Crystal distortion and magnetic structure of ?-MnPt alloys

We have made X-ray and neutron-diffraction experiments and magnetic susceptibility measurements for gamma-MnPt alloys. The alloy containing 8 at. % Pt shows a face-centered orthorhombic structure with a = 3.807, b = 3.748 and c = 3.685 Angstrom and a non-collinear antiferromagnetic structure with mu(a) = 0, mu(b) = 1.27 and mu(c) = 2.001 mu(B)/Mn atom at 10 K. The crystal transition occurs from the orthorhombic structure to a tetragonal structure with c/a < 1 at 450K, and finally to a cubic structure at 510K, which is nearly equal to the Neel temperature. Another alloy containing 12 at. % Pt shows a face-centered tetragonal structure with c/a > 1 and mu = 2.58 muB/Mn atom at 10 K, and transforms to a cubic structure at 325 K, which is lower than the Neel temperature (= 500 K). We have determined a phase diagram for gamma-MnPt alloys containing 7-20 at. % Pt.