K.R.A. Ziebeck

The crystal structure and phase transitions of the magnetic shape memory compound Ni2MnGa

High resolution neutron powder diffraction and single crystal measurements on the ferromagnetic shape memory compound Ni2MnGa 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 L21 structure, and orders ferromagnetically at TC ≈ 365 K. On cooling below ~ 260 K a super-structure, characterized by tripling of the repeat in one of the 110cubic directions, forms. This phase, known as the pre-martensitic phase, persists down to the structural phase transition at TM ≈ 200 K and can be described by an orthorhombic unit cell with lattice parameters aortho = 1/√2acubic, bortho = 3/√2acubic, cortho = acubic and space group Pnnm. Below TM the compound has a related orthorhombic super-cell with bortho ≈ 7/√2acubic, which can be described within the same space group. The new modulation appears abruptly at TM and remains stable down to at least 20 K.

The magnetization distributions in some Heusler alloys proposed as half-metallic ferromagnets

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.

Direct observation of a band Jahn-Teller effect in the martensitic phase transition of Ni2MnGa

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.

Magnetic structure of epitaxially grown MnAs on GaAs(001)

We investigate in detail the occurrence of magnetic domains in epitaxially grown MnAs films on GaAs(001) by magnetic force microscopy (MFM). 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.

The influence of atomic order on the magnetic and structural properties of the ferromagnetic shape memory compound Ni2MnGa

The effect of atomic order on the martensitic phase transition and magnetic properties of stoichiometric Ni2MnGa has been investigated in a sample quenched from 1000 °C. Magnetization, resistivity and x-ray diffraction measurements indicate that the structural phase transition occurs at ~ 103 K, substantially lower than the value reported for samples quenched from 800 °C and ordered in the Heusler L21 structure. A small reduction in the ferromagnetic moment was also observed, although the Curie temperature remained largely unaffected. The electronic Sommerfeld coefficient obtained from heat capacity measurements is enhanced but smaller than that observed for the 800 °C quenched sample. The results are consistent with band structure calculations and the electronic changes brought about by atomic disorder.

Observations of ferromagnetic correlations at high temperatures in paramagnetic iron

Abstract Polarised neutron scattering with polarisation analysis has been used to obtain a unique measurement of the paramagnetic fluctuations in iron at temperatures between 1273 and 1573 K. The results clearly demonstrate almost complete ferromagnetic correlation over distances up to 15 A. The average moment per atom taking part in the correlation and giving rise to paramagnetic scattering is about 1.3μ B . These findings should lead to a better understanding of paramagnetism in metals.

Spatial correlation of magnetisation in the paramagnetic phases of iron and nickel

Abstract The diffuse scattering of polarised neutrons with polarisation analysis has revealed the presence of ferromagnetic short range order in the paramagnetic phases of iron and nickel. Unlike the residual long range correlations which occur close to T C , the short range order arises from the itinerant nature of magnetic electrons.

Ferromagnetic correlations in both the α and γ-phases of paramagnetic iron

Abstract Paramagnetic scattering experiments using polarised neutrons and polarisation analysis in the paramagnetic phase of iron are reported. The measurements have been performed at 1120 K in the α-bcc phase and 1320 K in the γ-fcc phase. Iron moments to respectively 1.3 (0.1)μ B and 0.9 (0.1)μ B have been shown to persist at these temperatures. In both phases, ferromagnetic correlations are present and are especially strong in the α-phase. The ferromagnetic correlations also determine the magnetic character of iron in the γ-phase, contrary to previous suggestions of antiferromagnetic behaviour postulated in the literature on the basis of indirect experimental arguments.

The paramagnetic response of a localised metallic ferromagnet

Abstract Static magnetisation and susceptibility measurements indicate that Pd2MnSn has localised magnetic properties. This conclusion is supported by the success of the Heisenberg localised model in accounting for the spin dynamics and for the absence of spin waves in the paramagnetic phase. Polarised neutron and polarisation analysis measurements in the paramagnetic phase have established the absence of spatial magnetic correlations, and have shown the atomic moment to be (3.9 ± 0.2)μB per manganese atom, in close agreement with the value of (4.0 ± 0.1)μB obtained from static susceptibility measurements.

Paramagnetic scattering studies of the short‐range order above TC in 3d transition metal compounds and pure iron

For 3d metals the atomic moments derived from Curie‐Weiss susceptibility are often significantly larger than the value obtained in the ferromagnetic state from the saturation magnetization. Indeed the Heisenberg model is known to be inappropriate for metallic systems which in general are better described by band theory. The ground state properties of metallic magnets, particularly the occurrence of nonintegral moments, are well accounted for using the Stoner theory, but the model is less satisfactory at elevated temperatures. The persistence of spin waves above TC and the observation of paramagnetic moments indicate that the 3d band remains split in the paramagnetic phase in contradiction with mean field theory. A direct way of establishing the existence of paramagnetic moments is by the paramagnetic scattering of neutrons. The technique is particularly enhanced if polarization analysis is employed to obtain a unique estimate of the magnetic cross section. In the present paper, we report the results of re...