Phase transformation in nanocrystalline Sm₂Co₁₇ permanent magnet material

Single-phase ultrafine nanocrystalline Sm₂Co₁₇, a candidate material for permanent magnets, was prepared by a novel simple route based on a home-built `oxygen-free' in situ fabrication system. The as-prepared nanocrystalline Sm₂Co₁₇ has the stable hexagonal Th₂Ni₁₇-type (2:17 H) crystal structure at room temperature, which is distinctly different from the conventional polycrystalline Sm₂Co₁₇ having the rhombohedral Th₂Zn₁₇-type (2:17 R) crystal structure at room temperature. Phase transformations in the nanocrystalline Sm₂Co₁₇ alloy were investigated systematically by a series of annealing experiments. It was found that, along with the nanograin growth in the heat-treatment process, the crystal structure of the room-temperature phase transforms from 2:17 H to 2:17 R. The magnetization behavior of Sm₂Co₁₇ alloys in different structural states was characterized. The relationship between the magnetic performance and the structure characteristics was analyzed. Understanding the phase transformation of Sm₂Co₁₇ nanocrystals facilitates the development of nanocrystalline Sm₂Co₁₇-type magnets for high-temperature applications.