Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1998-10-08
2002-05-14
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S900000, C360S112000, C324S252000
Reexamination Certificate
active
06387548
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exchange coupling film composed of an antiferromagnetic layer and a ferromagnetic layer, the magnetization direction of the antiferromagnetic layer being fixed along a prescribed direction due to an exchange anisotropic magnetic field generated at the interface between the antiferromagnetic layer and ferromagnetic layer. Especially, the present invention relates to an exchange coupling film being allowed to obtain a larger exchange anisotropic magnetic field when the antiferromagnetic layer is formed of an antiferromagnetic material containing an element X (for example, Pt or Pd) and Mn, and to a magnetoresistive element (spin-valve type thin film element; AMR (anisotropic magnetoresistive) element) using this exchange coupling film.
2. Description of the Related Art
The spin-valve type thin film element belongs to one of GMR (giant magnetoresistive) elements making use of a giant magnetoresistance effect for sensing recording magnetic field from a recording medium such as a hard disk unit.
This spin-valve type thin film element has a relatively simple construction among the GMR elements along with having some features that its resistance can be varied under a weak magnetic field.
The spin-valve type thin film element described above has a most simple construction composed of an antiferromagnetic layer, a pinned magnetic layer, a non-magnetic conductive layer and a free magnetic layer.
The antiferromagnetic layer is formed in direct contact with the pinned magnetic layer and the magnetization direction of the pinned magnetic layer is fixed along a prescribed direction forming a single magnetic domain due to the exchange anisotropic magnetic field generated at the interface between the antiferromagnetic layer and pinned magnetic layer.
Magnetization of the free magnetic layer is aligned along the direction to cross with the magnetization direction of the pinned magnetic layer by being affected by bias layers formed at both sides of the free magnetic layer.
Usually, a film of a Fe—Mn (iron-manganese) alloy or Ni—Mn (nickel-manganese) alloy is used for the antiferromagnetic layer, a film of a Ni—Fe (nickel-iron) alloy is used for the pinned magnetic layer and free magnetic layer, a Cu (copper) film is used for the non-magnetic conductive layer
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and a film of a Co—Pt (cobalt-platinum) alloy is used for the bias layer.
In this spin-valve type thin film element, electric resistance is changed depending on the direction of the pinned magnetic field of the pinned magnetic layer related to variation of the magnetization direction of the free magnetic field caused by leakage magnetic field from the magnetic medium such as a hard disk unit. The leakage magnetic field can be thus sensed from voltage changes ascribed to this electric resistance changes.
Although the films of the Fe—Mn alloy or Ni—Mn alloys are used for the antiferromagnetic layer as described above, the film of the Fe—Mn alloy has drawbacks of low corrosion resistance, small exchange anisotropic magnetic field and a blocking temperature as low as about 150° C. Low blocking temperature causes a problem that the exchange anisotropic magnetic field is quenched by the temperature increase of the element during the manufacturing process of the head or in the head under operation.
The film of the Ni—Mn alloy has, on the contrary, a relatively large exchange anisotropic magnetic field as well as a blocking temperature of as high as about 300° C. Therefore, it is preferable to use the film of the Ni—Mn alloy rather than using the film of the Fe—Mn alloy for the antiferromagnetic layer.
B. Y. Wong, C. Mitsumata, S. Prakash, D. E. Laughlin and T. Kobayashi (Journal of Applied Physics, vol. 79, No. 10, p. 7896-7904 (1996)) reported the interface structure between the antiferromagnetic layer and pinned magnetic layer (the film of the Ni—Fe-alloy) when the film of the Ni—Mn alloy is used for the antiferromagnetic layer in.
The report describes “The film grows by keeping a crystal coherency at the NiFe/NiMn interface so that both {111} planes of NiFe and NiMn are parallel to the film surface. Coherent strain at the interface is relaxed by introducing a large number of twins having twining planes parallel to the film surface. However, ordering of the NiMn in the vicinity of the interface is suppressed due to remaining interface strain, making the degree of order high at the site spaced apart from the interface.”
The term “coherent” refers to a state where atoms in the antiferromagnetic layer and pinned magnetic layer at the surface exist in 1:1 correspondence with each other and, conversely, the term “incoherent” refers to a state where atoms in the antiferromagnetic layer and pinned magnetic layer at the interface are not located to form respective pairs between the layers.
A heat treatment allows an exchange anisotropic magnetic field to generate at the interface between the NiMn alloy and pinned magnetic field when the antiferromagnetic layer is formed of the NiMn alloy, because the NiFe alloy is transformed from a disordered lattice to an ordered lattice by applying a heat treatment.
While the crystal structure of the NiMn alloy assumes a face centered cubic lattice in which Ni and Mn atoms are distributed at random prior to subjecting to the heat treatment, the crystal structure is transformed from the face centered cubic lattice to the face centered tetragonal lattice after the heat treatment with ordering of the atomic sites (referred to a ordered lattice hereinafter). The ratio (c/a) of the lattice constant a to the lattice constant c of the film of the Ni—Mn alloy when the crystal structure is transformed into a perfectly ordered lattice is 0.942.
Since the lattice constant ratio c/a in the film of the MiMn alloy having a perfectly ordered lattice is relatively close to 1, the lattice strain at the interface generated during the modification from the disordered lattice to the ordered lattice becomes relatively small. Accordingly, the NiMn alloy is transformed from the disordered lattice to the ordered lattice by subjecting the alloy to a heat treatment even if the interface structure between the film of the NiMn alloy and the pinned magnetic layer assumes a coherent state, thereby generating an exchange anisotropic magnetic field.
The lattice strain at the interface is somewhat relaxed by forming twins, as described in the foregoing paper.
As hitherto described, the NiMn alloy has relatively large exchange anisotropic magnetic field as well as a blocking temperature of as high as 300° C., exhibiting superior characteristics to the conventional FeMn alloys. However, the alloy is not sufficient with respect to corrosion resistance as in the FeMn alloys.
Accordingly, X—Mn alloys (X=Pt, Pd, Ir, Rh, Ru and Os) using platinum group elements has been recently noticed for the antiferromagnetic material that is excellent in corrosion resistance along with being able to generate higher exchange anisotropic magnetic field and having a higher blocking temperature.
Using the X—Mn alloy containing platinum group elements as the antiferromagnetic layer makes it possible to improve conventional reproduction output, besides substantially eliminating the drawbacks that the reproduction characteristics are deteriorated by quenching the exchange anisotropic magnetic field due to temperature increase of the element in the magnetic head under operation.
Meanwhile, a heat treatment after deposition is required, as in the case when the NiMn alloy is used for the antiferromagnetic layer, for allowing the exchange anisotropic magnetic field to generate when the X—Mn alloy containing platinum group elements is used for the antiferromagnetic layer.
Although the foregoing paper describes that the interface structure between the NiMn alloy and the pinned magnetic layer (NiFe alloy) remains to be coherent, it was made clear that the exchange anisotropic magnetic field was hardly generated after the heat treatment when the interface structure with the
Hasegawa Naoya
Makino Akihiro
Oominato Kazuya
Saito Masamichi
Yamamoto Yutaka
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Kiliman Leszek
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