Chapter 4 - The Anisotropic Magneto-Resistive Effect

Abstract

The electrical resistance of an iron bar changes as its magnetization is altered. This phenomenon is called the anisotropic magnetoresistive (AMR) effect, and it remained a mere academic curiosity until Bob Hunt invented the anisotropic magneto-resistive head (AMRH). In a thin film of magnetic material, the magnetization can be single domain and not be subdivided by domain walls into the multi domain state. In the thin-film single-domain state, the AMR effect can be described very simply. The fundamental idea in all AMRHs is that the magnetic field produced by the written or recorded magnetization pattern in the tape or disk rotates the magneto-resistive element (MRE) magnetization angle. In all magneto-resistive heads (MRHs), Ohm's law gives the output signal voltage. When the magnetization angle in the MRE increases, magnetic poles are generated in the top and bottom regions of the sensor. When no vertical bias is used, the sensitivity, or slope, of the resistance change versus field characteristics is zero. It is important to realize that even when the optimum vertical bias is used, the output of an MRE is inherently nonlinear even for small tape or disk fields. To keep the single-domain magnetization in the MRE stable and free from hysteresis, it is usually necessary to provide a horizontal bias field in addition to the vertical bias field. The magneto-resistive coefficient is the maximum fractional change in the resistivity. For many purposes, this coefficient serves as a figure of merit of the MR material. It is remarkable that not only first AMRHs were made of permalloy, but also that permalloy still remains the material of choice in both conventional AMR heads.