2PO1-20 Flux pinning effect of embedded carbon nanotubes in Bi₂Sr₂CaCu₂O₈

Donald H. Galvan 1,2, Jun-Ho Kim 1, M.B. Maple 1, G.A. Hirata 2, and E. Adem 3. 1 Physics Department and Institute for Pure and Applied Physical Sciences, UCSD, La Jolla, CA. 2 Centro de Ciencias de la Materia Condensada-UNAM, Apdo. Postal 2681, C.P. 22800, Ensenada, B.C., Mexico. 3 Instituto de Fisica-UNAM, Apartado Postal 20-364, C.P. 01000, Mexico, D.F.

Presenting Author: D.H. Galvan

We investigate flux pinning behavior of Bi₂Sr₂CaCu₂O₈ (BSCCO) embedded with carbon nanotubes. The superconductors were prepared by means of solid-state reaction, while the carbon nanotubes were obtained when graphite was irradiated using a 2MeV Van de Graaf accelerator (High Voltage Engineering Corp.). The irradiation conditions were the following: 1.3 MeV voltage, 5 micro A current, 25 kGy/min dose rate, and 1000 kGy total dosage. Characterization of the samples was performed by High Resolution Transmission Electron Microscopy (HRTEM), SEM, X-ray analisis, magnetic susceptibility and magnetization loops in order to study the flux pinning of vortices in pure BSCCO and BSCCO with embedded carbon nanotubes. Magnetization measurements yielded values for J_c of 76.2 x 10⁶ A/cm² for pure BSCCO and 89.8 x 10(6) A/cm² for BSCCO with embedded carbon nanotubes. The enhancement of J_c for BSCCO with embedded carbon nanotubes compared to pure BSCCO indicates that the carbon nanotubes are effective flux pinning centers.

2PO1-21 Can the collective pinning approach be applied to YBa₂Cu₃O_7-d superconductors?

Franz M. Sauerzopf, Martin Zehetmayer, Anke Köhler, and Harald W. Weber. Atominstitut, TU Wien, A-1020 Vienna, Austria

Presenting Author: F.M. Sauerzopf

The collective pinning approach is tested for the evaluation of the critical current density J_c of YBa₂Cu₃O_7-d (Y-123) single crystals. However, the field dependence of J_c at low temperatures, before and after annealing and reactor neutron irradiation, indicates strong pinning. Collective effects are expected at high densities of pinning centers, but the standard weak pinning model fails. A comparison of data on untreated and neutron irradiated crystals shows that strong pinning also prevails in the as-grown material. Further results on related single crystal samples (RE-123 and Y-124) support these findings.

2PO1-22 Vortex Dynamics in Low Magnetic Fields in Single Crystal Bi₂Sr₂CaCu₂O_8+d

Jovan Mirkovic 1 and Kasuo Kadowaki 2,3. 1 Faculty of Sciences, University of Montenegro, 81000 Podgorica, Montenegro, Yugoslavia. 2 Institute of Materials Science, University of Tsukuba, Tsukuba 305-8573, Japan. 3 CREST, Japan Science and Technology Corporation (JST), Japan.

Presenting Author: J. Mirkovic

In-plane resistivity measurements were performed for H//c in single crystal Bi₂Sr₂CaCu₂O_8+d . In order to examine the vortex dynamics related to pure bulk properties of superconductor without influence of the surface barriers the measurements were done using Corbino disc geometry. The first-order vortex lattice melting transition was identified by the sharp resistance drop. The different vortex phases were clearly distinguished in the H-T phase diagram even as low as 2.5 Oe.

The resistance level r T_m where the sharp drop of resistance occurs, does not depend on magnetic fields in the low field region in contrast with strong field dependence previously reported from the measurements performed on the platelets samples. The new experimental findings of bulk properties of superconductor are in a nice accordance with the theoretical model proposed by Rojo et al. based on the universalities of the structure factor at the freezing transition and Verlet criterion.

