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

TLDR

Ni₂Mn₁.₄₄Sn₀.₅₆ adopts a high‑temperature cubic L2₁ structure with excess Mn on Sn sites and transforms to an orthorhombic Pmma phase via a Bain‑type distortion at low temperatures. Magnetization and neutron diffraction show the compound is ferromagnetic below 319 K, undergoes a structural transition at 221 K on cooling (239 K on warming), with Mn moments of 1.88 μB at 4(a) sites and 0.53 μB at 4(b) sites, and a ~0.5 % volume change that reduces spontaneous magnetization.

Abstract

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 TM = 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) μB but it was only 0.53(18) μB/Mn at the 4(b) sites. The low temperature phase stable below TM has an orthorhombic structure with space group Pmma related to the cubic phase through a Bain transformation aortho = (acub+bcub)/2; bortho = ccub; cortho = (acub− bcub). 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.