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

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)(2)Fe14B 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. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The magnetic properties and thermal stability of Tb-diffused Nd-Fe-B magnets were investigated, showing that diffusion temperature plays a key role in controlling thermal stability. Magnets fabricated at high diffusion temperatures exhibited good coercivity but poor thermal stability, while those prepared at lower temperatures showed the best thermal stability. The microstructure with diffused Tb atoms was found to be mainly responsible for the enhanced thermal magnetic stability in the magnets.