Klaus-Ulrich Neumann

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.

Atomic and magnetic order in the shape memory alloy Mn2NiGa.

Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory alloy Mn(2)NiGa have confirmed that it is ferromagnetic with a Curie temperature above 500 K. The compound undergoes a broad structural phase transformation ΔT ∼ 90 K with a mean transition temperature T(M) ∼ 270 K. The high temperature parent phase is cubic (a = 5.937 Å) and has a modified L 2(1) structure. At 500 K the ordered magnetic moment essentially all on the 4a site is 1.35 μ(B)/Mn. The low temperature martensite has space group I4/mmm and is related to the cubic phase through a Bain transformation a(tet) = (a(cub) + b(cub))/2, b(tet) = (a(cub) - b(cub)) and c(tet) = c(cub) in which the change in cell volume is < 2.6%. In this structure at 5 K the ordered moment of ≈2.3 μ(B) is again found to be confined to the sites with full Mn occupation and is aligned parallel to c. Neutron diffraction patterns obtained at 5 K suggested the presence of a weak incommensurate antiferromagnetic phase characterized by either a ((1/3)0(1/3)) or (00(1/3)) propagation vector.

Microstructure formation during MnAs growth on GaAs(0 0 1)

Abstract A comprehensive study of the GaAs(0xa00xa01)/MnAs interface formation and of the MnAs layer evolution during MBE growth has been performed in situ by RHEED and RDS and after growth by TEM. For low growth rates and high As4/Mn flux ratios we find a perfect (1xa01.0) epitaxial orientation with [00.1]MnAs‖[1xa01xa00]GaAs (A-orientation). The formation of B-domains with 90° azimuthal rotation, detected during growth, is restricted to thicknesses up to 7xa0nm. TEM studies on the as-grown samples only reflect the A-orientation connected with an atomically abrupt interface structure. The large lattice mismatch between MnAs and GaAs is accommodated by the generation of misfit dislocations with different character along perpendicular directions. Magnetization measurements revealed promising magnetic properties.

Magnetization distribution in CoS2; is it a half metallic ferromagnet?

Polarized neutron diffraction has been used to determine the spatial distribution of the magnetization in CoS2 and its temperature dependence. Although the previously reported ground state moment 0.84??B is non-integral, the compound has sometimes been classified as a half metallic ferromagnet. The present results show that the ferromagnetic phase cannot have half metallic properties. The magnetization distribution around the Co2+ ions in the ferromagnetic phase at 1.8?K is nearly spherical, indicating that, contrary to electronic structure calculations, electrons both of eg and t2g character contribute to the observed magnetic moment. The orbital contribution to the moment is found to be small, a factor of two less than that determined from magnetic circular dichroism measurements. A small moment, approximately 2% of that on the cobalt ions, resides on the sulfur atoms at 1.8?K. The magnetization distribution induced in the paramagnetic phase by a field of 9.6?T at 150?K is significantly different from that in the ferromagnetic state and has the asphericity characteristic of eg electrons. On heating to 300?K and then to 400?K the t2g character of the magnetization increases continuously and becomes predominant above the anomaly in the susceptibility at 350?K.

Atomic and magnetic order in the weak ferromagnet CoVSb: is it a half-metallic ferromagnet?

Magnetization and neutron powder diffraction measurements have been carried out on CoVSb. Both the transport and magnetic properties are predicted by band theory to depend upon the nature of the atomic order in the C1b lattice. Calculations in which the cobalt atoms occupy the (4b) site predict half-metallic behaviour and a ground state moment of ~1 μB per formula unit with 1.19 μB on the vanadium atoms and −0.19 μB on the cobalt atoms. However, calculations in which the Sb or V atoms occupy the (4b) site predict larger moments, ~0.7 μB, on the cobalt atoms, and either zero or a negative moment on the vanadium atoms. In contrast, the magnetization measured in fields of up to 5.5 T is small, amounting to a ferromagnetic moment per formula unit of 0.17 μB at 2 K. Both the field dependence of the magnetization in the ordered phase below Tc~48 K and the large effective moment obtained from the Curie–Weiss susceptibility above Tc are consistent with weak itinerant ferromagnetism. Confirmation of the small ground state moment and hence the absence of any significant moment on the vanadium atoms was obtained from neutron powder diffraction measurements, which also enabled the crystallographic structure to be determined as C1b with the cobalt atoms occupying the (4b) sites. The results show that CoVSb is not a half-metallic ferromagnet as predicted by band theory.

A study of the structural phase transformation in the shape memory alloy Co2NbSn

Resistivity, specific heat and magnetization measurements on the shape memory compound Co2NbSn corroborate the presence of ferromagnetic order below 119 K and a structural phase transition at 235 K. High-resolution neutron powder diffraction measurements confirmed that the high-temperature cubic phase has the Heusler L21 structure and established that the low-temperature phase is orthorhombic with space group Pmma. The thermal variation of the specific heat at low temperatures indicates a high density of states consistent with electronic structure calculations and a band Jahn-Teller mechanism for the transformation.