RARE EARTH-TRANSITION METAL PERMANENT MAGNETS

摘要

High ordering temperature and large uniaxial anisotropy are basic magnetic properties at the origin of coercivity in permanent magnets. An optimum combination of these may be obtained in rare earth-transition metal compounds. Among binary compounds, they characterize the SmCo5 phase, but none of the R-Fe phase. They were recently observed in the ternary Nd2Fe14B compound. In such systems, two main metallurgical processes may allow to develop coercivity. SmCo5 and Nd2Fe14B magnets are elaborated by sintering of powders. Coercivity is determined by nucleation of reverse domains at grain boundaries. Structural defects induce a severe reduction of coercivity, especially in SmCo5. In Sm(Co-Cu)5 and Sm(Co, Fe, Cu, Zr)7-8 magnets, pinning of domain walls occurs on structural defects, which are associated with the coexistence of 2 phases with different magnetic properties. In all systems, the decrease of coercivity as temperature increases is larger than that of anisotropy. This reveals that thermal activation effects, particularly enhanced in Sm(Co-Cu)5, where pinning of walls occurs on defects at a very small scale, must be taken into account.