Single Crystalline SrTiO3 Research Articles

Giant magnetoresistance in oxide-based metallic multilayers

The authors report on the first measurement of low-field giant magnetoresistance in metallic multilayers of perovskite oxides. The authors performed in-plane measurements of the magnetoelectric transport properties in La0.75Sr0.25MnO3∕LaNiO3 trilayers and succeeded in distinguishing the giant magnetoresistance effect from other contributions to the total magnetoresistance. The samples were grown on single-crystalline SrTiO3 substrates by dc sputtering.

Thin Films of Single-Crystalline SrTiO3 Nanorod Arrays and Their Surface Wettability Conversion

Thin films of single-crystalline SrTiO3 nanorod arrays were synthesized by ion-exchange and annealing titanate nanotube arrays. The diameter of the SrTiO3 nanorods ranged from 10 to 20 nm, and the long length axis of these nanorods was oriented in the [110] direction of the perovskite phase. These nanorod arrays were optically semitransparent and adhered well to the substrates. Due to their surface nanostructures, these nanorod surfaces exhibited superhydrophilicities with water contact angles (θ) below 3°. In addition, these surfaces turned into high-hydrophobic states (θ = 140°) when oleic acid was applied. UV illumination caused these hydrophobic thin films to become superhydrophilic states because single-crystalline SrTiO3 nanorods have strong photocatalytic oxidative activities, which decomposed the surface-adsorbed oleic acid.

Irreversible Magnetic Properties of Neutron Irradiated HLPE Coated Conductors

Hybrid liquid phase epitaxy (HLPE) is among the most promising high rate techniques for fabricating coated conductors. In this paper we report on the irreversible magnetic properties of HLPE thick films grown on single crystalline SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> at temperatures of 860degC and 890degC. Magnetic measurements in the temperature range from 5 to 90 K were carried out on these samples to determine the critical current density. Furthermore, neutron irradiation which introduces effective pinning centers of known density provides us with additional information on the field dependence of pinning in HLPE coated conductors.

Can the kink-pair theory apply for the yield stress of single crystalline SrTiO 3 at high temperatures?

The kink-pair theory of the flow stress is tentatively applied on the temperature and strain-rate dependence of the critical resolved shear of SrTiO 3 single crystals with 〈0 0 1〉 orientation of the compression axis in the high-temperature regime. The results suggest that both a glide process as well as climb process can be used for an interpretation.

Transmission-electron-microscopy study of quasi-epitaxial tungsten-bronze (Sr2.5Ba2.5Nb10O30) thin film on perovskite (SrTiO3) single crystal

Strontium barium niobate (Sr2.5Ba2.5Nb10O30) thin films were deposited on (001) SrTiO3 single-crystalline substrates by pulsed laser deposition. The growth nature was investigated by transmission electron microscopy (TEM). Selected-area electron diffraction and high-resolution transmission electron microscopy revealed the existence of six types of grains. These grains grew on the substrate in a partially epitaxial fashion. Geometrical models were built, which were confirmed by TEM observations. Based on the TEM results and geometrical analysis, a crystallographic model was developed. The strain nature resulting from the growth columns is discussed in this report.

Ferroelectric properties of compressively strained epitaxial SrTiO 3 films on sapphire

Magnetron sputtered and laser deposited SrTiO 3 thin films are deposited on CeO 2 buffered sapphire substrates. Their structural and ferroelectric properties are analyzed and possible (mutual) correlations between these properties are investigated. It is shown, that the biaxial compressive strain imposed by the substrate on the ferroelectric films leads to a considerable increase of the Curie temperature, as well as the dielectric constant and the tenability of these films in technically relevant temperature regimes. Generally, the dielectric constant and the tuning decreases with increasing strain. However, the ferroelectric phase transition of the SrTiO 3 films is shifted to higher temperatures compared to that of single crystalline SrTiO 3. As a consequence, the dielectric constant of the films is larger than that of undistorted SrTiO 3 single crystals for small strain (Δ a/ a < 0.005) and temperatures above the Curie temperature. Furthermore, a linear dependence of the loss tangent and the tunability on the dielectric constant is observed, which indicates, that all three properties are affected by the same mechanism that itself is affected by the lattice strain.

