On the temperature-dependent coercivities of anisotropic Nd-Fe-B magnet

We have studied the thermal stability of coercivity in anisotropic Nd-Fe-B magnet from 300 K to 500 K by combining the micromagnetic simulation with the numerical estimation of coercivity reduction caused by thermal activation. The numerically achieved coercivities agree well with the measured value of a conventional sintered magnet. The concave shape of H_c(T) which is different from linear shape of H_A(T) in anisotropic Nd-Fe-B magnets, as well as the break of linearity in the fitting of H_c/M_s vs. H_A/M_s are found both originated from the temperature dependence of magnetization of the ferromagnetic grain boundary phase. This curving of H_c(T) can be eliminated via transformation of grain boundary magnetism into non-ferromagnetic. We have demonstrated this hypothesis experimentally in the hot-deformed magnet heavily infiltrated with Nd–Cu alloy. The parameters, α and N_eff, achieved in the fitting of Kronmüller equation are found to be largely deviated from those achieved in micromagnetic method, which is due to the ignorance of temperature dependent grain boundary magnetization and the thermal activation effect in the fitting.