Relationship between the thermal stability of coercivity and the aspect ratio of grains in Nd-Fe-B magnets: Experimental and numerical approaches

The influence of the aspect ratio of grains on the thermal stability of coercivity of Nd-Fe-B hot-deformed magnets was systematically investigated by experimental and numerical approaches. With increasing amount of Nb doping from 0.2 at.% to 0.6 at.% in the hot-deformed magnets, the aspect ratio of grains, defined as the ratio of the width to the height of a Nd₂Fe₁₄B grain (D_c/D_ab), decreased while the average grain size was preserved. No change of temperature coefficient of coercivity (β) was observed in the as hot-deformed samples with different grain aspect ratios. However, the reduction of aspect ratio (increase of the height and reduction of the width of platelet shaped Nd₂Fe₁₄B grains) improves β for the sample infiltrated with a Nd-Cu eutectic alloy; i.e., from β = −0.42 %/°C to −0.40 %/°C for the aspect ratio reduction from 4.7 to 3.1. Micromagnetic simulations indicated that the grain aspect ratio is not a dominant factor to the thermal stability of coercivity for exchange-coupled anisotropic polycrystals. While smaller aspect ratio reduces demagnetizing field caused by magnetically isolated grains, resulting in the improvement of the thermal stability of the coercivity for Nd-Cu infiltrated hot-deformed magnets.