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A novel approach for optimization of the thermoelectric properties of p-type Heusler compounds with a C1b structure was investigated. A successful recipe for achieving intrinsic phase separation in the n-type material based on the TiNiSn system is isoelectronic partial substitution of Ti with its heavier homologues Zr and Hf. We applied this concept to the p-type system MCoSb0.8Sn0.2 by a systematic investigation of samples with different compositions at the Ti position (M = Ti, Zr, Hf, Ti0.5Zr0.5, Zr0.5Hf0.5, and Ti0.5Hf0.5). We thus achieved an approximately 40% reduction of the thermal conductivity and a maximum figure of merit ZT of 0.9 at 700 °C. This is a 80% improvement in peak ZT from 0.5 to 0.9 at 700 °C compared to the best published value of an ingot p-type half-Heusler compound. Thus far, comparable good thermoelectric p-type materials of this structure type have only been realized by a nanostructuring process via ball milling of premelted ingot samples followed by a rapid consolidation method, like hot pressing. The herein-presented simple arc-melting fabrication method reduces the fabrication time as compared to this multi-step nanostructuring process. The high mechanical stability of the Heusler compounds is favorable for the construction of thermoelectric modules. The Vickers hardness values are close to those of the n-type material, leading to good co-processability of both materials. [ABSTRACT FROM AUTHOR]

Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometric Co-based Heusler alloy shows an intense dependency of the tunnel magnetoresistance (TMR) on the Mn composition, demonstrating high TMR ratios of above 1000 % at 4.2 K (Yamamoto et al. J. Phys. Condens. Matter 22:164212, ). This work reports on the electronic structure of nonstoichiometric CoMnGe 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 nonstoichiometric CoMnGe films close to the Fermi energy indicate a shift of the spectral weight compared to bulk CoMnGe. 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 CoMnGe, together with additional Mn, is an appropriate technique to adjust the stoichiometry of the CoMnGe film composition. The resulting changes of the electronic structure within the valence band will allow one to tune the magnetoresistive characteristics of CoMnGe-based tunnel junctions as verified by the calculations and photoemission experiments. [ABSTRACT FROM AUTHOR]

Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attention due to its potential applications in various spintronic devices. However, the interfacial magnetic and electronic structure of a prototypical realization of this in Fe/MgO/Fe and the effective band gap of the MgO layer are still under debate. In order to resolve these issues, we have employed standing-wave excited core and valence photoemission, as well as core-level magnetic circular dichroism (MCD) in photoemission, to study the Fe/MgO interface with subnanometer depth resolution. For our synthetic procedure, we show that the Fe/MgO interface is linearly intermixed in composition over a length of ~8 Å (~4 monolayers) and that there is a magnetic dead layer ~2-3 Å thick. The unambiguous extraction of depth-resolved density of states (DOS) reveals that the interfacial layer composition is mostly metallic and nonmagnetic FeOx, with x ≅ 1, which accounts for a smaller magnetoresistance compared to theoretical predictions. The formation of the magnetic dead layer (FeO) at the interface should also reduce the tunneling spin polarization. The analysis of our data also shows a clear valence band edge of ultrathin MgO layer at ~3.5 eV below the Fermi level (EF) that is very close to that of single crystal bulk MgO. An analysis that does not consider the interdiffused region separately exhibits the valence band edge for MgO layer ~1.3 eV below EF, which is significantly closer to the MgO barrier height estimated from magnetotransport measurements and further suggests that the Fe/MgO interdiffusion effectively reduces the MgO band gap. [ABSTRACT FROM AUTHOR]

57Fe Mössbauer spectroscopic studies of Co2Mn1 − x Fe x Al based Heusler compounds are presented. The cubic structure type was found for all samples of the series in XRD studies. However, a mixture of the cubic B2 and L21 structure types are not unambiguously distinguished by XRD. The present study gives evidence for a mixture of the L21 structure with additions of a large amount of B2 like disorder. The isomer shifts and hyperfine magnetic fields on 57Fe atoms are compared with theoretical values from CPA calculations. [ABSTRACT FROM AUTHOR]

