2PO1-10 The Specific Damage Behaviors in 180-MeV Fe-irradiated Bi₂Sr₂CaCu₂O_x crystals

Daxiang Huang 1, Yukich Sasaki 2, Izumi Hirabayashi 1, and Yuich Ikuhara 3. 1 Superconductivity Research Laboratory, ISTEC, 2-4-1 Mutsuno, Atsuta, Nagoay 456-8587, Japan. 2 Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta, Nagoay 456-8587, Japan. 3 Department of Materials Science, University of Tokyo, Tokyo 113, Japan.

Presenting Author: D.X. Huang

Bi₂Sr₂CaCu₂O_x single crystals were irradiated by the relative light ion (Fe) beams with incident energies of ~ 180 MeV. The damage morphologies and their distribution along the ion traces were investigated using cross-sectional transmission electron microscopy. Comparing with the cases of heavy ion irradiation, several specific damage behaviors have been observed. At first, long columnar defects were found with a constant columnar size of ~ 3 nm in diameter in a large penetration depth region from the bombarded surface to a depth of about 13.5 mm. Secondly, the peak of damage efficiency that usually appeared in the heavy ion irradiation system was disappeared in the present light-ion irradiation system. Thirdly, the threshold of stopping power to produce a columnar defect in Fe- Bi₂Sr₂CaCu₂O_x irradiation system was decreased to be ~ 9 keV/nm, which is much lower than that in Au-Bi₂Sr₂CaCu₂O_x irradiation system (~16 keV/nm). A valid method has been developed for qualitatively analyzing the changing law of the damage size with the ion velocity. The applications on both Fe- and Au-Bi₂Sr₂CaCu₂O_x irradiation systems indicate that the appearance of long columnar defects with constant column size is a specific phenomenon in light ion irradiation. A qualitative analysis proves that the threshold of stopping power for generating columnar defects is not a material constant but decreases as the weight of incident ion decreases, which further explains the lower threshold of stopping power in light ion irradiation.

2PO1-11 Thickness effects on the pinning by columnar defects

Alejandro Silhanek, Julio Guimpel and Leonardo Civale, Comisión Nacional de Energía Atómica, Centro Atómico Bariloche de Instituto Balseiro, 8400 Bariloche, Río Negro, Argentina

Presenting Author: A.V. Silhanek

We measured the angular dependence of the irreversible magnetization (M_i) in GdBa₂Cu₃O₇ thin films (~150nm) with and without columnar defects, tilted with respect to the c axis. In the as grown film at T=35K for H parallel to the c axis we observe a peak due to the pinning of intrinsic correlated defects. For angles higher than 50 the anisotropic behavior is recovered. These features remain in the irradiated film at T= 35K but an evident broad peak superimposed to the intrinsic signal appears at the tracks’ direction, as a consequence of the uniaxial pinning. At T=60K the signature of the correlated defects disappears. However, at high fields the maximum at the tracks’ direction is still visible. At low fields it appears only as a perturbation to the material anisotropy related angular dependence. A straightforward comparison with a thicker (4mm) YBa₂Cu₃O₇ single crystal with columnar defects at the same angle, dose and ion source, shows a huge difference in the tracks’ related enhancement of the critical currents. In this paper we present a qualitative discussion which interprets these results as a consequence of competing effects between pinning correlated structures and surface conditions (thickness effect).

2PO1-12 Interplay of anisotropy and competing correlated pinning mechanisms in the angular dependence of the irreversible magnetization of YBa₂Cu₃O₇ single crystals

Alejandro V. Silhanek and Leonardo Civale, Comisión Nacional de Energía Atómica-Centro Atómico Bariloche and Instituto Balseiro, 8400 Bariloche, Argentina

Presenting Author: A.V. Silhanek

We present measurements of the angular dependence of the irreversible magnetization Miq of YBa₂Cu₃O₇ single crystals with columnar defects (CD) inclined with respect to the c-axis. At high fields and temperatures we observe a sharp maximum centered at the tracks' direction, while as field decreases the maximum shifts toward the c-axis due to material anisotropy effects. We show that the combined effects of CD, twins and intrinsic pinning generates a variety of vortex staircase structures, and that correlated disorder dominates pinning for all field orientations. At low temperatures (T £ 20K) the behavior is quite different. At high fields a broad maximum at the tracks' direction is still visible, but its height is very small and it appears only as a perturbation to the angular dependence due to material anisotropy. However, by performing the usual anisotropic rescaling (Blatter et al., Phys. Rev. Lett. 68, 875, 1992) we recover the strong unidirectional effects due to CD's. As field decreases M_i(q ) becomes progressively more symmetric with respect to the c-axis, until no hint of the uniaxial pinning of the CD is seen. In this limit the anisotropic scaling fails because the internal field B is not equal to the applied field H.

