Fabrication of Artificial Washboard Pinning Structures in Thin Niobium Films

We demonstrate a possibility to realize various types of the anisotropic washboard periodic pinning potentials in thin niobium films sputtered onto ??-alumina (11 $\bar{2}$ 0) substrates. Arrays of highly periodic channels as well as ferromagnetic cobalt lines were fabricated using focused ion-beam etching and gas-assisted focused electron beam deposition techniques, respectively. Three different nanostructure profiles which could provoke guided vortex motion were fabricated. By this way either uniaxial or bianisotropic, or asymmetric (ratchet) pinning structures have been realized. The results of scanning electron and atomic force microscopy investigations of the fabricated nanopatterns are discussed.     

Asymmetry of the Pinning Force in Thin Nb Films in Parallel Magnetic Field

The pinning force, F _p, is studied in Nb films of different thickness in parallel magnetic field H. The asymmetry in the magnetic field dependence of F _p has been observed for two opposite directions of the transport current. The effect is less pronounced for thin and thick films where, respectively, single vortex pinning and pinning of the internal vortices, is relevant. At intermediate thickness, where the pinning mechanism is mostly caused by surface effects, an asymmetry in the F _p(H) dependence is clearly visible. The different surface barriers that vortices should overcome to enter the sample from opposite sides of the film explain the effect, as confirmed by numerical calculations. These have been obtained by solving the Ginzburg?CLandau equations with asymmetric boundary conditions which take into account the different superconducting properties of the film?Csubstrate and film?Cvacuum interface. Such difference can also explain the reduction of the critical current usually observed in thin films as a function of their thickness.     

Vortex-Antivortex Lattices in Superconducting Films with Magnetic Pinning Arrays

No Heading We found a rich variety of vortex structures when a thin superconducting film (SC) is covered with a lattice of out-of-plane magnetized magnetic dots (MDs). The vortices are confined to the MD regions, and antivortices nucleate into regular lattices which relax through several second order transitions towards single-dot vortex-antivortex configurations with increasing period of the magnetic array. Creation of vortex-antivortex pairs with increasing MD-magnetization is controlled by a single quantity - the superfluid velocity. We demonstrate that due to the vortex-antivortex pairs and the supercurrents induced by the MDs, the critical current in the sample actually increases if exposed to a homogeneous external magnetic field, contrary to conventional SC behavior. We explain further experimental implications, such as the magnetic field-induced-superconductivity.PACS numbers: 74.78.–w, 74.25.Op, 74.25.Qt, 74.25.Dw.     

Fluxon Pinning by Artificial Magnetic Arrays

The field dependent transport properties of superconducting niobium films, which are modulated by a regular array of non-magnetic and magnetic normal-metal inclusions (dots), have been investigated. Strong peaks in the critical current are seen for fields at which the vortex density in the superconductor is some rational or sub-rational multiple of the dot density. This commensurate peak effect is enhanced for the magnetic dot arrays when the dots are magnetized in a direction parallel to the applied magnetic field and suppressed when they are anti-aligned. Qualitative information on the strength of this dot-vortex interaction is inferred from the commensurate peaks present and missing for different regular lattices.     

Artificial Pinning Centers on MgB2 Superconducting Thin Films Coated by FeO Nanoparticles

MgB₂ thin films were fabricated on MgO (100) single crystal substrates. First, deposition of boron was performed by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850?°C in magnesium vapor. In order to investigate the effect of FeO nanoparticles on magnetic properties of MgB₂ thin films, the films were coated with different concentrations of FeO nanoparticles by spin coating process. The magnetic field dependence of the critical current density $J_{\mathrm{c}}$ was calculated from the M?CH loops and also magnetic field dependence of the pinning force density $f_{\mathrm{p}}(b)$ was determined for the films containing different concentrations of FeO nanoparticles. The values of the critical current density $J_{\mathrm{c}}$ in zero field at 5?K was found to be around 1×10⁶?A/cm² for pure MgB₂ film, 1.4×10⁶ for MgB₂ film coated with 25?%, 7.2×10⁵ for MgB₂ film coated with 33?%, 9.1×10⁵ for MgB₂ film coated with 50?% and 1.1×10⁶?A/cm² for MgB₂ film coated with 100?%. It?was?found that the film coated with 25?% FeO nanoparticles has slightly enhanced critical current density and it can be noted that especially the film coated with 25?% FeO became stronger in the magnetic field. The films coated with FeO were successfully produced and they indicated the presence of artificial pinning centers created by FeO nanoparticles. The superconducting transition temperature of the film coated with 25?% FeO nanoparticles was determined by moment?Ctemperature (M?CT) measurement to be 34?K which is 4?K higher than that of the pure film.     