Spintronic is a multidisciplinary field and a new research area. New materials must be found for satisfying the different types of demands. The search for stable half-metallic ferromagnets and ferromagnetic semiconductors with Curie temperatures higher than room temperature is still a challenge for solid state scientists. A general understanding of how structures are related to properties is a necessary prerequisite for material design. Computational simulations are an important tool for a rational design of new materials. The new developments in this new field are reported from the point of view of material scientists. The development of magnetic Heusler compounds specifically designed as material for spintronic applications has made tremendous progress in the very recent past. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% in magnetic tunnel junctions. High Curie temperatures were found in Co2-based Heusler compounds with values up to 1120K in Co2FeSi. The latest results at the time of writing are a tunnelling magnet resistance (TMR) device made from the Co2FeAl0.5Si0.5 Heusler compound and working at room temperature with a (TMR) effect higher than 200%. Good interfaces and a well-ordered compound are the precondition to realize the predicted half-metallic properties. The series Co2FeAl1-x Six is found to exhibit half-metallic ferromagnetism over a broad range, and it is shown that electron doping stabilizes the gap in the minority states for x = 0.5. This might be a reason for the exceptional temperature behaviour of Co2FeAl0.5Si0.5 TMR devices. Using x-ray diffraction (XRD), it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For the compounds Co2FeGa or Co2FeGe, with Curie temperatures expected higher than 1000 K, the standard XRD technique using laboratory sources cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the data indicated that both compounds crystallize in the L21 structure which makes these two compounds suitable new candidates as materials in magnetic tunnel junctions. [ABSTRACT FROM AUTHOR]

Mn2NiGa is reported to be a shape memory material with a martensite-austenite phase transition. Temperature dependent measurements of the transport and magnetic properties reveal the martensitic transition close to room temperature with a thermal hysteresis of about 27 K. The electronic structure of the valence band in both phases was studied by hard x-ray photoelectron spectroscopy. The results clearly indicate that strong changes in the electronic structure appear at the phase transition. [ABSTRACT FROM AUTHOR]

This work reports on the structural and physical properties of the Heusler alloy (Zr0.5Hf0.5)1-xNbxNiSn with varying Nb concentrations. The structure of the (Zr0.5Hf0.5)1-xNbxNiSn solid solution was investigated by means of X-ray diffraction. It is found that the alloys exhibit the C1b structure for all Nb concentrations. The physical properties were studied using the physical properties measurement system from low temperature to room temperature. It was shown that the thermoelectric properties like the dimensionless Figure of Merit are increased five times by substituting (Zr0.5Hf0.5) with Nb to 0.09 at 300 K and the Powerfactor is increased 10 times to 1.8 mW/K2 m at 300 K. [ABSTRACT FROM AUTHOR]

The effect of Ti substitution by Sc on the thermoelectric properties of the Heusler compounds NiTi0.3-xScxZr0.35Hf0.35Sn (where 00.3Zr0.35Hf0.35Sn showed n-type conductivity with a Seebeck coefficient of -288 μV/K at 350 K, while under Sc substitution the system switched to p-type behavior. A maximum Seebeck coefficient of +230 μV/K (350 K) was obtained by 4% Sc substitution, which is the highest value for p-type thermoelectric compounds based on Heusler alloys. The electronic structure was studied by photoelectron spectroscopy excited by hard x-ray synchrotron radiation. Massive in gap states are observed for the parent compound. This proves that the electronic states close to the Fermi energy play a key role on the behavior of the transport properties. Especially, they are responsible for the high, negative Seebeck coefficient of the parent compound. [ABSTRACT FROM AUTHOR]

This work reports on the magnetostructural properties of Co2-xFe1+xSi with 00. Changes in the relative intensities indicate the well ordered substitution of Co by Fe atoms [ABSTRACT FROM AUTHOR]

This work reports on the detection of the valence band of buried Heusler compounds by means of hard x-ray photoemission spectroscopy. The measurements have been performed on the so-called “half” tunnel junctions that are thin films of Co2MnSi underneath MgO. Starting from the substrate, the structure of the samples is MgO(buffer)–Co2MnSi–MgO(tMgO)–AlOx with a thickness tMgO of the upper MgO layer of 2 and 20 nm. The valence band x-ray photoemission spectra have been excited by hard x rays of about 6 keV energy. The valence band spectra have been used to estimate the mean free path of the electrons through the MgO layer to be 17 nm at kinetic energies of about 6 keV. In particular, it is shown that the buried Co2MnSi films exhibit the same valence density of states as in bulk samples. [ABSTRACT FROM AUTHOR]

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 55Mn NMR measurements unambiguously demonstrate the random distribution of Mn and Fe on the 4b Wyckoff position in Co2Mn0.5Fe0.5Si. [ABSTRACT FROM AUTHOR]

This work reports about the structural and magnetic properties of the quaternary Heusler alloy Co2FeAl1-xSix with varying Si concentration. The structure of the Co2FeAl1-xSix solid solution was investigated by means of x-ray diffraction and differential scanning calorimetry. It is found that the alloys exhibit the L21 structure for x>=0.4. Depending on the Si concentration x, a structural phase transition is found at temperatures between 1000 and 1200 K. The transition temperature decreases with increasing Si content x. The magnetic properties were studied using low temperature magnetometry. For x>=0.4, the magnetization of the compounds is in agreement with a Slater-Pauling-like behavior and with the values obtained from ab initio calculations. For x≤0.4, enlarged magnetic moments are found due to disorder. The enlarged values are explained by an increase of the Co spin moments and enhanced orbital moments [ABSTRACT FROM AUTHOR]

