Ultrasonic Wave Velocity in Ferromagnets with Δ E Effect

In some polycrystalline ferromagnets, the stress-strain relation is nonlinear in a small stress regime. Since the apparent Young's modulus is reduced compared with the non-magnetoelastic case in this regime, the phenomenon is called the Δ E effect. The Δ E effect is most striking in the demagnetized state of the materials, so that it could not be understood by the earlier theory of magnetoelastic interactions based on micro-magnetics theory. The authors have recently proposed a new model which accounts for the Δ E effect in demagnetized states introducing the relative value of the magnetic domain volume as the fundamental variable. This paper analyzes the dependence of ultrasonic wave velocities on the applied stresses. Comparisons are made between predicted velocities by the model and the results reported by earlier experiments. The ultrasonic wave velocities change nonlinearly with applied stress in the small stress regime, and saturate to the usual linear dependence on stress in large stress regimes.