0000000000033810
AUTHOR
Stanislav Chadov
Hall effect and electronic structure of films
Abstract Tunneling experiments have shown that in order to retain half-metallicity at room temperature not only a large gap is required but also a Fermi energy considerably distant from the minority band edges. We correlate the position of the Fermi energy in the spin minority gap obtained from band structure calculations to Hall effect experiments. As a model system we chose Co 2 Fe x Mn 1 - x Si , where the Fermi energy was calculated to move from the valence band edge of the minority states to the conduction band edge with increasing x . On high quality laser ablated epitaxial films we observe a sign change of both the normal and the anomalous Hall effect with doping. The experimental da…
Topological Insulators from a Chemist's Perspective
Topology and chemistry are deeply entangled subjects, whichmanifests in the way chemists like to think and approachproblems. Although not at first glance, topology allows thecategorizationoffundamentalinherentpropertiesofthehugenumber of different chemical compounds, carving out theunique features of a class of materials of different complexity,a topic which Turro worked out in his treatise on geometricaland topological thinking in chemistry.
Half-metallic compensated ferrimagnetism with a tunable compensation point over a wide temperature range in the Mn-Fe-V-Al Heusler system
The cubic Heusler compound Mn1.5FeV0.5Al with the L21 Heusler structure is the first fully compensated half-metallic ferrimagnet with 24 valence electrons. The ferrimagnetic state can be tuned by changing the composition such that the compensation point appears at finite temperatures ranging from 0 K up to 226 K, while retaining half-metallicity in the system. In this paper, the structural, magnetic and transport properties of the Mn-Fe-V-Al system are discussed. Magnetic reversal and a change of sign of the anomalous Hall effect were observed at the compensation point, which gives rise to a sublattice spin-crossing. These materials present new possibilities for potential spintronic devices…
Quaternary Heusler Compounds without Inversion Symmetry: CoFe 1+ x Ti 1– x Al and CoMn 1+ x V 1– x Al
We report the quaternary Heusler compound derivatives CoFe1+xTi1–xAl and CoMn1+xV1–xAl, which do not have centers of inversion. Classical T2T′M (T, T′ = transition metal, M = main group element) Heusler compounds (prototype: Cu2MnAl) crystallize in the L21 structure, space group Fmm (225) that exhibits a center of inversion. Replacing one of the T2 atoms by another transition element (T″) results in a quaternary TT′T″M compound with F3m symmetry (Y; structure type LiMgPdSn) without center of inversion. In the case of “quasi closed shell” compounds with 24 valence electrons in the primitive cell, one expects the absence of ferromagnetism according to the Slater–Pauling rule. Increasing the n…
Direct observation of half-metallicity in the Heusler compound $Co_{2}MnSi$
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 experimen…
Pressure induced insulator/half-metal/metal transition in a strongly correlatedp-electron system
Mixed-valent Rb${}_{4}$O${}_{6}$ provides an exceptional prototype material for studying the interplay between local correlations (Hubbard $U$) and electron kinetic energy ($W$) in the open $sp$-electron shell. Based on a first-principles calculation we show that depending on $U/W$ ratio, when tuned by external pressure, Rb${}_{4}$O${}_{6}$ exhibits a surprising sequence of phase transitions between strongly correlated antiferromagnetic insulator, ferromagnetic insulator ($U/W\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gg}\phantom{\rule{-0.16em}{0ex}}1$), moderately correlated ferromagnetic half-metal ($U/W\phantom{\rule{-0.16em}{0ex}}\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}1$), and fin…
Localized magnetic moments in the Heusler alloy Rh2MnGe
X-ray magnetic circular dichroism (XMCD) of core-level absorption (x-ray absorption spectroscopy, XAS) spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloy Rh2MnGe at the Rh M3,2 and Mn L3,2 edges. The ratio of Rh and Mn spin moments amounts to 0.05 which is smaller than the ratio of 0.1 determined by a local density approximation electronic band structure calculation. We have found that the orbital moments of the Rh 4d and Mn 3d states are very small. The observed Rh 2p XAS spectrum can be understood on the basis of the Rh 3d partial density of unoccupied states as is typical for metals. The observed features of the Mn 2p XAS and XMCD spectra are dominated…
Topological Insulators from a Chemist’s Perspective
Topology and chemistry are deeply entangled subjects, whichmanifests in the way chemists like to think and approachproblems. Although not at first glance, topology allows thecategorizationoffundamentalinherentpropertiesofthehugenumber of different chemical compounds, carving out theunique features of a class of materials of different complexity,a topic which Turro worked out in his treatise on geometricaland topological thinking in chemistry.
