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

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 (K
₁
and K
₂
) 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 × l
_s
)] to the rigid pipe, where l
_s
is the length of the rigid pipe. In highly uniform field (H), the force F
_x
(= 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 K
₁
and K
₂
from 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 l
_s
. K
₁
and K
₂
for the sintered magnet of Nd
_13.6
FebalCo
_1.1
Al
_0.3
B
_5.7
are 4.6 and 0.37 MJ/m
³
at 298 K, respectively. K
₁
is close to the published value for the Nd
₂
Fe
₁₄
B single crystal. It is required to use highly oriented samples for the measurement. Both K
₁
and K
₂
decrease 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.