Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-04-12
2003-06-24
Renner, Craig A. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S324120, C360S314000
Reexamination Certificate
active
06583969
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pinned layer structure having a nickel iron (NiFe) film for reducing the coercivity of a free layer structure in a spin valve sensor and, more particularly, to the nickel iron (NiFe) film acting as a seed layer for improving the sensitivity of a magnetic moment of the free layer structure to signal fields from a rotating magnetic disk.
2. Description of the Related Art
A spin valve sensor is employed by a read head 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 structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic 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 structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures 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 the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering 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 layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
A read head in a magnetic disk drive of a computer includes the spin valve sensor as well as nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
Because of the interfacing of the pinning layer and the pinned layer structure the pinned layer structure is exchange coupled to the pinning layer. A unidirectional orientation of the magnetic spins of the pinning layer pins the magnetic moment of the pinned layer structure in the same direction. The orientation of the magnetic spins of the pinning layer are set by applying heat close to or above a blocking temperature of the material of the pinning layer in the presence of a field that is directed perpendicular to the ABS. The blocking temperature is the temperature at which all of the magnetic spins of the pinning layer are free to rotate in response to an applied field. During the setting, the magnetic moment of the pinned layer structure is oriented parallel to the applied field and the magnetic spins of the pinning layer follow the orientation of the pinned layer structure. When the heat is reduced below the blocking temperature the magnetic spins of the pinning layer pin the orientation of the magnetic moment of the pinned layer structure. The pinning function is effective as long as the temperature remains substantially below the blocking temperature.
As stated hereinabove, the magnetic moment of the free layer structure is free to rotate in response to signal fields from a rotating magnetic disk. The degree with which the magnetic moment is free to rotate in response to these signal fields equates to the sensitivity of the spin valve sensor. Accordingly, if the magnetic moment is stiff in its rotation by rotating only a small amount in response to a signal field the signal amplitude of the spin valve sensor is low because there has been a small amount of relative rotation between the pinned layer structure and the free layer structure. A measure of the stiffness of the free layer structure is by its easy axis coercivity H
C
or uniaxial anisotropy H
K
. The easy axis coercivity is the amount of field required to switch the orientation of the magnetic moment of the free layer structure 180° along its easy axis while uniaxial anisotropy H
K
is the amount of field required to rotate the magnetic field 90° from its easy axis. A free layer structure which consists entirely of a nickel iron (NiFe) free layer can have a low coercivity H
C
of only 1 or 2 Oe. It has become desirable, however, to combine a cobalt iron (CoFe) nanolayer (NL) with the nickel iron (NiFe) layer for the purpose of increasing the magnetoresistive coefficient dr/R of the spin valve sensor. Unfortunately, however, this addition raises the coercivity H
C
of the free layer structure up to about 10 Oe, thereby increasing the stiffness of the free layer structure in response to signal fields. There is a strong-felt need to maintain the improved magnetoresistive coefficient dr/R and reduce the coercivity H
C
of the free layer structure with the cobalt iron (CoFe) layer. In a dual spin valve sensor the free layer structure typically employs a nickel iron (NiFe) free layer between first and second cobalt iron (CoFe) layers. Accordingly, in the dual spin valve sensor the free layer structure is even more stiff in its operation because of the additional cobalt iron (CoFe) layer. Another problem that occurs when the coercivity H
C
of the free layer structure is high is that the rotation of the magnetic moment of the free layer structure in response to the signal field is not smooth. When this occurs the rotation is referred to as having jumps which causes noise in the playback system. The degree of coercivity H
C
of the pinned layer structure is also important which is discussed next.
In the presence of some magnetic fields the magnetic moment of the pinned layer structure can be rotated antiparallel to the pinned direction. The question then is whether the magnetic moment of the pinned layer structure will return to the pinned di
Johnston Ervin F.
Renner Craig A.
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