The magnetic and structural properties of the magnetic shape memory compound Ni₂Mn_1.44Sn_0.56

Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory compound Ni₂Mn_1.44Sn _0.56 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 L 2₁ structure, a ≤ 5.973 Å, 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) μ_B but it was only 0.53(18) μ_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 ≈0.5%, suggesting that the atomic moments are unchanged although the spontaneous magnetization drops significantly.