Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1999-03-30
2001-07-31
Letscher, George J. (Department: 2754)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06268985
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a read head having a spin valve sensor with an improved capping layer and, more particularly, to a capping layer which has at least two thin films for improving the magnetoresistance of the read head, the first thin film being ruthenium (Ru) and the second thin film being a metal oxide with the ruthenium (Ru) thin film being located between the metal oxide thin film and a ferromagnetic free layer in the spin valve sensor.
2. Description of the Related Art
A magnetic head assembly typically includes write and read heads wherein the write head writes magnetic bits of information into a rotating magnetic disk in a disk drive and the read head reads the magnetic bits of information from the rotating disk. The write head includes first and second pole piece layers which have a yoke region between a pole tip region and a back gap region. An insulation stack with a write coil layer embedded therein is located between the first and second pole piece layers in the yoke region, the first and second pole piece layers are separated by a nonmagnetic write gap layer at an air bearing surface (ABS), which faces the rotating disk, and the first and second pole piece layers are magnetically connected in the back gap region. The insulation stack includes multiple photoresist layers which are baked at a high temperature. This processing step can seriously degrade the performance of the read head which will be discussed in more detail hereinafter.
The read head includes a spin valve sensor and first and second lead layers that are connected to first and second side edges of the spin valve sensor for conducting a sense current therethrough. The spin valve sensor and the first and second leads are located between nonmagnetic electrically insulative first and second read gap layers and the first and second read gap layers are, in turn, located between ferromagnetic first and second shield layers. In a merged magnetic head assembly the second shield layer and the first pole piece layer are a common layer whereas in a piggyback type magnetic head assembly these are separate layers which are separated by a nonmagnetic layer.
The spin valve sensor includes a nonmagnetic electrically conductive spacer layer which is located between a ferromagnetic pinned layer and a ferromagnetic free layer. The free layer typically has a magnetic moment which is oriented parallel to the ABS and which can be rotated up or down in response to the magnetic bits of information from the rotating disk. The magnetic moment of the pinned layer is typically pinned perpendicular to the ABS by exchange coupling with an antiferromagnetic pinning layer. The free layer has a magnetoresistance which varies according to the function cos &thgr;, where &thgr; is the angle between the magnetic moments of the free and pinned layers. When the magnetic moments of the free and pinned layers are parallel, the magnetoresistance is zero, however, when the magnetic moments of the free and pinned layer are antiparallel the magnetoresistance is at a maximum. Assuming that the magnetic moment of the pinned layer is perpendicular and toward the ABS, positive and negative magnetic bits of information sensed by the sensor from the rotating disk rotate the magnetic moment of the free layer upwardly and downwardly respectively causing an increase and a decrease respectively in the magnetoresistance of the sensor. With a constant sense current the changes in magnetoresistance cause corresponding changes in potential which can be processed by processing circuitry as digital signals.
It is important that the magnetoresistance capability of the spin valve sensor be preserved during processing steps following the making of the spin valve sensor. The read head is typically made before the making of the write head. After constructing the spin valve sensor a reset function may be performed which is annealing the spin valve sensor at a temperature such as 230° C. for a short period of time, such as 5 minutes, in the presence of a field that is perpendicular to the ABS for setting the magnetic spins of the pinning layer in the direction of the field. After this initial reset the spin valve sensor has a magnetoresistive coefficient which is expressed as dr/R wherein dr is the change in magnetoresistance of the free layer and R is the resistance of the free layer before the change in magnetoresistance. Unfortunately, this magnetoresistive (MR) coefficient can be seriously degraded by subsequent processing steps, in particular, processing steps which construct the write head. As mentioned hereinabove, the various photoresist layers of the insulation stack in the write head are baked at a high temperature so that they will harden. This baking is typically at 230° C. to 250° C. for a period of 11 hours in a field which is perpendicular to the ABS. The field is perpendicular to the ABS for the purpose of maintaining the magnetic spins of the pinning layer perpendicular to the ABS. Unfortunately, this high temperature for such a long period of time significantly degrades the MR coefficient of the spin valve sensor in spite of the field applied perpendicular to the ABS for preserving the integrity of the pinning layer.
Accordingly, there is a strong-felt need for making spin valves that do not lose their magnetoresistance capability after baking of photoresist layers in the insulation stack of the write head.
SUMMARY OF THE INVENTION
An oxide capping layer is highly desirable since it causes what is known in the art as specular reflection. Specular reflection means that the oxide layer functions like a mirror and reflects conduction electrons toward the free layer back into a mean free path of the conduction electrons that extends between first and second side edges of the sensor. The mean free path is the distance between scattering events of the conduction electrons between the first and second side edges and may be on the order of 100 angstroms (Å). It is the scattering events at the interfaces of the free layer that changes the magnetoresistance of the sensor. The greater the scattering events the greater the magnetoresistance of the sensor. Accordingly, when conduction electrons are reflected back into the free layer by the reflective layer more electrons are involved in the scattering events which enhances the MR coefficient (dr/R).
Unfortunately, other aspects of the oxide capping layer make it undesirable. If the oxide capping layer interfaces a nickel iron (NiFe) free layer the uniaxial anisotropy (H
K
) of the free layer is significantly increased. Uniaxial anisotropy is the amount of applied field that is required to rotate the magnetic moment of the free layer from a parallel easy axis position to a position that is perpendicular to the ABS. An increase in the uniaxial anisotropy means that the free layer is more stiff in its rotation response to applied magnetic bits of information from a rotating magnetic disk. Another common problem with an oxide capping layer is that the MR coefficient is significantly reduced after the aforementioned annealing step wherein photoresist layers of the insulation stack in the write head are baked at a high temperature. I have found that an oxide capping layer with a MR coefficient (dr/R) of greater than 4% after an initial reset of the pinning layer can be reduced to literally zero after the annealing step.
I have discovered a capping layer which overcomes the aforementioned problems of an oxide capping layer. The present capping layer does not stiffen the magnetic moment of the free layer and the MR coefficient (dr/R) of the sensor is not serious degraded after the aforementioned annealing step. The present capping layer is a double thin film structure wherein a first thin film is ruthenium (Ru) and a second thin film is a metallic oxide, such as nickel oxide (NiO), tantalum oxide (Ta
2
O
3
) or aluminum oxide (Al
2
O
5
). With this capping layer an MR coefficient (dr/R) of greater than 4% after resetting the pinning layer is decrea
Gray Cary Ware & Freidenrich
International Business Machines - Corporation
Johnston Ervin F.
Letscher George J.
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