At room temperature, a kink in negative fields appears in the hysteresis loops of Nd-Fe-B permanent magnets. This phenomenon can be related to the fact that the grains at the surface of the magnet exhibit a strongly reduced coercivity compared to those of the bulk. Below 130 K a kink of another nature develops. It is strictly reversible and amounts to about 30% of the spontaneous magnetization. It is caused by a combined effect of the spin reorientation occurring in the Nd2Fe14B phase and the incomplete alignment of the particles in the magnets.
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Givord et al. (1986) studied this question.