Accurate Measurement of Magnetic Properties of Nd-Fe-B Sintered Magnets With High Coercivity

摘要

To obtain the accurate magnetic properties of Nd–Fe–B sintered magnets with high coercivity $(H_{rm cJ})$, we have compared a pulsed-field magnetometer (PF) method with a hysteresis graph (HG) method using an extra large-sized electromagnet. To prevent the effects of machining on magnetic properties in small samples when making measurements of the accurate hysteresis and demagnetization curves, cylindrical samples used in the experiment were kept at a constant common diameter of 10 mm. This was necessary to correct the demagnetizing field and the effect of eddy current for the PF and the magnetization distortion for the HG methods. Reliable calibrations and corrections of the magnetization and the magnetic field were very important. Furthermore, it was essential that the longer length $(L)$ of the sample be magnetized uniformly for both the methods. To correct the demagnetizing field and obtain exact differential susceptibility near $H_{rm cJ},$ $L$ of the sample had to be more than 14 mm for the PF method. Because the saturation magnetization of the Nd–Fe–B sintered magnet was almost equal to that of the cast anisotropic AlNiCo magnet, the correction of magnetization distortion for the HG method using the AlNiCo magnet of $L = 21$ mm, which was the same size of the sample, was most suitable in the experiment compared with the other conditions.