Transmission electron microscopy of single-phase Co₅Sm permanent magnets

Abstract

The microstructure and magnetic domain structure of single-phase sintered Co₅Sm permanent magnets has been studied by means of transmission electron microscopy. The materials contain coherent inclusions of different sizes within single-crystal Co₅Sm grains. Inclusions with diameters less than 200 nm are characterized as Co₇Sm₂. The bordering α fringes correspond to a boundary fault vector of . Electron-diffraction analysis reveals that the large inclusions (> 300 nm) consist of a rhombohedral Co₁₇Sm₂ phase. Besides inclusions, grains with numerous fringe patterns perpendicular to the [0001] direction are also observed. These grains contain overlapping stacking faults with different crystallographic shear vectors of the type . The coercivity of the single-phase Co₅Sm magnets is limited by the nucleation of reverse domains at soft magnetic inclusions. It is found that precipitates act as weak pinning centres for domain walls, and grain boundaries as strong ones.