Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W38: Flux Pinning and Critical Currents |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Judy Wu, University of Kansas Room: Baltimore Convention Center 341 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W38.00001: Theory for the Dependence on Thickness Shown by the Critical Current vs. Magnetic Field in Films of PLD-YBCO Martin P. Maley, Jose P. Rodriguez The theoretical consequences of the proposal that the vortex lattice induced by perpendicular magnetic field in films of PLD-YBCO is in a thermodynamic Bose glass state are explored. Attention is focused on the high-field regime at the extreme type-II limit, in which case only a small fraction of the vortex lines are localized at the dislocations that thread the film along the c axis, and in which case the pinning of the vortex lattice is collective. The critical current density along the film is predicted to follow an inverse square-root power law as a function of external magnetic field in the collective-pinning regime. It gives a fair account of the critical current density at kG magnetic fields in films of PLD-YBCO that are microns thick, at liquid nitrogen temperature. It fails, however, for much thinner films at lower temperature. This failure is corrected by including the effect of point pins along the interstitial vortex lines that lie in between the correlated pins. They contribute an inverse dependence on film thickness to the critical current density in magnetic field oriented near the c axis. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W38.00002: The growth mechanism of pinning-effective nanostructures embedded in YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) superconducting thin film A. Gapud, A. Khan, M. Paranthaman, D. Christen Following up on previous success in modifying the pulsed-laser-deposition (PLD) film processing to introduce self-assembled pinning defects in films of various high-temperature superconductors (HTSC), specifically the case of self-assembled columnar arrays of oxide nanodots in YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) thin film [\textit{Superconductor Science and Technology} \textbf{18}, 1533 (2005)] using a nanodot-doped YBCO target, a careful and systematic examination of the growth mechanism is yet to be conducted on this and similar systems. This study examines (1) how the oxide nanodots retain their character during the deposition and (2) how the nanodots both influence, and are influenced by, the local potential-energy landscape that promotes spontaneous assembly into coherent stacks. This is done by growing the film subject to slight variations in the processing parameters which may influence the nature of the heterogeneity of any given layer in the film. Particular attention is paid to the influence of varying laser-pulse frequency which determines the time duration by which the potential-energy landscape of a pulsed layer is consolidated in time for the next pulsed layer. The mechanism is also tested for a system in which the embedded nanostructures are made from a non-oxide material (gold). Results consist of microstructure (cross-sectional HRTEM, XRD, surface SEM), transport properties (critical temperature and critical current), and magnetic susceptibility. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W38.00003: Magnetic coupling between vortices in superconductors and adjacent magnetic layers Masaki Suenaga, Qiang Li, Qing Jie We presented a study of magnetic coupling between vortices in superconductors and adjacent magnetic layers in two systems: superconductor/magnetic multilayers and HTS films on magnetic substrates. The flux motion in superconductor and the behavior of magnetic layer (or magnetic substrate) were captured by quantitative Magneto-optical imaging (MOI) technique with an external magnetic field applied perpendicular to the sample surface and varied along a whole hysterisis loop cycle. Bulk dc SQUID magnetization, ac loss, and direct transport measurements were performed to compliment the MOI studies. It was found that magnetic substrate has limited effect on transport properties of HTS films, although some enhancement of $J_{c}$ was observed near $T_{c}$ in the multilayers due to the magnetic coupling. However, magnetic substrate did result in significant reduction of the ac losses. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W38.00004: Time resolved magneto-optical imaging of ac currents in YBCO conductor. Andrea Lucarelli, Gunter Luepke, Timothy Haugan, George Levin, Paul Barnes The use of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (YBCO) in ac applications, whether with applied ac currents or alternating magnetic fields, is strictly related to the availability of long-length ac-tolerant YBCO coated conductor. These ac applications, such as fully superconducting generators and motors or transformers, may operate with ac currents in a range of frequencies from tens of Hz up to a thousand Hz. We present a newly developed time-resolved magneto-optical imaging (MOI) technique for studying high-temperature superconductors (HTS) with applied alternating currents (AC) in the frequency range 30 -- 1000 Hz. The evolution of the magnetic flux density distribution in YBCO thin films and coated conductors is studied as a function of the phase of the applied AC current. Time- and spatially-resolved images of the magnetic flux profiles are presented for a detailed series of values of the phase. A quantitative analysis of the data allows us to calculate the current density profiles at different phases. We observe for the first time that the maxima of the AC current density is shifted from the edges further inside the sample which may be caused by the higher self-induced field in that region. