We present here an experimental and theoretical study of the Ti-ferrite (TiFe2O4, ulvospinel). The theoretical study was performed in the framework of density functional theory using the full-potential linearized augmented plane waves method and employing different approximations for the exchange and correlation potential. In order to discuss the magnetic ordering and the electronic structure of the system, we considered different distributions of the Fe/Ti atoms in the two cationic sites of the structure and, for each distribution, different spin arrangements (ferromagnetic, ferrimagnetic and antiferromagnetic cases). We found that the equilibrium structure corresponds to an inverted spinel structure with an antiferromagnetic spin configuration in which the magnetic moments of the Fe ions in both A and B sublattices are ferromagnetically ordered, while the magnetizations of these two sublattices are antiparallel with respect to each other. Our calculations predict that TiFe2O4 is a wide-band gap semiconductor (band gap in the order of 2.3 eV) and successfully describe the hyperfine properties (isomer shift, magnetic hyperfine field, and quadrupole splitting) at the Fe sites that are seen by Mossbauer spectroscopy (MS) experiments at 4.2 K reported in the literature and MS performed at 300 K in the present study. We also measured and simulated the X-ray absorption near-edge spectroscopy (XANES) spectra of TiFe2O4 at both Ti and Fe K-edges. Our calculations correctly reproduce the XANES spectra and enable us to separate the contribution of each site to the experimental spectra. All these studies enable us to obtain a complete structural, electronic, magnetic, and hyperfine characterization of TiFe2O4.

We present here an ab initio study of the structural, magnetic, and hyperfine properties of Fe-doped rutile SnO2 for different concentrations and distributions of the Fe atoms and oxygen vacancies in the SnO2 host. The calculated results are compared with experimental ones obtained by Mössbauer spectroscopy and X-ray absorption techniques. This comparison enables us to characterize the local structure around Fe atoms and to identify the different hyperfine interactions that are observed in samples prepared by different methods. It is concluded that oxygen vacancies are fundamental for the ferromagnetic response of Fe-doped SnO2. The ab initio calculations show that two Fe ions sharing an oxygen vacancy are coupled ferromagnetically, forming a bound magnetic polaron (BMP), and that two neighbor BMPs are aligned antiparallel to each other. Electron doping plays a fundamental role mediating the magnetic coupling between the BMP inducing ferromagnetic alignment between the BMPs.

In this work, we present an experimental and theoretical study of structural and magnetic properties of Fe doped rutile TiO2 nanopowders. We show that Fe-doping induces the formation of oxygen vacancies in the first-sphere coordination of iron ions, which are in +2 and +3 oxidation states. We found that Fe ions form dimers that share one oxygen vacancy in the case of Fe3+ and two oxygen vacancies in the case of Fe2+. The saturation magnetization is almost independent of iron concentration and slightly increases with the relative fraction of Fe2+. Ab initio calculations show that two Fe ions sharing an oxygen vacancy are coupled ferromagnetically, forming a bound magnetic polaron (BMP), but two neighbor BMPs are aligned antiparallel to each other. Extra electron doping plays a fundamental role mediating the magnetic coupling between the ferromagnetic entities: carriers, possibly concentrated at grain boundaries, mediate between the BMP to produce ferromagnetic alignment.

We investigate the structural, electronic, and magnetic properties of a particular interface in the oxide heterostructures LaAlO3/SrTiO3 (LAO/STO) and TiO2-anatase/LaAlO3 (TiO2/LAO), namely the interface of AlO2 facing TiO2, which is the energetically preferred one in the presence of interfacial oxygen vacancies. The optimum stacking for the ground state is different for each heterostructure with the interfacial Ti atoms being located either at hollow or bridge sites facing the AlO2 surface layer. This structural property determines the electronic character of the interface and as a consequence, in LAO/STO it is metallic while in TiO2/LAO it can be semiconducting and magnetic for a large concentration of vacancies. In addition, we find that cation interdiffusion at this interface is an energetically favored defect in both heterostructures with interfacial vacancies. Its main effect is to increase the size of the band gap in the semiconducting case and to open up a gap in the metallic one, thus allowing for a tuning of a metal to insulator transition. [GRAPHICS] . (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The melting and soldering processes of two iron filled carbon nanotubes is explored by means of classical molecular dynamics, in order to develop an understanding of the underlying mechanisms that govern the dynamics of nano-soldering. Molten Fe flows from the open end of the two CNTs, leading to a liquid junction, and eventually to a solid contact. This soldering process is accompanied by partial or total healing of the carbon nanotubes, which after cooling and relaxation form just a single unit which encapsulates the iron, depending on the relative separation, diameters and axial offset of the nanotubes. This makes for a promising scenario for CNT soldering, repairing and healing, and a variety of different tools in the field of nanoelectronics.

