The effect of thermal ageing up to annealing temperatures of 660 °C on the coercive force jHc at room temperature and on the susceptibility χ as function of temperature are investigated for a sintered NdFeB magnet. The coercive force is drastically reduced after ageing in the temperature range from 300 to 500 °C. The initial value of jHc may be recovered by subsequent thermal ageing above 600 °C. The value of jHc depends critically on the rate of cooling from the annealing temperature. The observed behaviour of jHc may be caused by structural changes of the intergrain boundary region and the surface region of the Nd2Fe14B grains.
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Eckert et al. (1987) studied this question.
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