Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2001-05-30
2002-05-28
Dinh, Son T. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S145000, C365S171000, C360S112000
Reexamination Certificate
active
06396734
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic head to record and reproduce information on and from a magnetic recording medium, and more particularly to an improved giant magnetoresistive sensor and a magnetic recording/reproducing apparatus equipped with said sensor.
2. Description of the Related Art
The increasing magnetic recording density requires a highly sensitive magnetic head for reproduction. The one meeting this requirement is described in “Giant magnetoresistance in soft magnetic multi-layer film”, Physical Review B, vol. 43, pp. 1297-1300. It is constructed such that two magnetic layers are separated by one non-magnetic layer and an exchange bias magnetic field is applied to one of the magnetic layers from an antiferromagnetic layer. This type of multi-layer film has resistance R with a component varying in proportion to cos&thgr;, with &thgr; being an angle between the directions of magnetization of the two magnetic layers, according to the aforesaid thesis. This phenomenon is referred to as giant magnetoresistance (GMR).
A conventional giant magnetoresistive sensor is shown in FIG.
7
. It consists of a substrate
5
and several layers sequentially formed thereon. Adjacent to the substrate are a magnetic shield layer
10
and a magnetic gap layer
20
. On the magnetic gap layer
20
is a magnetoresistive film
30
, which consists of a ferromagnetic film (free layer)
35
, a copper layer
40
, a ferromagnetic film (pinned layer)
65
, and an antiferromagnetic film
70
, which are formed sequentially one over another. The arrow
55
indicates the direction of magnetization. With the magnetoresistive film
30
patterned, there are arranged an electrode film
90
and a permanent magnet layer
80
at each side thereof. The top is covered with a magnetic gap layer
100
and a magnetic shield layer
110
. The magnetoresistive film mentioned above is characterized in that the pinned layer has its magnetization pinned in the direction of element height (depth) by the exchange bias magnetic field from the antiferromagnetic layer. In general, the free layer has the axis of easy magnetization parallel to the cross-track direction (z direction) of the head.
In the case of the head mentioned above, it is desirable that the magnetization in the entire free layer be kept parallel to the cross-track direction of the head so that the free layer does not suffer magnetic saturation when a signal magnetic field from the medium is applied upward and downward in the direction of the element height of the head. Unfortunately, the magnetization in the free layer does not become uniformly parallel to the cross-track direction of the head because the free layer receives a static magnetic field which occurs as the pinned layer (orienting vertically to the medium surface) becomes magnetized. The result is that the head becomes sensitive unequally to the positive and negative magnetic fields and reproduces a large peak asymmetry of read-back waveform. This not only adversely affects the improvement of error rate by signal processing such as PRML (partial response sampling plus maximum likelihood detection) but also lowers the output. The peak asymmetry of read-back waveform is defined as follows.
Asym.=|
V
+
−V
−
|/|V
+
+V
−
|
(where V
+
denotes the peak value of the positive output and V
−
denotes the peak value of the negative output.)
There is disclosed in Japanese Patent Laid-open No. 169026/1995 a giant magnetoresistive sensor designed to reduce the peak asymmetry of read-back waveform. As shown in
FIG. 8
, it has a magnetoresistive film
30
consisting of a ferromagnetic film (free layer)
35
, a copper layer
40
, a composite ferromagnetic film (pinned layer)
50
, and an antiferromagnetic film
70
. The pinned layer
50
, which is a composite ferromagnetic film, consists of two ferromagnetic films
51
and
53
(of Co or the like) and a non-magnetic layer
52
(or Ru or the like), the former having their magnetization strongly coupled in the antiparallel direction through the latter. The two ferromagnetic films produce magnetic moments aligning in the antiparallel direction, thereby canceling out each other. The result is a reduction of static magnetic field applied to the free layer from the pinned layer. The second ferromagnetic film
53
of the pinned layer
50
has its magnetization pinned by the antiferromagnetic film
70
.
There is disclosed in Japanese Patent Laid-open No. 7235/1996 another giant magnetoresistive sensor designed to reduce the peak asymmetry of read-back waveform. As shown in
FIG. 9
, it has a magnetoresistive film
30
consisting of a ferromagnetic film (free layer)
35
, a copper layer
40
, and a composite ferromagnetic film (pinned layer)
50
. The pinned layer
50
, which is a composite ferromagnetic film, consists of two ferromagnetic films
51
and
53
(of Co or the like) and a non-magnetic layer
52
(or Ru or the like), the former having their magnetization strongly coupled in the antiparallel direction through the latter, like the aforesaid head. The two ferromagnetic films
51
and
53
should have an adequate thickness, so that the pinned layer has a large effective coercive force for it to be of self-pinned type. The result is a reduction of static magnetic field applied from the pinned layer and obviation of the antiferromagnetic film to fix the pinned layer. The advantage is a reduction of the entire film thickness of the head and a reduction of the gap length.
On the other hand, there is disclosed in Japanese Patent Laid-open No. 347013/1993 and U.S. Pat. No. 5,287,238 a giant magnetoresistive sensor designed to increase its reproducing output. As shown in
FIG. 10
, it has a magnetoresistive film
30
consisting of a first antiferromagnetic layer
70
, a first pinned ferromagnetic film
65
, a non-magnetic film
40
, a free ferromagnetic film
35
, a non-magnetic layer
40
, a second pinned ferromagnetic film
66
, and a second antiferromagnetic film
71
. The multi-layer structure, with the free layer being held between the pinned layers, causes electrons to scatter over a larger area of interface. This tends to a larger relative change of magnetoresistance (&Dgr;R/R in percent) and a larger output of reproduction. This type of giant magnetoresistive sensor is called dual spin valve (SV) head.
Another type of dual spin valve (SV) head is disclosed in Japanese Patent Laid-open No. 225925/1995. As shown in
FIG. 11
, it has a magnetoresistive film
30
consisting of a first free magnetic film
35
, a non-magnetic film
40
, a first ferromagnetic pinned film
65
, an antiferromagnetic film
70
, a second ferromagnetic pinned layer
66
, a non-magnetic film
40
, and a second free magnetic film
36
. As in the foregoing head, the multi-layer structure, with the antiferromagnetic film being held between the pinned layers and the free layers, causes electrons to scatter over a larger area of interface. This tends to a larger relative change of magnetoresistance (&Dgr;R/R in percent).
SUMMARY OF THE INVENTION
The disadvantage of the aforesaid structure, with the free layer or antiferromagnetic film being held between two upper and lower pinned layers, is that the static magnetic field applied to the thickness of the free layer from the pinned layer increases as the pinned layer increases. Consequently, the direction of magnetization of the free layer deviates from the direction of the track width of the head, with the result that the peak asymmetry of reproduced signals becomes larger. The larger the asymmetry, the lower the read-back output.
A dual spin valve film to remedy the peak asymmetry of read-back waveform is described in “PtMn dual spin valve film with a Co/Ru/Co laminated ferri pinned magnetic layer”, Synopsis of the 22
nd
Lecture Meeting of Japan Institute of Applied Magnetism, p. 309. As shown in
FIG. 12
, it has a magnetoresistive film
30
consisting of a first antiferromagnetic film
70
, a first co
Ishikawa Chiaki
Suzuki Yoshio
Totsuka Kaori
Dinh Son T.
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
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