The magnetic and structural properties of the magnetic shape memory compound Ni2Mn1.44Sn0.56

Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory compound Ni2Mn1.44Sn0.56 have confirmed that it is ferromagnetic below 319 K and undergoes a structural phase transition which takes place at T-M = 221 K on cooling and 239 K on warming. The high temperature phase has the cubic L2(1) structure, alpha = 5.973 angstrom, with the excess manganese atoms occupying the 4(b) tin sites. In the cubic phase at 245 K the manganese moments at both sites were found to be ferromagnetically aligned. The magnetic moment at the 4(a) sites was 1.88(10) mu(B) but it was only 0.53(18) mu(B)/Mn at the 4(b) sites. The low temperature phase stable below T-M has an orthorhombic structure with space group Pmma related to the cubic phase through a Bain transformation a(ortho) = (a(cub) + b(cub))/2; b(ortho) = c(cub); c(ortho) = (a(cub)-b(cub)). The change in cell volume in the transition is only approximate to 0.5%, suggesting that the atomic moments are unchanged although the spontaneous magnetization drops significantly.