ORAL SESSION 3B4: Vortex Pinning

Wednesday, Feb. 23, 10:00 a.m. – 12:00 p.m., General Assembly Hall A (GRB)

Chairs: L. Krusin-Elbaum (IBM Yorktown), V.V. Moshchalkov (Katholiek U Leuven), A. Weber (NSF)

3B4.1 Vortex Confinement by Regular Pinning Arrays

Y. Bruynseraede, Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium

Presenting Author: Y. Bruynseraede

The pinning of vortices by a regular array of sub-micron Co dots with in-plane magnetization and Co/Pt dots with out-of-plane magnetization is studied in superconducting Pb films. Matching anomalies are observed in the magnetization curves at integer and rational multiples of the first matching field, corresponding to stable vortex configurations recently also revealed by Lorentz microscopy measurements and by molecular dynamics simulations. The magnetic domain structure of the dots is revealed by MFM measurements at room temperature; the distribution of the local fields generated by the dots and the vortices is determined by scanning Hall-probe microscopy above and below T_c. These measurements indicate the presence of stable vortex lattices and give evidence of the pinning of multi-quanta vortices at lower temperature. Finally, in superconducting Pb films with a square lattice of microholes, Shapiro voltage steps have been observed in the voltage-current characteristics in the presence of rf-radiation. These steps clearly reveal the presence of mobile interstitial vortices coexisting with immobile vortices strongly pinned at the microholes.

This work is performed in collaboration with M.J. Van Bael, L. Van Look, J. Bekaert, M. Lange, G. Güntherodt, K. Temst, and V.V. Moshchalkov, and supported by the Belgian IUAP and the Flemish GOA and FWO Programs.

3B4.2 Flux Pinning in a Superconductor by Arrays of Submicron Magnetic Dots

Ivan K. Schuller 1, A. Hoffmann 1, Y. Jaccard 1, M-C. Cyrille 1, F. Sharifi 2, J. Martin 1,3, M. Velez 1,3, J. Vicent 1,3, J. Nogues 1, J-M. George 1, M. Grimsditch 4, M.J. Van Bael 5, K. Temst 5, C. Van Haesendonck 5, V.V. Moshchalkov 5, and Y. Bruynseraede 5. 1 University of California, San Diego, La Jolla, CA 92093. 2 University of Florida, Gainesville, FL 32611. 3 Universidad Complutense, 28040 Madrid, Spain. 4 Argonne National Labs, Argonne, IL. 60439. 5 Katholieke Universiteit Leuven, B-3030-Leuven, Belgium.

Presenting Author: I.K. Schuller

Electron beam lithography is a powerful technique which allows the preparation and control of arrays of small magnetic dots. In this way, we have fabricated, triangular and square lattices, lines etc. of submicrometer magnetic dots (Ni, Co and Fe). We have studied the magnetic and transport properties of these arrays using a variety of techniques including magnetotransport, magnetization, Magnetic Force Microscopy and light scattering.

The interaction between an ordered array of small magnetic (Ni, Co, Fe) particles and a superconducting (Nb) thin film can lead to important pinning effects due to the synchronized interaction with the vortex lattice. The resistivity vs. magnetic field curves present sharp minima close to the transition temperature, whereas the transport critical currents exhibit pronounced maxima. These minima and maxima appear at constant field intervals , clearly related with the lattice parameter of the vortex lattice array. I will describe a series of experiments and comparisons to address many of the aspects of this interesting collective matching phenomenon.

Supported by the US-DOE, UC-CLC and AFOSR.

3B4.3 Static and Dynamic Properties of Vortex Structures under the Influence of Weak Periodic Pinning Potentials

F. de la Cruz 1, M. Menghini 2, and Y. Fasano 2. 1 Centro Atomico Bariloche, CNEA. 2 Instituto Balseiro. UNC, 8400 Bariloche, RN, Argentina.

