Estimation of Time-Dependent Polarization Losses in Sintered Nd-Fe-B Permanent Magnets

The thermal stability of sintered NdFeB magnets can be described by the temperature coefficient of remanent flux density α only on condition that demagnetization due to rising temperature or field and oxidation of the magnets are totally avoided. This also requires the control of time-dependent demagnetization at elevated temperatures. We have studied the time-dependent polarization losses in sintered NdFeB magnets with different coercivities by determining temperature T₀ for each magnet (Pc=1.1). T₀ is the maximum temperature at which the total polarization loss even after 30 years of exposure is estimated to be less than 2%. Temperature T₀ for four magnets produced from different materials are presented in this work. Flux losses at temperatures higher than T₀ after different exposure times are also described. The estimated flux loss after 30 years is found to be roughly twice the loss after 1 h, except in the vicinity of T₀. At temperature T₀+10°C , it could be as high as four times the loss after 1 h.