Spin-resolved low-energy and hard x-ray photoelectron spectroscopy of off-stoichiometric Co2MnSi Heusler thin films exhibiting a record TMR
Half-metallic Co2MnSi-based Heusler compounds have attracted attention because they yield very high tunnelling magnetoresistance (TMR) ratios. Record TMR ratios of 1995% (at 4.2 K) are obtained from off-stoichiometric Co2MnSi-based magnetic tunnel junctions. This work reports on a combination of band structure calculations and spin-resolved and photon-polarisation-dependent photoelectron spectroscopy for off-stoichiometric Heusler thin films with the composition Co2Mn1.30Si0.84. Co and Mn are probed by magnetic dichroism in angle-resolved photoelectron spectroscopy at the 2p core level. In contrast to the delocalised Co 3d states, a pronounced localisation of the Mn 3d states is deduced fro…
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 multifunct…
Probing the electronic states of high-TMR off-stoichiometric Co2MnSi thin films by hard x-ray photoelectron spectroscopy
The tunnel magnetoresistance ratio (TMR) of fully epitaxial magnetic tunnel junctions with an off-stoichiometric Co${}_{2}$MnSi Heusler alloy has been shown to exhibit a systematic dependence on Mn content, reaching 1135% at 4.2 K for Co${}_{2}$Mn${}_{1.29}$Si. In this paper, we explain the behavior of 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 Co…
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. Starting with the discovery of two dimensional TIs, the HgTe-based quantum wells, many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families, the HgTe family and the Bi2Se family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and the…
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. Starting with the discovery of two dimensional TIs, the HgTe-based quantum wells, many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families, the HgTe family and the Bi2Se family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and the…
Tetragonal Heusler Compounds for Spintronics
With respect to the requirements of spin torque transfer (STT) materials, one the most promising materials families are the tunable tetragonal Heusler compounds based on Mn2YZ (Y=Co,Fe,Ni,Rh,...; Z=Al, Ga, Sn). They form the inverse cubic Heusler structure with three distinct magnetic sublattices, which allows a fine tuning of the magnetic properties. Starting with the stoichiometric Mn3Ga compound, we explored the complete phase diagram of Mn3-xYxZ (Y=Co, Fe, Ni and Z=Ga ). All series exhibit thermally stable magnetic properties. As we demonstrate, Mn3-xFexGa series, which are tetragonal over the whole range of compositions, are good as hard magnets, whereas magnetically more weak Mn3-xNix…
Correlation effects in the total energy, the bulk modulus, and the lattice constant of a transition metal: Combined local-density approximation and dynamical mean-field theory applied to Ni and Mn
We present an accurate implementation of total-energy calculations into the local-density approximation plus dynamical mean-field theory $(\text{LDA}+\text{DMFT})$ method. The electronic structure problem is solved through the full-potential linear muffin-tin orbital and Korringa-Kohn-Rostoker methods with a perturbative solver for the effective impurity suitable for moderately correlated systems. We have tested the method in detail for the case of Ni, and investigated the sensitivity of the results to the computational scheme and to the complete self-consistency. It is demonstrated that the $\text{LDA}+\text{DMFT}$ method can resolve a long-standing controversy between the LDA/generalized …
Electronic structure, localization, and spin-state transition in Cu-substitutedFeSe:Fe1−xCuxSe
We report density-functional studies of the ${\text{Fe}}_{1\ensuremath{-}x}{\text{Cu}}_{x}\text{Se}$ 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 ${d}^{10}$ 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 $x\ensuremath{\approx}0.12$. We associate this with the experimentally observed jump near this concentration. Our results are …
Iron-based Heusler compounds Fe2YZ: Comparison with theoretical predictions of the crystal structure and magnetic properties
The present work reports on the new soft ferromagnetic Heusler phases Fe${}_{2}$NiGe, Fe${}_{2}$CuGa, and Fe${}_{2}$CuAl, which in previous theoretical studies have been predicted to exist in a tetragonal Heusler structure. Together with the known phases Fe${}_{2}$CoGe and Fe${}_{2}$NiGa these materials have been synthesized and characterized by powder x-ray diffraction, ${}^{57}$Fe M\"ossbauer spectroscopy, superconducting quantum interference device, and energy-dispersive x-ray measurements. In particular M\"ossbauer spectroscopy was used to monitor the degree of local atomic order/disorder and to estimate magnetic moments at the Fe sites from the hyperfine fields. It is shown that in con…
Dirakův kužel a pseudogapped hustota stavů v topologické polovině Heuslerovy sloučeniny YPtBi
Topologické izolátory (Tis) jsou zajímavé materiály, které vykazují nebývalé vlastnosti. . Zde jsme prozkoumali sloučeniny YPtBi jako příklad ze třídy polovu-Heuslerových materiálů. Topological insulators (TIs) are exciting materials, which exhibit unprecedented properties, such as helical spinmomentum locking, which leads to large torques for magnetic switching and highly efficient spin current detection. Here we explore the compound YPtBi, an example from the class of half-Heusler materials, for which the typical band inversion of topological insulators was predicted. We prepared this material as thin films by conventional cosputtering from elementary targets. By in situ time-of-flight mo…
Spin Polarimetry and Magnetic Dichroism on a Buried Magnetic Layer Using Hard X-ray Photoelectron Spectroscopy
The spin-resolved electronic structure of buried magnetic layers is studied by hard X-ray photoelectron spectroscopy (HAXPES) using a spin polarimeter in combination with a high-energy hemispherical electron analyzer at the high-brilliance BL47XU beamline (SPring-8, Japan). Spin-resolved photoelectron spectra are analyzed in comparison with the results of magnetic linear and circular dichroism in photoelectron emission in the case of buried Co2FeAl0.5Si0.5 layers. The relatively large inelastic mean free path (up to 20 nm) of fast photoelectrons enables us to extend the HAXPES technique with electron-spin polarimetry and to develop spin analysis techniques for buried magnetic multilayers a…
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.