However, as the magnetic field is further increased and melting transition decreases below approximately 55 K the resistance level r T_m sharply drops while the resistance anomaly itself loses sharpness and gradually disappears. This means that in this temperature region melting transition may change its character from the 1st order to 2nd order phase transition or something dramatic happens in vortex phases.

2PO1-23 SQUID and magneto-optic investigations of the flux turbulence in the critical state

M.R. Koblischka 1, M. Murakami 2, T.H. Johansen 3, M. Baziljevich 3, and T. Wolf 4. 1 Condensed Matter Physics and Chemistry Department, Risø National Laboratory, DK-4000 Roskilde, Denmark. 2 SRL/ISTEC, 1-16-25, Shibaura, Minato-ku, Tokyo 105, Japan. 3 Department of Physics, University of Oslo, N-0316 Oslo, Norway. 4 ITP, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany.

Presenting Author: M.R. Koblischka

Instabilities of the critical state are observed in various NdBa₂Cu₃O_y single crystals by means of direct magneto-optic (MO) imaging while warming up flux states containing vortices of opposite polarity. Using the same samples as in the MO investigations, we observe for the first time characteristic steps in the integral magnetization measured by means of a SQUID system, which allows a constant temperature sweep. A direct correlation between the steps and jumps in the M(T) curves, which are recorded in 100 mK steps, and the MO images could be established. The integral measurement technique enables the study of the instability phenomenon ("turbulence") in a much wider range of temperatures and fields than the MO investigations, and also in the orientations parallel c and perpendicular c. As a result, we find turbulence not only in a narrow window of temperatures (65 < T < 80 K), but also in a window of applied negative fields ranging between 30 and 80 mT. Furthermore, we present tests for the flux turbulence also in various other high-T_c superconductors.

2PO1-24 The vortex lattices in LuNiBC(1221) single crystals.

L. Ya. Vinnikov 1, T.L. Barkov 1, K.O. Cheon 2, P.C. Canfield 2, and V.G. Kogan 2. 1 Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow dist.,142432 Russia. 2 Ames Laboratory, U.S. Department of Energy and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011-3020, USA.

Presenting Author: L.Ya. Vinnikov

Vortex lattices in LuNiBC(1221) have been investigated by the decoration method at fields 20-1000 Oe at liquid helium temperatures and in a field cooling regime. At small fields of 20-200 Oe we have observed vortex lattice domains with different lattices orientations. Domains size increased with magnetic field increase. At low fields vortex lattices closepacked directions in small domains do not correlate with crystal directions and the correlations become stronger at greater fields. One of the closepacked vortex arrays directions within a domain becomes parallel to <110> or <100> directions.

The vortex lattices themselves evolve at larger fields. The vortex lattice is aligned with the <100> direction and its rhombic cell apex angle b increases from 64 degrees at 180 Oe to 75-90 degrees at 1000 Oe.

2PO1-25 Flux Penetration and Critical Current Limited by Material Inhomogeneities in a YBCO Superconducting Strip

Alexander Nikolaevich Grigorenko 1, Simon J. Bending 1, and Richard G. Humphreys 2. 1 Department of Physics, University of Bath, Bath, BA2 7AY, United Kingdom. 2 DERA Electronics Division, St. Andrew's Road, Malvern, Worcestershire WR14 3PS, United Kingdom.

Presenting Author: A.N. Grigorenko

The onset of resistance in a current-carrying superconducting wire or strip represents a demanding theoretical problem with many important practical applications. Using a state-of-the-art scanning Hall probe microscope we have measured flux profiles across one 20mm wide 'wire' of a long YBCO thin film meander line as a function of both transport current density and applied magnetic field. We find that flux trapped in the sample upon cooling does not play a significant role in the flux dynamics when the penetration field or the critical curerent are exceeded. We observe instead that flux bundles nucleate at the edges of the wire near material inhomogeneities where the current flow (transport current or Meissner current) deviates from the ideal straight-line behaviour generating local magnetic field maxima. We speculate that this may be a rather universal mechanism which limits J_c or H_p in narrow wires of this type at temperatures close to T_c. At low temperatures we find that the influence of inhomogeneities is less pronounced and we are able to image thin film Bean-like profiles of flux penetration and exit after various magnetic histories. These will be compared with existing theoretical models.

