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

To obtain a more accurate hysteresis curve for large Nd-Fe-B sintered magnets with high coercivity (H
_cJ
), we employed a superconducting magnet-based vibrating sample magnetometer (SCM-VSM), which is independent of the drift. The demagnetization curves around H
_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/d
_Hex
), 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 (d
_J
/dH
_eff
) near H
_cJ
using different shapes (cylinders and sphere) and sizes of magnets, where H
_eff
is the effective field. The squareness ratio (H
_k
/H
_cJ
) and d
_J
/dH
_eff
near H
_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
_cJ
(≥2 MA/m), the abnormality of J on the demagnetization curve had direct effects upon d
_J
/dH
_eff
near H
_cJ
and H
_k
/H
_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 d
_J
/dH
_eff
near H
_cJ
and H
_k
/H
_cJ
were influenced by the correction of the eddy current in the PF method.