A ferromagnetic-antiferromagnetic model for copper-manganese and related alloys

Abstract

The microscopically inhomogeneous magnetic structure conceived for CuMn, AgMn and other related alloys at very low temperatures is simulated by a simple model in which the magnetic unit is a small ensemble of mutually-interacting ferromagnetic and antiferromagnetic domains. In the ground state of the hypothetical system each domain-ensemble has zero net magnetization, but when the system is cooled in a magnetic field some of its domain-ensembles are forced into a different state (with a non-zero magnetization) which is stabilized by the growth of a strong anisotropy ( k A ) in the antiferromagnetic domains. In this field-cooled state, the magnetic hysteresis loops of the system are displaced asymmetrically from the origin, and its torque curves even at very high fields indicate a single easy direction of magnetization. These and related properties and their variation with field applied during cooling and with temperature, as predicted by the model, are in excellent qualitative agreement with the field-cooled behavior of CuMn, AgMn and other alloys. According to this model, the temperature of maximum susceptibility for each of these alloys is to be interpreted not as a Néel temperature but as simply the point at which the anisotropy k A disappears; the alloy does not become paramagnetic until a significantly higher temperature ( T c ) is reached. It is predicted that at T c there will be a kink in the inverse susceptibility vs. temperature curve, and this has been observed experimentally in various Cu-Mn and Ag-Mn alloys. Also consistent with this prediction is the electrical resistivity anomaly (of a type suggestive of a magnetic transition) found at this higher temperature ( T c ) in each of these materials.