Chemical order in off-stoichiometric Ni-Mn-Ga ferromagnetic shape-memory alloys studied with neutron diffraction

The chemical order of three off-stoichiometry Ni-Mn-Ga compositions has been measured in the austenitic phase using powder and single-crystal neutron diffraction. The compositions studied, 48-52 at.% nickel, having excess manganese and deficient in gallium, are of technical interest due to the observed large room-temperature, magnetic-field-induced strain. It has been determined that compositions with less than 50% nickel have the excess Mn atoms occupying Ni and Ga sites. Compositions enriched in nickel are best fit with Ni atoms in excess of 50% occupying Mn sites while the excess and displaced Mn occupy Ga sites. The saturation magnetic moments calculated from the site occupations determined here and using Ni and Mn moments reported for Ni2MnGa, agree within 4% with the low-temperature measured moments.