M. Velez

Temperature Dependence and Mechanisms of Vortex Pinning by Periodic Arrays of Ni Dots in Nb Films

Pinning interactions between superconducting vortices in Nb and magnetic Ni dots were studied as a function of current and temperature to clarify the nature of pinning mechanisms. A strong current dependence is found for a square array of dots, with a temperature-dependent optimum current for the observation of periodic pinning, which decreases with temperature as

Order in driven vortex lattices in superconducting Nb films with nanostructured pinning potentials

(1\ensuremath{-}{T/T}_{C}{)}^{3/2}.

Flux pinning and weak links in the behavior of the critical current of a‐axis and c‐axis EuBa2Cu3O7 superconducting thin films

This same temperature dependence is found for the critical current at the first matching field with a rectangular array of dots. The analysis of these results allows one to narrow the possible pinning mechanisms to a combination of two: the interaction between the vortex and the magnetic moment of the dot and the proximity effect. Moreover, for the rectangular dot array, the temperature dependence of the crossover between the low-field regime with a rectangular vortex lattice to the high-field regime with a square configuration has been studied. It is found that the crossover field increases with decreasing temperature. This dependence indicates a change in the balance between elastic and pinning energies, associated with the dynamical effects of the vortex lattice in the high-field range.

Critical currents and pinning forces in a‐axis oriented EuBa2Cu3O7/PrBa2Cu3O7 superlattices

Driven vortex lattices have been studied in a material with strong pinning, such as Nb films. Samples in which natural random pinning coexists with artificial ordered arrays of defects (submicrometric Ni dots) have been fabricated with different geometries (square, triangular and rectangular). Three different dynamic regimes are found: for low vortex velocities, there is a plastic flow regime in which random defects frustrate the effect of the ordered array; then, for vortex velocities in the range 1-100 m/s, there is a sudden increase in the interaction between the vortex lattice and the ordered dot array, independent on the geometry. This effect is associated to the onset of quasi long range order in the vortex lattice leading to an increase in the overlap between the vortex lattice and the magnetic dots array. Finally, at larger velocities the ordered array-vortex lattice interaction is suppressed again, in agreement with the behavior found in numerical simulations.

Magnetic flux pinning and microstructure, a special case : a-axis oriented EuBa2Cu3O7/PrBa2Cu3O7 superconducting superlattices

Pure a‐axis and c‐axis oriented EuBa2Cu3O7 films have been grown by a dc sputtering technique. The a‐axis films show critical current values typically one order of magnitude lower than c‐axis films. The Jc(T) behavior could be governed, in a‐axis films, by the microstructure of these films with small domains and 90° boundaries, but Jc(B) does not seem to be limited by weak links. The magnetic‐field dependence of Jc shows a similar behavior in a‐axis and c‐axis films with a leading role of pinning in controlling the critical current in the high‐field range. Jc(B) is limited by flux creep with a similar distribution of pinning energies in both types of films, in spite of the very different microstructure.

Critical currents and thermal activation in a-axis oriented EuBa/sub 2/Cu/sub 3/O/sub 7/ thin films

a‐axis oriented EuBa2Cu3O7/PrBa2Cu3O7 (EBCO/PBCO) superlattices have been grown by dc sputtering. The critical current (JC) behavior is modified by the pinning force arising from the insulating PrBa2Cu3O7 layers, which in a‐axis oriented superlattices are perpendicular to the Cu–O planes. Depending on the EBCO and PBCO layer thicknesses the pinning forces deviate from the usual scaling laws as a function of temperature observed in single films. When B is perpendicular to the sample, an exponential dependence of JC(B) appears as the PBCO thickness is increased (coupling is reduced), and also as the thickness of the EBCO layer is reduced.

Interplay between the vortex lattice and arrays of submicrometric pinning centers

a-Axis oriented EuBa 2 Cu 3 O 7 films and a-axis oriented EuBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices have been grown by d.c. magnetron sputtering on (100) SrTiO 3 substrates. These films and multilayers have been characterized by X-ray diffraction, transmission and scanning electron microscopies, and atomic force and scanning tunnelling microscopies. These techniques show a remarkable microstructure with the Cu-O planes perpendicular to the substrate plane and with these planes arranged in small domains separated by 90° boundaries. This microstructure reduces the critical current in comparison with c-axis oriented films, but they do not act as weak links. The competition between the natural layered character of these high-temperature superconductors and the artificially layered structure introduced by the PrBa 2 Cu 3 O 7 layers allows us to modify at will the anisotropy of the system and the vortex motion. Therefore the activation energies and the anisotropy in the flux pinning could be changed playing with the superlattice modulation lengths and the applied magnetic field.

Fabrication and properties of c-axis and a-axis oriented EuBa2Cu3O7/SrTiO3 superlattices

a-axis oriented EuBa/sub 2/Cu/sub 3/O/sub 7//SrTiO/sub 3/(100) thin films have been grown by DC magnetron sputtering. Critical current and resistivity have been measured as a function of temperature, down to 15 K, and field, up to 9 T. In spite of the existence of 90/spl deg/ grain boundaries, the critical current is not limited by weak links. The lower part of the resistivity transition and the field dependence of the pinning force may be understood in terms of thermally activated flux creep with activation energy U(T,H)=U/sub 0/(T)/H and U/sub 0/ values of several hundreds of meV.<<ETX>>

Magnetic flux pinning and microstructure, a special case: a-axis oriented superconducting superlattices

Abstract Ordered arrays of submicrometric magnetic pinning centers have been fabricated in superconducting Nb films by electron beam lithography. Periodic pinning effects, observed when the vortex lattice matches the array of dots, have been studied as a function of temperature, current and array geometry in order to analyze the physics of the interaction between the vortex lattice and the periodic array of microscopic pinning centers. These results have allowed pointing to the magnetic origin of the relevant pinning mechanism in this system and to dynamic ordering effects of the vortex lattices. The effect of the relative direction of the driving force with respect to the pinning potential is dependent on the anisotropy configuration of the artificial submicrometric array.

Hall effect and longitudinal resistivity of 123 superconducting thin films: Scaling relations

Copyright (c) 1997 Elsevier Science S.A. All rights reserved.c-Axis oriented multilayers of superconducting EuBa 2 Cu 3 O 7 and insulating SrTiO 3 (EBCO/STO) have been grown by d.c. and r.f. magnetron sputtering, respectively, on (100) LaAlO 3 (LAO) and (100) SrTiO 3 (STO) substrates. These superconducting multilayers show good X-ray diffraction rocking curves and X-ray satellite peaks coming from the superlattice structure. In these superlattices the CuO 2 planes are parallel to the substrate plane. The modification of the growing conditions allows us to obtain pure a-axis oriented EBCO/STO superlattices with the CuO 2 planes running perpendicular to the substrate plane. The decrease of the oxygen pressure in the chamber and the decrease of the substrate temperature during deposition produce textured, good quality a-axis superlattices, with X-ray diffraction satellite peaks and superconducting transitions close to the liquid nitrogen temperature.