Magnetic order and phase transformation in Ni₂MnGa

Abstract Magnetization, magnetic susceptibility, optical microscopy, X-ray diffraction and neutron diffraction measurements have been made on a quenched sample of Ni2MnGa. Room-temperature neutron diffraction patterns indicate a highly ordered Heusler alloy, L21 type, structure. The alloy is ferromagnetic with a Curie temperature of 376 K, and a magnetic moment of 4·17μB largely confined to the Mn sites, but probably with a small moment <07·3μB associated with the Ni sites. A martensitic phase transition to a complex tetragonal structure occurs on cooling below 202 K. Neutron diffraction oscillation photographs taken using the D12 facility at Institut Laue Langevin, Grenoble, and low-temperature powder neutron diffraction data enabled details of the low temperature crystallographic superlattice and magnetic structure to be determined. Reasons for the structural transformation are discussed.