Electricity: measuring and testing – Magnetic – Magnetic information storage element testing
Reexamination Certificate
1999-03-05
2001-09-25
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetic information storage element testing
C324S246000, C324S252000
Reexamination Certificate
active
06294911
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of measuring magnetization direction of magnetoresistive effect (MR) devices biased by anti-ferromagnetic material layer and to a measuring apparatus based on such method especially for giant magnetoresistive effect (GMR) devices such as spin valve effect MR devices, and tunneling magnetoresistive effect (TMR) devices which are applied to hard disk drive (HDD) units as read sensors.
DESCRIPTION OF THE RELATED ART
Recently, MR thin film read sensors based on spin valve effect of GMR characteristics are proposed (United States 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 thin film structure includes first and second thin film layers of a ferromagnetic material separated by a thin film layer of non-magnetic 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 measurement 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 magnetoresistive effect greatly changes and GMR 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 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 by the exchange coupling between the layer and adjacently formed anti-ferromagnetic layer.
In this kind of spin valve effect MR read sensor structure, the direction of the magnetization of 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. 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 under an external magnetic field with a specific direction (pin annealing) is implemented. The pin annealing is done as follows, first the temperature is elevated up to the Neel point at which temperature magnetization order in the anti-ferromagnetic material layer will be destroyed, and then cooled down to room temperature under a certain magnetic field strength with a specific direction for the exchange coupling. 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 MR 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 direction of the magnetization in the pinned layer is different from that of the magnetic field (Hhm) by the hard magnet. And hence the direction of the magnetization of pinned layer which is contacted with the anti-ferromagnetic layer is slightly rotated toward the direction of Hbm (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 area disappear and then cooled down to usual room temperature, some microscopic areas whose Neel temperatures are less than T is effectively annealed and the direction of the magnetization is rotated to &thgr;p. The total amount of the &thgr;p and the rotated amount component will change the exchange coupling state between the anti-ferromagnetic layer and the adjacent ferromagnetic layer to determine the new pinned direction of the magnetization of the magnetic structure. The new pinned direction will vary depending upon the period of time kept at high temperature because the magnetic characteristics of the ferromagnetic layer is changing over with time under the high temperature.
As stated in the above paragraph, usage of such spin valve effect MR read sensor at high temperature may cause a change of the pinned direction of the magnetization in the pinned layer, and the electrical output characteristics of the sensor are degraded in signal levels, and waveform symmetry.
In order to prevent the above-mentioned problems from occurring, it had been desired that material having a high blocking temperature and possible to provide smaller Neel temperature distribution is used for the anti-ferromagnetic layer.
As stated in the above paragraphs of the background explanation, it is very important to investigate how the thermal stability of the magnetization direction of the pinned layer depends upon the applied magnetic materials and annealing process conditions in the material development phase of MR devices, and hence a method of precise measurement of the magnetization direction of the pinned layer is required. And also the data of the magnetization direction of the pinned layer is important for device evaluation and qualification in device manufacturing. Therefore, it is very necessary to measure the magnetization direction precisely and quickly.
However, there has been no well-established way to measure the magnetization direction of the pinned layer of GMR devices such as spin valve effect MR devices and of TMR devices with good accuracy.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention is to provide a method and apparatus for measuring magnetization direction of a pinned layer of a MR device biased by anti-ferromagnetic material.
According to the present invention, a method of measuring magnetization direction of a MR device, includes a first step of obtaining both maximum electrical resistance values under positive and negative applied measurement magnetic fields onto the MR device biased by anti-ferromagnetic material, a second step of relatively rotating a basic axis of the MR device against a direction of the applied measurement magnetic field until both the maximum resistance values become comparatively the same within ±1% error, and a third step of obtaining a relative rotation angle between the basic axis of the MR device and the direction of the applied measurement magnetic field.
The magnetization direction of the pinned layer of a MR device biased by anti-ferromagnetic material like a spin valve effect MR read sensor device is determined by measuring the relative rotation angle between the basic axis of the MR device and the direction of applied magnetic field which is rotated until both the maxim
Araki Satoru
Hachisuka Nozomu
Ohta Manabu
Sasaki Tetsuro
Shimazawa Koji
Andersen Henry S.
Arent Fox Kintner & Plotkin & Kahn, PLLC
TDK Corporation
Williams Hezron
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