Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
1999-12-07
2001-04-10
Phan, Trong (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
Reexamination Certificate
active
06215695
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetoresistance element having magnetization stability and showing high magnetoresistance ratio. The present invention relates also to a magnetic memory device (nonvolatile memory device) employing the magnetoresistance element.
2. Related Background Art
The magnetoresistance effect type memory device conducts recording by magnetization direction of a magnetic layer corresponding to digital information. This type of memory device does not require energy supply from the outside for memory retention, and can be produced by a simple process in comparison with a semiconductor memory device without limitation of the substrate material. Therefore, this type of memory device is promising as an inexpensive nonvolatile memory device having a large capacity.
FIG. 1
is a schematic sectional view of an example of constitution of a conventional magnetoresistance effect type memory device. This magnetoresistance effect type memory device as shown in
FIG. 1
has basically a sandwich structure having nonmagnetic layer
52
between two ferromagnetic layers
51
,
53
. The process for detecting the recorded information is classified roughly into two types. The function of the ferromagnetic layers
51
,
53
differs depending on the type of the detection process.
A first type of the process is described below.
In the first type of process, two ferromagnetic layers
51
,
53
are constituted to be different in the coercivity: the layer of lower coercivity serving as a detection layer, and the other layer of higher coercivity serving as a memory layer. The coercivity of ferromagnetic layers
51
,
53
is differentiated usually by changing the constituting chemical elements or composition of the layers or by changing the layer thickness.
The recording is conducted by application of a recording magnetic field (Hw) greater than the coercivity of the memory layer to parallelize the magnetization direction of the memory layer to the recording magnetic field (Hw). This process is explained by reference to
FIGS. 2A and 2B
.
FIGS. 2A and 2B
are schematic drawings illustrating the states of the recorded information in a conventional magnetoresistance effect type memory device. In
FIGS. 2A and 2B
, the device comprises memory layer
61
having higher coercivity, nonmagnetic layer
62
, and detection layer
63
having ferromagnetism of lower coercivity than memory layer
61
.
In
FIG. 2A
, for example, the symbol “0” denotes the state that memory layer
61
under nonmagnetic layer
62
is magnetized in a − direction (leftward direction), and as shown in
FIG. 2B
, the symbol “1” denotes the state that memory layer
61
is magnetized in a + direction (rightward direction). Immediately after the recording, the magnetization directions of the both layers are parallel, since the coercivity of detection layer
63
is less than that of memory layer
61
.
The detection is conducted by applying direct electric current at a prescribed intensity to the memory device, applying simultaneously thereto a magnetic field (Ha) less intense than the coercivity of memory layer
61
, and measuring the potential change caused by reversal of magnetization in detection layer
63
. In the state of the parallel magnetization of both magnetic layers, the resistivity of the element is lower than that in the state of the antiparallel magnetization.
The states are shown in
FIGS. 3A
to
3
C through FIG.
6
.
FIGS. 3A
to
3
C and
FIGS. 5A
to
5
C are schematic drawings for explaining the change of magnetization of the respective layers by application of a magnetic field. In these drawings, the same symbols as in
FIGS. 2A and 2B
are used for the corresponding members without special explanation.
FIGS. 4 and 6
are respectively a timing chart showing the potential change by application of a magnetic field: the abscissa showing the time t, and the ordinate showing the electric potential V.
As shown in
FIGS. 3A
to
3
C, on application of a detecting magnetic field of +Ha to a memory device having a record of the state “0”, for example, as shown in
FIG. 3A
, the magnetization directions of the both magnetic layers (memory layer
61
, and detection layer
63
) become antiparallel as shown in
FIG. 3B
to raise the electric potential, and on subsequent application of the magnetic field of −Ha, the magnetization directions become parallel to lower the potential.
FIG. 4
shows the change of the electric potential.
Similarly, as shown in
FIGS. 5A
to
5
C, on application of detecting magnetic field of +Ha to a memory device having a record of state “1”, as shown in
FIG. 5A
, the magnetization directions of the both magnetic layers become parallel as shown in
FIG. 5B
to lower the electric potential, and on subsequent application of the magnetic field of −Ha, the magnetization directions become antiparallel to raise the electric potential.
FIG. 6
shows the change of the electric potential. In this type of the detection, detected signal depends only on the magnetization direction of the memory layer independently of the magnetization direction of the detection layer before the detection, which enables precise detection of the information recorded in the memory layer.
Another type of process is described by reference to
FIGS. 7A and 7B
.
FIGS. 7A and 7B
show schematically magnetization directions of magnetoresistance effect type memory device. In this process, as shown in
FIGS. 7A and 7B
, two ferromagnetic layers holds a nonmagnetic layer
72
. One of the two ferromagnetic layers is employed as fixed-magnetization layer
71
which is magnetized in a fixed direction, and the other one is employed as memory layer
73
. The magnetization of memory layer
73
is forced to be parallel to the direction of the applied magnetic field. The fixed-magnetization layer
71
can be formed by giving coercivity greater than the recording magnetic field to the ferromagnetic layer. The magnetization-reversing field in the fixed magnetization direction can be intensified by formation of exchange-coupling with an antiferromagnetic layer.
FIG. 8
shows shift of the magnetization loop of a film of “ferromagnetic layer/antiferromagnetic layer” formed by exchange-coupling of a ferromagnetic layer and an antiferromagnetic layer.
FIG. 8
shows the rightward shift corresponding to Hex (exchange-coupling magnetic field). As shown in
FIG. 8
, the magnetization-reversing field is Hex±Hc (where Hc is the coercivity of the ferromagnetic layer). Therefore, the stronger magnetic field is required for reversing the magnetization by application of a magnetic field in a direction of the shift of the magnetization loop than that for a ferromagnetic single layer.
In the case where the shift of the magnetization loop is greater than the coercivity, namely Hex>Hc, the magnetization is directed, under zero magnetic field, invariably to a fixed direction. In such a case, even if the magnetization is reversed by some cause, the magnetization returns to the original state, requiring no initialization.
With such a magnetoresistance effect type memory device, information is detected by applying a magnetic field, to the memory device in a state of absence of application of a magnetic field, at a magnetic field intensity for reversing the magnetization of memory layer
73
in a prescribed direction, and measuring the change of the output voltage. This process is shown in
FIGS. 10A and 10B
through FIG.
13
.
FIGS. 10A and 10B
and
FIGS. 12A and 12B
illustrate schematically the change of magnetization of the respective layers on application of a magnetic field. In these drawings, the same symbols as in
FIGS. 7A and 7B
are used for denoting the corresponding members without explanation.
FIGS. 11
,
12
A,
12
B and
13
are respectively a timing chart for illustrating the change of the electric potential on application of a magnetic field with the abscissa representing time t, and the ordinate representing an electric potential V.
I
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Phan Trong
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