Precise Measurements of the Hysteresis Curves of High-Coercivity Nd-Fe-B Sintered Magnets

摘要

This paper reports the effects of the thermal fluctuation aftereffect and the eddy current on the measurement of hysteresis curves for high-coercivity Nd-Fe-B sintered magnets. Hysteresis curves were measured with a vibrating sample magnetometer using a superconducting magnet (SCM-VSM) and a pulsed field magnetometer (PFM) over a long period in the same maximum field of 4 MA/m. Spherical specimens with a diameter of 6.0 mm were used, and the demagnetization field was compensated for by using a demagnetizing factor N= 1/3. In this experiment, the anisotropy field value obtained from the intersection of the magnetization curves of an M-type Sr ferrite magnet sintered at 1240 deg C was used for common magnetic field normalization. In the case of the SCM-VSM method, where the waiting time varied from 2 to 20 s, the difference in the H_(cJ) value was small. However, the difference in H_k was 40-90 kA/m. In a comparison of the SCM-VSM and PFM methods, the values of H_(cJ) and H_k in the PFM method showed increases of 30 and 90-190 kA/m, respectively. The eddy current influence strongly affected the measured value of H_k in the case of the PFM method. Hysteresis curves swelled under the influence of the eddy current.