Reversible tuning of a series of intergrowth phases of the Ruddlesden–Popper type SrO(SrTiO3)n in an (001) SrTiO3 single-crystalline plate by an external electric field and its potential use for adaptive X-ray optics

Reversible structural changes of near-electrode regions of an (001) SrTiO3 (STO) single-crystal plate with perovskite-type of structure in an external electric field are described as the tunable and reversible formation of a significant volume of domains containing Ruddlesden–Popper (RP) phases of composition SrO(SrTiO3)n with variable n. Compositional changes are discussed to be caused by electromigration of ion complexes in the static electric field, i.e., by solid state electrolysis of the perovskite STO. Electromigration is initiated by the electric field which then stabilizes the RP domains formed. The RP domains can be described as coherently intergrown with the surrounding perovskite matrix. The X-ray scattering behaviour of these intergrowths, compared with the perovskite STO, suggests the technical use of the phenomenon for adaptive X-ray optics.

Oriented growth of Sr n+1Ti nO 3n+1 Ruddlesden–Popper phases in chemical solution deposited thin films

Oriented thin films of perovskite-related Sr n +1Ti n O 3 n +1 Ruddlesden–Popper phases ( n=1, 2, 3) were grown on (001) single-crystalline SrTiO 3 substrates. Preparation of the films was carried out by wet chemical deposition from metalorganic Sr–Ti solutions (rich in Sr) and subsequent conversion into the crystalline state by thermal treatment in air atmosphere at a maximum temperature of 700 °C. Solutions were prepared by a modified Pechini method. The films were investigated by wide-angle X-ray scattering and high-resolution transmission electron microscopy. The phase content of powders prepared from the dried solutions and annealed under similar conditions differed from that present in the films, i.e. only polycrystalline SrTiO 3 was detected together with oxides of Ti and Sr.

Frequency-Modulated Non-contact Atomic Force Microscopy Study of Heat-Treated Oxide Surface with Organic Molecules

The morphology of the (100) surface of single-crystalline SrTiO3 was studied using frequency-modulated non-contact atomic force microscopy (FM-NCAFM) at a temperature of 100 K. After appropriate etching and heating treatments, unit-cell steps with atomically flat terraces have been successfully obtained. These surfaces were found to be quite stable under ultrahigh vacuum conditions as long as the temperature was maintained below 600 K. A specially tailored porphyrin-based molecule with carboxyphenyl substituent was dispersed on the substrates, and strong suppression of their mobility on the surface was found by FM-NCAFM observations.

Discrete deposition as a powerful tool to govern magnetoresistance of the doped manganite films

A peculiar preparation routine, namely, a discrete deposition, is proposed for a goal-oriented change of electric and magnetotransport properties of the doped manganite films. The distinguishing feature of such procedure is a division of the whole deposition process into several cycles consisting of the deposition itself and subsequent annealing of the sample in the deposition chamber. The investigations were carried out on a series of (La,Sr)MnO3 films with different numbers of deposition cycle n (1⩽n⩽5), as well as on a single-layer reference film. All the films were deposited on single-crystalline SrTiO3 (001) substrates held at a temperature of 700°C in Ar–O2 mixture using reactive magnetron sputtering technique and subjected to a postdeposition annealing at 750°C. It has been shown that the discrete deposition procedure results in the increase of the magnetic transition temperature, substantial drop of the resistivity, and reduction of the extrinsic magnetoresistance. It is shown that the proposed procedure strongly affects the dynamics of the film growth, minimizes the effects caused by substrate-induced stress, and leads to the improvement of the structural and magnetic homogeneities of the film layers, which are not adjacent to the substrate. All these effects become especially efficient for n⩾2.

