Structure and magnetic properties of bulk nanocrystalline Nd-Fe-B permanent magnets prepared by hot pressing and hot deformation

Structure and magnetic properties were studied for bulk nanocrystalline Nd-Fe-B permanent magnets that were prepared at 650 °C for 3 min under 300 MPa using the SPS-3.20-MK-V sintering machine and the hot pressed magnets were then submitted to hot deformation with height reduction of 50%, 60%, 70%, 80%, and 85%. Effects of height reduction (HR) and deformation temperature on the structure and magnetic properties of the magnets were investigated. The crystal structure was evaluated by means of X-ray diffraction (XRD) and the microstructure was observed by transmission electron microscopy (TEM). The magnetic properties of the magnets were investigated by vibrating sample magnetometer (VSM). As the height reduction increased, the remanence (B_r) of the magnets increased first, peaks at 1.3 T with HR=60%, then decreased again, and the coercivity (H_ci) of the magnets decreased monotonically. On the other hand, as the deformation temperature increased, the B_r of the magnets increased first, peaks at 1.36 T with HR=60%, then decreased again, and the H_ci of the magnets decreased monotonically. Under optimal conditions, the hot deformed magnet possessed excellent magnetic properties as B_r=1.36 T, H_ci=1143 kA/m, and (BH)_max=370 kJ/m³, suggesting the good potential of the magnets in practical applications.