The ferromagnetic-antiferromagnetic properties of copper-manganese and silver-manganese alloys

Abstract

Magnetic hysteresis loops displaced from their symmetrical positions about the origin are observed when the alloys of about 5–30 per cent Mn in Cu and 10–25 per cent Mn in Ag are cooled to 1.8°K in a magnetic field. The two remanent magnetizations from each loop are equal both in magnitude and direction and represent a ferromagnetic alignment of a significant fraction of the Mn atomic moments. For each alloy, the hysteresis loop displacement decreases with increasing temperature and its disappearance is attended by large hysteresis losses. At a somewhat higher temperature, the losses vanish and the magnetic susceptibility reaches a maximum. However, the only anomaly in the electrical resistivity vs. temperature curve, indicative of a magnetic transition, occurs at a still higher temperature. All these properties and their dependence on alloy composition are shown to be qualitatively consistent with a microscopic exchange anisotropy model. According to this model, the coexistence of antiferromagnetic and ferromagnetic interactions between Mn atoms of different separation, together with the statistical fluctuations in the local Mn concentration, gives rise to unusual magnetic states in these atomically-disordered alloys.