Effects of Particle Size on the Spin Reorientation Transition in R sub 2 Fe sub 14 B (R = Neodyium, Erbium) Hard Magnets

Spin reorientation transitions (SRTs) were observed via ac-susceptibility measurements in powdered and as-cast R sub 2 Fe sub 14 B (R = Nd, Er) alloys. The temperature dependence of the susceptibility was found to be largely structure sensitive: powdered samples showed a moderate increase before the transition and a sharp decrease following the transition, while a sharp cusp in the susceptibility preceded the transition in bulk samples. At the same time, the transition temperature and the relative intensity of the effects did not show any frequency dependence in the 0.1-10 kHz range. The results were compared with existing theoretical models assuming different magnetization mechanics such as rotation of the magnetization vector, domain wall bowing, and domain wall displacement. Domain wall contributions to the overall susceptibility cannot be neglected in either sample. At the same time, the rotational susceptibility is more significantly influenced by the sign reversal of the anisotropy constant, and this is the main contribution to the susceptibility maximum at the SRT. In powders, the expected sharp increase of the rotational susceptibility is counterbalanced by magnetostatic surface effects (" mu * effect"). In bulk materials, the influence of surface effects is less significant and the cusp can be observed.