Magnetocrystalline anisotropy in FeNi invar alloys under high magnetic fields

Torque measurements were made for FeNi Invar alloys at 4.2 K in high magnetic fields up to 47 kOe by using a split pair type superconducting magnet. It was found that the magnetocrystalline anisotropy constant, K₁, determined from the observed torque curves increases monotonically with increasing the field. The increasing rate, $\text{d}\text{K}{}_1\text{d}\text{H}$, was extraordinally high and could not be explained by simply considering the Zener's power law. It was suggested that some other terms, such as the Fermi-level dependent one, play an important role in this quantity.