Presenting Author: F. de la Cruz

We discuss the presence of a vortex liquid in the low field range of the mixed state of Bi₂Sr₂CaCu₂O_8-d for fields applied in the c direction. Strong evidence that the first order vortex melting transition is suppressed at low fields is provided through a systematic study of vortex structures, plus the analysis of the response of the vortex system to the presence of engineered superficial weak pinning centers, induced by the Bitter technique. This is analyzed in terms of two possible scenarios: a disordered induced critical point in the low field range or the existence of a reentrant vortex liquid above the Meissner state.

3B4.4 Natural strong pinning of vortices in high-T_c films

R. Griessen, B. Dam, R.J. Wijngaarden, J. Huijbregtse, F.C. Klaassen, R. Surdeanu, J.H. Rector, R. van der Geest, and E. Visser, Faculty of Sciences, Division of Physics, Vrije Universiteit, De Boelelaan 1081, NL-1081 HV Amsterdam, The Netherlands

Presenting Author: B. Dam

In thin high-T_c films natural linear defects are responsible for:

a) large critical currents (j_c up to 10¹² A/m²) due to strong pinning of vortices at screw and edge dislocations [1],

b) a plateau in the field dependence of j_c as a result of the self-organised short-range order of these natural linear defects in films, and

c) the kinetic roughening of magnetic flux penetration fronts. The corresponding growth and roughening exponents are remarkably similar to those of the combustion front in burning paper [2]. Roughening is best seen in high quality films (e.g. epitaxial YBCO films on NdGaO₃)

These examples show that thin epitaxial high-T_c films provide attractive possibilities to investigate the behaviour of vortex matter (e.g. non-linear diffusion) in presence of self-organised strong pinning linear defects. The short range order of this natural pinning landscape is in sharp contrast with the fully random distribution of artificial columnar defects in single crystals.

[1] B. Dam et al., Origin of high critical currents in YBCO superconducting thin films, Nature 399 (1999) 439.

[2] R. Surdeanu, et al. Kinetic roughening of penetrating flux front in high-T_c thin film superconductors, Phys. Rev. Lett. 83 (1999) 2054.

3B4.5 Interaction between Columnar Defects and Pancake Vortices in Bi₂Sr₂CaCu₂O_8+y Studied by Josephson Plasma Resonance

T. Shibauchi 1, *, M. Sato 1, N. Kameda 1, S. Ooi 1, T. Tamegai 1, and M. Konczykowski 2. 1 Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan. 2 Laboratorie des Solides Irradies, CNRS URA-1380, Ecole Polytechnique, 91128 Palaiseau, France. *Present address: IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA, and MST-STC, Los Alamos National Laboratory, Los Alamos, NM87545, USA.

Presenting Author: T. Shibauchi

Line vortices can be pinned effectively by topologically matched "columnar defects" introduced by heavy-ion irradiation. An important question then is whether decoupled two-dimensional (pancake) vortices in highly anisotropic superconductors can be localized in the vicinity of columnar defects. One of the most powerful probes for the c-axis pancake vortex arrangement is Josephson plasma resonance (JPR). We studied JPR in Bi₂Sr₂CaCu₂O_8+y with and without columnar defects in a wide range of matching field BF (5G ~ 40 kG). In unirradiated crystals, we found that the vortices are decoupled just above the vortex-lattice melting transition [1]. In irradiated crystals, the c-axis coherence is found to be drastically enhanced above a characteristic (temperature-independent) field H* in the vortex liquid state above the melting line [2], which we interpreted as recoupling of pancake vortices by columnar defects. The recoupling field H* scales as (0.2-0.3)BF at high doses (BF>500 G), but at lower doses it is almost constant (~60 G). At around 60 G, the intervortex distance becomes 21 (~600 nm) suggesting that the recoupling occurs when vortex-vortex interactions are significant at low doses. At high doses we found that (1)H* does not depend on oxygen content of the crystal(i.e. strength of Josephson coupling) and (2) when columnar defects are introduced at an angle q from the c-axis, H* is scaled as (0.2-0.3)BFcosq . These results indicate that the recoupling at high doses is controlled only by the density of defects in each CuO₂ layers.

[1] T. Shibauchi et al., Phys. Rev. Lett. 83, 1010 (1999).

[2] M. Sato et al., Phys. Rev. Lett. 79, 3759(1997); M. Kosugi et al., Phys. Rev. B59, 8970(1999).