Fe precipitation at the grain boundaries in Zn-reacted Nd–Fe–B magnets: Experimental and first-principles studies

Traces of non-magnetic atoms at the grain boundaries can effectively increase the coercivity of sintered Nd–Fe–B magnets. These atoms also have chance to occupy crystallographic Fe sites in NdFeB, which may force the unbound Fe to precipitate at the grain boundaries and thus destroy the nonmagnetic properties of the grain boundaries. In order to provide the thermodynamic evidences of Fe migration to the grain boundaries, one Zn diffusion reaction in NdFeB is investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy experiments, which confirm that 90 atomic percent of Fe is expelled to the grain boundaries. Magnetization measurements show that when Fe atoms precipitate at the grain boundaries, magnet's coercivity rapidly declines. The first-principle calculations using linearized augmented plane wave method show that Zn atoms in 16k1, 8j2, and 4e sites have positive substitution energies and unfavorable for the Zn substitution at these sites, while the substitution energies are negative values and favorable for the Zn substitution at the sites of 8j1, 4c, and 16k2. Our experimental and theoretical calculations indicate that the doped atoms have significant effect on the formation of the Fe-enriched layer at the grain boundary.