Christian G. F. Blum

Effects of random distribution of Mn,Fe in Co2Mn1-xFexSi Heusler compounds probed by 55Mn nuclear magnetic resonance

Recent spin echo nuclear magnetic resonance (NMR) revealed that the half-metallic Co2Mn1−xFexSi Heusler alloys exhibit the L21 structure with random distribution of Mn and Fe on the 4b Wyckoff position. This random distribution is most prominent in the third coordination shell of the Mn55 nuclei, as this shell is the first magnetically active shell, leading to a line splitting in the Mn55 NMR spectra. A comparison, as presented in this paper, of all Mn55 NMR satellite lines shows that the resonance frequencies corresponding to the same number of Fe atoms in the third shell of Mn55 shift to slightly different resonance frequencies with increasing overall Fe content x. This shift is related to the constructive contribution of higher shells to the hyperfine field. These higher shells affect also the satellite linewidth, leading to a parabolic behavior with increasing Fe concentration x. This parabolic behavior is modified by the frequency spacing related to the replacement of Mn by Fe.

Probing the random distribution of half-metallic Co2Mn1-xFexSi Heusler alloys

Co2Mn1−xFexSi Heusler alloys crystallize in the L21 structure. This structure type requires random distribution of Mn and Fe in case of the mixed alloys. The spin echo nuclear magnetic resonance (NMR) technique probes the direct local environments of the active atoms and is thus able to resolve next neighboring shells providing a unique tool to verify the random distribution of Mn and Fe in Co2Mn1−xFexSi. Exemplarily, the half-metallic quaternary Heusler alloy Co2Mn0.5Fe0.5Si was investigated by means of NMR. The Mn55 NMR measurements unambiguously demonstrate the random distribution of Mn and Fe on the 4b Wyckoff position in Co2Mn0.5Fe0.5Si.

Off-stoichiometry in Co2FeSi thin films sputtered from stoichiometric targets revealed by nuclear magnetic resonance

Co2FeSi is predicted to be a half-metallic ferromagnet with an extraordinary high magnetic moment and Curie temperature. However, a low tunnel magneto-resistance ratio, a lower spin polarization and a lower magnetic moment were experimentally observed in thin film samples. Consequently, thin Co2FeSi films of different groups were studied using spin-echo nuclear magnetic resonance (NMR). NMR probes the local hyperfine fields of the active atoms, which strongly depend on the local environment. NMR is thus able to reveal the next neighbouring shells of the 59Co nuclei in the Co2FeSi thin films. As expected, our NMR study shows the main resonance line corresponding to 59Co nuclei in the L21 environment but also additional resonance lines at the high frequency site of the main line with a spacing between adjacent resonance lines of 32?MHz. The additional resonance lines correspond to 59Co with more Fe next neighbours than expected for the L21 type ordering, which is interpreted as the formation of an off-stoichiometric film yielded by sputtering from a stoichiometric target. The analysis of the NMR data and a comparison with off-stoichiometric bulk samples reveals the composition of the thin films to be about Co2Fe(Si0.92Fe0.08). This off-stoichiometry might explain the observed deviations from the expected behaviour.

Peculiarities of anisotropic electrical resistivity in Lu2PdSi3 single crystals

Lu2PdSi3 represents a non-magnetic member of the class of R2PdSi3 intermetallic compounds (R = rare earths) with hexagonal AlB2-type crystallographic structure. The electrical resistivity of single crystalline Lu2PdSi3 is highly anisotropic and reflects metallic behaviour. Its temperature dependence follows the Bloch–Gruneisen law for c-axis orientation [001]. For [100] orientation in the perpendicular plane the observed deviation from the Bloch–Gruneisen law can be explained by the Ioffe–Regel criterion. The relatively low Debye temperature ΘD of the order of 200 K fairly well agrees with that derived from specific heat data of Lu2PdSi3. Notably, Lu2PdSi3 allows extracting the magnetic entropy contribution of isostructural R2PdSi3 compounds.

Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds

Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics.Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 − xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals.