Measurement of magnetostriction in self-biased shielded magnetoresistive heads

We report a method of measuring the magnetostriction of self-biased shielded magnetoresistive (MR) elements. In this technique the optimum bias current for the MR element, defined as the point where the head delivers the maximum output per unit current, is determined by measuring the head output from a flux source while varying the bias current fed to the element. A mechanical fixture is then used to bend the head and strain the MR film. Because of magnetoelastic effects, the change in stress alters the MR element anisotropy and causes a shift of the optimum bias point. The optimum bias point is measured as a function of the stress applied to the MR element and the dependence modeled using a nonlinear transmission-line model for a shielded MR head with shunt and/or offset bias. Parameters for the model are set by fitting the full output voltage versus bias current curve. The changes in the value of the MR element anisotropy required to fit the experimental data are attributed to changes in the magnetoelastic anisotropy and the magnetostriction calculated. This measurement technique does require the removal of one of the shields in order to guarantee a uniform and reproducible strain of the MR film when the head is bent