2PO1-13 Interplay between two families of columnar defects in Bi-2212 single crystals

Sylvie Hebert, Garry K. Perkins, Mohamed Abdel-Salam, and A. David Caplin, Experimental Solid State Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BZ, United Kingdom

Presenting Author: S.A. Hebert

The pinning of vortices by columnar defects has been investigated in Bi-2212 single crystals, irradiated by 6 Gev Pb ions along one or two directions tilted by 45° from the c axis. Angular vector magnetic measurements were performed on a Vibrating Sample Magnetometer.

At high temperatures, the pinning of vortices by one set of columnar defects is highly anisotropic: the maximum current densities and the lowest creep rate are obtained when the magnetic field is applied along the defect direction. On the other hand, when two families of defects are introduced, the maximum pinning efficiency is achieved when the magnetic field is applied along the defect mid-direction, i.e. along the c axis. Furthermore, no peak of current densities is observed along the defect directions, thus demonstrating that the two families of defects do not behave simply in an additive way. This will be discussed taking into account the different pinning mechanisms of vortices in this defect geometry and the influence of defect splay.

We would like to thank V. Hardy, G. Villard, Ch. Simon and J. Provost (Laboratoire CRISMAT, Caen, France) for their collaboration and for providing the crystals.

This work has been supported by UK EPSRC.

2PO1-14 Gigantic surface pinning created by columnar defects

Mikhail V. Indebom 1, Cornelis J. Van Der Beek 2, Marcin Konczykowski 2, and Frederic Holtzberg 3. 1 Institute for Solid State Physics RAS, 142432 Chernogolovka, Russia. 2 Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France. 3 I.B.M. Thomas J. Watson Research Center, Yorktown Heights, NY 10598 USA.

Presenting Author: M.V. Indenbom

The observation of very large critical currents after heavy ion irradiation of high-T_c superconductor is of significant theoretical and technological interest. The columnar tracks created by the ions are considered to be the most efficient defects since they pin the vortices over their whole length. However, recent magneto-optical observations and local Hall-probe measurements of the magnetization of YBCO single crystals with columnar defects reveal that at low and moderate fields the shielding current due to pinning by the columns is not proportional to the sample thickness, as it should be in the case of bulk pinning. We conclude that, in contradiction to what is commonly believed, the vortices are pinned by the columnar defects only at the surface. Additional experiments supporting this idea are reported and the consequences of the gigantic surface pinning are discussed.

2PO1-15 Interplay between columnar defects and twin planes in YBa₂Cu₃O₇ crystals

Mihail V. Indenbom 1, Marcin Konczykowski 2, Cornelis J. Van Der Beek 2, and Frederic Holtzberg 3. 1 Institute of Solid State Physics, Chernoglovka, Russia. 2 Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France. 3 IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598, USA.

Presenting Author: M.V. Indenbom

Columnar defects introduced by swift heavy ion irradiation into YBa₂Cu₃O₇ provide strong flux pinning and a huge enhancement of magnetic irreversibility. The main limitation of the persistent current in such material is flux creep mediated by vortex kink nucleation at the crystal surface at low fields, and vortex interactions impeding the motion of interstitial vortices at high fields. We demonstrate that the presence of pre-existing twin planes and point defects can be determinant for the ultimate vortex velocity in the preence of columnar defects. This conclusion is inferred from measurements of magnetic hysteresis performed on a series of crystals containing single twin families and irradiated at various angles. This leads to various mutual arrangements between twin boundaries and columnar defects. At temperatures above 40 K, flux creep is enhanced when twin planes provide easy sliding paths for vortex kinks. In contrast creep is impeded in crossed twin boundary-column arrangements. Below 40 K the bulk pinning force exerted by point defects exceeds that from twin planes; then, the slow motion of vortex kinks through the pinning potential created by point pins becomes the limiting factor. As a result creep rate becomes insensitive to twin plane columnar defects arrangement.

2PO1-16 Role of correlated defects near the plastic regime phase-boundary in melt textured YBCO

R. Gerbaldo 1, G. Ghigo 1, L. Gozzelino 1, E. Mezzetti 1, B. Minetti 1, G. Krabbes 2, P. Schaetzle 2, E. Carlino 3, G. Cuttone 4, and A. Rovelli 4. 1 INFM- UdR Torino-Politecnico; INFN - Sez. Torino, Department of Physics, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Torino, Italy. 2 Institute of Solid State and Materials Research Dresden (IFW), Dresden, Germany. 3 C.N.R.S.M - PA.S.T.I.S., Brindisi, Italy. 4 I.N.F.N. Laboratori Nazionali del Sud, Catania.

