Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1999-11-05
2002-06-11
Ometz, David L. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06404606
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a seed layer structure for a platinum manganese pinning layer of a spin valve sensor and more particularly to a seed layer structure that promotes a higher exchange coupling field H
ex
and a higher pinning field H
p
between the pinning layer and a pinned layer of the 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) which 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 typically 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, 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 antiparallel magnetic moments.
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 at 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 this orientation. 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.
Nickel oxide (NiO) is a desirable material for the aforementioned pinning layer structure in a bottom spin valve. Unfortunately, nickel oxide (NiO) has a relatively low blocking temperature which is about 220° C. In a magnetic disk drive the operating temperature may exceed 150° C. A portion of the magnetic spins of the nickel oxide (NiO) pinning layer rotate below the blocking temperature because of a blocking temperature distribution below the blocking temperature where portions of the magnetic spins of the pinning layer commence to rotate. Accordingly, a portion of the magnetic spins of the nickel oxide (NiO) pinning layer can rotate at operating temperatures in the presence of a magnetic field, such as a signal field from the rotating magnetic disk, the write field from the write head or an unwanted electric static discharge (ESD) caused by contact with a statically charged object. The problem is exacerbated when the slider contacts an asperity on the magnetic disk which can raise the temperature above the disk drive operating 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, 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 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 performance of the pinned layer structure relative to the free layer will be degraded. The exchange coupling field H
ex
and the pinning field H
p
will be discussed in more detail in the detailed description.
A desirable antiferromagnetic pinning layer material is platinum manganese (PtMn) since it has a higher blocking temperature than nickel oxide (NiO) and it will perform satisfactorily with less thickness than nickel oxide (NiO). The higher blocking temperature improves thermal stability and the thinner layer improves the read gap. While nickel oxide (NiO) has a blocking temperature of about 220° C. and requires a thickness of about 425 Å, platinum manganese (PtMn) has a blocking temperature of a about 350° C. and requires a thickness of about 175 Å. Unfortunately, platinum manganese (PtMn) has demonstrated a low exchange coupling field H
ex
and a low pinning field H
p
in bottom spin valves. If these paramet
Johnston Ervin F.
Ometz David L.
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