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The Slater-Pauling rule states that L21 Heusler compounds with 24 valence electrons do never exhibit a total spin magnetic moment. In case of strongly localized magnetic moments at one of the atoms (here Mn) they will exhibit a fully compensated half-metallic ferrimagnetic state instead, in particular, when symmetry does not allow for antiferromagnetic order. With aid of magnetic and anomalous Hall effect measurements it is experimentally demonstrated that Mn1.5V0.5FeAl follows such a scenario. The ferrimagnetic state is tuned by the composition. A small residual magnetization, that arises due to a slight mismatch of the magnetic moments in the different sublattices results in a pronounced change of the temperature dependence of the ferrimagnet. A compensation point is confirmed by observation of magnetic reversal and sign change of the anomalous Hall effect. Theoretical models are presented that correlate the electronic structure and the compensation mechanisms of the different half-metallic ferrimagnetic states in the Mn-V-Fe-Al Heusler system., Comment: Under review

Diminution of the thermal conductivity is a crucial aspect in thermoelectric research. We report a systematic and significant reduction of the cross-plane thermal conductivity in a model system consisting of DC sputtered TiNiSn and HfNiSn half-Heusler superlattices. The reduction of $\kappa$ is measured by the 3$\omega$ method and originates from phonon scattering at the internal interfaces. Heat transport in the superlattices is calculated based on Boltzmann transport theory, including a diffusive mismatch model for the phonons at the internal interfaces. Down to superlattice periodicity of 3 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the reduction of $\kappa$. For very thin individual layers the interface model breaks down and the artificial crystal shows an enhanced $\kappa$. We also present an enhanced ZT value for all investigated superlattices compared to the single TiNiSn and HfNiSn films., Comment: 5 pages, 4 figures plus supplementary data with one additional figure

Improvements in the thermoelectric properties of Half-Heusler materials have been achieved by means of a micrometer-scale phase separation that increases the phonon scattering and reduces the lattice thermal conductivity. A detailed study of the p-type Half-Heusler compounds Ti(1-x)Hf(x)CoSb0.85Sn0.15 using high-resolution synchrotron powder X-ray diffraction and element mapping electron microscopy evidences the outstanding thermoelectric properties of this system. A combination of intrinsic phase separation and adjustment of the carrier concentration via Sn substitution is used to realize a record thermoelectric figure of merit for p-type Half-Heusler compounds of ZT around 1.15 at 710C in Ti0.25Hf0.75CoSb0.85Sn0.15. The phase separation approach can form a significant alternative to nanostructuring processing time, energy consumption and increasing the thermoelectric efficiency., Comment: 13 pages, 6 figures, 1 table

Half-Heusler (HH) compounds have shown high Figure of merits up to 1.5. The key to these high thermoelectric efficiencies is an intrinsic phase separation, which occurs in multicomponent Half-Heusler compounds and leads to an significantly reduction of the thermal conductivity. For commercial applications, compatible n- and p-type materials are essential and their thermal stability under operating conditions, e.g. for an automotive up to 873 K, needs to be guaranteed. For the first time, the long-term stability of n- and p-type HH materials is proved. We investigated HH materials based on the Ti0.3Zr0.35Hf0.35NiSn-system after 500 cycles (1700 h) from 373 to 873 K. Both compounds exhibit a maximum Seebeck coefficient of S around 210 muV/K and an intrinsic phase separation into two HH phases. The dendritic microstructure is temperature resistant and maintained the low thermal conductivity values (kappa less than 4 W/Km). Our results emphasize that phase-separated HH compounds are suitable low cost materials and can lead to enhanced thermoelectric efficiencies beyond the set benchmark for industrial applications., Comment: 10 pages, 4 figures, submitted to Phys. Chem. Chem. Phys

Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometric Co2-based Heusler alloy shows a intense dependency of the tunnel magnetoresistance (TMR) on the Mn composition, demonstrating giant TMR ratios of up to 1995% at 4.2 K for 1. This work reports on the electronic structure of non-stoichiometric CoxMnyGez thin films with a fixed Co/Ge ratio of x : z = 2 : 0.38. The electronic structure was investigated by high energy, hard X-ray photoelectron spectroscopy combined with first-principles calculations. The high-resolution measurements of the valence band of the non-stoichiometric CoxMnyGez films close to the Fermi energy indicate a shift of the spectral weight compared to bulk Co2MnGe. This is in agreement with the changes in the density of states predicted by the calculations. Furthermore it is shown that the co-sputtering of Co2MnGe together with additional Mn is an appropriate technique to adjust the stoichiometry of the CoxMnyGez film composition. The resulting changes of the electronic structure within the valence band will allow to tune the magnetoresistive characteristics of CoxMnyGez based tunnel junctions as verified by the calculations and photoemission experiments.

This work reports the measurement of magnetic dichroism in angular-resolved photoemission from in-plane magnetized buried thin films. The high bulk sensitivity of hard X-ray photoelectron spectroscopy (HAXPES) in combination with circularly polarized radiation enables the investigation of the magnetic properties of buried layers. HAXPES experiments with an excitation energy of 8 keV were performed on exchange-biased magnetic layers covered by thin oxide films. Two types of structures were investigated with the IrMn exchange-biasing layer either above or below the ferromagnetic layer: one with a CoFe layer on top and another with a Co$_2$FeAl layer buried beneath the IrMn layer. A pronounced magnetic dichroism is found in the Co and Fe $2p$ states of both materials. The localization of the magnetic moments at the Fe site conditioning the peculiar characteristics of the Co$_2$FeAl Heusler compound, predicted to be a half-metallic ferromagnet, is revealed from the magnetic dichroism detected in the Fe $2p$ states.

