The structure and properties of the alloy Cu2MnIn

The ferromagnetic Heusler alloy Cu2MnAl has a body-centred cubic structure with an ordered arrangement of the atoms. Considerations of valency and atomic diameters suggested that if the aluminium were replaced by indium the superlattice would be more stable, and that a homogeneous alloy would be obtained if the indium content were slightly less than that required by the formula Cu2MnIn. An alloy of composition Cu2.057Mn0.975In0.965 was prepared and was ferromagnetic in the as-cast state, and also after a variety of heat treatments. The intensities of magnetization were measured in field strengths between 7250 and 17,150 oersteds at temperatures between — 183° C and the Curie point. X-ray powder photographs show that the indium alloy has the same structure as the aluminium Heusler alloy but with a larger lattice spacing. This increase in the size of the unit cell means an increase in the R/r value for the closest approach of manganese atoms from 2.84 in Cu2MnAl to 2.98 in Cu2MnIn, and corresponds with a fall in Curie point of 97° C. The two alloys are strictly analogous as regards structure and valencies, and the results suggest that in the indium alloy the critical value of R/r is being approached at which ferromagnetism will disappear. Reasons are given for supposing that the ferromagnetism is due to interaction between manganese atoms, in which case the Bohr magneton number per atom of manganese is approximately 4, and manganese behaves as a univalent element, in agreement with conclusions reached independently from the study of phase-boundaries in ternary Cu-Mn-Al alloys.