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Along with recent advancements in thin-film technologies, the engineering of complex transition metal oxide heterostructures offers the possibility of creating novel and tunable multifunctionalities. A representative complex oxide is the perovskite strontium titanate (STO), whose bulk form is nominally a centrosymmetric paraelectric band insulator. By tuning the electron doping, chemical stoichiometry, strain, and charge defects of STO, it is possible to control the electrical, magnetic, and thermal properties of such structures. Here, we demonstrate tunable magnetism in atomically engineered STO thin films grown on STO (001) substrates by controlling the atomic charge defects of titanium (VTi) and oxygen (VO) vacancies. Our results show that the magnetism can be tuned by altering the growth conditions. We provide deep insights into its association to the following defect types: (i) VTi, resulting in a charge rearrangement and local spin polarization, (ii) VO, leading to weak magnetization, and (iii) VTi–VO pairs, which lead to the appearance of a sizable magnetic signal. Our results suggest that controlling charged defects is critical for inducing a net magnetization in STO films. This work provides a crucial step for designing magnetic STO films via defect engineering for magnetic and spin-based electronic applications. [ABSTRACT FROM AUTHOR]

The topological Hall effect is used extensively to study chiral spin textures in various materials. However, the factors controlling its magnitude in technologically-relevant thin films remain uncertain. Using variable-temperature magnetotransport and real-space magnetic imaging in a series of Ir/Fe/Co/Pt heterostructures, here we report that the chiral spin fluctuations at the phase boundary between isolated skyrmions and a disordered skyrmion lattice result in a power-law enhancement of the topological Hall resistivity by up to three orders of magnitude. Our work reveals the dominant role of skyrmion stability and configuration in determining the magnitude of the topological Hall effect. Previous studies of skyrmions in thin film architectures have shown widely-varying magnitudes of the topological Hall effect. Here, Raju et al. show that this variation follows a power-law behaviour driven by chiral spin fluctuations at the phase transition between isolated and lattice skyrmions. [ABSTRACT FROM AUTHOR]

Ultrafast spin dynamics on femto- to picosecond timescales is simulated within a density-operator approach for a Co/Cu bilayer. The electronic structure is represented in a tight-binding form; during the evolution of the density operator, optical excitation by a femtosecond laser pulse, coupling to a bosonic bath as well as dephasing are taken into account. Our simulations corroborate the importance of interfaces for ultrafast transport phenomena and demagnetisation processes. Moreover, we establish a reflow from Cu d orbitals across the interface into Co d orbitals, which shows up prominently in the mean occupation numbers. On top of this, this refilling manifests itself as a minority-spin current proceeding several layers into the Cu region. The present study suggests that the approach captures essential ultrafast phenomena and provides insight into microscopic processes. [ABSTRACT FROM AUTHOR]

The Ohmic spin diode (OSD) is a recent concept in spintronics, which is based on half-metallic magnets and spin-gapless semiconductors (SGSs). Quaternary Heusler compounds offer a unique platform to realize the OSD for room temperature applications as these materials possess very high Curie temperatures as well as half-metallic and spin-gapless semiconducting behavior within the same family. Using state-of-the-art first-principles calculations combined with the nonequilibrium Green's function method, we design four different OSDs based on half-metallic and spin-gapless semiconducting quaternary Heusler compounds. All four OSDs exhibit linear current–voltage (I–V) characteristics with zero threshold voltage VT. We show that these OSDs possess a small leakage current, which stems from the overlap of the conduction and valence band edges of opposite spin channels around the Fermi level in the SGS electrodes. The obtained on/off current ratios vary between 30 and 105. Our results can pave the way for the experimental fabrication of the OSDs within the family of ordered quaternary Heusler compounds. [ABSTRACT FROM AUTHOR]

Complex oxides show extreme sensitivity to structural distortions and defects, and the intricate balance of competing interactions which emerge at atomically defined interfaces may give rise to unexpected physics. In the interfaces of non-magnetic complex oxides, one of the most intriguing properties is the emergence of magnetism which is sensitive to chemical defects. Particularly, it is unclear which defects are responsible for the emergent magnetic interfaces. Here, we show direct and clear experimental evidence, supported by theoretical explanation, that the B-site cation stoichiometry is crucial for the creation and control of magnetism at the interface between non-magnetic ABO3-perovskite oxides, LaAlO3 and SrTiO3. We find that consecutive defect formation, driven by atomic charge compensation, establishes the formation of robust perpendicular magnetic moments at the interface. Our observations propose a route to tune these emerging magnetoelectric structures, which are strongly coupled at the polar-nonpolar complex oxide interfaces. In the interfaces of non-magnetic complex oxides, it is unclear which defects are responsible for the magnetic interfaces. Here, the authors find the B-site cation stoichiometry is crucial for the creation and control of magnetism at the interface between non-magnetic ABO3-perovskite oxides. [ABSTRACT FROM AUTHOR]

