Easy Determination of Large Anisotropy Constants for Nd–Fe–B Sintered Magnets Using Strain Gauges in Unidirectional High Fields

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We have developed a new torque measurement that uses the strain gauges to sense the force (F) acting on the sample and measured the quadratic (first) and quartic (second) anisotropy constants (K1and K2) of Nd-Fe-B sintered magnet in the temperature range of 298-473 K. The sample is mounted on the free end of the rigid pipe in the superconducting magnet. It applies the mechanical torque [L(= F × ls)] to the rigid pipe, where lsis the length of the rigid pipe. In highly uniform field (H), the force Fx(= M sinφ × ∂ H/∂x) based on the magnetic moment (M) does not act on a sample because a gradient of H (∂ H/∂x) can be disregarded, where φ is the angle of inclination. Thus, L is caused exclusively by the magnetic torque L(θ), where θ is the angle between the easy axis and the direction of saturation magnetization. To obtain K1and K2from the results of F, we make φ of π/4 and π/9 to the perpendicular unidirectional H (4.8-7.2 MA/m). The torque L(φ) obtained from F is equivalent to the values of the L(θ) curve at θ of π/4 and π/9. L was calibrated using the product of mass of the weight and ls. K1and K2for the sintered magnet of Nd13.6FebalCo1.1Al0.3B5.7are 4.6 and 0.37 MJ/m3at 298 K, respectively. K1is close to the published value for the Nd2Fe14B single crystal. It is required to use highly oriented samples for the measurement. Both K1and K2decrease as the temperature rises. Despite the very simple mechanism, this method can measure the large anisotropy constants of highly oriented Nd-Fe-B sintered magnets.