Spin-dependent tunneling sensor suitable for a magnetic memory

Details Spin-dependent tunneling sensor suitable for a magnetic memory Spin-dependent tunneling sensor suitable for a magnetic memory

2001-10-02

2002-07-09

Tran, Andrew Q. (Department: 2824)

Static information storage and retrieval

Systems using particular element

Magnetic thin film

C365S171000, C365S158000

Reexamination Certificate

active

06418048

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to magnetic systems, and more particularly to a method and system for providing spin-dependent tunneling sensors suitable for use as cells in a magnetic memory.
BACKGROUND OF THE INVENTION
Because of their high magnetoresistance ratio, spin dependent tunneling sensors are currently of interest for use in a variety of devices, including magnetic memories such as magnetic random access memories (MRAM).
FIG. 1
depicts a conventional bottom spin-dependent tunneling sensor
10
. The conventional spin-dependent tunneling sensor
10
includes a seed layer
12
, an antiferromagnetic layer
14
, a pinned layer
16
, a tunneling barrier
18
, a free layer
20
and a capping layer
22
. The pinned layer
16
and the free layer
20
are ferromagnetic. The antiferromagnetic layer
14
fixes, or pins, the magnetization of the pinned layer
16
in a particular direction. The magnetization of the free layer
20
is free to rotate in response to a writing field provided at the conventional spin-dependent tunneling sensor
10
. The tunneling barrier
18
is an insulator such as alumina and is thin enough to allow charge carriers to tunnel between the free layer
20
and the pinned layer
16
. Based on the orientation of the magnetizations of the free layer
20
and the pinned layer
16
, the resistance and thus the current through the conventional spin-dependent tunneling sensor
10
changes.
FIG. 2
depicts another conventional spin-dependent tunneling sensor
30
. The conventional spin-dependent tunneling sensor
30
includes a seed layer
32
, an antiferromagnetic layer
34
, a conventional synthetic pinned layer
36
, a tunneling barrier
44
, a free layer
46
and a capping layer
48
. The conventional synthetic pinned layer
36
includes two ferromagnetic layers
38
and
42
separated by a nonmagnetic spacer layer
40
. The conventional spin-dependent tunneling sensor
30
functions similarly to the conventional spin-dependent tunneling sensor depicted in FIG.
1
.
Although the conventional spin-dependent tunneling sensors
10
and
30
function, one of ordinary skill in the art will readily recognize that the conventional spin-dependent tunneling sensors
10
and
30
do not have a symmetric response to an external magnetic field.
FIG. 3
depicts a hysteresis loop
50
of the conventional spin-dependent tunneling sensor
10
or
30
. The hysteresis loop
50
indicates the magnetization of the conventional spin-dependent tunneling sensor
10
or
30
versus external field applied to the conventional spin-dependent tunneling sensor
10
or
30
. The hysteresis loop
50
is shifted from being symmetric about a zero external field. This occurs because of an interlayer coupling (known as orange peel coupling) between the free layers
20
and
46
and the pinned layers
16
and
36
, respectively. The tunneling barriers
18
and
44
have a relatively rough upper surface because they have an antiferromagnetic layer
14
beneath them. Thus, the free layers
20
and
46
also have rough surfaces. The rough interfaces of the free layers
20
and
46
and the conventional pinned layers
16
and
36
result in a high interlayer coupling. The conventional spin-dependent tunneling sensors
10
and
30
thus behave as though there is an additional field applied to the free layers
20
and
46
. The response of the conventional spin-dependent tunneling sensors
10
and
30
are thus shifted from being symmetric about a zero external field. Because the magnetization of the conventional spin-dependent tunneling sensors
10
and
30
are asymmetric with respect to an external applied field, the magnetoresistance of the conventional spin-dependent tunneling sensors
10
and
30
is also asymmetric.
FIG. 4
depicts a conventional top pinned spin-dependent tunneling sensor
60
. The asymmetry of the conventional spin-dependent tunneling sensors
10
and
30
can be addressed using a conventional top pinned spin-dependent tunneling sensor
60
. The conventional top pinned spin-dependent tunneling sensor
60
includes a seed layer
62
, a free layer
64
, a tunneling barrier
66
, a conventional synthetic pinned layer
68
, an antiferromagnetic layer
76
and a capping layer
78
. The conventional synthetic pinned layer includes two ferromagnetic layers
70
and
74
separated by a thin, nonmagnetic spacer layer
72
. The free layer
64
and conventional synthetic pinned layer
68
operate in a manner that is analogous to the conventional spin-dependent tunneling sensors
10
and
30
.
Because the free layer
64
is not formed on the thick layer including a tunneling barrier, a pinned layer and an antiferromagnetic layer, the conventional spin-dependent tunneling device
60
is not subject to a high interlayer coupling. However, the conventional spin-dependent tunneling device
60
has a poorly pinned conventional synthetic pinned layer
68
. The conventional synthetic pinned layer
68
and the antiferromagnetic layer
76
are grown above the tunneling barrier
66
, which is amorphous. As a result, the antiferromagnetic layer
76
may be of poor quality. Consequently, the exchange coupling between the antiferromagnetic layer
76
and the conventional synthetic pinned layer
68
is reduced. As a result, the magnetization of the conventional synthetic pinned layer
68
is poorly pinned and may move in response to a writing field. Thus, the signal from the conventional spin-dependent tunneling device
60
may be unreliable.
Accordingly, what is needed is a system and method for providing an improved spin-dependent tunneling sensor. The present invention addresses such a need.
SUMMARY OF THE INVENTION
The present invention provides a method and system for providing a top pinned spin-dependent tunneling sensor. The method and system comprise providing a free layer, a tunneling barrier, a synthetic pinned layer and an antiferromagnetic layer. The free layer is ferromagnetic. The tunneling barrier is an insulator. The tunneling barrier is disposed between the free layer and the synthetic pinned layer. The synthetic pinned layer is ferromagnetic and includes a ferromagnetic top layer. The synthetic pinned layer is between the tunneling barrier and the antiferromagnetic layer. The ferromagnetic top layer acts as a seed layer for the antiferromagnetic layer.
According to the system and method disclosed herein, the present invention provides a top pinned spin-dependent tunneling sensor that has improved pinning, between the synthetic pinned layer and the antiferromagnetic layer.


REFERENCES:
patent: 5650958 (1997-07-01), Gallagher et al.
patent: 5764567 (1998-06-01), Parkin
patent: 5792510 (1998-08-01), Farrow et al.
patent: 5871622 (1999-02-01), Pinarbasi
patent: 6297983 (2001-10-01), Bhattacharyya

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