1. Field of the Invention
The present invention relates to a spin valve sensor with an improved antiparallel (AP) pinned layer and more particularly to an AP pinned layer that has reduced current shunting and lower coercivity.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS to cause the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field across the gap between the pole pieces. This field fringes across the gap at the ABS for the purpose of writing the aforementioned magnetic impression in tracks on moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to an air bearing surface (ABS) of the head and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons 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 magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. A spin valve is also know as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer). A pinning layer in a bottom spin valve is typically made of nickel oxide (NiO).
Another type of spin valve sensor is an antiparallel (AP) spin valve sensor. The AP pinned spin valve sensor differs from the simple spin valve sensor, described above, in that the pinned layer of the AP pinned spin valve sensor comprises multiple thin layers, which are collectively referred to as an antiparallel (AP) pinned layer. The AP pinned layer has a ruthenium (Ru) spacer layer sandwiched between first and second ferromagnetic thin layers. The first ferromagnetic thin layer has its magnetic moment oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second ferromagnetic thin layer is immediately adjacent to the free layer and is antiparallel coupled to the first thin layer because of the minimal thickness (in the order of 8 .ANG.) of the spacer layer between the first and second ferromagnetic thin layers. The magnetic moment of the second ferromagnetic thin layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first ferromagnetic layer.
The AP pinned layer is preferred over the single layer pinned layer. The magnetic moments of the first and second layers of the AP pinned layer subtractively combine to provide a net pinning moment of the AP pinned layer. The direction of the net moment is determined by the thicker of the first and second thin layers. The thicknesses of the first and second thin layers are chosen to reduce the net moment. A reduced net moment equates to a reduced demagnetization (demag) field from the AP pinned layer. Since the antiferromagnetic exchange coupling is inversely proportional to the net pinning moment, this increases exchange coupling between the first ferromagnetic film of the AP pinned layer and the pinning layer. The high exchange coupling promotes higher stability of the head. When the head encounters elevated thermal conditions caused by electrostatic discharge (ESD) from an object or person, or by contacting an asperity on a magnetic disk, the blocking temperature (temperature at which magnetic spins of the layer can be easily moved by an applied magnetic field) of the antiferromagnetic layer can be exceeded, resulting in disorientation of its magnetic spins. The magnetic moment of the pinned layer is then no longer pinned in the desired direction. A reduced demag field also reduces the demag field imposed on the free layer which promotes a symmetry of the read signal. The AP pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein.
The first and second ferromagnetic layers of the AP pinned spin valve sensor are typically made of cobalt (Co). Unfortunately, cobalt has high coercivity, high magnetostriction and low resistance. When the first and second ferromagnetic layers are formed they are sputtered deposited in the presence of a magnetic field that is oriented perpendicular to the ABS which sets the easy axis (e.a.) of the ferromagnetic films perpendicular to the ABS. During operation of the head the AP pinned layer is subjected to extraneous magnetic fields that have components parallel to the ABS, such as components of the write field. These extraneous fields, combined with heating of the pinning layer, can cause the pinning layer to lose its pinning strength (exchange coupling) and allow the magnetic moments of the ferromagnetic layers to switch from being perpendicular to the ABS to some other direction. If the coercivity of the ferromagnetic films is higher than the exchange field that urges the magnetic moments of the ferromagnetic layers back to their original positions the magnetic moments of the ferromagnetic layers will remain in the wrong direction. This renders the read head inoperable.
Cobalt (Co) has a high negative magnetostriction. The negative sign determines the direction of any stress induced anisotropy. When a magnetic head is lapped, which is a grinding process, nonuniform compressive stresses occur in the layers of the sensor. Because of the magnetostriction and the stresses the cobalt (Co) ferromagnetic films acquire a stress induced anisotropy that is parallel to the ABS. This is the wrong direction. The stress induced anisotropy may rotate the magnetic moment of the first and second ferromagnetic layers of the AP pinned layer to some extent from perpendicular to the ABS in spite of the exchange coupling field tending to maintain the perpendicular position. This condition can cause read signal asymmetry.
The low resistance of the cobalt (Co) ferromagnetic films of the AP pinned layer causes a portion of the sense current to be shunted past the free and spacer layers. This causes a loss of read signal.
Efforts continue to increase the spin valve effect of GMR heads. An increase in the spin valve effect equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. Promoting read signal symmetry is also a consideration. This is accomplished by reducing the magnetic influences on the free layer. A search still continues to lower the coercivity, substantially eliminate magnetostriction and increase the resistance of some of the critical layers of the spin valve sensor.