This work reports about the structural and magnetic properties of the quaternary Heusler alloy Co2FeAl1-xSix with varying Si concentration. The structure of the Co2FeAl1-xSix solid solution was investigated by means of x-ray diffraction and differential scanning calorimetry. It is found that the alloys exhibit the L21 structure for x>=0.4. Depending on the Si concentration x, a structural phase transition is found at temperatures between 1000 and 1200 K. The transition temperature decreases with increasing Si content x. The magnetic properties were studied using low temperature magnetometry. For x>=0.4, the magnetization of the compounds is in agreement with a Slater-Pauling-like behavior and with the values obtained from ab initio calculations. For x≤0.4, enlarged magnetic moments are found due to disorder. The enlarged values are explained by an increase of the Co spin moments and enhanced orbital moments [ABSTRACT FROM AUTHOR]

This work reports on the structure of Fe containing, Co2-based Heusler compounds that are suitable for magnetoelectronic applications. The compounds Co2FeZ (where Z=Al, Si, Ga, and Ge) were investigated using the x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) techniques. Using XRD, it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For compounds containing Ga or Ge, the XRD technique cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the EXAFS data indicated that both compounds crystallize in the L21 structure. [ABSTRACT FROM AUTHOR]

This work reports about the electronic, magnetic, and structural properties of the binary compound Mn3Ga. The tetragonal DO22 phase of Mn3Ga was successfully synthesized and investigated. It has been found that the material is hard magnetic with an energy product of Hc×Br=52.5 kJ m-3 and an average saturation magnetization of about 0.25μB/at. at 5 K. The saturation magnetization indicates a ferrimagnetic order with partially compensating moments at the Mn atoms on crystallographically different sites. The Curie temperature is above 730 K where the onset of decomposition is observed. The electronic structure calculations indicate a nearly half-metallic ferrimagnetic order with 88% spin polarization at the Fermi energy. [ABSTRACT FROM AUTHOR]

The present work elaborates on the correlation between the amount and ordering of the free carbon phase in silicon oxycarbides and their charge carrier transport behavior. Thus, silicon oxycarbides possessing free carbon contents from 0 to ca. 58 vol.% (SiOC/C) were synthesized and exposed to temperatures from 1100 to 1800 °C. The prepared samples were extensively analyzed concerning the thermal evolution of the sp2 carbon phase by means of Raman spectroscopy. Additionally, electrical conductivity and Hall measurements were performed and correlated with the structural information obtained from the Raman spectroscopic investigation. It is shown that the percolation threshold in SiOC/C samples depends on the temperature of their thermal treatment, varying from ca. 20 vol.% in the samples prepared at 1100 °C to ca. 6 vol.% for the samples annealed at 1600 °C. Moreover, three different conduction regimes are identified in SiOC/C, depending on its sp2 carbon content: (i) at low carbon contents (i.e., <1 vol.%), the silicon oxycarbide glassy matrix dominates the charge carrier transport, which exhibits an activation energy of ca. 1 eV and occurs within localized states, presumably dangling bonds; (ii) near the percolation threshold, tunneling or hopping of charge carriers between spatially separated sp2 carbon precipitates appear to be responsible for the electrical conductivity; (iii) whereas above the percolation threshold, the charge carrier transport is only weakly activated (Ea = 0.03 eV) and is realized through the (continuous) carbon phase. Hall measurements on SiOC/C samples above the percolation threshold indicate p-type carriers mainly contributing to conduction. Their density is shown to vary with the sp2 carbon content in the range from 1014 to 1019 cm−3; whereas their mobility (ca. 3 cm2/V) seems to not depend on the sp2 carbon content. [ABSTRACT FROM AUTHOR]

Diluted magnetic semiconductors were prepared by substituting titanium in the semiconducting compound CoTiSb with iron. The structural, electronic, and magnetic properties of the pure and doped materials were investigated. It was found that substitution of up to 10% Ti by Fe does not affect the crystalline structure. Self-consistent calculations of the electronic structure predict the material to be a half-metallic ferromagnet. The Curie temperature of the Fe substituted alloy is far above room temperature (>700 K), thus making that material a serious candidate for future electronic applications, in particular, for magnetoelectronics and spintronics. [ABSTRACT FROM AUTHOR]

