Antiferromagnetism of γ-Phase Manganese Alloys Containing Ni, Zn, Ga and Ge

Manganese-rich γ-phase alloys of Mn-Ni, Mn-Zn, Mn-Ga and Mn-Ge have been investigated with X-ray and neutron diffraction and specific heat measurements. The Mn-Zn, Mn-Ga and Mn-Ge alloys exhibit antiferromagnetism below the transition temperature, at which the cubic lattice is transformed to the tetragonal lattice (the axial ratio c / a <1). The Mn-Ni alloys containing less than 15 at. % Ni become tetragonal ( c / a <1) at the Néel temperature, while those containing more than 15 at. % Ni are cubic even below the Néel temperature and become tetragonal ( c / a >1) at a lower temperature. The magnetic structure for the axial ratio c / a >1 is different from that for the axial ratio c / a <1. The integrated values of the anomalous specific heat are less than the values predicted by the localized moment model of antiferromagnetism.