Formulation of energy loss due to magnetostriction to design ultraefficient soft magnets

The mechanism of energy loss due to magnetostriction in soft magnetic materials was analytically formulated, and our experiments validated this formulation. The viscosity of magnetic materials causes the resistive force acting on magnetic domain walls through strain due to magnetostriction, and magnetic energy is eventually dissipated by friction even without eddy currents. This energy loss mechanism explains the frequency dependence of the excess loss observed in the experiments, and the excess loss is dominated by the contribution of magnetostriction when the magnetostriction constant exceeds approximately 20 ppm. The random anisotropy model was extended by considering the effect of local magnetostriction as a correction to the magnetocrystalline anisotropy. The effect of magnetostriction was considerably suppressed by the exchange-averaging effect. The estimated effective random magnetoelastic anisotropy for nanocrystalline α-Fe reached as low as 18.6 J/m3, but this static effect could not explain the high excess loss at high frequencies observed in the experiments. The results of this research could provide new design criteria for high-performance soft magnetic materials based on low magnetostriction to reduce the excess loss. The energy loss mechanism due to magnetostriction was clarified by analytical formulation considering the viscosity of magnetic materials. Effects of magnetostriction have been focused on contributions to magnetic anisotropy. However, our formulation shows that the magnetic anisotropy due to magnetostriction cannot explain excess losses in nanocrystalline soft magnetic materials, and the viscosity causes resistance forces acting on domain wall motions. This viscous resistance dissipates the magnetic energy and generates the energy loss, which has the same frequency dependence as anomalous eddy current loss. The results of this research provide new design criteria for ultra-efficient soft magnetic materials.