Peculiarities of Pinning and Microwave Absorption Hysteresis in Thin Superconducting Films

The results of experimental and theoretical studies of the hysteretic microwave absorption (MWA) in the superconducting Bi₂Sr₂CaCu₂O₈ thin films are presented. It has been found experimentally that the hysteresis loop manifests some unusual features such as the non-monotone hysteresis variation and the change of a hysteresis sign. We have shown that such unusual behavior is due to the special nature of the bulk pinning in a superconducting film with a thickness comparable with the field penetration depth. The theoretical model of the MWA hysteresis has been developed taking into account the inhomogeneous distribution of centers with different symmetry of a pinning potential and their variation with the magnetic field value.     

Pinning Energies and Relaxation Rates in Laser Ablated Al-Doped YBCO Superconducting Thin Films

Systematic magnetization measurements were performed on laser ablated Al-doped YBCO thin films in the temperature range from 4 to 80 K and fields up to 12 T. The magnetization as a function of logarithmic time in the temperature range from 10 to 80 K generally showed a linear behavior. This type of performance allows us to determine the temperature and field dependence of the apparent pinning potential on the basis of the Kim–Anderson model for thermal activation of flux motion. The relaxation rate values are calculated from the time-dependent magnetization curves. The relaxation rate dependence on temperature, magnetic field, and doping concentrations has been investigated for different Al concentrations and applied fields up to 5 T. A strong nonlinearity was observed for the relaxation rate dependence on both temperature and applied fields. The apparent pinning potential values, U _o, are estimated from the dependence of relaxation rates temperature curves using U _o=k _B T/S expression. The dependence of U _o on both temperature and fields is elaborated and discussed in detail. The pinning potential, U _o, increases with temperature up to 40 K, however, above 40 K, a rapid decrease of U _o has been observed. Besides, a noticeable decrease in U _o as the applied field increases up to 0.1 T for temperatures below 40 K, and it is almost field independent for temperatures exceeding 40 K.     

Self-Demagnetizing Effects in Thin Films with Finite Lateral Dimensions

To analyze the shape effects on magnetic properties in films, it is common to assume some direction as very large in relation to another if the aspect ratio reaches 20 times. Thus, models based on oblate ellipsoids are assumed to estimate the format factor of the given geometry. In the present work, an experimental analysis complemented by simulations was carried out in 50 nm Permalloy (Py) thin films with 2000 nm circular-shaped area. Another geometry contemplates 50 nm Py films covering an ellipse-like format area, with 1000 nm of minor axes and 4000 nm of major axis. Magnetization curves, with the external magnetic field at different orientations, show differences in results if compared the two geometries. Experimental results and numerical simulations show that, even being large areas, the films format is relevant for aspect ratio (thickness to lateral dimensions) of 40 times.

     

The Effects of Pinning on Critical Currents of Superconducting Films

Using molecular dynamics simulations, we analyze the effects of artificial periodic arrays of pinning sites on the critical current of superconducting thin films as a function of vortex density. We analyze two types of periodic pinning array: hexagonal and Kagomé. For the Kagome pinning network we make calculations using two directions of transport current: along and perpendicular to the main axis of the lattice. Our results show that the hexagonal pinning array presents higher critical currents than the Kagomé and random pinning configuration for all vortex densities. In addition, the Kagomé networks show anisotropy in their transport properties.     

Laser-induced Effects in Te-Ge Multilayer Thin Films

Experiments on the interaction of an excimer and argon lasers with multilayer configurations in the Ge-Te system are performed. The use of similar laser energy densities with very different irradiation pulse lengths and areas induces widely differing damage effects.     

