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

Abstract

To obtain a more accurate hysteresis curve for large Nd-Fe–B sintered magnets with high coercivity ( $H_{{\mathrm {cJ}}}$ ), we employed a superconducting magnet-based vibrating sample magnetometer (SCM-VSM), which is independent of the drift. The demagnetization curves around $H_{{\mathrm {cJ}}}$ 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 ( $H_{d}$ ) for the cylindrical samples using a magnetometric demagnetizing factor dependent on the aspect ratio ( $L/D$ ) and differential coefficient of the measured curve ( $dJ/dH_{{\mathrm {ex}}}$ ), where $L$ , $D$ , $J$ , and $H_{{\mathrm {ex}}}$ 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 ( $dJ/dH_{{\mathrm {eff}}}$ ) near $H_{{\mathrm {cJ}}}$ using different shapes (cylinders and sphere) and sizes of magnets, where $H_{{\mathrm {eff}}}$ is the effective field. The squareness ratio ( $H_{k}$ / $H_{{\mathrm {cJ}}}$ ) and $dJ/dH_{{\mathrm {eff}}}$ near $H_{{\mathrm {cJ}}}$ obtained from the SCM-VSM method were found to be superior to those obtained from the PF method, where $H_{k}$ is the knee field. For the sample with a higher $H_{{\mathrm {cJ}}}$ ( $\geq 2$ MA/m), the abnormality of $J$ on the demagnetization curve had direct effects upon $dJ/dH_{{\mathrm {eff}}}$ near $H_{{\mathrm {cJ}}}$ and $H_{k}$ / $H_{{\mathrm {cJ}}}$ 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 $dJ/dH_{{\mathrm {eff}}}$ near $H_{{\mathrm {cJ}}}$ and $H_{k}$ / $H_{{\mathrm {cJ}}}$ were influenced by the correction of the eddy current in the PF method.