Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2001-03-22
2002-05-28
Nguyen, Tan T. (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S171000, C257S421000
Reexamination Certificate
active
06396735
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic memory capable of magneto-resistive reproduction of recorded information and a manufacturing method thereof, and in particular to a magnetic memory element in which stable magnetization exists in a storage layer despite high density, a magnetic memory and the manufacturing method thereof.
BACKGROUND OF THE INVENTION
In recent years, an application of elements such as an Anisotropy Magneto Resistive (AMR) element, a Giant Magneto Resistive (GMR) element and a Magnetic Tunnel Junction (MTJ) element to an HDD reproducing head and a magnetic memory has been proposed. The magnetic memory, like a semiconductor memory, is a solid-state memory having no operation sections, and when compared to the semiconductor memory, the magnetic memory has a number of merits such that (a) it loses no information upon power-down, (b) it is available for the unlimited number of repeated use, and (c) it prevents the storage content thereof from being destroyed by an incident x-ray.
Particularly, the MTJ element can change to a large extent a resistance rate of change depending on directions of magnetization in a pair of ferromagnetic layers which form the MTJ element. The use of the MTJ element in a memory cell has been expected.
A structure of a conventional MTJ element is disclosed, for example, in Japanese Unexamined Patent Publication No. 106514/1997 (Tokukaihei 9-106514 published on Apr. 22, 1997).
A MTJ element
50
, as shown in
FIG. 33
, is made up of an antiferromagnetic layer
51
, a ferromagnetic layer
52
, an insulating layer
53
and a ferromagnetic layer
54
, which are stacked.
The antiferromagnetic layer
51
is made of an alloy such as FeMn, NiMn, PtMn and IrMn. The ferromagnetic layers
52
and
54
are made of Fe, Co or Ni, or an alloy thereof. Further, as a material of the insulating layer
53
, the use of various oxides or nitrides has been examined, among which the use of an Al
2
O
3
film is known to produce the highest magneto-resistive (MR) ratio.
Furthermore, other than the foregoing, there has been proposed a MTJ element which utilizes a difference in coercive force between the ferromagnetic layers
52
and
54
in a structure excluding the antiferromagnetic layer
51
.
The principles of the MTJ element
50
when used as a magnetic memory are shown in FIG.
34
.
Magnetization in both the ferromagnetic layers
52
and
54
is in-plane magnetization, which is subject to effective uniaxial magnetic anisotropy that directs magnetization either parallel or anti-parallel. In addition, magnetization of the ferromagnetic layer
52
is virtually fixed in one direction due to exchange coupling with the antiferromagnetic layer
51
. Further, recording is retained in a direction of magnetization in the ferromagnetic layer
54
which flexibly varies within a range of the uniaxial magnetic anisotropy. Note that, “anti-parallel” refers to a state of magnetization of the ferromagnetic layers
52
and
54
being parallel to each other and directed opposing each other.
The magnetization of the ferromagnetic layer
54
to be a storage layer has a characteristic that a resistance value of the entire MTJ element
50
varies according to which direction is taken, parallel or anti-parallel to the magnetization of the ferromagnetic layer
52
.
Accordingly, when reproducing, the resistance value is detected so as to retrieve information data stored in the MTJ element
50
.
Further, when recording, a magnetic field generated by a current wire disposed in a vicinity of the MTJ element
50
is utilized to change the direction of magnetization in the ferromagnetic layer
54
, thereby performing writing of data to the MTJ element
50
.
Meanwhile, the MTJ element
50
having the foregoing structure generates magnetic poles at both ends, since the ferromagnetic layers
52
and
54
are magnetized in the in-plane direction. As a result, when forming a memory array using the MTJ element
50
, magnetostatic interaction occurs between the MTJ element
50
and an adjacent MTJ element. This means that a condition of the adjacent MTJ element has an effect on a characteristic of an individual MTJ element, thus making it difficult to reduce a spacing between the MTJ elements and increase a recording density.
In view of the foregoing problems, Japanese Unexamined Patent Publication No. 161919/1999 (Tokukaihei
11-161919
published on Jun. 18, 1999) discloses a method of reducing an effect of edge magnetic poles.
A structure of the MTJ element
60
which reduces the effect on the edge magnetic poles is shown in FIG.
35
. In
FIG. 34
, a ferromagnetic layer (fixed layer)
62
, the direction of magnetization is fixed by being coupled with an antiferromagnetic layer
61
, and a ferromagnetic layer (flexible layer)
64
which can flexibly rotate with respect to an external magnetic field are stacked so as to sandwich an insulating layer
63
. Furthermore, the ferromagnetic layer
62
has a structure such that a pair of ferromagnetic layers
71
and
73
which are antiferromagnetically coupled sandwich a non-magnetic metallic layer
72
. Likewise, the ferromagnetic layer
64
has a structure such that a pair of ferromagnetic layers
74
and
76
which are antiferromagnetically coupled sandwich a non-magnetic metallic layer
75
, thereby reducing magnetic poles generated on the edges of both the ferromagnetic layer
64
as the flexible layer and ferromagnetic layer
62
as the fixed layer.
However, the conventional magnetic memory as above has the following problems.
The ferromagnetic layer (flexible layer)
64
which does not adjoin the antiferromagnetic layer is composed of NiFe layer/Ru layer/NiFe layer, and flexibly rotates when an external magnetic field is applied. In the prior art document, non-magnetic metallic layer (Ru layer)
75
has a film the thickness of which is set so that the pair of ferromagnetic layers (NiFe layers)
74
and
76
have the maximum antiferromagnetic coupling strength and slightly different film thicknesses therebetween. When a magnetic field is applied from the outside, the ferromagnetic layer
64
as the flexible layer rotates net magnetization generated due to a difference between the film thicknesses of the pair of ferromagnetic layers (NiFe layers)
74
and
76
.
However, a film thickness of the non-magnetic metallic layer (Ru layer)
75
is set so that the pair of ferromagnetic layers (NiFe layers)
74
and
76
have the maximum antiferromagnetic coupling strength therebetween. Therefore, the film thickness of the non-magnetic metallic layer (Ru layer)
75
ranges from 4 Å to 8 Å, that is considerably thin. In this arrangement, formation of a pin hole works in reverse and induces ferromagnetic coupling, and it is thus difficult to obtain stable antiferromagnetic coupling strength. In addition, in order to allow an external magnetic field to reverse a direction of magnetization, the pair of ferromagnetic layers (NiFe layers)
74
and
76
are required to have different film thicknesses. More specifically, when apparent magnetization of the two layers is 0, it is difficult to reverse magnetization, and therefore, magnetization requires to be generated by changing a film thickness. However, a difference in the film thicknesses of the two layers prevents the net magnetization of the externally viewed MTJ element
60
from being reduced to zero. Accordingly, there has been a problem that the conventional magnetic memory cannot provide high-density magnetic memory because a magnetic pole which is generated on an edge of a ferromagnetic layer adversely affects an adjacent magnetic memory element.
Further, when using the MTJ element
60
as a magnetic memory element, a magnetic field which is required to reverse magnetization is generated by the passage of electric current through adjacent conductive wires. However, in the prior art document, no arrangements to reduce power consumption is disclosed.
Furthermore, in the conventional magnetic memory, when adopting the MTJ element
60
as a magnetic head, the MTJ e
Hayashi Hidekazu
Michijima Masashi
Minakata Ryoji
Conlin David G.
Dike, Bronstein, et al.
Nguyen Tan T.
Sharp Kabushiki Kaisha
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