A comparison of magnetic anisotropy constants and anisotropy fields of permanent magnets determined by various measuring methods

Uni-axial anisotropy constants K-A and uni-axial anisotropy fields H-A of oriented polycrystalline magnets were obtained from the field-dependence of the magnetization by three methods and from torque measurements. The results from torque measurements are found to be superior to those from the intersections of magnetization curves for fields parallel and perpendicular to the preferred axis, the integral of the perpendicular magnetization and the maximum in the second derivative of the perpendicular magnetization, called the singular point detection (SPD) method. Sintered Sr ferrite magnets can be considered as quasi-single crystals because of their high degree oi orientation and their large crystal grain sizes in the range 20-400 mu m. A value of K-A = 3.60 x 10(5) J m(-3) was obtained by extrapolation to high field of the K-1 sin 2 theta term determined by Fourier analysis of unsaturated torque curves. This is closer to the result for single crystals than the other methods. The SPD method was used to analyse data recorded in high pulsed fields (12 MA m(-1)) to determine the large values of H-A for sintered Nd-Dy-Fe-B magnets with various compositions. There is an increase in H-A that is linear in the Dy content.