Microstructures and phase evolution in YBa2Cu3O7−x films grown on various substrates fabricated via a non-fluorine sol–gel route

YBa2Cu3O7−x films were grown on polycrystalline Ag and single crystalline YSZ, SrTiO3, and MgO substrates using non-fluorine sol–gel and spin coating methods. The effects of heat treatment conditions on the phase evolution and microstructures were investigated using optical microscope, X-ray diffraction, and atomic force microscope. For Ag substrates, Y123 phase started to form at 750°C and higher sintering temperatures improved the degree of (001) texture. Mirror-like surfaces without any cracks were achieved for sintering at 750–900°C. However, voids were observed for films grown on the Ag substrates at temperatures higher than 810°C and their size and density increased as the temperature increased. For the films grown on single crystal substrates sintered at 800°C, numerous microcracks with large crack widths were observed, while cracking is less of a problem for films grown on Ag substrates. Epitaxial films without any cracks were achieved for films grown on single crystalline substrates under optimized conditions. Possible mechanism for the formation of cracks is discussed.

Electrical Properties of La0.7Ca0.3MnO3 Thin Films Obtained by Metal-Organic Deposition (MOD) using Excimer Laser and Thermal Annealing

Epitaxial La0.7Ca0.3MnO3 (LCMO) thin films were prepared by thermal and excimer laser (EL) metal-organic deposition (MOD). Using simple thermal annealing (STA) at 800–1000°C, the LCMO films were well epitaxially grown on single-crystalline SrTiO3 substrates. When, using a KrF or an ArF excimer laser irradiation, the LCMO films were also epitaxially grown at a substrate temperature of 500°C. The films produced by STA exhibited metal–insulator transition temperatures Tp's ranging from 282 to 290 K while those obtained by EL irradiation had a Tp of approximately 205 K. For bolometric applications, the temperature coefficient of resistance (TCR) of the films was calculated. The LCMO films obtained by STA have a TCR of 8.5%/K at 228 K. On the other hand, using the EL irradiation, the TCR values are 5.8%/K at 165 K and 6.5%/K at 128 K for the KrF and ArF excimer lasers, respectively.

An electrical field-induced structural effect in strontium titanate at room temperature

An electric field-induced reversible structural distortion at room temperature is observed at an as-cut single-crystalline SrTiO3 [001] wafer. The structural changes have been characterised in situ by means of X-ray diffraction. It is concluded that the effect requires a distortion of the cubic crystal structure, as it was present only at the rough unpolished side of the wafer. The appearance of the phenomenon was found to depend on the direction of the electric field. Structural changes, depending on the electric field strength, were completely reversible. Regarding the possible electromechanical coupling phenomena, the piezoelectric and electrostrictive effects have to be considered. Cubic SrTiO3 belongs to the centrosymmetric point group \(m\overline{3}m\) ; hence, no piezoelectric effect is present, whereas second-order effects like electrostriction may occur. Peculiarities arising as a result of the special constraints are discussed.

Nucleation of YBa2Cu3O7−x on buffered metallic substrates in thick precursor films made by theBaF2 process

We present experimental data on direct measurements of area densities ofYBa2Cu3O7−x, YBCO, nuclei in fluorinated precursor films which are onCeO2 buffered metallicand single-crystalline SrTiO3 substrates. The area density of the YBCO nuclei was measured by polarized-light microscopyafter the nuclei had grown to the surface of the film. The density was found to depend stronglynot only on processing conditions, but also on the type of the substrate. We also established acorrelation between the area density of the nuclei and the nucleation of the randomly andc-axis oriented YBCO grains in the films. A model which was based on classical nucleationtheory predicted that the nucleation was a result of collective interactions among theexisting nuclei. Also, it predicted qualitatively some aspects of the functionaldependence of the nucleus density on processing parameters. It was found thatJc values ofvery-large-grained c-axis-oriented YBCO films were low due to weak connectivity at grain boundaries.

Thermoelectric properties of reduced and La-doped single-crystalline SrTiO 3

The thermoelectric properties of single-crystalline reduced and La-doped SrTiO 3 were investigated along the [1 0 0]-axis from room temperature to 773 K. The reduced sample showed lower electrical conductivity and thermal conductivity, which indicated that oxygen vacancies acted as strong scattering centers for both electrons and phonons. The thermal conductivity decreased about 40% by the reduction at room temperature. The sound velocity measured at room temperature was almost unaffected by the reductions and La doping, thus the decrease was caused only by the decrease of the mean free path. The reduced and La-doped sample showed almost the same figure of merit in the temperature range considered.

