14 research outputs found
Emergence of a Dynamic Super-Structural Order Integrating Antiferroelectric and Antiferrodistortive Competing Instabilities in EuTiO3
Microscopic structural instabilities of EuTiO3 single crystal were
investigated by synchrotron x-ray diffraction. Antiferrodistortive (AFD) oxygen
octahedral rotational order was observed alongside Ti derived antiferroelectric
(AFE) distortions. The competition between the two instabilities is reconciled
through a cooperatively modulated structure allowing both to coexist. The
electric and magnetic field effect on the modulated AFD order shows that the
origin of large magnetoelectric coupling is based upon the dynamic equilibrium
between the AFD - antiferromagnetic interactions versus the electric
polarization - ferromagnetic interactions
Seebeck coefficients of half-metallic ferromagnets
In this report the Co2 based Heusler compounds are discussed as potential
materials for spin voltage generation. The compounds were synthesized by
arcmelting and consequent annealing. Band structure calculations were performed
and revealed the compounds to be half-metallic ferromagnets. Magnetometry was
performed on the samples and the Curie temperatures and the magnetic moments
were determined. The Seebeck coefficients were measured from low to ambient
temperatures for all compounds. For selected compounds high temperature
measurements up to 900 K were performed.Comment: accepted contribution o the Special Issue "Spin Caloritronics" of
Solid State Communication
Tuning cationic composition of La:EuTiO3−δ films
Eu1−xLaxTiO3−δ (x = 0, 0.3, 0.5) films were deposited in a p(Ar(96%)/H2(4%)) = 4 × 10−4 mbar atmosphere on (LaAlO3)0.3-(Sr2AlTaO6)0.7 vicinal substrates (0.1°). Reflection high-energy electron diffraction oscillation characteristics of a layer-by-layer growth mode were observed for stoichiometric and Ti-rich films and the laser fluence suited to deposit stoichiometric films was identified to be 1.25 J/cm2 independent of the La content. The variety of resulting film compositions follows the general trend of Eu-enrichment for low laser and Ti-enrichment for high laser fluence. X-ray diffraction confirms that all the films are compressively strained with a general trend of an increase of c-axis elongation for non-stoichiometric films. The surfaces of non-stoichiometric films have an increased roughness, the highest sheet resistances, exhibit the presence of islands, and are Eu3+ rich for films deposited at low laser fluence
RF-plasma assisted pulsed laser deposition of nitrogen-doped SrTiO3 thin films
ISSN:0947-8396ISSN:1432-0630ISSN:0340-379
Optical Properties of Nitrogen-Substituted Strontium Titanate Thin Films Prepared by Pulsed Laser Deposition
Perovskite-type N-substituted SrTiO3 thin films with a preferential (001) orientation were grown by pulsed laser deposition on (001)-oriented MgO and LaAlO3 substrates. Application of N2 or ammonia using a synchronized reactive gas pulse produces SrTiO3-x:Nx films with a nitrogen content of up to 4.1 at.% if prepared with the NH3 gas pulse at a substrate temperature of 720 °C. Incorporating nitrogen in SrTiO3 results in an optical absorption at 370-460 nm associated with localized N(2p) orbitals. The estimated energy of these levels is ≈2.7 eV below the conduction band. In addition, the optical absorption increases gradually with increasing nitrogen content
Bandgap tuning in by anionic-lattice variation
Polycrystalline SrTiO3 and SrTi(O,F)3 powders were synthesized by a solid-state reaction. A partial substitution of oxygen by nitrogen was subsequently carried out using thermal ammonolysis resulting in SrTi(N,O)3 and SrTi(N,O,F)3. Powder X-ray diffraction (XRD) revealed a cubic perovskite structure with space group Pm-3m for all samples. The thermal ammonolysis slightly increased the lattice parameters, crystallite sizes and strain. As a result from the co-substitution of oxygen with nitrogen and fluorine for SrTi(N,O,F)3, highly distorted TiO6 octahedra were detected using X-ray absorption near edge structure (XANES) spectroscopy. The weakening of all active modes of the Raman spectra after thermal ammonolysis also indicated enhanced distortions in the local crystal structure. SrTi(N,O,F)3 has the largest amount of nitrogen as well as fluorine among all four samples as determined by thermogravimetric analysis (TGA), elemental analysis and X-ray photoelectron spectroscopy (XPS). In the UV–vis spectra a distinctive shift of the absorption-edge energy was observed exclusively for the SrTi(N,O,F)3 sample from 390 to 510 nm corresponding to a bandgap narrowing from 3.18 to 2.43 eV
Investigation of the thermoelectric properties of LiAlSi and LiAlGe
The compounds LiAlSi and LiAlGe were synthesized and their thermoelectric properties and temperature stability were investigated. The samples were synthesized by arc melting of the constituent elements. For the determination of the structure type and the lattice parameter, x-ray powder diffraction was used. Both compounds were of the C1 b structure type. The stability of the compounds was investigated by differential thermal analysis and thermal gravimetry. The Seebeck coefficient and the electrical resistivity were determined in the temperature range from 2 K to 650 K. All compounds showed p-type behavior. The thermal conductivity was measured from 2 K to 400 K. The evaluation of the thermal conductivity yielded values as low as 2.4 W m -1 K -1 at 400 K for LiAlGe. The low values are ascribed to high mass fluctuation scattering and a possible rattling effect of the Li atoms
Surface deformations as a necessary requirement for resistance switching at the surface of SrTiO3:N
Atomic force microscopy (AFM), conductive AFM and electrochemical strain microscopy were used to study the topography change at the defect surface of SrTiO3:N, breakdown in the electrical conduction of the tip/sample/electrode system and ionic motion. The IV curves show resistance switching behavior in a voltage range ±6 V ± 6 V), revealing the additional necessity (albeit insufficient due to 50% yield of working cells) of surface protrusion formation for resistance switching memory