Neutron diffraction study of gamma-phase Mn-Ir single crystals

Neutron diffraction was measured on single crystals of the disordered γ-phase Mn 1- x Ir x ( x =0.128, 0.170, 0.204 and 0.256). Diffraction patterns were interpreted by an antiferromagnetic two-sublattice structure with tetragonal symmetry. The (110) magnetic reflection was measured up to about 800 K for the determination of the temperature dependence of sublattice magnetization and the Néel temperature. The critical index for magnetization was found to be 0.38 for the x =0.170 alloy. The Néel temperature increases remarkably with increasing Ir at a rate of about 9 K/at%Ir. The obtained magnetic form factors are well expressed by f =(1/0.9)( f 3d -0.1 f 4s ) constructed from the scattering functions calculated by Freeman and Watson. The average magnetic moment per Mn atom increases from 2.2µ B to 2.6µ B with increasing Ir from x =0 to x =0.204. Diffuse scattering due to partial atomic order of Mn 3 Ir was observed in the x =0.256 alloys.