Study on thermal demagnetizing mechanisms of high-energy product NdFeB permanent magnet

The demagnetization mechanism of high energy product NdFeB permanent magnet was analyzed.Firstly,changes of open-circuit magnet flux with temperature were studied.And then high-temperature metallurgical phase as well as surface composition changes were observed and analyzed using multi-functional metallographic microscope,thermal analysis instrument and X-ray diffraction,respectively.Finally we studied the phase magnetic domain structure of sintered NdFeB permanent magnet after magnetization and high-temperature treatment using magnetic force microscopy(MFM).The experimental results showed that in addition to changes in color of microstructure morphology,no obvious change occurs in grain shape and size below 320℃.XRD analysis showed that the color change is due to surface oxidation,which indicates that thermal demagnetization below 320℃ has nothing to do with the metallurgical structure.XRD and metallographic analysis also showed that surface oxidation becomes serious above 320℃,especially at 580℃ melting Nd-rich phase precipitates,the grain boundary became fuzzy and large grain annexes small one.However,magnetic domain observation showed that,magnetization direction changes after thermal demagnetization below 320℃,which is the fundamental reason for thermal demagnetization.