Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-11-04
2002-07-02
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C029S603080
Reexamination Certificate
active
06413325
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a thin-film magnetic head equipped with a spin valve magnetoresistive effect (SVMR) element utilizing the giant magnetoresistive effect (GMR), used for a hard disc drive (HDD) unit.
DESCRIPTION OF THE RELATED ART
Recently, thin-film magnetic heads with magnetoresistive effect (MR) read elements based on spin valve effect of GMR characteristics are proposed (U.S. Pat. Nos. 5,206,590 and 5,422,571) in order to satisfy the requirement for ever increasing data storage densities in today's magnetic storage systems like HDD units. The SVMR element includes first and second thin-film layers of a ferromagnetic material separated by a thin-film layer of non-magnetic and electrically conductive material, and an adjacent layer of anti-ferromagnetic material is formed in physical contact with the second ferromagnetic layer to provide exchange bias magnetic field by exchange coupling at the interface of the layers. The magnetization direction in the second ferromagnetic layer is constrained or maintained by the exchange coupling, hereinafter the second layer is called “pinned layer”. On the other hand the magnetization direction of the first ferromagnetic layer is free to rotate in response to an externally applied magnetic field, hereinafter the first layer is called “free layer”. The direction of the magnetization in the free layer changes between parallel and anti-parallel against the direction of the magnetization in the pinned layer, and hence the magneto-resistance greatly changes and GMR characteristics are obtained.
The output characteristic of the SVMR element depends upon the angular difference of magnetization between the free and pinned ferromagnetic layers. The direction of the magnetization of the free layer is free to rotate in accordance with an external magnetic field. That of the pinned layer is fixed to a specific direction (called as “pinned direction”) by the exchange coupling between this layer and adjacently formed anti-ferromagnetic layer.
In order to provide the exchange coupling between the pinned and anti-ferromagnetic layers, a process of temperature annealing under an external magnetic field with a specific direction (pin annealing or pin anneal process) is implemented. The pin annealing is done by elevating the temperature up to the Neel point at which magnetic regulation in the anti-ferromagnetic layer will be lost, and thereafter cooling down under application of magnetic field toward a desired magnetization direction.
In this kind of SVMR element, the direction of the magnetization in the pinned layer may change in some cases by various reasons. If the direction of the magnetization changes, the angular difference between the pinned and free layers changes too and therefore the output characteristic also changes. Consequently, controlling the direction of the magnetization in the pinned layer to a correct direction is very important.
However, the various characteristics of the SVMR element may be changed under actual high temperature operation of a HDD unit, even if the pin anneal processing is properly implemented. This change is caused by magnetic anisotropy change in the free layer due to the high temperature stress during operation of the HDD unit and due to the magnetic field by a hard magnet layer used for giving a bias magnetic field to the free layer.
The detail of this phenomenon is as follows.
(1) During fabricating process of the SVMR element, the free layer is deposited under application of magnetic field toward the track width direction. Thus, axis of easy magnetization of the free layer orients to the track width direction.
(2) The pin anneal process is done under application of magnetic field toward the pinned direction which is perpendicular to the easy magnetization axis of the free layer. Thus, the magnetic anisotropy of the free layer after the pin annealing may be weakened in comparison with that just after the deposition of the free layer, or the easy magnetization axis and the hard magnetization axis of the free layer may be reversed with each other.
(3) When the magnetic head with such a SVMR element is used under the state of high temperature, the magnetic anisotropy of the free layer changes again in accordance with magnetic field toward the track width direction from the hard magnet so that the axis of easy magnetization of the free layer gradually orients the track width direction.
(4) When actually using the magnetic head, magnetic field from the magnetic disk will be applied to the free layer in the direction perpendicular to the track width direction. Therefore, the change of the easy magnetization axis of the free layer toward the track width direction as described in
(3) results that any magnetization change in the free layer becomes difficult and that the reproduction output from the magnetic head lowers or deteriorates.
Thus, the magnetic anisotropy of the free layer immediately after the pin anneal process changes as the magnetic head is actually used, and as a result degradation of the head reproduction output and degradation of the symmetry of output wave may arise.
In order to reduce degradation which may be occurred at the time of such actual use, a performing of a free layer annealing wherein the SVMR element is additionally heated under application of magnetic field toward the track width direction so as to strongly fix the easy magnetization axis of the free layer in the track width direction has been proposed in Japanese patent unexamined publication No.10-223942.
However, if such the free layer annealing is performed, the pinned direction of the pinned layer may change due to the heat treatment under application of the magnetic field toward the track width direction and lowering of the head reproduction output may be brought as a result.
The detail of this degradation phenomenon is as follows.
(a) The pinned direction of the magnetization in the pinned layer is different from that of the magnetic domain control field that is generated by the hard magnet (track width direction). And hence the direction of the magnetization of the pinned layer which is contacted with the anti-ferromagnetic layer is slightly rotated toward the direction of the magnetic domain control field (hereinafter the direction of the magnetization of the pinned layer is expressed as &thgr;p).
(b) In the anti-ferromagnetic material layer, the Neel point temperature differs from location to location inside the layer from macroscopic point of view, and it is distributed in a certain range of temperature. Even if the temperature is less than the “bulk” Neel point (average Neel point), there could be small area whose micro Neel point temperature is low and where the exchange coupling with the pinned layer disappears.
(c) When such SVMR element is operated at a high temperature T, which is equal to or less than the blocking temperature at which the exchange couplings of all microscopic areas disappear, and then cooled down to usual room temperature, some microscopic area whose Neel temperatures are less than T is effectively annealed again and the direction of the magnetization is rotated to &thgr;p.
(d) The total amount of the &thgr;p rotated area by the temperature cycle determines the magnetic structure of the anti-ferromagnetic layer and also the new direction of the magnetization of the pinned layer.
As stated in the above paragraph, performing of the free layer annealing may cause a change of the pinned direction in the pinned layer, and the electrical output characteristics of the SVMR element are degraded in signal levels, and waveform symmetry.
Hereinafter, the degradation of the output characteristics of the SVMR element due to the rotation of the pinned direction will be described with reference to drawings.
The SVMR element operates by detecting change in its electrical resistance depending upon an angle between directions of magnetization in the pinned and free layers as aforementioned. The electrical resistance R is expressed by R=(1−co
Araki Satoru
Morita Haruyuki
Shimazawa Koji
Oltmans Andrew L.
Sheehan John
TDK Corporation
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