Neutron diffraction study of short range order in γ MnCu alloys

A detailed study of the diffuse scattering in the neutron diffraction patterns of single crystals of γ MnCu alloys containing 10, 15, 25 and 40 at% Mn has shown that diffuse peaks occur at the 1½0 type positions, and are due to both atomic and magnetic short range ordering. Analysis of the diffuse scattering has yielded the atomic and magnetic short range order parameters and a value of S = 2.0 ± 0.3 for the localized spin on the manganese site. Neutron diffraction measurements of polycrystalline specimens have also confirmed that dilute γ MnCu alloys containing 3, 8, 15 and 25 at% Mn show no evidence of co-operative magnetic ordering at temperatures down to 4.2K. It is suggested that earlier results from Mössbauer and electron spin resonance experiments can be reconciled with the neutron diffraction results by invoking a model in which there is a non-zero temporal average of the spins rather than long range spatial magnetic ordering.