Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-03-16
2003-02-25
Davis, David (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06525911
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to permeability offset of shield layers for correcting the bias of a free layer structure in a spin valve sensor and, more particularly, to such shield layers which exert a net image current field H
IM
for counterbalancing other fields acting on the free layer structure.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinning layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos &thgr;, where &thgr; is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk. The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
An improved spin valve sensor, which is referred to hereinafter as antiparallel pinned (AP) spin valve sensor, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. The AP spin valve differs from the spin valve described above in that the pinned layer comprises multiple thin films, hereinafter referred to as AP pinned layer. The AP pinned layer has a nonmagnetic spacer film which is sandwiched between first and second ferromagnetic thin films. The first thin film, which may comprise several thin films, is immediately adjacent to the antiferromagnetic layer and is exchange-coupled thereto, with its magnetic moment directed in a first direction. The second thin film is immediately adjacent to the free layer and is exchange-coupled to the first thin film by the minimal thickness (in the order of 6 Å) of the spacer film between the first and second thin films. The magnetic moment of the second thin film is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first film. The magnetic moments of the first and second films subtractively combine to provide a net moment of the AP pinned layer. The direction of the net moment is determined by the thicker of the first and second thin films. The thicknesses of the first and second thin films are chosen so that the net moment is small. A small net moment equates to a small demagnetizing (demag) field exerted on the free layer by the AP pinned layer. Since the antiferromagnetic exchange coupling is inversely proportional to the net moment, this results in a large exchange coupling between the pinning and pinned layers.
The transfer curve for a spin valve sensor is defined by the aforementioned cos &thgr; where &thgr; is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced.
Readback asymmetry is defined as
V
1
-
V
2
max
(
V
1
or
V
2
)
For example, +10% readback asymmetry means that the positive readback signal V
1
is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in many applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. In thes
Davis David
Johnston Ervin F.
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