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 Ni ₂Mn _1.14Ga _0.86 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(α0γ)00 superspace group symmetry with the following fundamental lattice parameters: a = 4.255(4) b = 5.613(4) c = 4.216(3) a commensurate periodicity of 5M, and a modulation wave vector of q - =2/5c - . 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 μ _B per Ni ₂Mn _1.14Ga _0.86 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. © 2012 American Physical Society.