Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1999-08-05
2001-07-03
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C360S327330
Reexamination Certificate
active
06255814
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a measurement method and an apparatus to measure bias magnetic field for controlling magnetic domain of a magnetoresistive effect (MR) element biased with magnetic domain control field, particularly to a measurement method and an apparatus to measure bias magnetic field for controlling magnetic domain of spin valve MR elements and tunnel magnetoresistive effect (TMR) elements, which are biased with exchange coupling magnetic field by anti-ferromagnetic material.
DESCRIPTION OF THE RELATED ART
Recently, MR thin-film read sensors based on spin valve effect of giant magnetoresistance effect (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 thin-film structure includes first and second thin-film layers of a ferromagnetic material separated by a thin-film layer of non-magnetic metal material, and an adjacent layer of anti-ferromagnetic material which 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 ferromagnetic 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 ferromagnetic 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 magnetoresistance effect (GMR) characteristics are obtained.
The output characteristics of the spin valve MR read sensor depends upon the angular difference of magnetization between the free and pinned layers. Assuming the angular difference is &thgr;, the relative output of the MR read sensor is given by Output=(1−cos&thgr;)/2. By this equation, the best output waveform with good symmetry can be obtained at the condition of &thgr;=90 degrees as an initial condition of the angular difference. Therefore, in a spin valve MR element the magnetization directions of the pinned and free layers are adjusted to be orthogonal each other by applying a bias magnetic field. In an initial condition, the magnetization direction of the free layer is set toward the track width direction by the bias magnetic field for controlling the magnetic domains in the free layer. On the other hand, the magnetization direction of the pinned layer is set toward the direction perpendicular to the track width direction (MR height direction) by the exchange coupling magnetic field by the anti-ferromagnetic material layer.
In a MR element, to suppress Barkhousen noise due to non-continuous magnetization by fluctuations or displacements of the magnetic domain boundaries, the bias magnetic field (longitudinal bias magnetic field) for controlling the magnetic domains is applied toward the longitudinal direction (track width direction). If this longitudinal bias magnetic field applied to the free layer is small, Barkhousen noise will be easily occurred. If the longitudinal bias magnetic field is large, the change in the magnetization direction of the free layer becomes difficult causing the sensing sensitivity of the spin valve MR element to be degraded. Therefore, it is necessary to apply an optimum amount of the longitudinal bias magnetic field in a spin valve MR element. In order to implement it, it is very important to know the strength of the longitudinal bias magnetic field of each element and the distribution on a wafer.
There are well known methods of measuring such longitudinal bias magnetic field in anisotropic magnetoresistive (AMR) elements.
FIGS. 1
a
to
1
d
show a measurement method of the longitudinal bias magnetic field in the AMR element.
FIG. 1
a
shows the magnetization direction of MR layer
10
at an initial condition with no sense current I
s
. The MR layer
10
is magnetized toward the direction of the longitudinal bias magnetic field applied by hard magnets
11
and
12
.
As shown in
FIG. 1
b
, when a sense current I
s
is forced to flow, the magnetic field induced by the sense current I
s
magnetizes a SAL (soft adjacent layer)
13
. The static magnetic field (transverse bias magnetic field) by the magnetized SAL
13
is applied to the MR layer
10
, and hence the magnetization direction of the MR layer
10
gradually shifts toward the MR height direction (transverse direction) in accordance with the increase of the sense current I
s
. Therefore, as shown in this figure, the output resistance (OUTPUT/I
s
) of the AMR element does strongly depend on the sense current I
s
.
As shown in
FIG. 1
c
, when an external magnetic field in opposite direction to the longitudinal bias magnetic field by the hard magnets
11
and
12
is applied, the longitudinal bias magnetic filed is offset and the MR layer is much more affected by the transverse bias magnetic field due to the sense current I
s
, and the magnetization direction of the MR layer
10
is much more rotated toward the MR height direction. Thus, the output resistance dependency on the sense current I
s
becomes smaller.
As shown in
FIG. 1
d
, if the externally applied magnetic field becomes much larger, an AMR element's output resistance dependency on the sense current I
s
becomes very small or minimum. Therefore, applying an external magnetic field in opposite direction to the longitudinal bias magnetic field and measuring the output resistance under comparatively small sense current, and by adjusting the strength of the external magnetic field so that the output resistance becomes minimum, the strength of the longitudinal bias magnetic field can be determined from the applied external magnetic field at that point.
This conventional measurement method of the longitudinal bias magnetic field is effective for the AMR element. However, it is impossible to apply this method to GMR elements like spin valve MR elements by the structure of pinned and free layers of which principles of operation are quite different from that of the conventional AMR elements. Also, this conventional method can be applied to only a MR element with no shield layer. In case of a MR element with shield layers, since the shield layers absorb the external magnetic field, correct value of the longitudinal bias magnetic field cannot be measured.
As explained above, there has been no way to measure the longitudinal bias magnetic field strength of GMR elements like spin valve MR elements and of TMR elements.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and an apparatus for accurate measuring a bias magnetic field (longitudinal bias magnetic field) for controlling magnetic domains in a MR element biased with exchange coupling magnetic field by anti-ferromagnetic material.
Another object of the present invention is to provide a method and an apparatus for accurate measuring a bias magnetic field (longitudinal bias magnetic field) for controlling magnetic domains in a MR element with shield layers.
According to the present invention, a method for measuring bias magnetic field for controlling magnetic domain (longitudinal bias magnetic field) of a MR element has the step of applying an external measurement magnetic field onto the MR element which is biased with the magnetic filed for controlling the magnetic domain (longitudinal bias magnetic field) in parallel to the direction of the bias magnetic field, the step of measuring &rgr;-H loop of the MR element (output resistance of MR element versus magnetic field strength loop) under the application of the external measurement magnetic field, and the step of determining a shifted amount of the m
Araki Satoru
Shimazawa Koji
Arent Fox Kintner & Plotkin & Kahn, PLLC
Patidar Jay
TDK Corporation
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