Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-11-29
2003-01-07
Cao, Allen (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06504688
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exchange coupling film made up of an antiferromagnetic layer and a ferromagnetic layer, in which the direction of magnetization of the ferromagnetic layer is pinned in a certain direction by an exchange (anisotropic) coupling magnetic field generated at the interface between the antiferromagnetic layer and the ferromagnetic layer. More particularly, the present invention relates to an exchange coupling film capable of providing a strong exchange coupling magnetic field, a magnetoresistive sensor (spin-valve type magnetoresistive sensor or AMR sensor) using the exchange coupling film, and a thin-film magnetic head using the magnetoresistive sensor.
2. Description of the Related Art
A spin-valve type magnetoresistive sensor is one of GMR (giant magnetoresistive) sensors utilizing the giant magnetoresistive effect, and is employed to detect a recording magnetic field on a recording medium such as a hard disk.
Of the GMR sensors, a spin-valve type magnetoresistive sensor has several advantages in that the structure is relatively simple and resistance can be changed with a weak magnetic field.
The spin-valve type magnetoresistive sensor comprises, in the simplest structure, an antiferromagnetic layer, a pinned magnetic layer, a nonmagnetic intermediate layer, and a free magnetic layer. The antiferromagnetic layer and the pinned magnetic layer are formed in contact with each other. Because an exchange coupling magnetic field is generated at the interface between the antiferromagnetic layer and the pinned magnetic layer, the pinned magnetic layer is put into a single domain state and a direction of magnetization thereof is pinned in a certain direction. The magnetization of the free magnetic layer is aligned by bias layers formed on both sides thereof in a direction substantially crossing the direction of magnetization of the pinned magnetic layer. The magnetization of the free magnetic layer varies depending on a magnetic field leaked from a recording medium, whereby electrical resistance is changed based on the relationship with respect to the magnetization of the pinned magnetic layer. As a result, the leaked magnetic field is reproduced.
It is known that the exchange coupling magnetic field is generated upon transformation of the antiferromagnetic layer from irregular lattices (face-centered-cubic lattices) to regular lattices (face-centered-tetragonal lattices) when the antiferromagnetic layer and the pinned magnetic layer are formed as a multilayer and subjected to a heat treatment.
In this connection, the inventors have found that, when the interior of crystals of the antiferromagnetic layer is under the condition, described below, after the above two layers have been formed as a multilayer and subjected to heat treatment, the antiferromagnetic layer is not satisfactorily transformed to regular lattices, and the exchange coupling magnetic field generated between the antiferromagnetic layer and the ferromagnetic layer is very weak.
FIG. 19
is a schematic illustration of an electron micrograph that is obtained by imaging, with an electron microscope, a section of a conventional multilayered structure of an antiferromagnetic layer and a pinned magnetic layer taken along the direction of film thickness. Note that
FIG. 19
represents the state after being subjected to heat treatment.
Referring to
FIG. 19
, an antiferromagnetic layer
53
is formed of, for example, a PtMn alloy and a ferromagnetic layer
54
is formed of, for example, a NiFe alloy. Grain boundaries
55
are formed in the antiferromagnetic layer
53
and extend from the interface to an upper surface. Crystal grains formed on both sides of the grain boundaries
55
have crystal azimuths different from each other.
Further, a twin
56
is formed in the antiferromagnetic layer
53
. Herein, the term “twin” means one solid in which two or more single crystals of one substance are combined with each other in accordance with a particular symmetrical relationship. In the structure of
FIG. 19
, the twin
56
includes twin boundaries
57
extending in a direction (X-direction indicated in
FIG. 19
) parallel to the interface between the antiferromagnetic layer
53
and the ferromagnetic layer
54
. The twin
56
has an atomic array being mirror-symmetrical about the twin boundaries
57
.
It has been expected that a strong exchange coupling magnetic field can be obtained upon formation of the twin boundaries
57
. Such an expectation has been based on an assumption that, with the formation of the twin boundaries
57
, an atomic array is changed to become mirror-symmetrical in a relevant area and lattice strains caused upon transformation to regular lattices are relaxed, whereby the transformation to regular lattices is promoted satisfactorily.
It has been found, however, that the exchange coupling film shown in
FIG. 19
provides only a very weak exchange coupling magnetic field. The reason is presumably that the twin boundaries
57
are formed to extend in the direction parallel to the interface between the antiferromagnetic layer
53
and the ferromagnetic layer
54
. In other words, the twin boundaries
57
extending in the direction parallel to the interface between both the layers are formed to relax lattice strain created in a direction of film thickness (Z-direction indicated in FIG.
19
), and the lattice strain in the direction parallel to the interface are not relaxed. Eventually, the transformation to regular lattices is not promoted satisfactorily.
Thus, in
FIG. 19
, atoms of the antiferromagnetic layer
53
at the interface are in a state tightly bound to the crystal structure of the ferromagnetic layer
54
, and the transformation to regular lattices is not developed satisfactorily at the interface. This results in a very weak exchange coupling magnetic field.
SUMMARY OF THE INVENTION
With the view of overcoming the above-mentioned problems in the related art, it is an object of the present invention to provide an exchange coupling film in which a twin boundary is not formed in an antiferromagnetic layer parallel to the interface between the antiferromagnetic layer and a ferromagnetic layer, and hence is capable of producing a strong exchange coupling magnetic field. Another object is to provide a magnetoresistive sensor using the exchange coupling film, and a thin-film magnetic head using the magnetoresistive sensor.
To achieve the above objects, according to the present invention, an exchange coupling film is made up an antiferromagnetic layer and a ferromagnetic layer formed in contact with each other, and the direction of magnetization of the ferromagnetic layer is held in a certain direction by an exchange coupling magnetic field generated at an interface between the antiferromagnetic layer and the ferromagnetic layer. In the antiferromagnetic layer, crystal planes other than an equivalent crystal plane represented by {111} plane are oriented at least partly among crystal planes lying in a direction parallel to the interface. A twin is formed in at least a part of the antiferromagnetic layer, and a twin boundary is formed in at least a part of the twin that is not parallel to the interface.
The present invention is based on the findings as follows. When the above-mentioned twin boundary appears after forming and heat-treating the exchange coupling film, atoms of the antiferromagnetic layer are not in a bound condition to the crystal structure of the ferromagnetic layer in the film forming stage. With such weakening of binding forces at the interface, the antiferromagnetic layer is more likely to transform from irregular lattices (face-centered-cubic lattices) to regular lattices (face-centered-tetragonal lattices) under the heat treatment.
The transformation, however, causes lattice strains. Unless those lattice strains are satisfactorily relaxed, it is impossible to develop the transformation effectively. In the transformation process, it is envisaged that atoms of the antiferromagnetic layer are rearran
Hasegawa Naoya
Saito Masamichi
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Cao Allen
LandOfFree
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