Current Density Of Thin Films Research Articles

Controlling particle properties in nanocomposites by combining PLD with an inert gas condensation system

The critical current density in thin films, which limits their application in external magnetic fields, can be enhanced by the introduction of artificial pinning centers such as non-superconducting nanoparticles inducing additional defects and local strain in the superconducting matrix. To understand the correlation between superconductivity, defect structures and particles, a controlled integration of particles with adjustable properties is essential. A powerful technique for the growth of isolated nanoparticles in the range of 10 nm is dc-magnetron sputtering in an inert gas flow. The inert gas condensation (IGC) of particles allows for an independent control of both the particle diameter distribution and the areal density. We report on the integration of such gas-phase-condensed nanoparticles into pulsed laser deposited (PLD) thin film multilayers with a combined PLD-IGC system. The particles and the structure of the multilayers are analyzed by transmission electron microscopy on cross-sectional FIB lamellae. As a result of the IGC particle implementation, randomly as well as biaxially oriented precipitates are formed in the thin films. With as few as three interlayers of nanoparticles, the pinning force density is enhanced in the low-field region.

Frontiers of Research on Iron-Based Superconductors toward Their Application

The recent discovery of iron-based superconductors has evoked enthusiasm for extensive research on these materials because they form the second high-temperature superconductor family after the copper oxide superconductors and impart an expectation for materials with a higher transition temperature (Tc). It has also been clarified that they have peculiar physical properties including an unconventional pairing mechanism and superconducting properties preferable for application such as a high upper critical field and small anisotropy. This paper reviews the research on thin films, Josephson junctions, and superconducting wires and tapes made from iron-based superconductors, which has been performed toward the realization of future applications. Though there are many technical hurdles toward the practical application of these materials, some promising features such as a high critical current density in thin films under high magnetic fields and advantageous grain boundary properties over copper oxides have been clarified.

Dielectric and ferroelectric properties of Ba(Sn 0.15Ti 0.85)O 3 thin films grown by a sol–gel process

Ferroelectric Ba(Sn 0.15Ti 0.85)O 3 (BTS) thin films were deposited on LaNiO 3-coated silicon substrates via a sol–gel process. Films showed a strong (1 0 0) preferred orientation depending upon annealing temperature and concentration of the precursor solution. The dependence of dielectric and ferroelectric properties on film orientation has been studied. The leakage current density of thin films at 100 kV/cm was 7 × 10 −7 A/cm 2 and 5 × 10 −5 A/cm 2 and their capacitor tunability was 54 and 25% at an applied field of 200 kV/cm (measurement frequency of 1 MHz) for the thin films deposited with 0.1 and 0.4 M spin-on solution, respectively. This work clearly reveals the highly promising potential of BTS compared with BST films for application in tunable microwave devices.

Critical current density in thin films due to the surface barrier

For superconducting films thinner than the penetration depth λ, the critical current density when an applied external magnetic field is parallel to the film surface is expected to be higher than the critical current density due to the bulk pinning. This increase is due to the effect of the surface barrier. In this study, we considered the distribution and stability of vortices in thin films within the framework of the London theory, and derived an expression for the critical current density in a parallel magnetic field as a function of the magnetic field and the film thickness. The resulting expressions showed that the critical current density decreases logarithmically with increasing magnetic field, and is inversely proportional to the film thickness. These results showed good agreement with observed data previously reported for NbTi films.

Reproducibility of transition temperature and critical current density of thin films of YBa2Cu3O7 on (100) LaAlO3

Submicrometer epitaxial films of YBa2Cu3O7(YBCO) on (100) LaAlO3 were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (Tc) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (Jc) at 77 K has an average value of 2×105 A cm−2, with a tendency for decreasingJc with increasing film thickness (0.2 to 0.8μm). Variations inJc are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJc. If the film composition is enriched in Y relative to Ba and Cu,Tc decreases by several degrees.

Epitaxial growth and critical current density of thin films of YBa2Cu3O7−x on LaAlO3 substrates

Thin films of the high-temperature superconductor YBa2 Cu3 O7−x have been produced on (100) LaAlO3 substrates by coevaporation and furnace annealing. A 14-μm-wide and 400-μm-long constriction patterned on a 0.8-μm-thick film had a zero resistance transition temperature of 90 K, a transition width of 1.5 K, and a critical current density of 8×104 A cm−2 at 77 K. Although x-ray diffraction shows a definite c-axis alignment normal to the substrate plane, further analysis reveals that c-axis alignment in the substrate plane is also present. The detailed microstructural picture is revealed by transmission electron microscopy: a continuous layer, about 0.2 μm thick adjacent to the substrate, with c axis normal to the substrate plane, and the remaining top portion of the film, with the c axis in the film plane. In spite of the bilayer structure, the film remains epitaxial (the axes of the superconductor are parallel to the 〈100〉 directions of the substrate).

Large differences of critical current density in thin films of superconducting YBa2Cu3O7−x

Thin films of YBa2 Cu3 O7−x with three types of c-axis alignment have been prepared by evaporation: unaligned films on oxidized silicon with zero-resistance transition temperatures as high as 88 K (the highest value reported for thin films of this superconductor on this substrate), films with regions aligned along each of the three 〈100〉 directions of the (100) SrTiO3 substrate, and films with the c-axis perpendicular to the (100) SrTiO3 plane. Typical values of critical current density (A cm −2 ) at 77 K are 102 , 3×104 , and 106, respectively. The temperature dependence of the critical current density is similar for the three types of films; it increases linearly with decreasing temperature, which is suggestive of a flux creep-limited model.

Critical temperature gradient and critical current density in thin films of a type I superconductor

Measurements of the critical temperature gradient and the critical current density in superconducting lead films in a transverse magnetic field indicate that the critical current flows predominantly along the surface of the films and that the critical surface currents contribute only very little to the Lorentz force on a fluxoid.