Magnetic microstructures of a high coercivity Nd–Fe–B sintered magnet in remanent and incomplete thermal demagnetization states

Magnetic microstructures of a high coercivity Nd–Fe–B sintered magnet in remanent and incomplete thermal demagnetization states have been revealed by using magnetic force microscopy (MFM) with high coercivity tips. MFM results indicate that specimens in a remanent state are single domain and their magnetizations align with the direction of the magnetizing field. The evolution of the magnetic domains with annealing temperatures shows that the thermal demagnetization process consists of four stages. Nd–Fe–B should be heat-treated at about 120–170 °C to make its magnetic state stable before practical applications.