Stanislav Chadov

Electron correlations in Co2Mn1−xFexSi Heusler compounds

This study presents the effect of local electronic correlations on the Heusler compounds Co2Mn1−xFexSi as a function of the concentration x. The analysis has been performed by means of first-principles band-structure calculations based on the local approximation to spin-density functional theory (LSDA). Correlation effects are treated in terms of the dynamical mean-field theory and the LSDA + U approach. The formalism is implemented within the Korringa–Kohn–Rostoker Greens function method.In good agreement with the available experimental data the magnetic and spectroscopic properties of the compound are explained in terms of strong electronic correlations. In addition, the correlation effects have been analysed separately with respect to their static or dynamical origin. To achieve a quantitative description of the electronic structure of Co2Mn1−xFexSi both static and dynamic correlations must be treated on an equal footing.

Half-Heusler compounds: novel materials for energy and spintronic applications

Half-Heusler compounds are an impressive class of materials with a huge potential for different applications such as future energy applications and for spintronics. The semiconducting Heusler compounds can be identified by the number of valence electrons. The band gap can be tuned between 0 and 4 eV by the electronegativity difference of the constituents. Magnetism can be introduced in these compounds by using rare-earth elements, manganese or ‘electron’ doping. Thus, there is a great interest in the fields of thermoelectrics, solar cells and diluted magnetic semiconductors. The combination of different properties such as superconductivity and topological edge states leads to new multifunctional materials, which have the potential to revolutionize technological applications. Here, we review the structure, the origin of the band gap and the functionalities of semiconducting half-Heusler compounds.

Design scheme of new tetragonal Heusler compounds for spin-transfer torque applications and its experimental realization.

Band Jahn-Teller type structural instabilities of cubic Mn(2)YZ Heusler compounds causing tetragonal distortions can be predicted by ab initio band-structure calculations. This allows for identification of new Heusler materials with tunable magnetic and structural properties that can satisfy the demands for spintronic applications, such as in spin-transfer torque-based devices.

Direct observation of half-metallicity in the Heusler compound Co2MnSi

Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of () % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure.

Large zero-field cooled exchange-bias in bulk Mn2PtGa.

We report a large exchange-bias effect after zero-field cooling the new tetragonal Heusler compound Mn(2)PtGa from the paramagnetic state. The first-principles calculation and the magnetic measurements reveal that Mn(2)PtGa orders ferrimagnetically with some ferromagnetic inclusions. We show that ferrimagnetic ordering is essential to isothermally induce the exchange anisotropy needed for the zero-field cooled exchange bias during the virgin magnetization process. The complex magnetic behavior at low temperatures is characterized by the coexistence of a field-induced irreversible magnetic behavior and a spin-glass-like phase. The field-induced irreversibility originates from an unusual first-order ferrimagnetic to antiferromagnetic transition, whereas the spin-glass-like state forms due to the existence of antisite disorder intrinsic to the material.

Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias

Rational material design can accelerate the discovery of materials with improved functionalities. This approach can be implemented in Heusler compounds with tunable magnetic sublattices to demonstrate unprecedented magnetic properties. Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. Our design approach is also demonstrated on a second material with a magnetic transition above room temperature, Mn-Fe-Ga, exemplifying the universality of the concept and the feasibility of room-temperature applications. These findings may lead to the development of magneto-electronic devices and rare-earth-free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.

Electronic structure localization and spin-state transition in Cu-substituted FeSe: Fe1-xCuxSe

We report density-functional studies of the Fe{sub 1-x}Cu{sub x}Se alloy done using supercell and coherent-potential approximation methods. Magnetic behavior was investigated using the disordered local moment approach. We find that Cu occurs in a nominal d{sup 10} configuration and is highly disruptive to the electronic structure of the Fe sheets. This would be consistent with a metal-insulator transition due to Anderson localization. We further find a strong crossover from a weak moment itinerant system to a local moment magnet at x{approx}0.12. We associate this with the experimentally observed jump near this concentration. our results are consistent with the characterization of this concentration-dependent jump as a transition to a spin glass.

Efficient spin injector scheme based on Heusler materials.

We present a rational design scheme intended to provide stable high spin polarization at the interfaces of the magnetoresistive junctions by fulfilling the criteria of structural and chemical compatibilities at the interface. This can be realized by joining the semiconducting and half-metallic Heusler materials with similar structures. The present first-principles calculations verify that the interface remains half-metallic if the nearest interface layers effectively form a stable Heusler material with the properties intermediately between the surrounding bulk parts. This leads to a simple rule for selecting the proper combinations.

Topological insulators and thermoelectric materials

Topological insulators (TIs) are a new quantum state of matter which have gapless surface states inside the bulk energy gap [1–4]. Starting with the discovery of two-dimensional TIs, the HgTe-based quantum wells [5, 6], many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families [7], the HgTe family and the Bi2Se3 family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials [8–10]. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and thermoelectrics, and give examples of compound classes where both good thermoelectric properties and topological insulators can be found. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Topological Insulators in Ternary Compounds with a Honeycomb Lattice

We investigate a new class of ternary materials such as LiAuSe and KHgSb with a honeycomb structure in Au-Se and Hg-Sb layers. We demonstrate the band inversion in these materials similar to HgTe, which is a strong precondition for existence of the topological surface states. In contrast with graphene, these materials exhibit strong spin-orbit coupling and a small direct band gap at the Γ point. Since these materials are centrosymmetric, it is straightforward to determine the parity of their wave functions, and hence their topological character. Surprisingly, the compound with strong spin-orbit coupling (KHgSb) is trivial, whereas LiAuSe is found to be a topological insulator.