A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.

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.

Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in the voltage-current characteristics of a superconducting film with a square lattice of perforating microholes (antidots)in the presence of radiofrequent radiation. These equidistant steps appear at the second matching field H_2 when the flow of the interstitial vortex lattice in the periodic potential created by the antidots and the vortices trapped by them, is in phase with the applied rf frequency. Therefore, the observation of Shapiro steps clearly reveals the presence of mobile intersitial vortices in superconducting films with regular pinning arrays. The interstitial vortices, moved by the driving current, coexist with immobile vortices strongly pinned at the antidots.Comment: 6 pages text, 3 EPS figures, RevTeX, accepted for publication in PRB Rapid Communication

The pinning of flux lines by two different types of regular arrays of submicron magnetic dots is studied in superconducting Pb films; rectangular Co dots with in-plane magnetization are used as pinning centers to investigate the influence of the magnetic stray field of the dots on the pinning phenomena, whereas multilayered Co/Pt dots with out-of-plane magnetization are used to study the magnetic interaction between the flux lines and the magnetic moment of the dots. For both types of pinning arrays, matching anomalies are observed in the magnetization curves versus perpendicular applied field at integer and rational multiples of the first matching field, which correspond to stable flux configurations in the artificially created pinning potential. By varying the magnetic domain structure of the Co dots with in-plane magnetization, a clear influence of the stray field of the dots on the pinning efficiency is found. For the Co/Pt dots with out-of-plane magnetization, a pronounced field asymmetry is observed in the magnetization curves when the dots are magnetized in a perpendicular field prior to the measurement. This asymmetry can be attributed to the interaction of the out-of-plane magnetic moment of the Co/Pt dots with the local field of the flux lines and indicates that flux pinning is stronger when the magnetic moment of the dot and the field of the flux line have the same polarity.Comment: 7 pages including figures; submitted for publication in Physica C (Proceedings ESF-Vortex Conference, 18-24 Sept. 1999, Crete, Greece

Superconducting structures with the size of the order of the superconducting coherence length ξ(T ) have a critical temperature T c , oscillating as a function of the applied perpendicular magnetic field H (or flux Φ). For a thin-wire superconducting loop, the oscillations in T c are perfectly periodic with H (this is the well-known Little-Parks effect), while for a singly connected superconducting disk the oscillations are pseudoperiodic, i.e. the magnetic period decreases as H grows. In the present paper, we study the intermediate case: a loop made of thick wires. By increasing the size of the opening in the middle, the disk-like behaviour of T c (H) with a quasi-linear background (characteristic for '3-dimensional' (3D) behaviour) is shown to evolve into a parabolic T c (H) background ('2D'), superimposed with perfectly periodic oscillations. The calculations are performed using the linearized Ginzburg-Landau theory, with the proper normal/vacuum boundary conditions at both the internal and the external interface. Above a certain crossover magnetic flux Φ, T c (Φ) of the loops becomes quasi-linear, and the flux period matches with the case of the filled disk. This dimensional transition is similar to the 2D-3D transition for thin films in a parallel magnetic field, where vortices enter the material as soon as the film thickness t > 1.8 ξ(T ). For the loops studied here, the crossover point appears for w ≈ 1.8 ξ(T ) as well, with w the width of the wires forming the loop. In the 3D regime, a "giant vortex state" establishes, where superconductivity is concentrated near the sample's outer interface. The vortex is then localized inside the loop's opening. 74.60.Ec, 74.25.Dw, 74.20.De, Typeset using REVT E X