2PO1-27 Magnetic relaxation in the "Bragg-glass" phase in BSCCO

Cornelis J. Van Der Beek 1, Sylvain Colson 1, Marcin Konczykowski 1, Mikhail V. Indenbom 2, Roelof J. Drost 3, and Peter H. Kes 3. 1 Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France. 2 Institute of Solid State Physics, 142432 Chernogolovka, Russia. 3 Kamerlingh Onnes Laboratorium, Rijksuniversiteit Leiden, The Netherlands.

Presenting Author: C.J. Van Der Beek

Magnetic relaxation in overdoped Bi₂Sr₂CaCu₂O₈ (BSCCO) crystals (T_c = 80 K) was investigated using time-resolved magneto-optical visualisation of the flux distribution. In these crystals, the vortex lattice disordering transition at the second magnetization peak lies B_sp ~ 600 G, much in excess of the first penetration field H_p ~ 100 G. The applied field regime over which the low-field vortex phase (which displays long-range translational order) can be accessed is therefore rather wide. This has permitted us to study magnetic relaxation in this phase, and from this, to extract the current-voltage characteristic. The E(j) curve can be well described by a power-law, although fits to a stretched exponential E ~ exp(-j_c/j)µ with µ ~0.5 are possible at long times (in excess of 100 s).

2PO1-28 Flux Avalanches in Superconducting Films with Periodic Arrays of Holes

Vitalii K. Vlasko-Vlasov 1,2, Ulrich Welp 1, Vitaly V. Metlushko 1, and George W. Crabtree 1. 1 Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL. 2 Institute of Solid State Physics RAS, 142432 Chernogolovka, Russia.

Presenting Author: V.K. Vlasko-Vlasov

The magnetic flux behavior in Nb films with 1x1 and 2x2 µm square lattices of holes is studied using the magneto-optical technique. An anisotropy of the flux motion along [100], [010], and [110] preferred directions (with respect to the hole lattice) for vortex penetration and exit is revealed. The smooth flux entry along quasi-periodically spaced channels with distances between them much larger than the hole spacing is observed at high temperatures and small fields. Possible mechanisms and the role of magnetostatic interactions for the flux motion in the hole lattice are discussed. At temperatures close to T_c~6.7K the periodic formation of critical state like induction patterns is found at matching fields. A model explaining observed flux gradients at the matching fields is suggested.

It is found that at lower temperatures and larger fields avalanches dominate the flux dynamics. These correspond to macroscopic flux jumps as revealed in macroscopic magnetization curves. The avalanches form channels of dense flux lines much wider than the hole spacing with irregularly turning branches along [100], [010], and [110] directions of the hole array. A study of their shape and size distribution as well as the behavior of successive flux jumps will be presented. Flux avalanches are treated in terms of thermo-magnetic instabilities in superconductors.

The work was supported by the U.S.DOE, BES-Materials Sciences, under Contract No.W-31-109-ENG-38.

2PO1-29 Imaging of the First Order Spin Flop Transition in the Layered Manganite La_1.4Sr_1.6Mn₂O₇

Ulrich Welp, Andreas Berger, Dean J. Miller, Vitalii K. Vlasko-Vlasov, Ken E. Gray, and John F. Mitchell, Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA

Presenting Author: U. Welp

The spin-flop transition in the antiferromagnetic layered manganite La_1.4Sr_1.6Mn₂O₇ was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. Within the MnO₂-planes the magnetization process proceeds predominantly through nucleation of polarized domains at crystal defect sites and not through the expansion of polarized domains due to domain wall motion. Along the c-axis the polarized domains grow with increasing applied field until they stretch through the entire sample thickness. A small magnetic hysteresis is caused by the difference between the mechanisms of nucleation and annihilation of domains in the mixed state. These results establish a direct link between the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.

This work was supported by the U.S. Department of Energy, BES - Materials Sciences, under contract No. W-31-109-ENG-38.