The relationship of thermal expansion to magnetocrystalline anisotropy, spontaneous magnetization, and T-c for permanent magnets

The materials studied here include SmCo5, GdCo5, Sm2TM17, Gd2TM17, Nd9.4Pr4 .6Fe80B6, Alnico, SrFe12O19, and PtCo (TM=transition metal). For the first time, the thermal expansion data alpha(perpendicular to) and alpha(parallel to) for the eight materials from as low as -180 degrees C to as high as 12 50 degrees C have been reported. alpha(perpendicular to) is the thermal exp ansion coefficient in the base plane and alpha(parallel to) is the coeffici ent in the c axis. All anisotropic materials tested show thermal expansion anisotropy (TEA). The degree of TEA is defined as gamma=alpha(perpendicular to)/alpha(parallel to). The gamma has been observed to be related to the d egree of magnetocrystalline anisotropy field H-A. SmCo5 has the highest H-A (350 kOe) and the highest g (2.5). All the materials show thermal anomaly with a changing slope for alpha(parallel to) near T-c and near phase transf ormation temperatures. The magnitude of thermal anomaly has been observed t o be related to the magnitude of spontaneous magnetization (SMZ). The most distinct thermal anomalies are observed for Nd9.4Pr4.6Fe80B6, in which both alpha(perpendicular to) and alpha(parallel to) show a deep valley near T-c , and a peak at around the spin-tilt temperature. Nd-Fe-B type material has the highest SMZ and the largest thermal anomaly of all the permanent magne ts. Spontaneous magnetostriction occurs mainly in the c axis for RE-Co allo ys (RE=rare earth), and in all directions for Nd-Fe-B type alloys. A rubber band model is used to describe the spontaneous magnetostriction force. (C) 1999 American Institute of Physics. [S0021-8979(99)44408-1].