Magnetic structure of single-domain antiferromagnetic FCT γ-Mn-Cu and γ-Mn-Fe alloys

Detailed neutron diffraction studies both by double axis and triple axis spectrometer are reported for the first time for fct single-domain single crystals of γ-Mn-Cu and γ-Mn-Fe. The main purpose of these studies was to elucidate further the nature of the diffuse (001) peak, forbidden by the AF1 magnetic structure, established earlier in this group of γ-Mn alloys. Line shapes and their temperature dependence were measured for (001), (100) and (010) reflections in Mn-Fe and in Mn—Cu, in the latter case both for homogeneous (quenched) and chemically decomposed (aged) samples. Only (001) reflection was found in Mn-Fe, while in Mn-Cu samples diffuse peaks were also observed at (100) and (010). Together with their temperature dependence and the results of energy analysis the data suggest a new model of the magnetic structure of these alloys. In Mn-Fe the spins deviate by a fixed angle from the c axis and this perturbation has an infinite correlation range and life time. In Mn-Cu a more complex set of perturbations is required to interpret the data.