Daniel Ebke

Large tunnel magnetoresistance in tunnel junctions with Co2MnSi∕Co2FeSi multilayer electrode

Two kinds of magnetic tunnel junctions with Co2FeSi electrodes are compared. Using Co2FeSi single layers a tunnel magnetoresistance of 52% is reached, whereas the magnetization of the Co2FeSi is only 75% of the expected value. By using [Co2MnSi∕Co2FeSi]x10 multilayer electrodes the magnetoresistance can be increased to 114% and the full bulk magnetization is reached. All junctions show an inverse tunnel magnetoresistance, when the electrons are tunneling from the Co–Fe into the Heusler compound electrode. This results from a special band structure feature of full Heusler compounds, which is robust even for atomic disorder in the films.

Loss of anisotropy in strained ultrathin epitaxial L10 Mn-Ga films

We have investigated the magnetization and loss of anisotropy in ultrathin strained and unstrained Mn-Ga films at room temperature. Two Mn-Ga compositions, one of which is doped with Co, were grown on Cr buffered MgO (001) substrates. Films with a thickness below 10 nm are highly strained and the ratio c/a vs. thickness is depending on composition. The perpendicular magnetic anisotropy is shown to be drastically reduced with decreasing thickness and increasing strain. These findings should be considered when generalizing and downscaling results obtained from films > 20 nm. The strain can effectively be reduced by introducing an additional Pt buffer and thus maintaining a high perpendicular magnetic anisotropy for a thickness as low as 6 nm.

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.

Inverted spin polarization of heusler alloys for spintronic devices

A magnetic logic concept overcomes the major limitations of field programmable gate arrays while having a 50% smaller unit cell than conventional designs utilizing magnetic tunnel junctions with one Heusler alloy electrode. These show positive and negative tunneling magnetoresistance values at different bias voltages at room temperature which generally add an additional degree of freedom to all spintronic devices.

Structural, electronic, and magnetic properties of perpendicularly magnetised Mn2RhSn thin films

Epitaxial thin films of Mn2RhSn were grown on a MgO(0 0 1) substrate by magnetron co-sputtering of the constituents. An optimised range of temperature for heat treatment was used to stabilise the tetragonal structure and to prevent the capping Rh layer from diffusing into the Heusler layer. Electronic and magnetic properties were analysed by hard x-ray photoelectron spectroscopy as well as field- and temperature-dependent Hall and resistivity measurements. The measured valence spectra are in good agreement with the calculated density of states. The measured saturation magnetisation corresponds to a magnetic moment of 1 μB in the primitive cell. The magnetisation measurements revealed an out-of-plane anisotropy energy of 89 kJ m−3 and a maximum energy product of 3.3 kJ m−3. The magnetoresistance ratio is 2% for fields of 9 T. The lattice parameter of the compound has a very small mismatch with MgO, which makes it promising for coherent electron tunnelling phenomena.

On the influence of bandstructure on transport properties of magnetic tunnel junctions with Co2Mn1-xFexSi single and multilayer electrode

The transport properties of magnetic tunnel junctions with different (110)-textured Heusler alloy electrodes such as Co2MnSi, Co2FeSi or Co2Mn0.5Fe0.5Si, AlOx barrier, and Co–Fe counterelectrode are investigated. The bandstructure of Co2Mn1−xFexSi is predicted to show a systematic shift in the position of the Fermi energy EF through the gap in the minority density of states while the composition changes from Co2MnSi toward Co2FeSi. Although this shift is indirectly observed by x-ray photoemission spectroscopy, all junctions show a large spin polarization of around 70% at the Heusler alloy/Al–O interface and are characterized by a very similar temperature and bias voltage dependence of the tunnel magnetoresistance. This suggests that these transport properties of these junctions are dominated by inelastic excitations and not by the electronic bandstructure.

Electric field induced reversible tuning of resistance of thin gold films

The change in resistance of nanostructured metals with respect to an applied field is believed to be due to a change in carrier concentration and hence a linear variation of resistance with the surface charge is expected. In this article, we propose a different approach to explain the resistance variation based on a change in the effective thickness of the film due to a shift of the electron density profile resulting from the applied surface charge. The change in effective thickness together with its effect on surface scattering of electrons account for the majority of the observed variation in resistance. The thin film geometry with different thicknesses and hence a controlled variation of the surface-to-volume ratio allows a deep quantitative understanding and interpretation of the observed phenomena. The model presented in this work shows that a nominal nonlinear response of the resistance of a metal on electrochemically applied surface charge does not necessarily indicate an onset of a redox reaction.

Tunneling spectroscopy and magnon excitation in Co2FeAl/MgO/Co–Fe magnetic tunnel junctions

Magnetic tunnel junctions with the Heusler compound Co2FeAl as the soft electrode are prepared. Pinned Co–Fe is used as the hard reference electrode. The junctions show a high tunnel magnetoresistance ratio of 273% at 13 K. The electronic transport characteristics are investigated by tunneling spectroscopy—dI/dV and d2I/dV2 are discussed. In the parallel magnetic state the tunneling spectra are asymmetric with respect to the bias voltage, with a pronounced bias-independent region. In the antiparallel state the dependence on bias voltage is much stronger and the curves are symmetric. The findings can be explained with a gap in the minority density of states of Co2FeAl.

Direct measurement of the spin polarization of Co2FeAl in combination with MgO tunnel barriers

It is a truth universally acknowledged that a Heusler compound in possession of a good order must be in want of a high spin polarization. In the present work, we investigated the spin polarization of the Heusler compound Co2FeAl by spin polarized tunneling through a MgO barrier into a superconducting Al–Si electrode. The measured spin polarization of P=55% is in good agreement with the previously obtained tunnel magnetoresistance values and compared to the data by other groups.

Evidence for band structure effects in the magnetoresistance of Co-based Heusler compounds

Magnetic tunnel junctions are prepared with Co-based Heusler compounds as one magnetic electrode and alumina as the tunnel barrier. The investigated compounds are Co2MnSi and Co2FeSi. An inversion of the tunnel magnetoresistance in the Co2FeSi as well as in the Co2MnSi case was found with alumina barriers at certain bias voltages at room and low temperatures. We present calculations of the density of states of these compounds based on fully relativistic spin-polarized KKR methods in order to elucidate the inversion.