High pressure effects on isotropic Nd2Fe14B magnet accompanying change in coercive field

Abstract

We investigated the effects of hydrostatic pressure on an isotropic Nd2Fe14B magnet (the exact chemical formula is Nd2.0Fe14.1B) consisting of nanocrystals, with the size of approximately 30 nm, by magnetization measurements at pressures (P's) up to 9.3 GPa and structural analyses up to 4.3 GPa. Magnetization curves were measured by using a miniature diamond anvil cell made of Ti alloy with spatially uniform magnetization. The initial value of coercive field Hc at 300 K is 840 kA/m (=10.6 kOe), and Hc initially increases to approximately 1180 kA/m (=15.0 kOe) almost linearly against the pressure. The increase in Hc, however, saturates at around P = 3 GPa. The change in Hc is understood by the decrease in the saturation magnetization Ms within the framework of the constant anisotropy of the single domain phase. The crystalline strain increases for P < 1 GPa. Afterward, the crystalline size (D) starts to decrease with increasing pressure, and the reduction tends to saturate at above approximately 3 GPa. Furthermore, the change in Ms is actually related with both the change in strain and that in D. The data on the temperature dependence of Hc at P = 0, 6.6, and 9.3 GPa exhibit pressure-induced suppression of the Curie temperature. The maximum energy product decreases with increasing pressure over the whole temperature range.