Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-03-19
2002-06-18
Sheehan, John P (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C029S608000, C148S121000, C360S112000
Reexamination Certificate
active
06406556
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a manufacturing method of a thin-film magnetic head with a magnetoresistive effect (MR) multi-layered structure using exchange coupling magnetic bias, such as spin valve effect MR sensor, used for HDD (Hard Disk Drive) units.
DESCRIPTION OF THE RELATED ART
Recently, thin-film magnetic heads with MR read sensors based on spin valve effect of giant MR (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 hard disk drive units. The spin valve effect multi-layered structure 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 giant magneto-resistance characteristics are obtained.
The output characteristic of the spin valve MR read sensor 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 this kind of spin valve effect MR read sensor structure, 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 stabilizing the direction of the magnetization in the pinned layer is very important.
In order to stabilize the direction of the magnetization by the strong exchange coupling between the pinned and anti-ferromagnetic layers, a process of temperature-annealing (pin anneal process) under an external magnetic field with a specific direction is implemented. The pin annealing is done as follows, first the temperature is elevated up to the Neel point under the magnetic field strength of 500 Oe to 3 k Oe, and held for about 30 minutes to 5 hours, and then cooled down to room temperature. By this pin anneal process, the exchange coupling is regulated at the interface of the pinned and anti-ferromagnetic layers toward the direction of the externally applied magnetic field.
However, the magnetoresistance characteristics may be changed under actual high temperature operation of a hard disk drive unit, even if the pin anneal processing is properly implemented. This degradation is caused by the high temperature stress during operation of the hard disk drive unit and by the magnetic field by a hard magnet layer used for giving a bias magnetic field to the free layer.
The detail of this degradation is as follows. The pinned direction of the magnetization in the pinned layer is different from that of the magnetic field (H
HM
) by the hard magnet. 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 H
HM
(hereinafter the direction of the magnetization of the pinned layer is expressed as &thgr;p). 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. When such spin valve effect MR read sensor is operated at a high temperature T, which is 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. The total amount of the &thgr;p rotated area by the temperature cycle determines the magnetic structure of the anti-ferromagnetic layer and the new direction of the magnetization of the pinned layer.
As stated in the above paragraph, usage of such spin valve MR read sensor at high temperature may cause a change of the pinned direction in the pinned layer, and the electrical output characteristics of the sensor are degraded in signal levels, and waveform symmetry.
Hereinafter, the degradation of the output characteristics of the sensor due to the rotation of the pinned direction will be described with reference to drawings.
The spin valve effect sensor operates by detecting change in its electrical resistance depending upon an angle between directions of magnetization in the pinned and free layers. The electrical resistance R is expressed by R=(1−cos &thgr;)/2+&bgr;, where &thgr; is the angle between directions of magnetization in the pinned and free layers and &bgr; is an electrical resistance (Rs) when the magnetization directions in the pinned and free layers are in parallel (&thgr;=0 degree) as illustrated in
FIG. 1
a
. When the magnetization directions in the pinned and free layers are in anti-parallel (&thgr;=180 degrees) as illustrated in
FIG. 1
b
, the electrical resistance becomes R=1+&bgr;. Also, when the magnetization directions in the pinned and free layers are orthogonal (&thgr;=90 degrees) as illustrated in
FIG. 1
c
, the electrical resistance becomes R=½+&bgr;.
As illustrated in
FIG. 2
, the spin valve effect sensor produces output voltage in response to the change in magnetization direction of the free layer caused by application of changing leakage magnetic field from the magnetic recording medium. Suppose that the direction of magnetization in the free layer rotates by +20 degrees (first magnetization state of the free layer) and by −20 degrees (second magnetization state of the free layer) due to the leakage magnetic field from the magnetic recording medium. If the pinned direction is normal, the resistance value across the sensor at the first magnetization state R
F1
is R
F1
=(1−cos 70°)/2=0.329 and the resistance value across the sensor at the second magnetization state R
F2
is R
F2
=(1−cos 110°)/2=0.671 as shown in
FIG. 3
a
. Thus, the difference &Dgr; R becomes as &Dgr;R=R
F2
−R
F1
=0.342. Whereas, if the pinned direction rotates by 20 degrees from the normal direction, the resistance value across the sensor at the first magnetization state R
F1
is R
F1
=(1−cos 500)/2=0.178 and the resistance value across the sensor at the second magnetization state R
F2
is R
F2
=(1−cos 90°)/2=0.500 as shown in
FIG. 3
b
. Thus, the difference &Dgr;R becomes as &Dgr;R=R
F2
−R
F1
=0.322. Therefore, 20 degrees rotation of the pinned direction results degradation of 5.8% in the sensor output.
It will be understood from the above-description that thin-film magnetic heads with good output characteristics can be fabricated by stabilizing the
Ohta Manabu
Sasaki Tetsuro
Shimazawa Koji
Arent Fox Kintner & Plotkin & Kahn, PLLC
Sheehan John P
TDK Corporation
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