Neutron Diffraction Study of γ-Phase Mn–Ir Single Crystals

Abstract

Neutron diffraction was measured on single crystals of the disordered γ-phase Mn_1−xIr_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−0.1f_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 Mn₃Ir was observed in the x=0.256 alloys.