Ab initio calculations were performed for a very thin iron nanowire, both free-standing and enclosed in a carbon nanotube with the same size and structure of available experiments. Our interest was to study the effects of low dimensionality and the influence of the Fe C interaction on the magnetic and hypefine properties of these systems. Our main finding was that the interfacial region between the nanowire and the carbon nanotube is of fundamental importance, as an iron atom close to the carbon atoms has a magnetic moment and a local hyperfine field very different from that at the surface of a free-standing iron nanowire. In fact, the properties of the calculated iron nanowire, of only 1 nm in diameter, when encapsulated inside a carbon nanotube result close to those of bulk iron.

Ab initio calculations using the local spin density approximation and also including the Hubbard U have been performed for three low-energy configurations of the interface between LaAlO 3 and TiO 2 anatase. Two types of interfaces have been considered: LaO/TiO 2 and AlO 2 /TiO, the latter with Ti termination and therefore a missing oxygen. A slab-geometry calculation was carried out, and all the atoms were allowed to relax in the direction normal to the interface. In all the cases considered, the interfacial Ti atom acquires a local magnetic moment, and its formal valence is less than +4. When there are oxygen vacancies, this valence decreases abruptly inside the anatase slab, while in the LaO/TiO 2 interface, the changes are more gradual.

Ab initio calculations using the local spin-density approximation plus Hubbard U (LSDA+U) method have been performed for the three reported phases of Ti4O7. Using the experimental structural parameters, we find that the electronic and magnetic properties are qualitatively different for each phase. The low-temperature structure is an antiferromagnetic semiconductor, with bipolarons arranged symmetrically in chains, separated by other nonmagnetic ion chains. The intermediate-temperature structure also contains bipolarons, but in a much more complicated order, in addition to unpaired magnetic Ti3+ ions and nonmagnetic Ti4+ ions. It has a smaller band gap than the low-temperature one. The high-temperature structure is metallic, and different distributions of Ti3+ and Ti4+ ions can be found that are almost degenerate.

In this paper we study the possible relation between the electronic and magnetic structure of the TiO2/LaAlO3 interface and the unexpected magnetism found in undoped TiO2 films grown on LaAlO$_3$. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO3 has a layered structure along the (001) direction with alternating LaO and AlO2 planes, with nominal charges of +1 and -1, respectively. As a consequence of that, an oxygen deficient TiO2 film with anatase structure will grow preferently on the AlO2 surface layer. We have therefore performed ab-initio calculations for superlattices with TiO2/AlO2 interfaces with interfacial oxygen vacancies. Our main results are that vacancies lead to a change in the valence state of neighbour Ti atoms but not necessarily to a magnetic solution and that the appearance of magnetism depends also on structural details, such as second neighbor positions. These results are obtained using both the LSDA and LSDA+U approximations.

In this article, we study the possible relation between the electronic and magnetic structures of the TiO2/LaAlO3 interface and the unexpected magnetism found in undoped TiO2 films grown on LaAlO3. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO3 has a layered structure along the (001) direction with alternating LaO and AlO2 planes, with nominal charges of +1 and −1, respectively. As a consequence of that, an oxygen-deficient TiO2 film with anatase structure will grow preferently on the AlO2 surface layer. We have therefore performed ab initio calculations for superlattices with TiO2/AlO2 interfaces with interfacial oxygen vacancies. Our main results are that vacancies lead to a change in the valence state of neighbor Ti atoms but not necessarily to a magnetic solution and that the appearance of magnetism depends also on structural details, such as second neighbor positions. These results are obtained using both the local spin density approximation (LSDA) and LSDA + U approximations.

Using density functional theory calculations together with the Monomer method for the search of saddle points (combined for the first time with an ab initio algorithm), we obtain the vacancy formation energies and the migration barriers for α-Zr self-diffusion and for the diffusion of interstitial impurities, including the ultra-fast diffuser Fe. Good agreement with measured diffusion coefficients is obtained, as a much lower energy barrier for the ultra-fast diffuser is found. We also suggest a possible mechanism for the increase in self-diffusion due to the Fe impurity, always present in the experimental samples.

In this work we explore the origin of the ferromagnetism appearing when a TiO2 film is grown on another non-magnetic oxide as a substrate such as LaAlO3 (001), concentrating on the role played by the oxygen vacancies in this phenomenon. Using Density Functional Theory ab-initio methods, we study the free-standing anatase film as well as the interfaces with either the LaO or AlO2 planes of LaAlO3. Our results show that the interface LaO/TiO2 is favored against the AlO2/TiO2 one if no oxygen vacancies are present in the interface where as the contrary happens when there are oxygen vacancies. In both cases, the cohesive energy is of the same order of magnitude but only at AlO2/TiO2 we found a magnetic solution.