Topological Insulators in Ternary Compounds with a Honeycomb Lattice
One of the most exciting subjects in solid state physics is a single layer of graphite which exhibits a variety of unconventional novel properties. The key feature of its electronic structure are linear dispersive bands which cross in a single point at the Fermi energy. This so-called Dirac cone is closely related to the surface states of the recently discovered topological insulators. The ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states. In contrast to graphene with two Dirac cones at K and K' points, these material…
Large Zero-Field Cooled Exchange-Bias in BulkMn2PtGa
We report a large exchange-bias (EB) effect after zero-field cooling the new tetragonal Heusler compound Mn2PtGa from the paramagnetic state. The first-principle calculation and the magnetic measurements reveal that Mn2PtGa orders ferrimagnetically with some ferromagnetic (FM) inclusions. We show that ferrimagnetic (FI) ordering is essential to isothermally induce the exchange anisotropy needed for the zero-field cooled (ZFC) EB 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…
The role of correlations in the high-pressure phase of FeSe
This study addresses the structural and electronic properties of the NiAs- and MnP-type phases dominating in FeSe at high pressures. The analysis is performed using first-principle band structure calculations within the framework of the B3LYP hybrid exchange-correlation functional. Based on the volume-pressure relation deduced from the available experimental data, we optimize the form and internal coordinates of the unit cell, which agree reasonably well with experiment. In particular, the present calculations resolve the structural NiAs-MnP phase transition which occurs at about 10 GPa. Both structures are found to be semiconducting at low pressures and metallizing at about 80-90 GPa. Usin…
Helical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 Fe$_{1-y}$Co$_y$Ge films
Epitaxial films of the B20-structure compound Fe1−yCoyGe were grown by molecular beam epitaxy on Si (111) substrates. The magnetization varied smoothly from the bulklike values of one Bohr magneton per Fe atom for FeGe to zero for nonmagnetic CoGe. The chiral lattice structure leads to a Dzyaloshinskii-Moriya interaction (DMI), and the films' helical magnetic ground state was confirmed using polarized neutron reflectometry measurements. The pitch of the spin helix, measured by this method, varies with Co content y and diverges at y∼0.45. This indicates a zero crossing of the DMI, which we reproduced in calculations using first-principles methods. We also measured the longitudinal and Hall r…
Quaternary Heusler compounds Co(2-x)Rh(x)MnZ (Z = Ga, Sn, Sb): crystal structure, electronic structure, and magnetic properties.
Within the huge family of Heusler compounds only a few quaternary derivatives are known that crystallize in the F43m space group. In this work, the yet unreported compounds CoRhMnZ (Z = Ga, Sn, Sb) and the alloy Co(0.5)Rh(1.5)MnSb were investigated in detail by experimental techniques and theoretical methods. The ab initio calculations predict the CoRhMnZ compounds to be half-metallic ferromagnets or to be close to the half-metallic ferromagnetic state. Calculations of the elastic constants show that the cubic structure is stable in compounds containing Mn. Both calculations and experiment reveal that Mn cannot be exchanged by Fe (CoRhFeGa). The low temperature magnetization of the compound…
Tunable multifunctional topological insulators in ternary Heusler compounds
Recently the Quantum Spin Hall effect (QSH) was theoretically predicted and experimentally realized in a quantum wells based on binary semiconductor HgTe[1-3]. QSH state and topological insulators are the new states of quantum matter interesting both for fundamental condensed matter physics and material science[1-11]. Many of Heusler compounds with C1b structure are ternary semiconductors which are structurally and electronically related to the binary semiconductors. The diversity of Heusler materials opens wide possibilities for tuning the band gap and setting the desired band inversion by choosing compounds with appropriate hybridization strength (by lattice parameter) and the magnitude o…
Test of band structure calculations for Heusler compounds by spin-resolved photoemission spectroscopy
The electronic density of states of epitaxial thin films of the Heusler compound Co${}_{2}$MnGa is probed in situ by spin-resolved ultraviolet photoemission spectroscopy. The experiments reveal several characteristic features in the intensity spectrum and a clear Fermi edge signature. A high spin polarization of $\ensuremath{\simeq}\phantom{\rule{-0.16em}{0ex}}55%$ at the Fermi edge is followed by a sign change at the binding energy of $\ensuremath{\simeq}$0.8 eV. Corresponding calculations of the band structure and the photoemission spectrum were performed employing a spin-polarized relativistic Korringa-Kohn-Rostoker code. Good agreement between the experimental data and calculations was …