Electron emission characteristics from a single crystalline ferroelectric material by an infrared light irradiation

An infrared sensor, based on the principle of electron emission from a thin Pb(Zr,Ti,)O3 (PZT) plate, is proposed in this study. The electron emission current from a thin, single crystalline SrTiO3 plate caused by infrared light irradiation was observed at an electric field of 1.6 MV/cm. To compare the electron emission characteristics, a sintered PZT and a single crystalline SrTiO3 emitter were measured under the same infrared light irradiation conditions. The peak emission current from the SrTiO3 plate was 1.6 times higher than that of the PZT emitter, and the response speed was faster. However, the emission current did not respond continuously to a pulsed infrared irradiation.

Ion beam studies of single crystalline manganite thin films

Colossal magnetoresistive manganites are oxide materials where the magnetic interactions and the electrical properties are mainly controlled by the Mn valence ratio (+3/+4). For thin films these properties present also a strong dependence on strain and microstructure. In this study we focus on doped La–Sr–MnO 3 and undoped stoichiometric LaMnO 3. The films were prepared by laser ablation using stoichiometric ceramic targets and were deposited at a substrate temperature of 770 °C on single crystalline SrTiO 3. All the films were submitted to the same in situ oxygen/heat treatment and the thicknesses were in the range of 20–500 nm. Rutherford backscattering spectrometry/channelling analysis shows the epitaxial growth of manganite films with an excellent single crystalline quality. For fully strained films, due to lattice mismatch with the substrate, we obtained minimum yields of 5% measured along the [1 0 0] growth direction. Strain relaxation leads to an increase of the minimum yield with the increase of thickness. Detailed angular scans were performed in selected films to study the misalignment along the tilt [1 1 0] and [1 1 1] directions in order to obtain an indication on the residual strain. The results were compared with X-ray diffraction and the correlation with respect to the structural quality of the films is remarkable. Moreover the magnetic measurements suggest a close relation between the transition temperature ( T c) and the strain in the films.

Large-scale synthesis of single-crystalline perovskite nanostructures.

Single-crystalline perovskite nanostructures with reproducible shape have been prepared using a simple, readily scaleable solid-state reaction in the presence of NaCl and a nonionic surfactant. Pristine BaTiO3 nanowires have diameters ranging from 50 to 80 nm with an aspect ratio larger than 25. Single-crystalline SrTiO3 nanocubes with a mean edge length of 80 nm have been produced using a similar procedure.

Superior Schottky electrode of RuO2 for deep level transient spectroscopy on anatase TiO2

An anatase TiO2 film was epitaxially grown on a conductive Nb-doped single-crystalline SrTiO3 (100) substrate by metalorganic chemical vapor deposition. RuO2 Schottky electrode was fabricated on the epitaxial anatase film by reactive dc magnetron sputtering. The dark I–V and capacitance–voltage characteristics indicated the good rectification and thermal stability of the RuO2/anatase junction. This RuO2/anatase junction enables the stable measurements of deep level transient spectroscopy in the high-temperature region and is a promising Schottky electrode to examine the origins of deep levels in the band gap of anatase.

Superconductivity of SrTiO $ \mathsf {_{3-\delta}}$

Superconducting SrTiO_{3-\delta} was obtained by annealing single crystalline SrTiO_3 samples in ultra high vacuum. An analysis of the V(I) characteristics revealed very small critical currents I_c which can be traced back to a unavoidable doping inhomogeneity. R(T) curves were measured for a range of magnetic fields B at I<<I_c, thereby probing only the sample regions with the highest doping level. The resulting curves B_{c2}(T) show upward curvature, both at small and strong doping. These results are discussed in the context of bipolaronic and conventional superconductivity with Fermi surface anisotropy. We conclude that the special superconducting properties of SrTiO_{3-\delta} can be related to its Fermi surface and compare this finding with properties of the recently discovered superconductor MgB_2.