Magnetoresistive measurement of the exchange constant in permalloy

A current passed through a thin ferromagnetic stripe, with magnetization along the stripe (easy) axis, will generate an internal field that tries to fan the magnetization in opposite directions through the film thickness, but which is opposed by exchange forces. Methfessel et al. observed this effect experimentally in Permalloy using inductive methods and determined a value for A, the exchange constant, of 0.55 × 10^-6ergs/cm. This is about half the value normally measured using spin wave resonance. We have extended the theoretical analysis and repeated the experiment on small, 3000 Å thick Permalloy stripes using dc magnetoresistive detection instead of induction. By comparing theoretical and experimental results, we have determined a value for A of 1.1 × 10^-6erg/cm ± 10%. No assumptions about surface anisotropy or magnetization inhomogenieties, required to explain spin wave modes, are needed. We believe this is the most reliable value reported for A in Permalloy.