In this work we perform an ab initio study of the electric field gradient (EFG) at the nucleus of Fe impurities in crystalline SnO. The Augmented Plane Waves plus Local Orbitals method is used to obtain the electronic structure of the doped system and the atomic relaxations introduced by the impurities in the SnO host in a fully self-consistent way. Most calculations are performed assuming that Fe ions replace the Sn atoms of the structure, in some cases including oxygen vacancies in order to discuss their role in the hyperfine interactions and in determining the local structure around Fe impurities. The case of interstitial Fe sites is also considered. Our predictions are compared with available Mossbauer spectroscopy results and also with theoretical and experimental results obtained for rutile SnO2 and TiO2. (C) 2009 Elsevier Ltd. All rights reserved.

We present an investigation of Fe-doped TiO2 anatase nanoparticles (2.8 and 5.4 at.% Fe) where Fe substitutes Ti atoms without the presence of other phases. In order to characterize these samples we used x-ray absorption experiments, 57Fe Mössbauer spectroscopy, ab initio calculations and magnetometry. Results from iron K-edge near-edge and extended x-ray absorption fine structure confirm that Fe3+ replaces Ti4+ in the TiO2 anatase structure increasing the metal-anion bond length. Mössbauer spectra recorded at room temperature show asymmetric Fe3+ broad doublets. These results agree with structural, hyperfine and magnetic properties calculated using density-functional theory, if oxygen vacancies are present in the iron–oxygen octahedra. Mössbauer and magnetic measurements indicate that samples are paramagnetic at room temperature. At low temperatures, two kind of magnetic species can be distinguished: (i) isolated paramagnetic Fe3+ ions and (ii) antiferromagnetically coupled Fe3+ ions. These results also show that substitutional Fe in nanosized anatase TiO2 does not induce ferromagnetic ordering.

In this work we study the electronic structure and magnetism of a TiO2 film grown on another non-magnetic oxide such as a LaAlO3 (001) substrate, concentrating on the role played by structural relaxation and oxygen vacancies. Using Density Functional Theory ab-initio methods, we study the free-standing anatase film as well as the interfaces with either the LaO or AlO2 planes of LaAlO3, focusing on the possibility of magnetic solutions. Our results show that the interface LaO/TiO2 is favored against the AlO2/TiO2 one if no oxygen vacancies are present in the interface whereas the contrary happens when there are oxygen vacancies. In both cases, the cohesive energy is of the same order of magnitude but only the AlO2/TiO2 interface presents an stable magnetic solution.

The role of Fe in the hcp Zr diffusion process is analyzed, given its ultra-fast diffusion (up to nine orders of magnitude higher than the self-diffusion in the temperature range 779–1128K) and the enhancement observed in the self and substitutional diffusion induced by its unavoidable presence as impurity. Ab-initio calculations using SIESTA and WIEN2K codes were performed in order to find the actual Fe minimum energy configuration within the hcp Zr matrix and its interaction with vacancies. Several off-centre quasi-interstitial positions with energies similar to substitutional Fe were encountered. The comparison with diffusion coefficient measurements and Mössbauer experiments allows us to discard the substitutional position of the Fe atom as well as to affirm that its presence creates a considerable lattice distortion together with an increment in the number of vacancies. The above effects could be responsible for the enhancement in the self and substitutional diffusion, whereas the large amount of quasi-interstitial positions for Fe could be, at least partially, responsible for the ultra-fast Fe diffusion.

We present an ab initio study of pure and doped TiO2 in the rutile and anatase phases. The main purpose of this work is to determine the role played by different defects and different crystal structures in the appearance of magnetic order. The calculations were performed for varying impurity and vacancy concentrations in both TiO2 structures. For Co impurities the local magnetic moment remained almost independent of the concentration and distribution while for Cu this is not the case, there is magnetism for low concentrations that disappears for the higher ones. Impurity–impurity interactions in both structures favor linear ordering of them. Magnetism in undoped samples appears for certain vacancy concentrations and structural strain.

Transparent pure and Fe-doped SnO2 thin films were grown by pulsed laser deposition technique on LaAlO3 substrates. X-ray diffraction shows that the films are polycrystalline and have the rutile structure. Surprisingly, the pure film presents magnetic-like behavior at room temperature with a saturated magnetization of almost one-third of the doped film (∼3.6 and 11.3emu/g, respectively) and its magnetization could not be attributed to any impurity phase. Taking into account the magnetic moment measured in the pure film, the effective contribution of the impurity in the doped one can be inferred to be ∼2μB per Fe atom. A large magnetic moment was also predicted by an ab initio calculation in the doped system, which increases if an oxygen vacancy is present near the Fe impurity.

A study of the structural and magnetic properties of carbon encapsulated iron nanowires is presented. The influence of carbon presence on iron magnetic ordering by means of an ab initio computer simulation has been studied. For wires tightly encapsulated, i.e. with large ratio of wires and nanotubes radii, the presence of carbon strongly alters Fe magnetic configuration of free standing wires, in some cases yielding antiferromagnetic ordering. The energy differences between ferromagnetic and antiferromagnetic phases are small enough to allow their coexistence in a nanowire, which is in agreement with the experimental evidence of exchange-bias in such systems.