High resolution neutron powder diffraction and single crystal measurements on the ferromagnetic shape memory compound Ni 2 MnGa have been carried out. They enabled the sequence of transformations which take place when the unstressed, stoichiometric compound is cooled from 400 to 20 K to be established. For the first time the crystallographic structure of each of the phases which occur has been determined. At 400 K the compound has the cubic L2 1 structure, and orders ferromagnetically at T C ≈ 365 K. On cooling below ∼260 K a super-structure, characterized by tripling of the repeat in one of the 110 cubic directions, forms. This phase, known as the pre-martensitic phase, persists down to the structural phase transition at T M ≈ 200 K and can be described by an orthorhombic unit cell with lattice parameters a ortho = 1 √ 2 a cubic , b ortho = 3 √ 2 a cubic , c ortho = a cubic and space group Pnnm. Below T M the compound has a related orthorhombic super-cell with b ortho ≈ 7 √ 2 a cubic , which can be described within the same space group. The new modulation appears abruptly at T M and remains stable down to at least 20 K.

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.

The magnetization distributions in a series of ternary intermetallic compounds based on the composition Co2 YZ where Y is Ti, Mn or Fe and Z a subgroup-B element have been determined from polarized neutron diffraction measurements. Comparison of the magnetic structure factors with model calculations shows that the magnetization is associated principally with those atoms which in their elemental state are themselves magnetic. The observed deviations of the magnetic moment distributions from spherical symmetry have been used to deduce which of the 3d sub-bands are active at the Fermi energy. A small moment close to the limits of resolution is observed at some of the Z sites, together with a small delocalized moment which in most cases is negative. The results have been compared with the predictions of band models, which indicate that the Fermi level falls in a broad minimum in the minority-spin density of d states. Although the identity of the bands active at the Fermi surface is in broad agreement with predictions of band-structure calculations (Ishida S, Akazawa S, Kubo Y and Ishida J 1982 J. Phys. F: Met. Phys. 12 1111), the results suggest that there is a finite density of states in the minority-spin d band of manganese. Hence the compounds cannot be classified as half-metallic ferromagnets.

Polarized neutron scattering has been used to determine the changes in the distribution of unpaired electrons which take place in the martensitic transition in Ni2MnGa. Ni2MnGa is a ferromagnetic Heusler alloy which undergoes a reversible transition at about 220 K from a high temperature cubic phase to a low temperature tetragonal one. It has been suggested, on the basis of band structure calculations, that the structural phase transition is driven by a band Jahn-Teller distortion involving redistribution of electrons between 3d sub-bands of different symmetries. The results of the neutron scattering experiments show that the transition from the cubic to the tetragonal phase is accompanied by a transfer of magnetic moment from Mn to Ni. The unpaired electrons in the cubic phase have overall eg symmetry. In the tetragonal phase, the degeneracy of the eg and t2g bands is raised and the unpaired electrons are redistributed in such a way that the sub-bands based on orbitals extending towards the c-axis are preferentially occupied. Although the experimental moments differ in detail from those expected from band structure calculations, the change in symmetry of the magnetization distribution is consistent with a band Jahn-Teller origin for the phase transition.

We investigate in detail the occurrence of magnetic domains in epitaxially grown MnAs films on GaAs͑001͒ by magnetic force microscopy ͑MFM͒. MnAs layers exhibit in their demagnetized state a very complex magnetic domain structure. High resolution MFM images reveal detailed information on the domain wall. Additionally, we imaged magnetic domains in the dependence on the applied magnetic field. This detailed investigation gives new insight into the correlation between film topography and magnetic domain structures. Systematic magnetization measurements in-plane and out-of-plane have shown high anisotropy in our films. The out-of-plane magnetization determined as a function of the applied field reveals that the direction of the magnetic moments in the domain walls are out-of-plane, thus the domain walls are determined as 180°Bloch type.