Structural modulations and magnetic properties of off-stoichiometric Ni-Mn-Ga magnetic shape memory alloys

A comparative description of the crystallographic structural details and the magnetic properties of Ni-Mn-Ga magnetic shape memory alloys are necessary to understand the physical origins of their magnetoelastic properties. The crystallographic structural details of an off-stoichiometric Ni2Mn1.14Ga0.86 alloy have been obtained from the refinement of high-resolution single-crystal neutron diffraction data, following a (3 + 1)-dimensional [(3 + 1)-D] superspace formalism. In particular, the structure adopts a P2/m(alpha 0 gamma)00 superspace group symmetry with the following fundamental lattice parameters: a = 4.255(4) angstrom, b = 5.613(4) angstrom, c = 4.216(3) angstrom, a commensurate periodicity of 5M, and a modulation wave vector of (q) over bar = 2/5 (c) over bar. The magnetic properties of the crystal are characterized from magnetic susceptibility measurements, as well as from neutron diffraction intensities. The overall magnetic moment per unit cell is 2.7 mu(B) per Ni2Mn1.14Ga0.86 formula unit and the magnetic moments are aligned along the b axis. The modulations of the atomic site displacements, site occupancies, and magnetic moments are elucidated from a (3 + 1)-D refinement of the neutron diffraction intensities. In addition to atomic displacements corresponding to shear waves along < 110 >, distortions of the Ni-centric tetrahedra are also evident. Physical interpretations for the different structural distortions and their relationship to magnetic properties are discussed.