More Accurate Hysteresis Curve for Large Nd–Fe–B Sintered Magnets Employing a Superconducting Magnet-Based Vibrating Sample Magnetometer

摘要

To obtain a more accurate hysteresis curve for large Nd-Fe-B sintered magnets with high coercivity (HcJ), we employed a superconducting magnet-based vibrating sample magnetometer (SCM-VSM), which is independent of the drift. The demagnetization curves around HcJ resulting from the SCM-VSM, the hysteresis graph (HG), and the pulsed-field (PF) magnetometer were compared. Attention was paid to the accurate correction of the demagnetizing field (Hd) for the cylindrical samples using a magnetometric demagnetizing factor dependent on the aspect ratio (L/D) and differential coefficient of the measured curve (dJ/dHex), where L, D, J, and Hex are the length and diameter of sample, magnetization, and applied field, respectively. The cylindrical samples used were kept at a D of 10 mm and an L of 7 and 14 mm. The hysteresis curves and magnetic properties obtained from the SCM-VSM method agreed very well with one another, except for the differential susceptibility (dJ/dHeff) near HcJ using different shapes (cylinders and sphere) and sizes of magnets, where Heff is the effective field. The squareness ratio (Hk/HcJ) and dJ/dHeff near HcJ obtained from the SCM-VSM method were found to be superior to those obtained from the PF method, where Hk is the knee field. For the sample with a higher HcJ (≥2 MA/m), the abnormality of J on the demagnetization curve had direct effects upon dJ/dHeff near HcJ and Hk/HcJ in the HG method. However, a larger L/D has an effect on improving these values as has been previously found in this paper. For large magnets, the values of dJ/dHeff near HcJ and Hk/HcJ were influenced by the correction of the eddy current in the PF method.