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.

Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory compound Ni(2)Mn(1.48)Sb(0.52) have confirmed that it is ferromagnetic below 350 K and undergoes a structural phase transition at T(M)≈310 K. The high temperature phase has the cubic L2(1) structure with a = 5.958 Å, with the excess manganese atoms occupying the 4(b) Sb sites. In the cubic phase above ≈310 K the manganese moments are ferromagnetically aligned. The magnetic moment at the 4(a) site is 1.57(12) μ(B) and it is almost zero (0.15(9) μ(B)) at the 4(b) site. The low temperature orthorhombic phase which is only fully established below 50 K has the space group Pmma with a cell related to the cubic one by a Bain transformation a(orth) = (a(cub) + b(cub))/2; b(orth) = c(cub) and c(orth) = (a(cub) - b(cub)). The change in cell volume is ≈2.5%. The spontaneous magnetization of samples cooled in fields less than 0.5 T decreases at temperatures below T(M) and at 2 K the magnetic moment per formula unit in fields up to 5.5 T is 2.01(5) μ(B). Neutron diffraction patterns obtained below ≈132 K gave evidence for a weak incommensurate magnetic modulation with propagation vector (2/3, 1/3, 0).

The mesoscopic and microscopic mechanisms giving rise to shape memory behaviour in single crystals of the ferromagnetic alloy Ni 54 Fe 19 Ga 27 have been investigated using polarized and unpolarized neutron diffraction. The measurements confirm that the Curie temperature T C coincides with the martensitic phase transition at T M = 296 K. At room temperature the crystal, as grown, had the tetragonal L1 0 structure with c/a ≈ 1.20. It transformed to the cubic Heusler L2 1 structure at ≈330 K. In subsequent heating and cooling cycles the transition took place at T M ≈ 295 K and it was found that applying a magnetic field raised T M by ≈0.3 K T −1 , making the material attractive for applications. The tetragonal structure has space group I 4/mmm and is related to the cubic parent phase by a Bain transformation. The change in cell volume at the transition is only ≈1%, suggesting that the atomic moments are unchanged, although the magnetization drops significantly. The polarized neutron results show that in the cubic phase the magnetic electrons at the iron-rich sites have predominantly e g symmetry (60(3)%), a distribution similar to that observed in Fe 3 Al and Fe 3 Si. A small transfer of magnetization from Fe to Ni is associated with the martensitic transition, but no significant redistribution of magnetic electrons between orbitals whose degeneracy is lifted, such as that predicated by the band Jahn-Teller mechanism, was observed.

Polarized neutron diffraction has been used to study the magnetization distribution in two isostructural inter-metallic compounds NiMnSb and PdMnSb. Band structure calculations have predicted that whereas the former should be a half metallic ferromagnet the latter should not. Measurements made at 5 K on different crystals show that disorder can occur between the A (Mn) and B (Sb) sites in both alloys and in the case of NiMnSb, by partial occupation of the void D sites by Ni. In all the crystals most of the moment was found on the Mn atoms in the A sites; in NiMnSb it is due to spin only but in PdMnSb there is evidence for a significant orbital contribution (g = 2.22). The magnitudes of the moments associated with each atom are in fair agreement with the theoretical values; however, the distribution of magnetization around the Mn atoms is found to have nearly spherical symmetry (40% e(g)) rather than the 50% e(g) character expected from the band structure.

The magnetic moment distributions in the ferrimagnetic (FI) and antiferromagnetic (AF) phases of Co substituted Mn 2 Sb (Mn 1.9 Co 0.1 Sb) have been determined using both polarized and unpolarized neutron diffraction. In Mn 1.9 Co 0.1 Sb a transition from a ferrimagnetic to an antiferromagnetic state takes place on cooling through T ms = 138 K. The antiferromagnetic structure has propagation vector (00 12 ), and contains (001) layers of Mn atoms with ferromagnetically coupled Mn moments aligned perpendicular to [001]. The two crystallographically distinct Mn atoms have moments 3.98 and 1.71 μ B at 4 K and are oppositely aligned within the unit cell. At 200 K the moments determined from polarized neutron scattering are 3.86 and 1.84 μ B . In both the AF and FI phases the magnetization associated with the high moment (Mn2) site is nearly spherically symmetric; that around the Mn1 site has predominantly t 2g character which is stronger in the AF than in the FI phase. At T ms the only bonds whose lengths change significantly are those between Mn and Sb establishing the 125 • Mn1-Sb-Mn2 interlayer exchange path as the dominant exchange interaction determining the type of long range magnetic order.