Presenting Author: L. Gozzelino

In the present paper the magnetic properties of high quality melt textured YBCO irradiated with 2 GeV Au-ions are studied. The irradiation only affects about 10% of the whole sample. The aim is to gain a basic understanding of correlation/competition processes in pinning mechanism between surface columnar defects (SCDs) and intrinsic defects near the vortex phase-boundary of the plastic regime. A study of the pinning force behavior by means of a modified Dew-Hughes model allows to separate the contribution of different kinds of defects to the pinning mechanism as a function of field and temperature [1]. The phase diagram in correspondence to and above the phase-point where a peak effect shows up is investigated in detail. In this paper we show that the correlated phase corresponding to the peak is damaged by further injection of SCDs. On the contrary as soon as the peak is suppressed before irradiation by the increasing temperature and some defects lose their effectiveness, a phase dominated by correlated defects is restored after irradiation, mainly due to the contribution of SCDs. This analysis allows to conclude that when strong correlated pinning is already active in the virgin sample, the SCDs promote destructive interference with the pre-existent correlated pinning mechanism and competition sets up. When SCDs are the dominating ordered pinning structure, they promote correlation with other kinds of defects. In this case an enhanced pinning force can be obtained near the dose equivalent field.

[1] L. Gozzelino et al., IWCC Simposium paper - Supercond. Sci. Technol., in press.

2PO1-17 Control of the flux regime in BSCCO tapes by means of surface columnar defects

R. Gerbaldo 1, G. Ghigo 1, L. Gozzelino 1, E. Mezzetti 1, B. Minetti 1, L. Martini 2, and G. Cutttone 3. 1 INFM- UdR Torino-Politecnico, INFN- Sez. Torino, Dept. of Physics, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Torino, Italy. 2 ENEL-SRI, Segrate (MI), Italy. 3 INFN- Laboratori Nazionali del Sud, Catania, Italy.

Presenting Author: R. Gerbaldo

Surface columnar defects are produced in bulk HTSC materials by irradiating them with 0.25 GeV gold ions only on a top layer up to 10% of the full volume. The ion beam is orthogonal to the tape plane. Such defects seem to be able to confine vortices along their length, inside "tubes" having the same diameter as the distance of surface columnar defects. This confinement shows itself in the enhancement of J_c, in the shift of irreversibility lines and in the decrease of the anisotropy [1,2]. In this paper we concentrate on some dynamical properties in the high current regime. The vortex dynamics, as investigated by a.c. susceptometry, exhibits features characteristic of vortex confinement in a wide range of frequencies. In particular, measurements of the activation energy, as deduced from Im(X) vs. T curves at different frequencies, reveal two different regimes, the first one, almost frequency independent, under the dose equivalent field, the second one above. The first one is characterized by a Bean-like low creep-rate, the second one exhibits a frequency dependent increase of the creep rate. The data are discussed in the framework of recent results concerning defect-tuned channeling of vortices in HTSC.

[1] E. Mezzetti et al., Phys. Rev B 59 (1999), 3890.

[2] E.Mezzetti et al., Proceedings of CEC/ICMC '99.

2PO1-18 Vortex States in Heavily Pb-Substituted Bi₂Sr₂CaCu₂O_8+y with Planar Defects

K. Itaka 1, H. Taoka 1, S. Ooi 1, T. Shibauchi 1, T. Tamegai 1, Z. Hiroi 2, and M. Takano 3 . 1 Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. 2 Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106-8666, Japan. 3 Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan.

Presenting Author: K. Itaka

Vortex states in heavily Pb-substituted Bi₂Sr₂CaCu₂O_8+y with planar defects (PDs) is studied using Hall probes. Heavily Pb-substituted Bi₂Sr₂CaCu₂O_8+y crystals with starting compositions of Bi_2.2-xPb_xSr_1.8CaCu₂O_8+y up to x = 0.7 are grown by floating zone method using infrared furnace. By annealing the crystals in controlled oxygen atmosphere, we change the doping level of the system from over-doped to under-doped region. AFM observation in crystals after annealing at 400 C confirmed that the PDs are not destroyed by the annealing. Angular dependence of magnetization reveal two characteristic features specific to the PDs. (1) Magnetization hysteresis is larger when the field is inclined in the PDs compared with that perpendicular to the PDs. (2) In a limited angle and field region, magnetization first increases when the field is tipped away from the c-axis and then decreases again. This is understood as a manifestation of the channeling of vortices along the PDs. In most of the crystals, we observe an enhancement of magnetization (peak effect) at nearly constant c-axis component of the field. The peak field has systematic correlation with the average spacing of PDs suggesting that a kind of matching effect is responsible for this phenomenon.

2PO1-19 Frozen Flux Superconductors

Igor F. Lyuksyutov and Donald G. Naugle, Dept. of Physics Texas A&M University College Station, TX 77843-4242

Presenting Author: I.F. Lyuksyutov

We propose a new class of composite superconductors with embedded magnetic nanoparticles/nanorods. These nanomagnets can create a thermodynamically stable, frozen, dense flux line network. We discuss the methods of fabrication of these novel materials. We show that due to the frozen flux created by magnetic nanorods the critical current in this composite superconductor can be higher than in the superconductor with a comparable density of columnar defects.