The interaction of ferrocene-1,1′-dithiol (FDT) with two parallel Ag(111) surfaces has been theoretically studied at density-functional level. The effect of surface defects on the energetic and electronic structure was investigated. The electronic transport properties are studied with the nonequilibrium Green’s function approach. The adsorption geometry has a strong effect on the electronic levels and conductivity. The presence of point defects strongly enhances the molecule-surface interaction but has a surprisingly small effect on the density of states near the Fermi energy. The FDT-surface bond is particularly strong near terraces or steps and leads to significant shifts of the molecular orbitals relative to the gas phase. For all considered defect types except the single adatom the electronic conductivity through the FDT molecule is decreased compared to adsorption on perfect surfaces. [ABSTRACT FROM AUTHOR]

The electronic structure of the organic semiconductor copper-phthalocyanine (CuPc) has been determined by a combination of conventional and resonant photoemission, near-edge x-ray absorption, as well as by the first-principles calculations. The experimentally obtained electronic valence band structure of CuPc is in very good agreement with the calculated density of states results, allowing the derivation of detailed site specific information. [ABSTRACT FROM AUTHOR]

We report first-principles study of the magnetic structure of Mn3Si. The Mn atoms form two sublattices. One of them is (MnI) characterized by large atomic magnetic moments and the other one (MnII) possesses induced moments. It is shown that in agreement with experiment the magnetic ground state is a spin spiral. We found that the spiral magnetic structure is formed as a result of the competition between direct antiferromagnetic interaction of the MnI magnetic moments and indirect ferromagnetic interaction of the same moments through the magnetic moment of the MnII sublattice. We demonstrate that this competition is strongly volume dependent and leads to volume dependence of the wave vector of the spin spiral. These properties are related to the volume dependence of the induced moments of MnII atoms. [ABSTRACT FROM AUTHOR]

The ferromagnetic oxides La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) couple antiferromagnetically, leading to a canted spin-structure at the interface’s proximity. Performing first-principles calculations in combination with Monte Carlo and finite temperature micromagnetic simulations, we explore possible magnetic phases and find a distinctive element specific hysteresis behavior whose dependence on external parameters is analyzed. The results reveal the interplay of magnetic anisotropy and exchange coupling at the interface for the appearance of interfacial spin canting and unusual magnetic switching which are of strong relevance for spintronic devices. In particular, the different coercive fields for SRO and LSMO allow performing selective switching of SRO and LSMO layers. [ABSTRACT FROM AUTHOR]

We present full ab initio calculations of giant magnetoresistance (GMR) in Co/Cu (001) multilayers including self-consistent impurity scattering potentials. Starting from density functional theory the electronic structure of the multilayer and the scattering at impurities are described by means of a new Green function method. It will be demonstrated that impurity scattering in magnetic multilayers is strongly anisotropic in comparison to impurity scattering in bulk systems. Concerning transport the anisotropy of scattering leads to a formation of highly conducting channels which give rise to short circuits and cause strong variation of GMR as a function of the impurity position in the multilayer. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present ab initio calculations of the giant magnetoresistance in magnetic multilayers. The electronic structure of the multilayers is calculated by spin density functional theory using a screened Korringa–Kohn–Rostoker method. The scattering of nanostructural defects in the multilayers is described by means of a Green’s function method. The scattering potentials are calculated self-consistently. The transport properties are treated quasiclassically solving the Boltzmann equation including the electronic structure of the layered system and the anisotropic scattering. The solution of the Boltzmann equation is performed iteratively taking into account both scattering out and scattering in terms (vertex corrections). Since we consider ferromagnetic systems a two current model is applied. Trends of residual resistivities and giant magnetoresistance ratios are discussed for Co/Cu multilayers with 3d transition metal defects. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The aim of this article is to present ab initio calculations of interlayer exchange coupling for Fe/Au (001) multilayers with the same spacer thickness as in recently performed experiments. The structure is characterized by a bcc–fcc transition. The calculations are based on density functional theory using a scalar relativistic screened Korringa–Kohn–Rostoker method. The results show that in case of ideally smooth interfaces the differences of the theoretically and experimentally obtained coupling strengths by one order of magnitude remain. It will be demonstrated that the usual explanation by means of a mesoscopic roughness fails. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Combining first-principles density functional theory and semiclassical Boltzmann transport, the anisotropic Lorenz function was studied for thermoelectric BiTe/SbTe superlattices and their bulk constituents. It was found that, already for the bulk materials BiTe and SbTe, the Lorenz function is not a clear function of charge carrier concentration and temperature. For electron-doped BiTe/SbTe superlattices, large oscillatory deviations of the Lorenz function from the metallic limit were found even at high charge carrier concentrations. The latter can be referred to quantum well effects, which occur at distinct superlattice periods. [ABSTRACT FROM AUTHOR]

