1. Technical Field
The present invention relates, in general, to sensors for reading magnetic flux transitions from magnetic media such as magnetic disks and magnetic tapes and, in particular, to methods and systems for resetting a GMR sensor head in particular. Still more particularly, the present invention relates to a method and system for controlling the application of a reset pulse to a GMR sensor such that existing data is not damaged or altered.
2. Description of the Related Art
Magnetic field sensors are in widespread commercial use in applications such as linear and rotary encoders, proximity detectors, speed and position sensors and magnetometers for sensing the earth's magnetic field. One common magnetic field sensor is based upon the Hall effect and is utilized to sense magnetic fields in the range of 100-10,000 Oe. Another common field transducer is the inductive coil which, although inexpensive, is relatively bulky and has poor low-frequency response. More recently, field sensors based upon anisotropic magnetoresistive (AMR) effect have been introduced. These materials show a change in electrical resistance as a function of the external magnetic field. Wheatstone bridge circuits made of AMR materials are used as magnetic field sensors to sense below approximately 50 Oe over a frequency range from DC to at least one megahertz.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures. The distinguishing feature of GMR is that there are at least two ferromagnetic layers separated by a thin, non-ferromagnetic metal layer. Application of an external magnetic field causes a variation in the relative orientation of magnetizations of the neighboring ferromagnetic layers. This, in turn, causes a change in the spin-dependent scattering of conduction electrons and, thus, in the electrical resistance of the structure.
A particularly useful GMR structure is a sandwich of two essentially uncoupled ferromagnetic layers. The magnetization of one of the ferromagnetic layers is fixed or "pinned" in a particular direction by exchange coupling with an adjacent antiferromagnetic layer, while the magnetization of the other ferromagnetic layer is free to rotate with an externally applied field. This multilayer sandwich is known as a "spin valve" and exhibits a change in resistance that is proportional to the cosine of the angle between the free and pinned layers. This relationship can be exploited to create devices with an inherently linear relationship between the applied magnetic field and output voltage, which are excellent candidates for devices such as magnetic recording heads and bridge sensors.
A GMR head designed for use in a magnetic recording device typically comprises two weakly coupled ferromagnetic layers separated by a non-magnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the ferromagnetic layer to be pinned on an antiferromagnetic layer, such as an iron-manganese layer, to create an interfacial exchange coupling between the two layers. The spin structure of the antiferromagnetic layer can be aligned 10 along a desired direction (in the plane of the layer) by heating beyond the blocking temperature of the antiferromagnetic layer and cooling in the presence of a magnetic field with a predetermined direction.
The blocking temperature is the temperature at which exchange anisotropy vanishes because the local anisotropy of the antiferromagnetic layer, which decreases with temperature, has become too small to anchor the antiferromagnetic spins to the crystallographic lattice. The unpinned or free ferromagnetic layer also may have the magnetization of its extensions (those portions of the free layer on either side of the central active sensing region) fixed, but in a direction perpendicular to the magnetization of the pinned layer so that only the magnetization of the central region of the free layer can rotate in the presence of an external field. The magnetization in the free-layer extensions may be fixed by longitudinal hard biasing or exchange coupling to an antiferromagnetic layer. However, if exchange coupling is used, the antiferromagnetic material is different from the antiferromagnetic material used to pin the pinned layer, and is typically nickel-manganese. This resulting structure is called a spin valve MR sensor and is referred to throughout this application as a "GMR" sensor.
In a GMR sensor, only the free ferromagnetic layer is able to rotate in the presence of an external magnetic field. U.S. Pat. No. 5,159,513 assigned to International Business Machines Corporation, the Assignee of the present application, discloses a GMR sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetization of the two ferromagnetic layers are maintained substantially perpendicular to each when an externally applied magnetic field is not present by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer.
U.S. Pat. No. 5,206,590, also assigned to International Business Machines Corporation, discloses a basic GMR sensor wherein the free layer is a continuous film having a central active region and end regions. The end regions of the free layer are exchange biased by exchange coupling to one type of antiferromagnetic material, and the pinned layer is pinned by exchange coupling to a different type of antiferromagnetic material.
GMR sensors are becoming a replacement for conventional MR sensors based upon the AMR effect. Such sensors have special potential for use as an external magnetic field sensor, such as in anti-lock braking systems and as read heads in magnetic recording systems, such as in rigid-disk drives. However, the GMR sensor, which is typically fabricated by depositing an antiferromagnetic layer of Fe--Mn onto the ferromagnetic pinned layer of cobalt or permalloy, suffers from the problem that the range of blocking temperatures for this interface is relatively low, i.e., it extends only from approximately 130.degree. C. to approximately 160.degree. C. These temperatures may be reached by certain thermal effects during operation of a disk drive, such as an increase in the ambient temperature inside the drive; heating of the GMR sensor due to the bias current; and rapid heating of the GMR sensor due to the head carrier contacting asperities on the disk. In addition, during assembly of the disk drive, the GMR sensor can be heated by an electrical current resulting from an electrostatic discharge, often referred to as electrical overstress.
If any of these thermal effects cause the GMR sensor to exceed the antiferromagnetic's blocking temperature, the magnetization of the pinned layer will no longer be pinned in the desired direction. This will lead to a change in the GMR sensor's response to an externally applied magnetic field and, thus, to errors in data read back from a magnetic disk.
The aforementioned cross-referenced co-pending patent application discloses a method and system for re-initializing a stack of GMR heads so that the GMR heads all have the same orientation, thereby precluding the inversion of half of the GMR heads facing oppositely from the rest. While this technique is highly desirable for providing re-initialization of GMR heads at different points during the manufacturing process, a need exists for a method and system which can re-establish a predetermined magnetic orientation of a GMR sensor in a magnetic disk drive storage system while protecting data stored therein during operation of that disk drive system.