Frictional Effects in Thin 3He Films

The recent high-precision torsional oscillator experiments of Casey et al. involving thin films of normal liquid ³He showed that the film decouples from the substrate with a time constant which is proportional to T ⁻¹ where T is the absolute temperature. We interpret this experiment by adapting a theory due to Meyerovich which was developed for dilute ³He-⁴He mixtures flowing between two relatively smooth plates. The analysis of the experiment confirms the central idea that varies as T ⁻¹. The variation of with film thickness, d, is affected by the change in the shape of the free surface of the film, due to van der Waals forces, as the film becomes thinner.     

Vortex Self-Organization in the Presence of Magnetic Pinning Arrays

Lateral nanostructuring is an efficient tool to control vortex confinement in superconductors. This will be illustrated by studying pinning phenomena in type-II superconducting Pb films with a lattice of submicron magnetic dots. We consider rectangular Co dots with in-plane magnetization and circular Co/Pt dots with out-of-plane magnetization. The domain structure of the Co dots can be changed from multi- to single-domain, resulting in an enhancement of their stray field. After covering this Co dot array with a Pb film, we demonstrate the influence of the local magnetic stray field of the dots on their flux pinning efficiency. The Co/Pt dots have a single-domain structure with their magnetic moment out of plane. Depending on the magnetic history, the magnetic moment of all dots can be aligned in positive or negative direction, or a random distribution of positive and negative magnetic moments of the dots can be achieved. For a Pb film covering this Co/Pt dot array, we observe an asymmetric magnetization loop due to the magnetic interactions between the vortices and the magnetic dots.     

Comparison of Flux Pinning Mechanism in Laser Ablated YBCO and YBCO:BaZrO3 Nanocomposite Thin Films

Thin films of YBCO and YBCO:BaZrO₃ (BZO) nanocomposite have been deposited using the pulsed laser deposition technique. Substantial increase in critical current density (J _C ) and pinning force density (F _p ) of the nanocomposite thin films was observed. The possible pinning mechanism in YBCO:BZO nanocomposite thin films has been explored and compared with the pinning mechanism in pure YBCO thin film by studying the variation of J _C with magnetic field (B) and temperature. In the intermediate field regime (0.1–1 T), J _C follows B ^−α with nearly similar values of α for YBCO and YBCO:BZO nanocomposite thin films indicating similar pinning mechanism in both thin films. The variation of J _C with reduced temperature (t=T/T _C ) has been studied for both the films and it was observed that the mechanism of pinning in both YBCO and YBCO:BZO thin films is similar (δT _C pinning). The observed enhanced values of J _C and F _p of the nanocomposite thin film is attributed to the presence of BZO nanoparticles, which induces more defects due to lattice mismatch between YBCO and BZO leading to improved flux pinning properties of the nanocomposite thin film.     

Surface Effects on the Dynamic Behavior of Vortices in Type II Superconducting Strips with Periodic and Conformal Pinning Arrays

Using molecular dynamics techniques, we simulate the vortex behavior in a type II superconducting strip in the presence of triangular and two types of conformal pinning arrays, one with a pinning gradient perpendicular to the driving force (C1) and the other parallel (C2), at zero temperature. A transport force is applied in the infinite direction of the strip, and the magnetic field is increased until the rate between the density of vortices (n _v ) and pinning (n _p ) reaches n _v /n _p =?1.5. Our results show a monotonic increase, by steps, of the vortex density with the applied magnetic field. Besides, each pinning lattice presents a different vortex penetration delay. For the triangular pinning array, different than the case of infinite films, here the system exhibits only one pronounced depinning force peak at n _v /n _p =?1. However, the depinning force peak is present for only one value of field, in the range of fields where n _v /n _p =?1 is stable. For the case of conformal pinning arrays, in analogy to what is observed in infinite films, no commensurability depinning force peaks were found. In the present case, the C1 array is more efficient at low fields, all arrays are equivalent in the intermediate fields, and the C2 array is more efficient for high fields. We also show that for the C1 array at high fields, vortices depin following the conformal arches, from the edge to the center. For the C2 array, the depinning force is higher when compared to that of C1, because this particular conformal structure prevents the formation of easy vortex flow channels.