An MR sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor, such as that used in the IBM "Corsair" disk drive, operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned. The physical origin is the same in all types of structures: the application of an external magnetic field causes a variation in the relative orientation of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure comprising two uncoupled ferromagnetic layers separated by a nonmagnetic metallic layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the layer onto an iron-manganese (Fe--Mn) layer to exchange couple the two layers. This results in a spin valve magnetoresistive (SVMR) sensor in which only the unpinned or free ferromagnetic layer is free to rotate in the presence of an external magnetic field. U.S. Pat. No. 5,206,590, filed Dec. 11, 1990, and assigned to IBM, discloses a basic spin valve MR sensor. U.S. Pat. No. 5,159,513, filed Feb. 8, 1991, and also assigned to IBM, discloses a spin valve MR sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each other at zero externally applied magnetic field by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer.
The spin valve MR sensor that has the most linear response and the widest dynamic range is one in which the magnetization of the pinned ferromagnetic layer is parallel to the signal field and the magnetization of the free ferromagnetic layer is perpendicular to the signal field. In the case where the spin valve MR sensor is to be used in a horizontal magnetic recording disk drive, this means that the plane of the sensor is perpendicular to the disk surface with the magnetization of the pinned layer oriented perpendicular to and the magnetization of the free layer oriented parallel to the disk surface. One difficulty in achieving this magnetization orientation is caused by the dipole field generated by the pinned layer. The pinned layer has a net magnetic moment and thus essentially acts as a macroscopic dipole magnet whose field acts on the free layer. In spin valve MR sensors, where the read element is relatively short, the result of this magnetostatic coupling is that the magnetization direction in the free layer is not uniform. This causes portions of the sensor to saturate prematurely in the presence of the signal field, which limits the sensor's dynamic range and thus the recording density and overall performance of the magnetic recording system.
What is needed is a spin valve MR sensor with an improved pinned ferromagnetic layer that has minimal magnetostatic coupling with the free ferromagnetic layer.