Exchange anisotropy in disordered nickel-manganese alloys

Abstract

Disordered polycrystalline nickel-manganese alloy specimens of about 20, 25 and 30 atomic per cent manganese were cooled from 300 to 1.8°K in a magnetic field, and their magnetic hysteresis loops measured parallel to this field are found to be displaced from their symmetrical positions about the origin. Upon warm-up, the displacement of the loops decreases monotonically, vanishing at about 25, 35 and 75°K for the 20, 25 and 30 atomic per cent manganese specimens, respectively. The disappearance of this asymmetry with increasing temperature is accompanied by large hysteresis losses. Torque measurements were made on the same specimens cooled to 4.2°K in a field perpendicular to the axis of rotation. For all compositions, the predominant anisotropy is unidirectional, with the easy direction the same as the direction of the field applied during cooling. The magnitude of this anisotropy decreases monotonically with increasing temperature, vanishing at the same temperatures as the asymmetry of the hysteresis loops. At these temperatures, the rotational hysteresis rises to a maximum. These effects are believed to be due to exchange-anisotropy interactions between very small regions of ferromagnetic and antiferromagnetic spin order in these alloys at low temperatures.