Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1999-08-05
2002-09-10
Tupper, Robert S. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06449134
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a read head with a file resettable dual spin valve sensor and more particularly to a dual spin valve sensor in which magnetic spins of first and second pinning layers can be reset with a current pulse from a sense current circuit.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. A typical sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces and is exchange coupled to the pinned layer for pinning a magnetic moment of the pinned layer 90° to an air bearing surface (ABS) where 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 typically oriented parallel to the ABS in a quiescent condition where the quiescent condition is when the sense current is conducted through the sensor in the absence of any signal fields. The magnetic moment of the free layer is free to rotate from the parallel position in response to signal fields from the rotating magnetic disk.
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 are scattered at the interfaces of the spacer layer with respect to the pinned layer and the free layer. 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. Changes in scattering, in response to field signals from a rotating disk, changes the resistance of the spin valve sensor as a function of cos &thgr;, where &thgr; is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layers and R is the resistance of the sensor when the moments are parallel.
The transfer curve (readback signal of the spin valve head versus applied signal from the magnetic disk) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos &thgr;. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback signal (voltage sensed by processing circuitry). With positive and negative signal fields from a rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the resistance of the spin valve sensor be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs a bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the free layer is parallel to the ABS in the quiescent state the bias point is located at zero and the positive and negative readback signals are equal when sensing positive and negative signal fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric which equates to reduced storage capacity of a magnetic disk drive.
The location of the bias point on the transfer curve is influenced by three major forces on the free layer, namely a demagnetization field (H
D
) from the pinned layer, a ferromagnetic coupling field (H
F
) between the pinned layer and the free layer, and sense current fields (H
l
) from all conductive layers of the spin valve except the free layer. When the sense current is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer and the first spacer layer, which are all on one side of the free layer, impose sense current fields on the free layer that rotate the magnetic moment of the free layer in a first direction. The ferromagnetic coupling field from the pinned layer further rotates the magnetic moment of the free layer in the first direction. The demagnetization field from the pinned layer on the free layer rotates the magnetic moment of the free layer in a second direction opposite to the first direction. Accordingly, the demagnetization field opposes the sense current and ferromagnetic coupling fields and can be used for counterbalancing.
A dual spin valve sensor employs a ferromagnetic free layer between first and second ferromagnetic pinned layers wherein a first spacer layer separates the first pinned layer from the free layer and a second spacer layer separates the second pinned layer from the free layer. The first and second pinned layers are pinned by first and second antiferromagnetic pinning layers. With this arrangement, the magnetoresistive coefficient is increased by a factor of approximately 1.4 due to the spin valve effect on each side of the free layer. The magnetic spins of the pinning layers are set by cooling from a high temperature in the presence of a magnetic field. In a standard dual spin valve, it is necessary that the magnetic spins of the first and second pinning layers be set parallel with respect to each other in order for the spin valve effect to be additive. This is the case of a simple free layer which has only one layer or layered structure. A problem with this design arises when the dual spin valve undergoes an in file reset. The current in the sensor applies a magnetic circumferential magnetic field to the sensor. This results in the magnetic field applied to the two pinned layers being in opposite directions, setting the moments antiparallel. The spin valve effect from the upper and lower portions of the spin valve are no longer additive but instead subtract. Thus, no signal is produced in this configuration for a spin valve with a simple free layer.
Over the years a significant amount of research has been conducted to improve the magnetoresistive coefficient dr/R of spin valve sensors without adversely affecting other performance factors such as biasing of the free layer and thermal stability of the pinning layers. These efforts have increased the storage capacity of computers from kilobytes to megabytes to gigabytes.
SUMMARY OF THE INVENTION
We have provided a dual spin valve sensor wherein first and second antiferromagnetic pinning layers of the sensor can be reset in a magnetic disk drive by a current pulse through the sense current circuit, which resetting is referred to hereinafter as file resettable. This has been accomplished by providing an antiparallel (AP) coupled free layer structure which is located between the first and second pinned layers with a first nonmagnetic conductive spacer layer between the first pinned layer and the free layer structure and a second nonmagnetic conductive spacer layer between the second pinned layer and the free layer structure. First and second antiferromagnetic pinning layers are exchange coupled to the first and second pinned layers for the purpose of setting the magnetic moments of the pinned layers. The file reset pulse sets these moments antiparallel with respect to each other, which is the proper orientation so that the spin valve effects, one on each side of the AP free layer structure, are additive. The free layer structure includes a nonmagnetic conductive antiparallel (AP) coupling layer which is located between first and second antiparallel (AP) coupled free layers. It is necessary that one of the first and second AP coupled free layers be thicker than the oth
Beach Robert Stanley
Carey Matthew
Gurney Bruce A.
Altera Law Group LLC
International Business Machines - Corporation
Tupper Robert S.
Watko Julie Anne
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