Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1999-07-23
2002-03-12
Davis, David (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06356419
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an antiparallel (AP) pinned read sensor with an improved magnetoresistance and, more particularly, to an AP pinned structure which shunts less sense current I
S
.
2. Description of the Related Art
An exemplary high performance read head employs a spin valve sensor for sensing magnetic signal fields from a moving magnetic medium, such as a rotating magnetic disk. The 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 the pinned layer for pinning the magnetic moment of the pinned layer 90° to an air bearing surface (ABS) which 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 which is typically parallel to the ABS, is the position of the magnetic moment of the free layer when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer is preferably parallel to the ABS. 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. The sensitivity of the sensor is quantified as 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 magnetoresistance. A spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve (magnetoresistive coefficient dr/R or 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 magnetic 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 read head be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs the 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, and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state, the bias point is located at zero and the positive and negative readback signals will be equal when sensing positive and negative magnetic 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.
The location of the bias point on the transfer curve is influenced by three major forces on the free layer, namely a ferromagnetic coupling field H
F
between the pinned layer and the free layer, a demag field H
demag
from the pinned layer, and sense current fields H
I
from all conductive layers of the spin valve except the free layer.
When the sense current I
S
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 toward a first direction perpendicular to the ABS. The pinned layer demagnetization field H
demag
further rotates the magnetic moment of the free layer toward the first direction counteracted by a ferromagnetic coupling field H
F
of the pinned layer that rotates the magnetic moment of the free layer toward a second direction antiparallel to the first direction.
Since the conductive material on the pinned layer side of the free layer far outweighs the conductive material on the other side of the free layer the sense current fields from the pinned layer side are a major force on the free layer which is difficult to counterbalance with the other magnetic forces acting on the free layer. Further, the conduction of the sense current I
S
through metallic layers of the spin valve sensor, other than the spacer layer, in effect shunts a portion of the sense current which reduces the amplitude of the signal detected by the read head. If less current is shunted through the conductive layers, other than the spacer layer, this can result in more sense current I
S
being conducted through the spacer layer to increase signal detection, or alternatively the sense current I
S
can be reduced to lower the generation of heat. If the pinned layer is an antiparallel (AP) pinned layer structure instead of a single pinned layer the aforementioned problems are exacerbated.
The AP pinned spin valve sensor differs from the simple spin valve sensor in that the AP pinned spin valve sensor has an AP pinned structure that has first and second AP pinned layers instead of a single pinned layer. An AP coupling layer is sandwiched between the first and second AP pinned layers. The first AP pinned layer has its magnetic moment oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second AP pinned layer is immediately adjacent to the free layer and is antiparallel coupled to the first AP pinned layer because of the minimal thickness (in the order of 8 Å) of the AP coupling layer between the first and second AP pinned layers. Accordingly, the magnetic moment of the second AP pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first AP pinned layer.
The AP pinned structure is preferred over the single pinned layer because the magnetic moments of the first and second AP pinned layers of the AP pinned structure subtractively combine to provide a net magnetic moment that is less than the magnetic moment of the single pinned layer. The direction of the net moment is determined by the thicker of the first and second AP pinned layers. A reduced net magnetic moment equates to a reduced demagnetization (demag) field H
demag
from the AP pinned structure. Since the exchange coupling between the pinned and pinning layers is inversely proportional to the net pinning moment a reduc
Castro Angel
Davis David
Gray Cary Ware & Freidenrich
International Business Machines - Corporation
Johnston Ervin F.
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