Time Dependence of Demagnetization and Flux Losses Occurring in Sintered Nd-Fe-B Permanent Magnets

It is a common impression that the measured B(H) curves of permanent-magnet materials indicate the temperature and the operating point at which flux losses start to occur. As long as the operating point stays above the knee of the B(H) curve, no losses will be expected. The demagnetization process is, however, a time-dependent process and losses occurring immediately are followed by a thermal aftereffect. Here, we report our study of the time dependence of flux losses in commercial permanent Nd-Fe-B magnets. We present flux losses as a function of temperature for different exposure times. We then compare the measured B(H) curves of the Nd-Fe-B material to the results of loss measurements. We found that B(H) curves will not always indicate the overall losses well. The measured B(H) curve represents the demagnetization state of the tested magnet more or less after only 10 ms of exposure to the elevated temperature. When the operating point of the magnet is approaching the B(H) curve knee, the time-dependent losses will become significant. To control the time-dependent losses in sintered Nd-Fe-B magnets, the operating point needs to be sufficiently far away from the knee of the B(H) curve.