This work reports on the electronic and crystalline structure and the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co2MnGe. The crystalline structure was examined in detail by extended x-ray absorption fine-structure spectroscopy and anomalous x-ray diffraction. The compound exhibits a well-ordered L21 structure as is typical for Heusler compounds with 2:1:1 stoichiometry. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab initio calculations. Transport measurements and hard x-ray photoelectron spectroscopy were performed to explain the electronic structure of the compound. The obtained valence band spectra exhibit small energy shifts that are the result of the photoexcitation process, whereas electron-electron correlation in the ground state is negligible. The vibration and mechanical properties of the compound were calculated. The observed hardness values are consistent to a covalent-like bonding of Co2MnGe. [ABSTRACT FROM AUTHOR]

The magnetic ground state of the Heusler compound Mn2CoSn was predicted to be nearly half-metallic ferrimagnetic with a high spin polarization by ab initio electronic structure calculations. Mn2CoSn was synthesized, and the magnetic behavior of the compound was studied using a superconducting quantum interference device and x-ray magnetic circular dichroism. The experimental values were found to be in fair accordance with the theoretical predictions. The electronic structure and the crystal structure of Mn2CoSn were characterized comprehensively using x-ray powder diffraction, 119Sn Mössbauer spectroscopy, nuclear magnetic resonance, and hard x-ray photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

The family of half-metallic ferromagnets Co2TiZ exhibits exceptional transport properties. The investigated compounds Co2TiAl(1− x )Si x (x =0.25, 0.5, 0.75) show Curie temperatures (T Cs) that vary between 250 and 350K, depending on the composition. Above T C the Seebeck coefficient remains constant. This makes them promising candidates for thermoelectric devices such as thermocouples with a tunable working range. The electrical resistivity data show an anomaly at T C which is attributed to changes in the electronic structure and therefore in the carrier concentration. [ABSTRACT FROM AUTHOR]

The tunnel magnetoresistance ratio (TMR) of fully epitaxial magnetic tunnel junctions with an off- stoichiometric Co2MnSi Heusler alloy has been shown to exhibit a systematic dependence on Mn content, reaching 1135% at 4.2 K for Co2Mn1.29Si. In this paper, we explain the behaviorof the observed TMR ratio using ab initio calculations and hard x-ray photoelectron spectroscopy (HAXPES). For the Mn-deficient samples, we show that the the drop of the TMR is caused by Co antisite atoms, which impose extra states into the minority-spin band gap. On the other hand, Mn-excess composition shows nearly half-metallic behavior. This result can be intuitively understood since both Co2MnSi and Mn2CoSi are theoretically predicted to be half-metallic ferromagnets. [ABSTRACT FROM AUTHOR]

Abstract: Due to their exceptional thermoelectric properties Half-Heusler alloys like MNiSn (M=Ti,Zr,Hf) have moved into focus. The growth of single crystalline thin film TiNiSn and Zr0.5Hf0.5NiSn by dc magnetron sputtering is reported. Seebeck and resistivity measurements were performed and their dependence on epitaxial quality is shown. Seebeck coefficient, specific resistivity and power factor for Zr0.5Hf0.5NiSn at room temperature were measured to be 63μVK−1, 14.1μΩm and 0.28mWK−2 m−1, respectively. Multilayers of TiNiSn and Zr0.5Hf0.5NiSn are promising candidates to increase the thermoelectric figure-of-merit by decreasing thermal conductivity perpendicular to the interfaces. The epitaxial growth of multilayers containing TiNiSn and Zr0.5Hf0.5NiSn is demonstrated by measuring satellite peaks in the X-ray diffraction pattern originating from the additional symmetry perpendicular to the film surface. [Copyright &y& Elsevier]

We investigate the phase separation of the solid solution CoTi(1− x )Mn x Sb into the two Heusler compounds CoTiSb and CoMnSb. Energy-dispersive X-ray spectroscopy measurements on the two-phase material reveal the presence of size- and shape-tunable CoTiSb regions in a CoMnSb matrix. We demonstrate that the formed phase and grain boundaries have a considerable influence on the phonon scattering processes, which leads to a reduction in the thermal conductivity by a factor of three compared to single-phase CoTiSb. [ABSTRACT FROM AUTHOR]

The Slater-Pauling rule states that L21 Heusler compounds with 24 valence electrons never exhibit a total spin magnetic moment. In the 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 the 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, which 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. [ABSTRACT FROM AUTHOR]

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 fulfil 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 behaviour. A large negative magnetoresistance (MR) of 55 per cent is observed for Co₂TiSn at room temperature in an applied magnetic field of μ(0)H=4T, which is comparable to the large negative MRs 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 -52 μVK(-1) 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.

Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.