Magnetic Recording Head Materials
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MRS BULLETIN/SEPTEMBER 1996
changes with the application of a magnetic field) using a simple two-point resistance measurement. The resolution of the head is determined by its size: delineated in the cross-track direction by the distance between the contacts on the sensor and along the track by the separation of the magnetic shielding. The contact separation is generally several micrometers, and typical shield-to-shield distances are 0.2-0.3 /xm. The areal density that can be achieved depends on the properties of the magnetic-recording medium (see the article by Doerner and White in this issue of MRS Bulletin), the distance between the medium and the read head, and the sensitivity of the recording head. Higher sensitivity allows smaller bits to be detected and loosens the restraint on flying height. The sensitivity is dependent on
the particular type of magnetoresistive material used. Read-Head Materials Anisotropic Magnetoresistive Materials Magnetoresistive heads in production today use permalloy (Ni8oFe2o) films as the magnetoresistor.1-2 The electrical response to a magnetic field is shown in Figure 2. The magnetoresistance (AR/R) is typically 2.5% in a field of 4-5 Oe at room temperature. In these materials, resistance varies as the cosine squared of the angle between the current and the magnetization direction. Since the sign of AR is dependent on the current direction (parallel or transverse to the magnetization), the effect is called anisotropic magnetoresistance. A practical limitation of this phenomenon is that the quadratic response about zero field requires the structure to be biased, such that the response is approximately linear in an applied field. This linearity is required for the high-speed electronics used in data transmission. Biasing is achieved by two primary methods: An adjacent magnetic film pinned in the transverse direction provides a field offset,1 or two differentially detected elements are used to mutually bias each other.2 Operation of the read head at high current densities3 is required for maximum sensitivity. The signal voltage produced across the read head depends not only on the magnitude of the magnetoresistance but also on the size of the sense current.
Figure 1. Geometry of a dual gap magnetoresistive (MR) recording head. The top shield of the read element also serves as the bottom pole of the write element.
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Magnetic Recording Head Materials
The maximum current that can be tolerated is determined by the power dissipation in the head, which in turn dictates the operating temperature of the head. Current densities of 2 X 1CT7 A/cm2 are commonly used, resulting in a temperature rise of a few tens of degrees above ambient. Smaller dimensions permit higher current densities3 but introduce concerns over electromigration issues. Engineering a read head requires considering not only the electrical and thermal characteristics of the materials but their mechanical and crystalline properties as well. For example the magnetostriction of the sensor material must be minimized since
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