COERCIVITY OF DIE UPSET ND-FE-B MAGNETS - A STRONG PINNING MODEL

We show that the temperature dependence of the intrinsic coercivity Hci(T) between 5 and 600 K in a die-upset NdFeB magnet is in good agreement with a model for strong domain wall pinning by a random array of pinning sites proposed by Gaunt [P. Gaunt, Phil. Mag. B48 (1983) 261]. The model includes both the temperature dependence of the intrinsic magnetic properties of the Nd2Fe14B phase and the effects of thermal activation of domain walls over the pinning barrier. The pinning sites are modeled as nonmagnetic planar inhomogeneities at the boundaries aetween platelet-shaped Nd2Fe14B grains. We develop an expression for the maximum pinning force per site, f, and derive the model prediction that (Hci/γHA) 1 2 varies linearly with (T/γ) 2 3, where HA and γ are the magnetocrystalline anisotropy and the domain wall energy per unit area of the Nd2Fe14B phase, respectively. The model is in good agreement with the observed Hci values over a broad temperature range from 200 to 477 K. Deviations from the model below 200 K are an artifact of the axial-to-conical spin reorientation in Nd2Fe14B at low temperature. Deviations at high temperature most likely occur because the strong pinning model is no longer valid close to the Curie temperature (585 K). © 1990.