Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-02-17
2001-11-13
Evans, Jefferson (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S324100
Reexamination Certificate
active
06317299
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a seed layer for improving the pinning field of a spin valve sensor and more particularly to a seed layer structure that improves the pinning field between pinning and pinned layers and promotes a higher magnetoresistive coefficient of a spin valve sensor.
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 and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning a 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 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 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 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 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 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. 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 layers. 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 rotates from a position parallel with respect to the magnetic moment of the pinned layer 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 first formed 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 at the bottom of the sensor next to the first read gap layer or at the top of the sensor closer 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 and pinned layers the pinned layer 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 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 is oriented parallel to the applied field and the magnetic spins of the pinning layer follow the orientation of the pinned layer. 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. The pinning function is effective as long as the temperature remains substantially below the blocking temperature.
In the presence of some magnetic fields the magnetic moment of the pinned layer can be rotated antiparallel to the pinned direction. The question then is whether the magnetic moment of the pinned layer will return to the pinned direction when the magnetic field is relaxed. This depends upon the strength of the exchange coupling field and the coercivity of the pinned layer. If the coercivity of the pinned layer exceeds the exchange coupling field between the pinning and pinned layers the exchange coupling field will not be strong enough to bring the magnetic moment of the pinned layer back to the original pinned direction. Until the magnetic spins of the pinning layer are reset the read head is rendered inoperative. Accordingly, there is a strong felt need to increase the exchange coupling field between the pinning layer and the pinned layer so that the spin valve sensor has improved thermal stability.
Another parameter that indicates the performance of the pinning of the pinned layer is the pinning field H
p
between the pinning and pinned layers. The pinning field, which is somewhat dependent upon the exchange coupling field H
ex
, is the applied field at which the magnetic moment of the pinned layer commences to rotate in a substantial manner. If the pinning field H
p
is low the orientation of the pinned layer will not be controlled thereby degrading performance of the read head. Accordingly, it is desirable to maximize the pinning field H
p
.
SUMMARY OF THE INVENTION
I have provided a seed layer structure for the pinning layer which increases the pinning field H
PIN
between the pinning layer and the pinned layer. In an example, which will be described in detail hereinafter, I obtained a pinning field H
PIN
of 600 Oe which is excellent in the spin valve sensor art. In the example the seed layer structure included a first seed layer of cobalt iron boron (CoFeB), a second seed layer of nickel manganese oxide (NiMnO) and a third seed layer of aluminum oxide (Al
2
O
3
) with the first seed layer interfacing the pinning layer and the second seed layer being located between the first and third seed layers. My invention also includes employing a seed layer structure which includes only the first seed layer of cobalt iron boron (CoFeB) since it directly interfaces the pinning layer and is primarily responsible for the improvement in the pinning field H
PIN
. The pinned layer can be a single pinned layer or an antiparallel (AP) pinned layer structure. The cobalt iron boron (CoFeB) seed layer provides a second significant function of at least partially counterbalancing the demagnetizing field from the pinned layer. Accordingly, the seed layer improves readback asymmetry of the read head by promoting a centering of a bias point of the spin valve sensor on its transfer curve. As will also be seen from the following example the spin valve sensor with the seed layer structure provided a magnetoresistive coefficient of 8.8% w
Evans Jefferson
International Business Machines - Corporation
Johnston Ervin F.
LandOfFree
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