording device or magnetic memory device with a high density that
is not effected by the limitation of miniaturization due to the magnetic interaction.
*
