B. Leibold

Anisotropy of thermoelectric properties in La2/3Ca1/3MnO3 thin films studied by laser-induced transient voltages

Abstract Calcium doped LaMnO3 thin films deposited on vicinal cut SrTiO3 substrates exhibit unexpected large laser-induced voltages at room temperature, in remarkable analogy to those measured for intrinsically anisotropic YBa2Cu3O7−x thin films. The experimental data for La2/3Ca1/3MnO3 thin films are consistent with the description of a generation of thermoelectric fields caused by off-diagonal elements of the Seebeck tensor. The results suggest that the anisotropy of the Seebeck tensor is due to long range co-operative Jahn–Teller distortions.

Y-Ba-Cu-O high temperature superconductor thin film preparation by pulsed laser deposition and RF sputtering: A comparative study

Abstract High temperature superconductor thin films of high quality have successfully been deposited on oxide substrates with good lattice match to the films by a variety of different techniques. Pulsed laser deposition [PLD] and sputtering have been appeared to be the most popular methods at least in the scope of thin film preparation for basic research and development. Both methods do not require any post deposition annealing steps if the conden-sation of the film is done at high substrate temperatures [e.g. 750° C] and high oxygen partial pressure [e.g. 10–200 Pa]. Films deposited by both methods using optimized preparation conditions show comparable super- conducting parameters like critical temperatures exceeding 90 K and critical currents larger than 10 6 A/cm 2 at 77K. The different growth mechanisms of the films prepared by the two methods give rise to some morphological differences of the films. The advantages of PLD vs sputtering are discussed and specified with respect to their potential in the research area as well as in the application oriented work. Some recent developments like large area in situ deposition by sputtering, epitaxial growth of YBCO films with oriented in plane c-axis by PLD and epitaxial growth of YBCO films on epitaxial buffer layers on silicon are discussed.

Thickness and roughness dependence of magnetic flux penetration and critical current densities in YBa2Cu3O7−δ thin films

Abstract As a result of the island growth mode all epitactic YBa 2 Cu 3 O 7−δ thin films show surface roughness. To investigate a possible surface pinning mechanism, combined magneto-optical and atomic force microscopy studies were carried out. Measurement of the spatial distribution of magnetic flux density on the surface of thin YBa 2 Cu 3 O 7−δ films by means of the magneto-optical Faraday effect (MOFE) under application of external magnetic fields allows an accurate determination of critical current densities j c . In order to have a quantitative comparison between the Bean model for thin films and experiment, a new nonlinear calibration technique for the flux densities was developed. For determining the thickness and roughness dependence of j c , samples with YBaCuO-strips of different thickness and roughness were patterned from one film. With the roughness determined experimentally by afm measurements, a satisfactory agreement between the measured and calculated thickness dependence of j c is achieved. Surface pinning is found to cause between 10%–30% of the critical current densities of epitactic YBa 2 Cu 3 O 7−δ thin films. Additionally microscopic deviations of the flux profiles from the Bean model are detected. Evidence for matching effects of the vortex line distribution with the density of surface pins is given.

Thickness dependent properties of La0.67Ca0.33MnO3 thin films

Abstract Epitaxial thin films of La 0.67 Ca 0.33 MnO 3 (LCMO) were prepared by pulsed laser deposition (PLD) on SrTiO 3 (100) substrates. After deposition the films were in situ annealed for 1 h in oxygen (10 5 Pa, 780°C). We studied the thickness dependence of the transport properties in order to address the relation between the microscopic structure and the metal–insulator transition temperature T MI . It could be demonstrated that T MI decreases systematically from the bulk value T MI =276 K with decreasing film thickness. Details of the temperature dependence of ϱ and its magnetic field dependence are discussed.

Anisotropy of perpendicular field penetration into high-Tc superconductors induced by strong longitudinal field

Abstract The geometry of penetration of a magnetic field oriented perpendicular to the surface of a superconductor is changed by the presence of a longitudinal field. This is studied directly by means of the magneto-optical technique in different high-Tc materials. In YBa2Cu3O7 single crystals Brandts mode of magnetization is observed, namely, penetration of an AC field only along the longitudinal field even if the corresponding dimension of the sample is much longer than the other two. This anisotropy is the evidence that the force-free configuration of current and vortices cannot be destroyed by cutting and reconnection of the vortices in this 3D superconductor. On the contrary, the absence of the induced anisotropy in Bi2Sr2CaCu2O8 single crystals reveals the independence of pancake vortices in CuO layers from longitudinal field which makes a realization of the force-free configuration impossible in the layered 2D superconductor. An intermediate effect in thin films indicates that vortices are almost normal to the film surface even if the applied field is nearly parallel to it. The described experiment can be used as a new tool to study the internal vortex structure in superconductors.

