Enhancement of thermal stability of Nd–Fe–B sintered magnets with tuned Tb-diffused microstructures via temperature control

Abstract

We investigate the magnetic properties and thermal stability of Tb-diffused Nd–Fe–B magnets prepared at various grain-boundary diffusion temperatures and additional heat-treatment temperatures. These heat-treatment processes improved the coercivity of Tb-diffused Nd–Fe–B magnets than that of the base magnets. The diffusion temperature was found to play a key role in controlling the magnetic thermal stability; temperature variations induced precise changes in the decoupled Tb-diffused microstructures in the magnets. The magnet fabricated at a high diffusion temperature showed the best coercivity at room temperature but poor thermal stability. This was due to the formation of Tb-rich (Tb, Nd)2Fe14B phases with high magnetocrystalline anisotropy produced inside the grains and less core–shell structures during diffusion at a higher temperature. The best thermal stability was observed for the magnet prepared at a lower diffusion temperature. This magnet had more well-formed core–shell structures than the remaining magnets. By analyzing its microstructure, using electron microscopy and a micromagnetic equation, it was found that a decoupled microstructure with diffused Tb atoms was mainly responsible for the better thermal magnetic stability. Results obtained herein suggest that an optimized diffusion temperature can provide a magnet with good thermal stability.