Magnetic Properties of Manganese-Gold Alloys and the Effect of Thermal and Mechanical Treatment

Abstract

Anomalies have been found in the variation with temperature of magnetic susceptibility, resistivity, and elasticity for AuMn alloys containing from 40 to 53 at.% Au. These anomalies occur with changes in magnetic order associated with the double martensitic bcc→bct structure transformations previously reported for these alloys. Large values and age dependence of susceptibility previously observed are shown to be due to precipitation at moderate temperatures of metamagnetic Au2Mn from solid solution. On this basis, AuMn 50 at.% behaves as an ordered solid solution. Deformation produces a stress-induced transformation from an ordered bcc structure to a fcc γ-Mn structure. Alloys containing more than 80 at.% Mn are shown by similar measurements, together with neutron diffraction, to have properties resembling those of γ-MnCu alloys. They exhibit long range antiferromagnetic order and a diffuse peak at the (100) position of the neutron-diffraction pattern is also observed. A structure transformation from fcc to fct occurs at the Néel temperature, but for alloys in the composition region around 90 at.% Mn, there is a second transformation to an orthohombic structure with little change in magnetic order. A double transformation in γ-MnNi and γ-MnPd alloys is also reported.