In this report the Co2 based Heusler compounds are discussed as potential materials for spin voltage generation. The compounds were synthesized by arcmelting and consequent annealing. Band structure calculations were performed and revealed the compounds to be half-metallic ferromagnets. Magnetometry was performed on the samples and the Curie temperatures and the magnetic moments were determined. The Seebeck coefficients were measured from low to ambient temperatures for all compounds. For selected compounds high temperature measurements up to 900 K were performed., Comment: accepted contribution o the Special Issue "Spin Caloritronics" of Solid State Communications

In this work the theoretical and experimental investigations of Co2TiZ (Z = Si, Ge, or Sn) compounds are reported. Half-metallic ferromagnetism is predicted for all three compounds with only two bands crossing the Fermi energy in the majority channel. The magnetic moments fulfill the Slater-Pauling rule and the Curie temperatures are well above room temperature. All compounds show a metallic like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting like behavior. A large negative magnetoresistance of 55% is observed for Co2TiSn at room temperature in an applied magnetic field of 4T which is comparable to the large negative magnetoresistances of the manganites. The Seebeck coefficients are negative for all three compounds and reach their maximum values at their respective Curie temperatures and stay almost constant up to 950 K. The highest value achieved is -52muV/K m for Co2TiSn which is large for a metal. The combination of half-metallicity and the constant large Seebeck coefficient over a wide temperature range makes these compounds interesting materials for thermoelectric applications and further spincaloric investigations., Comment: 4 pages 4 figures

This work reports on high resolution photoelectron spectroscopy for the valence band of Co2Mn(1-x)Fe(x)Si (x=0,0.5,1) excited by photons of about 8 keV energy. The measurements show a good agreement to calculations of the electronic structure using the LDA+U scheme. It is shown that the high energy spectra reveal the bulk electronic structure better compared to low energy XPS spectra. The high resolution measurements of the valence band close to the Fermi energy indicate the existence of the gap in the minority states for all three alloys., Comment: 14 pages, 5 figures, submitted to J. Phys. D: Appl. Phys

Polycrystalline samples of the half-metallic ferromagnet Heusler compound Co$_2$TiSn have been prepared and studied using bulk techniques (X-ray diffraction and magnetization) as well as local probes ($^{119}$Sn M\"ossbauer spectroscopy and $^{59}$Co nuclear magnetic resonance spectroscopy) in order to determine how disorder affects half-metallic behavior and also, to establish the joint use of M\"ossbauer and NMR spectroscopies as a quantitative probe of local ion ordering in these compounds. Additionally, density functional electronic structure calculations on ordered and partially disordered Co$_2$TiSn compounds have been carried out at a number of different levels of theory in order to simultaneously understand how the particular choice of DFT scheme as well as disorder affect the computed magnetization. Our studies suggest that a sample which seems well-ordered by X-ray diffraction and magnetization measurements can possess up to 10% of antisite (Co/Ti) disordering. Computations similarly suggest that even 12.5% antisite Co/Ti disorder does not destroy the half-metallic character of this material. However, the use of an appropriate level of non-local DFT is crucial., Comment: 11 pages and 5 figures

This work reports about bulk-sensitive, high energy photoelectron spectroscopy from the valence band of CoTiSb excited by photons from 1.2 to 5 keV energy. The high energy photoelectron spectra were taken at the KMC-1 high energy beamline of BESSY II employing the recently developed Phoibos 225 HV analyser. The measurements show a good agreement to calculations of the electronic structure using the LDA scheme. It is shown that the high energy spectra reveal the bulk electronic structure better compared to low energy XPS spectra., Comment: J. Electron Spectrosc. Relat. Phenom. accepted

Half-Heusler compounds with 18 valence electrons are semi-conducting. It will be shown that doping with electrons results in half-metallic ferromagnets, similar to the case of diluted semi-conductors. CoTiSb is known to be a semi-conducting Half-Heusler compound. Doping by Fe is expected to result in ferromagnetic order. It was found that Ti can be replaced by up to about 10% Fe while its crystal structure still remains C1b, which was proved by X-ray powder diffraction. SQUID magnetometry revealed a magnetic moment of 0.32 mB per unit cell at 5K., Comment: JMMM accepted

Electronic structure calculation were used to predict a new material for spintronic applications. Co2Mn0.5Fe0.5Si is one example which is stable against on-site correlation and disorder effects due to the position of the Fermi energy in the middle of the minority band gap. Experimentally the sample were made exhibiting L21 structure and a high magnetic order., Comment: 5 pages, 2 Figures, J. Magn. Magn. Mater. accepted

This work reports on the bulk properties of the quaternary Heusler alloy Co$_2$Mn$_{1-x}$Fe$_x$Si with the Fe concentration $x=$. All samples, which were prepared by arc melting, exhibit $L2_1$ long range order over the complete range of Fe concentration. Structural and magnetic properties of Co$_2$Mn$_{1-x}$Fe$_x$Si Heusler alloys were investigated by means of X-ray diffraction, high and low temperature magnetometry, M{\"o\ss}bauer spectroscopy, and differential scanning calorimetry. The electronic structure was explored by means of high energy photo emission spectroscopy at about 8 keV photon energy. This ensures true bulk sensitivity of the measurements. The magnetization of the Fe doped Heusler alloys is in agreement with the values of the magnetic moments expected for a Slater-Pauling like behavior of half-metallic ferromagnets. The experimental findings are discussed on the hand of self-consistent calculations of the electronic and magnetic structure. To achieve good agreement with experiment, the calculations indicate that on-site electron-electron correlation must be taken into account, even at low Fe concentration. The present investigation focuses on searching for the quaternary compound where the half-metallic behavior is stable against outside influences. Overall, the results suggest that the best candidate may be found at an iron concentration of about 50%., Comment: 26 pages, 9 figures Phys. Rev. B accepted