Magnetocrystalline anisotropy in Nd-Fe-B magnet

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Abstract
A torque magnetometer used in a 100-kOe superconducting magnet was designed to measure magnetic anisotropy constants for hard magnetic materials with high magnetocrystalline anisotropy. This magnetometer consists of a pulley system and a motor driven shaft, on which strain gauges are pasted to detect the torque exerted on the sample. Using this torque magnetometer, the magnetic torque curves in the (110) and (100) planes for a Nd2Fe14B single crystal sphere were measured in magnetic fields up to 100 kOe and over the temperature range from 4.2 to 300 K. The temperature and field dependencies of the anisotropy constants K1, K2, and K3 in tetragonal symmetry were obtained. The apparent anisotropy constants strongly depend on temperatures as well as magnetic fields. The intensity of magnetic field is insufficient to rotate the internal magnetization to the field direction. The demagnetization curve in the second quadrant for Nd15Fe77B8 permanent magnet showed stepwise decrease. An attempt to interpret this behavior in terms of the instability of the magnetization direction estimated from the observed anisotropy energy was unsuccessful.

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