A diffraction study of γ-Mn-Cu alloys

A study of some Mn-Cu alloys in the gamma -phase has been made using neutron and X-ray diffraction at low temperatures. For manganese-rich alloys the variation of magnetic moment with composition was measured and the critical concentration for antiferromagnetic ordering deduced to be x c =0.72. A measurement of the unit cell dimensions at 4.2K showed the antiferromagnetic order to be accompanied by the tetragonal cell distortion c/a<1, up to the critical concentration. The alternative cell distortions (c/a>1 and orthorhombic), observed with other solutes in manganese, were not detected in these measurements on gamma -Mn-Cu alloys. Using these results a magnetic phase diagram was drawn which could be compared with those diagrams obtained by calculation. The performance of the molecular field model of the martensitic transformation was also investigated for alloys close to the critical concentration. The model was found to be less successful here than when applied to alloys in which antiferromagnetism is well established. The diffuse magnetic neutron scattering from two alloys Mn 0.65 Cu 0.35 and Mn 0.60 Cu 0.40 was measured and analysed to give information on the spatial variation of magnetic short-range order for specimens close to the critical concentration. This short-range order (SRO) appears to be based on the type 3 antiferromagnetic structure, rather than on the type 1 antiferromagnetic structure, which exists as a long-range ordered state in the manganese-rich alloys. Finally an analysis of the diffuse nuclear neutron scattering from a Mn 0.65 Cu 0.35 specimen yielded values of atomic SRO parameters similar to those derived elsewhere, which suggested that the atomic SRO was only weakly developed in the specimens.