Unidirectional Properties in the Iron-Iron Sulfide System

The unidirectional properties and the nonvanishing rotational hysteresis for applied fields >2K/Is are ascribed to exchange anisotropy between ferromagnetic iron and antiferromagnetic ironsulfide. Torque curves, measured on samples cooled in a magnetic field, depended upon the percentage of iron, temperature, and applied field. Maximum torques at −196°C for 6000 oe increased from 2×106 to 1.5×107 d-cm/cm3 of ferromagnetic Fe for a decrease in iron percentage of 4.5–0.87%. This is attributed to a decrease in particle size with decreasing iron percentage, thereby enhancing the unidirectional properties. The decrease of the sinθ dependence of torque at high fields is explained by a reorientation of the moments of the antiferromagnetic iron sulfide. The fact that a sinθ term is present after a reorientation of the moments in the antiferromagnetic material indicates that the coupling between the moments of the iron and the iron sulfide across the interface remains, but has been realigned. Iron sulfide has a Néel temperature of 340°C and a reordering temperature near 140°C where the spins change their ordering direction by 90°. The significance of the reordering temperature is evident from the rapid decrease in rotational hysteresis as the reordering temperature is approached and also from the absence of an increase in maximum torqueupon cooling in a field from temperatures greater than 150°C.