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.

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.

The saturation magnetizations of very pure iron and nickel have been measured absolutely by a force method at room temperature. The values are 217.6 and 55.1 emu/g (217.6 and 55.1 J T -1 kg -1 ) respectively. The theory and reliability of the force method are investigated and discussed. New measurements have also been made by a relative method on the same materials at temperatures ranging from 4 K to above the respective Curie temperatures of iron and nickel. Reduced magnetization, temperature data have been derived and are compared with those already available for face centred cubic cobalt.

Long-range ferromagnetic coupling occurs in several dilute solid-solution alloys of the ferromagnetic transition elements in Pd or Pt. New measurements of magnetic moments and Curie temperatures of PdFe, PtFe, PtCo, and PdNi alloys are reported, and previous work on these and some of the more general properties of these alloys are briefly reviewed. Also, ferromagnetism occurs in dilute PdGd solid solutions. Dilute solutions of iron in gold are ferromagnetic, although below about 10% of Fe there seems to be no homogeneous bulk ferromagnetism.

No abstract