A new calibration technique for the determination of magnetic flux density with the magneto-optical Faraday effect

Abstract Using the high resolution magneto-optical Faraday effect the magnetic flux density distribution of circular shaped Y1Ba2Cu3O7−x thin films is investigated. A new calibration technique is developed to determine the z-component Bz(x,y) of the local magnetic flux density B (x,y) in the x,y-surface of the films with high accuracy. The observed flux density profiles agree well with calculations of B ( r ) using a simple model with homogeneous current distribution. The appearance of a negative magnetic flux and reversed flux lines at the edge of the films in the remanent state is caused by stray fields of the trapped flux. For a determination of the local critical current density jc, the local pinning forces pv and the magnetization of thin films from the observed magnetic flux density distribution B (x,y) the curvature of the magnetic field ∂Bz/∂r and ∂Br/∂z, respectively, has to be taken into account.

Thickness dependence of the thermoelectric voltages in YBaCuO7−δ thin films on tilted substrate of SrTiO3

Abstract We report the first measurements of the thickness (d) dependence of the thermoelectric voltages (V χ ) from YBa 2 Cu 3 O 7−δ (YBCO) films deposited on tilted SrTiO 3 (STO) substrate. It was found that the linear relation of V χ vs. 1/d predicted by the atomic layer thermopile(ALTP) model is followed only at rather thick film region; Instead of a monotonic increase, a maximum of the thermoelectric voltage was measured. A model, which takes the influence of the surface and interface defects into consideration, is put forward and discussed.

Investigations of the in-plane anisotropy and the critical behaviour of 10°-tilted YBa2Cu3O7−δ films grown upon (106) SrTiO3 substrates

Abstract We describe the electrical properties of 10°-tilted YBa 2 Cu 3 O 7− δ (YBCO) films grown epitaxially upon as-received (106) SrTiO 3 substrates. (106) substrates which have been annealed at sufficiently high temperatures may exhibit a regular “step-and-terrace” surface structure, while un-annealed substrates (UAS) have more irregular terracing. We show that both the normal state and the superconducting properties of films grown upon UAS films vary considerably from the reported properties of films grown upon annealed substrates (AS films). The normal state in-plane anisotropies of UAS films are close to that of a near-perfect single crystal, while the in-plane anisotropy falls smoothly towards unity as the temperature is lowered through the superconducting regime. The critical currents of UAS films are lower than those of AS films. The superconducting behaviour of UAS films is consistent with a reduced density of extended defects including anti-phase boundaries, defects to which many of the properties of AS films have been attributed. We have also studied the properties of the phase-transition-like electrical behaviour found in UAS films, and observed good scaling collapse of the data under an algorithm based on a continuous thermodynamic phase transition such as that between a vortex liquid and a vortex glass. We find that the transition temperature, T t , and the scaling exponents ν and z do not depend upon the direction of the measurement current, but that the scaling is not “universal” since the exponents depend upon magnetic field and upon film thickness. The values of the exponents are far higher than might be expected from a continuous thermodynamic phase transition. The scaling of data for the track of a film which has been etched shows that both the exponents and the scaling functions are strongly influenced by chemical treatment.

Microstructure and X-ray analysis on LaCaMnO thin film

Abstract La 2/3 Ca 1/3 MnO 3 thin films in the thickness range between 40–500 nm, have been deposited by an optimised pulsed laser deposition process on SrTiO 3 (STO) single crystal substrates. The surface morphology of the thin films was examined by AFM and SEM. X-ray investigations were applied to study the lattice parameters. XRD pole figures show an epitaxial growth of manganite thin films on (100) STO substrate. According to X-ray diffraction pattern, we could derive the dependence of the lattice parameters within the thickness of the film. It is shown that Rietveld refinement of XRD pattern is essential to assess the structure of the manganite phase. Lattice parameters are refined and distances and bond angels were determined. A correlation between structure, transport properties and thickness are discussed.

The quantitative determination of flux density gradients and volume pinning forces in patterned YBCO thin films using the magnetooptical Faraday effect

Abstract In circular patterned YBCO thin films the penetration of magnetic flux is observed directly and locally using the high resolution magneto-optical Faradayeffect in fields applied perpendicular to the film plane. The radial distribution of the z- component of magnetic flux B z (r) and its gradient ∂B z /∂r are measured. For the determination of the critical current density μ o j c = rotB the curvature of the radial component ∂B r /∂ z cannot be neglected, consequently, the determination of j c must be obtained by a comparison of measured and calculated B z (r) profiles. Values for j c determined by this procedure agree fairly well with the corresponding transport critical current as well as the magnetically determined j c .