Magnetoelastic anisotropy in NdFeB permanent magnets

Abstract

The influence of magnetoelastic effects on the anisotropy in NdFeB permanent magnets has been investigated. A relation between the magnetostriction coefficients and the magnetoelastic anisotropy is derived for crystals with tetragonal symmetry. Transmission electron microscopy experiments show that stress and strain are present at the boundaries of the Nd2Fe14B grains. The origin of the stress is discussed and an order-of-magnitude calculation of its effect on the anisotropy is provided for thermally induced stress. The calculations are in good agreement with the measured influence of stress on coercivity. Both experiment and calculations show that homogeneous stresses have only a small influence on the coercivity.