Atomic and magnetic order in the shape memory alloy Mn2NiGa

Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory alloy Mn2NiGa have confirmed that it is ferromagnetic with a Curie temperature above 500 K. The compound undergoes a broad structural phase transformation Delta T similar to 90 K with a mean transition temperature T-M similar to 270 K. The high temperature parent phase is cubic (a = 5.937 angstrom) and has a modified L2(1) structure. At 500 K the ordered magnetic moment essentially all on the 4a site is 1.35 mu B/Mn. The low temperature martensite has space group I4/mmm and is related to the cubic phase through a Bain transformation a(tet) = (a(cub) + b(cub))/2, b(tet) = (a(cub) - b(cub)) and c(tet) = c(cub) in which the change in cell volume is <2.6%. In this structure at 5 K the ordered moment of approximate to 2.3 mu(B) is again found to be confined to the sites with full Mn occupation and is aligned parallel to c. Neutron diffraction patterns obtained at 5 K suggested the presence of a weak incommensurate antiferromagnetic phase characterized by either a (1/3 0 1/3) or (00 1/3) propagation vector.