Flux pinning properties of holes and blind holes arranged periodically in a superconductor

in Physica C: Superconductivity (2004), 404(1-4), 298-301

We investigate the dynamic response of a vortex lattice in nanostructured superconducting Pb films by means of ac-magnetization measurements. We compare the flux pinning properties of superconducting Pb ... [more ▼]

We investigate the dynamic response of a vortex lattice in nanostructured superconducting Pb films by means of ac-magnetization measurements. We compare the flux pinning properties of superconducting Pb films with arrays of fully perforated holes (antidots) and partially drilled holes (blind holes). Our results show that an array of blind holes, characterized by a thin superconducting bottom layer, gives rise to a less efficient pinning potential than an array of antidots. On top of that, a lower number of flux quanta trapped per pinning site (the saturation number), is observed for the blind hole array. (C) 2004 Elsevier B.V. All rights reserved. [less ▲]

Dynamic regimes in films with a periodic array of antidots

in European Physical Journal B -- Condensed Matter (2004), 37(1), 19-24

We have studied the dynamic response of Pb thin films with a square array of antidots by means of ac susceptibility chi(T,H) measurements. At low enough ac drive amplitudes h, vortices moving inside the ... [more ▼]

We have studied the dynamic response of Pb thin films with a square array of antidots by means of ac susceptibility chi(T,H) measurements. At low enough ac drive amplitudes h, vortices moving inside the pinning potential give rise to a frequency- and h-independent response together with a scarce dissipation. For higher amplitudes, the average distance travelled by vortices surpasses the pinning range and a critical state develops. We found that the boundary h*(H,T) between these regimes smoothly decreases as T increases whereas a step-like behavior is observed as a function of field. We demonstrate that these steps in h*(H) arise from sharp changes in the pinning strength corresponding to different vortex configurations. For a wide set of data at several fields and temperatures in the critical state regime, we show that the scaling laws based on the simple Bean model are satisfied. [less ▲]

Guided vortex motion in superconductors with a square antidot array

in Physical Review. B : Condensed Matter (2003), 68(21),

We have measured the in-plane anisotropy of the vortex mobility in a thin Pb film with a square array of antidots. The Lorentz force, acting on the vortices, was rotated by adding two perpendicular ... [more ▼]

We have measured the in-plane anisotropy of the vortex mobility in a thin Pb film with a square array of antidots. The Lorentz force, acting on the vortices, was rotated by adding two perpendicular currents and keeping the amplitude of the net current constant. One set of voltage probes was used to detect the vortex motion. We show that the pinning landscape provided by the square antidot lattice influences the vortex motion in two different ways. First, the modulus of the vortex velocity becomes angular dependent with a lower mobility along the diagonals of the pinning array. Second, the vortex displacement is preferentially parallel to the principal axes of the underlying pinning lattice, giving rise to a misalignment between the vortex velocity and the applied Lorentz force. We show that this anisotropic vortex motion is temperature dependent and progressively fades out when approaching the normal state. [less ▲]

ANGULAR DEPENDENT VORTEX DYNAMICS IN SUPERCONDUCTORS WITH COLUMNAR DEFECTS

in Narlikar, Anant (Ed.) Irradiation effects and cationinc disorder in HTS (2003)

This book chapter described the vortex pinning properties of type II superconductors with columnar defects by means of magnetization and ac-susceptibility measurements.

Columnar defects acting as passive internal field detectors

in Physical Review. B : Condensed Matter (2002), 65(17),

We have studied the angular dependence of the irreversible magnetization of several YBa2Cu3O7 and 2H-NbSe2 single crystals with columnar defects tilted off the c axis. At high magnetic fields, the ... [more ▼]

We have studied the angular dependence of the irreversible magnetization of several YBa2Cu3O7 and 2H-NbSe2 single crystals with columnar defects tilted off the c axis. At high magnetic fields, the irreversible magnetization M-i(Theta) exhibits a well-known maximum when the applied field is parallel to the tracks. As the field is decreased below Hsimilar to0.02H(c2), the peak shifts away from the tracks' direction toward either the c axis or the ab planes. We demonstrate that this shift results from the misalignment between the external and internal field directions due to the competition between anisotropy and geometry effects. [less ▲]

Irreversible magnetization under rotating fields and lock-in effect on a ErBa2Cu3O7-delta single crystal with columnar defects

in Physical Review. B : Condensed Matter (2001), 64(14),

We have measured the irreversible magnetization (M-i) of an ErBa2Cu3O7-delta single crystal with columnar defects (CD), using a technique based on sample rotation under a fixed magnetic field H. This ... [more ▼]

