Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2000-12-08
2003-02-11
Mai, Son (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S158000, C365S173000
Reexamination Certificate
active
06519179
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic tunnel junction device, a magnetic memory adopting the magnetic tunnel junction device, a magnetic memory cell and an access method of the magnetic memory cell.
BACKGROUND OF THE INVENTION
Magnetic tunnel junction (MTJ) devices are known to output a signal of a higher level as compared to conventional anisotropic magnetoresistive (AMR) effect devices and giant magnetoresistive (GMR) effect devices. For this beneficial features of the magnetic tunnel junction (MTJ) devices, their applications to reproducing head for hard disk drives (HDDs) or magnetic memories have been considered.
Especially, in the magnetic memories, the MTJ devices are solid memory of no active section like the semiconductor memories. However, the MTJ devices are advantageous over the semiconductor memories for their beneficial features: a) information stored therein will not be lost in an event of power shut off, b) rewriting of information is permitted unlimited number of times, c) even with an applied radioactive ray, the information stored therein would not be lost.
An example structure of conventional MTJ device is shown in FIG.
11
. Such structure is, for example, disclosed by Japanese Unexamined Patent Publication No. 106514/1997 (Tokukaihei 9-106514, published on Apr. 22, 1997).
As shown in
FIG. 11
, an MTJ device
104
includes an antiferromagnetic layer
141
, a ferromagnetic layer
142
, an insulating layer
143
, and a ferromagnetic layer
144
which are laminated in this order. Both the ferromagnetic layer
142
and the ferromagnetic layer
144
have in-plane magnetizations which show such effective uniaxial magnetic anisotropy that the respective magnetizations are parallel to or antiparallel to one another. The magnetization of the ferromagnetic layer
142
is fixed in substantially one direction by an exchange coupling with the antiferromagnetic layer
141
, and thus information is recorded thereon in a magnetization direction of the ferromagnetic layer
144
.
For the antiferromagnetic layer
141
, FeMn, NiMn, PtMn or IrMn alloy may be adopted. For the ferromagnetic layer
142
and the ferromagnetic layer
144
, Fe, Co, Ni, or alloys thereof may be adopted. For the insulating layer
143
, various kinds of oxides, nitrides, etc., may be used. However, it is known that the highest magnetoresistance (MR) can be obtained when adopting an Al
2
O
3
film for the insulating layer
143
.
Another MTJ device has been proposed of a structure without the antiferromagnetic layer
141
, which utilizes a difference in coercive force between the ferromagnetic layer
142
and the ferromagnetic layer
144
.
An operation mechanism of a magnetic memory application of the described MTJ device
104
of
FIG. 11
is shown in FIGS.
12
(
a
) and
12
(
b
).
As described, both the ferromagnetic layer
142
and the ferromagnetic layer
144
have in-plane magnetization and show such effective uniaxial magnetic anisotropy that the respective magnetization are parallel to or antiparallel to one another.
The magnetization of the ferromagnetic layer
142
is fixed in substantially one direction by an exchange coupling with the antiferromagnetic layer
141
, and a recording is performed based on the magnetization direction of the ferromagnetic layer
144
.
Reading out operation is performed by detecting a resistance of the MTJ device
104
which differs depending on whether the magnetization of the memory layer of the ferromagnetic layer
144
and the magnetization of the ferromagnetic layer
142
are parallel to or antiparallel to each other.
FIG. 13
shows a schematic structure of a randomly accessible magnetic memory adopting the MTJ device of FIG.
11
. The magnetic memory includes a transistor
151
for selecting an MTJ device
152
when reading out. The respective information represented by “0” and “1” are recorded based on the magnetization direction of the ferromagnetic layer
144
of the MTJ device
104
shown in FIG.
11
. Then, information are read out utilizing the feature that the resistance value is low when the magnetization of the ferromagnetic layer
142
and the magnetization of the ferromagnetic layer
144
are parallel to one another, while the resistance value is high when the respective magnetization are antiparallel to one another.
On the other hand, recording is performed by inverting the magnetization direction of the ferromagnetic layer
144
by a synthetic magnetic field formed by a bit line
153
and a word line
154
. A reference numeral
155
in
FIG. 13
indicates a plate line.
In the described MTJ device
104
, however, magnetic poles generate at both end portions due to the in-plane magnetization of the ferromagnetic layer
142
and the ferromagnetic layer
144
. In order to meet a demand for high density magnetic memory, a miniaturization of the MTJ device
104
is necessary. However, the miniaturization of the MTJ device
104
results in a greater effect of the demagnetizing field caused by the magnetic poles generated at both end portions.
The ferromagnetic layer
142
which is exchange coupled with the antiferromagnetic layer
141
is not affected much by the described demagnetizing field. Further, as disclosed in U.S. Pat. No. 5,841,692, the magnetic poles generated at both end portions can be substantially eliminated.
However, the same cannot be applied to a memory layer of the ferromagnetic layer
144
, and as the pattern is miniaturized, the magnetization becomes more unstable due to the magnetic poles generated at both end portions, thereby making it difficult to maintain the initial recorded state.
In order to counteract the foregoing problem, attempts have been made to suppress the effects of the magnetic poles at both end portions by arranging the memory layer of the ferromagnetic layer
144
so as to have a closed magnetic circuit structure. In this state, by arranging such that both the bit line and the word line are formed in the closed magnetic circuit, the magnetization direction of the ferromagnetic layer
144
can be inverted effectively when recording. However, since the bit line and the word line are formed in the same direction within the MTJ device, it is difficult to adopt the simple orthogonal array as shown in FIG.
13
. The foregoing closed magnetic circuit structure is disclosed in, for example, Japanese Unexamined Patent Publication No. 302456/1998 (Tokukaihei 10-302456, published on Nov. 13, 1998), however, an optimal access method of the magnetic memory cell is not disclosed.
When applying the MTJ device to the magnetic head or the magnetic memory, it is important to attain a high resistance change ratio. However, an improvement in resistance change ratio by an optimal selection of a material is limited, and thus a method of improving resistance change ratio by modifying the film structure has been considered. For example, Japanese Unexamined Patent Publication No. 163436/1999 (Tokukaihei 11-163436, published on Jun. 18, 1999) realizes a high resistance change ratio by laminating a plurality of MTJ devices.
The structure of the MTJ device of Japanese Unexamined Patent Publication No. 163436/1999 is shown in FIG.
14
. As shown in
FIG. 14
, the MTJ device includes a first ferromagnetic layer
161
, a first insulating layer
162
, a second magnetic layer
163
, a second insulating layer
164
and a third magnetic layer
165
which are laminated in this order. The described MTJ device is mainly designed for the magnetic head; however, it can be also applied to magnetic memory.
In the case of applying the MTJ device of the structure shown in
FIG. 14
, the arrangement wherein the ferromagnetic layer
161
, the ferromagnetic layer
163
and the ferromagnetic layer
165
all show in-plane magnetization which show such effective uniaxial magnetic anisotropy that the respective magnetization are parallel to or antiparallel to one another.
The respective magnetization of the ferromagnetic layer
161
and the ferromagnetic layer
165
are fixed in substantially one direction by the exchange coupling wi
Hayashi Hidekazu
Michijima Masashi
Minakata Ryoji
Conlin David G.
Daley, Jr. William J.
Edwards & Angell LLP
Mai Son
Sharp Kabushiki Kaisha
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