We have studied the thermoelectric properties of porous silicon, a nanostructured, yet single-crystalline form of silicon. Using electrochemical etching, liquid-phase doping, and high-temperature passivation, we show that porous Si can be fabricated such that it has thermoelectric properties superior to bulk Si, for both n- and p-type doping. Hall measurements reveal that the charge carrier mobility is reduced compared to the bulk material which presently limits the increase in thermoelectric efficiency. [ABSTRACT FROM AUTHOR]

Magnetoelectric coupling allows the magnetic state of a material to be changed by an applied electric field. To date, this phenomenon has mainly been observed in insulating materials such as complex multiferroic oxides. Bulk metallic systems do not exhibit magnetoelectric coupling, because applied electric fields are screened by conduction electrons. We demonstrate strong magnetoelectric coupling at the surface of thin iron films using the electric field from a scanning tunnelling microscope, and are able to write, store and read information to areas with sides of a few nanometres. Our work demonstrates that high-density, non-volatile information storage is possible in metals. [ABSTRACT FROM AUTHOR]

The electronic structure and morphology of organic semiconducting cobalt-phtalocyanine (CoPc) films in situ prepared on the Au(001)-5×20 superstructure have been studied by a combination of experimental and theoretical work. The CoPc molecular film was characterized by photoemission spectroscopy (PES, valence band and core-level). The experimental results were simulated and have been explained in the framework of density functional theory (DFT) calculations. The C 1 s and N 1 s core level spectra were analyzed by taking into account the fact that both types of atoms have different nonequivalent positions in the molecule. And finally, the experimentally obtained electronic valence band structure of CoPc is in very good agreement with ab initio density of state results, allowing a detailed site-specific insight into the system. [ABSTRACT FROM AUTHOR]

We have studied the transport properties of Co/Ni superlattices, grown along the (111) direction of a face centered cubic lattice on the basis of the ab initio electronic structure calculated by the linear muffin-tin orbitals method. It was found that the density of states at the Fermi level for the minority spin shows oscillations as a function of the individual layer thickness. Within the approach based on the Boltzmann equation including the spin mixing, it was shown that these oscillations in the density of states can cause oscillations of the conductivity as obtained experimentally. [ABSTRACT FROM AUTHOR]

We developed a self-consistent screened-Korringa-Kohn-Rostoker (KKR) method in an ab initio tight-binding formulation. The reduction in numerical effort in comparison with that required for the standard KKR method is achieved by the choice of a suitable reference system. Following the idea of Zeller et al . (1995, Phys. Rev. B, 52 , 8807), reference systems with repulsive muffin-tin potentials of constant height are considered. This paper is dedicated to reference potentials of infinite height, so-called hard core potentials. It will be demonstrated that the advantage of the analytical solution to the scattering problem is accompanied by a reduced accuracy of the screened KKR method for close-packed hard spheres. To overcome this problem, we consider hard spheres with a reduced diameter. The best results also in comparison with reference potentials of finite height are obtained using hard spheres with a radius reduced to about 80% of the muffin-tin value. [ABSTRACT FROM AUTHOR]

Spin-dependent scattering is considered to be the origin of giant magnetoresistance in magnetic multilayers, although the role of bulk or interface defects is still under discussion. We present calculations of spin-dependent impurity scattering potentials as a function of the impurity position in a Co/Cu-multilayer. The calculations are performed within spin density functional theory using a Green function method (the tight binding Korringa-Kohn-Rostoker method). It is shown that the impurity potential depends strongly on the impurity position, which is reflected in local densities of states and local impurity moments. Furthermore, calculations of magnetic properties of impurity pairs at the interface are presented and compared with results for single impurities. [ABSTRACT FROM AUTHOR]

The resistivity and giant magnetoresistance (GMR) of (Cu3Ni3) n embedded in Cu(100), for n 11, that originates from the electronic structure of these finite, yet otherwise perfect, systems is calculated for currents in the plane of the layers (CIP) by using the Kubo-Greenwood formula for semi-infinite systems and the fully relativistic, spin-polarized screened Korringa-Kohn-Rostoker method. We find that for this particular type of repeated structure the CIP resistivity decreases from about 6 to 2muOmega cm as the number of repeats increases from 2 to 11, and the CIP-GMR while starting out at 4% for n = 2 goes up to 16% at n = 11. [ABSTRACT FROM AUTHOR]

Interlayer exchange coupling (IEC) is calculated for Co/Cu and Ni/Cu(100) multilayers. The calculations are based on density functional theory using two different Green function methods. Results of a supercell formulation with periodically repeated layers are compared to results for bilayers treated as planar perturbations. The coupling energy is obtained employing a frozen potential approximation. It will be shown that the results of both methods are in good agreement concerning the oscillation periods. Slight variations of the amplitude might be caused by additional multiple scattering effects in the superlattice. [ABSTRACT FROM AUTHOR]

Presents an abstract of the study 'Ab initio Calculations of Spin-Dependent Transport Properties,' by I. Mertig.