We have measured the irreversible magnetization (M-i) of an ErBa2Cu3O7-delta single crystal with columnar defects (CD), using a technique based on sample rotation under a fixed magnetic field H. This method is valid for samples whose magnetization vector remains perpendicular to the sample surface over a wide angle range-which is the case for platelets and thin films-and presents several advantages over measurements of M-L(H) loops at fixed angles. The resulting M-i(Theta) curves for several temperatures show a peak in the CD direction at high fields. At lower fields, a very well defined plateau indicative of the vortex lock-in to the CD develops. The H dependence of the lock-in angle phi (L) follows the H-1 theoretical prediction, while the temperature dependence is in agreement with entropic smearing effects corresponding to long range vortex-defects interactions. [less ▲]

Influence of nonlocal electrodynamics on the anisotropic vortex pinning in YNi2B2C

in Physical Review. B : Condensed Matter (2001), 64(1),

We have studied the pinning force density F-p of YNi2B2C superconductors for various field orientations. We observe anisotropies both between the c axis and the basal plane and within the plane that ... [more ▼]

We have studied the pinning force density F-p of YNi2B2C superconductors for various field orientations. We observe anisotropies both between the c axis and the basal plane and within the plane that cannot be explained by the usual mass anisotropy. For magnetic field H parallel toc, the reorientation structural transition in the vortex lattice due to nonlocality, which occurs at a field H(1)similar to1 kOe, manifests itself as a kink in F-p(H). When H perpendicular toc, F-p is much larger and has a quite different H dependence, indicating that other pinning mechanisms are present. In this case the signature of nonlocal effects is the presence of a fourfold periodicity of F-p within the basal plane. [less ▲]

Four-fold anisotropy of the basal plane magnetization of YNi2B2C: nonlocal effects

in Physica C: Superconductivity (2000), 341(Part 2), 1299-1300

We present studies of the vortex state magnetization M of a single crystal of the nonmagnetic borocarbide superconductor YNi2B2C. With magnetic field H applied in the square basal plane of the tetragonal ... [more ▼]

We present studies of the vortex state magnetization M of a single crystal of the nonmagnetic borocarbide superconductor YNi2B2C. With magnetic field H applied in the square basal plane of the tetragonal unit cell, M has a four-fold anisotropy. This pi /2 periodicity occurs deep in the superconductive mixed state, with an unusual field dependence, for a range of temperatures T < T-c. The observations are well explained by a generalized London model incorporating non-local electrodynamics, using parameters from complementary experiments. [less ▲]

Thickness effects on the pinning by columnar defects

in Physica C: Superconductivity (2000), 341(Part 2), 1217-1218

We compare the angular dependence of the pinning of equivalent columnar defects in GdBa2Cu2O7 thin films and a thicker YBa2Cu3O7 single crystal. The tracks' related enhancement of the critical currents is ... [more ▼]

We compare the angular dependence of the pinning of equivalent columnar defects in GdBa2Cu2O7 thin films and a thicker YBa2Cu3O7 single crystal. The tracks' related enhancement of the critical currents is much smaller for the case of the thin films. We interpret this thickness dependence as resulting from a competition between the pinning of the correlated structures, related to total sample thickness, and surface boundary conditions, which force the vortices to depin from the columnar defects near the sample surface. [less ▲]

Anomalous behavior of the irreversible magnetization and time relaxation in YBa(2)Cu(3)O(7) single crystals with splayed tracks

in Physical Review. B : Condensed Matter (1999), 60(18), 13189-13195

We have studied the angular dependence of the irreversible magnetization and its time relaxation in YBa(2)Cu(3)O(7) Single crystals with one or two families of columnar defects inclined with respect to ... [more ▼]

We have studied the angular dependence of the irreversible magnetization and its time relaxation in YBa(2)Cu(3)O(7) Single crystals with one or two families of columnar defects inclined with respect to the c axis. At high magnetic fields, the magnetization shows the usual maximum centered at the mean tracks' orientation and an associated minimum in the normalized relaxation rate. In contrast, at low fields we observe an anomalous local minimum in the magnetization and a maximum in the relaxation rate. We present a model to explain this anomaly based on the slowing down of the creep processes arising from the increase of the vortex-vortex interactions as the applied field is tilted away from the mean tracks' direction. [S0163-1829(99)11841-1]. [less ▲]