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W38.00005: Scanning Hall probe microscopy of AC losses in YBCO coated conductors Rafael Dinner, Kathryn Moler, M.~R. Beasley, D. Matthew Feldmann, David Larbalestier Magnetic imaging of current-induced vortex movement in superconducting films yields detailed information about dissipation and the path of an applied current. In our cryogenic scanning Hall probe microscope, a micro-Hall probe is rastered near the sample surface with submicron resolution and centimeter scan range. Hall probe time traces taken at each point are assembled into movies of the flux penetration as a function of time over a cycle of AC sample current. We image coated conductors---films of the high-temperature superconductor YBCO grown on metal tapes which give rise to grain boundaries. We then isolate the effects of the boundaries by imaging YBCO grown on bicrystal substrates that induce a single boundary at various angles to the current path. Current density, electric field, and dissipation distributions are reconstucted from the images. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W38.00006: Twins, Their Microstructure and Correlation to Critical Current Densities in Superconducting Melt-Textured Grown Y-Ba-Cu-O Siu-Wai Chan, Linfeng Mei Refinement of twin microstructure in melt-textured grown (MTG) Y-Ba-Cu-O (YBCO) is engineered by annealing at different temperatures. This isothermal method exploits the temperature dependence of twin-boundary-energy. The twin boundary energy is obtained by two independent methods: (1) twin spacing and (2) twin-tip shape. The twin boundary energy measurement is accomplished with transmission electron microscopy. Refinement of twin spacing with increasing temperature is confirmed. Twin spacing decreases from 190 nm to 54 nm as the temperature increases from 450 to 680$^{o}$C. Critical current measurements support that a sample with a higher density twin density often results in a higher pinning-force i.e. 3.4 x 10$^{8}$ N/m$^{3}$ versa 1.2 x 10$^{8}$ N/m$^{3}$. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W38.00007: Scanning Hall Probe Microscope and Imaging of Vortex Penetration into Nb A. Bove, N. Kundtz, A. M. Chang, V. Gusiatnikov, Art Lichtenberger We report on the construction of a scanning Hall probe microscope with 100 nm lateral resolution and a large scan range, which exceeds 40 $\mu$m at 4.2 K. The microscope is based on the beetle design and operates between room temperature and 1.5 K. The DSP-based control electronics achieves a high (100 kHz) sampling rate and a low noise. The system is capable of simultaneous tunneling and Hall signal acquisition. The Hall sensor for measuring local magnetic fields is fabricated on a GaAs heterostructure through standard EBL and wet etching. It has an active area of 300 nm x 300 nm and a sensistivity of ~ 0.2 $\Omega$/Gauss. We will present (1) a description of the microscope, and (2) progress on imaging the penetration of vortices and the growth of vortex dendritic patterns into thin Nb films \footnote{ Altshuler E. \textit{et al.}, Rev. Mod. Phys.\textbf{76}, 471 (April 2004)} and grid arrays \footnote{Hallen H. D. \textit{et al.}, Solid State Commumications \textbf{99} (9), 651-654 (SEP 1996)}. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W38.00008: Vortex Pinning in a YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ Thin Film Ophir M. Auslaender, Nicholas C. Koshnick, Kathryn A. Moler, Rob A. Hughes, John S. Preston Vortices, and the nanoscale structures that pin them, are important both fundamentally and for the development of high temperature superconductor technologies. We use a home-built magnetic force microscope (MFM) to determine the depinning forces required to move individual vortices in a 200nm thick YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ film. Our results indicate a wide distribution of depinning forces for different vortices. A comparison between distributions at various temperatures is underway, qualitatively showing a decrease of forces with increasing temperature. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W38.00009: Anomalous Slow Relaxation of the Magnetization in Y$_{1-x}$Pr$_{x}$Ba$_2$Cu$_3$0$_{7-\delta}$ P. Gyawali, V. Sandu, C. C. Almasan, B. J. Taylor, M. B. Maple We have investigated the time $t$ evolution of the irreversible magnetization $M_{irr}$ in a series of single crystals of Y$_{1-x}$Pr$_{x}$Ba$_2$Cu$_3$0$_{7-\delta}$, $x=0.13$ ($T_{c}=82$ K), $x=0.34$ (($T_{c}=50$ K), and $x=0.47$ (($T_{c}=34$ K), all displaying a second peak in magnetization. In all cases, $M_{irr}(t)$ follows the well known law of relaxation, $M_{irr}(t)\propto[\mu k_{B}T/U_{o}ln(t/t_{0})]^{-1/\mu}$. For fields/temperatures lower than the corresponding values of the second magnetization peak, $\mu$ is anomalously large, $2.5\leq\mu\leq4$, in contrast with theoretical predictions which gives a maximum value of 2.5, and with data reported for other cuprates. These large $\mu$ values in the vortex glass state, which give rise to a low relaxation rate, occur for all Pr doping and could be due to the presence of Pr ions. In contrast, at fields/temperatures above the second magnetization peak, $\mu$ decreases to values expected from theories of plastic vortex liquid and validated by most experimental reports. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W38.00010: Effective Action for Vortex Dynamics in Clean d-wave Superconductors Predrag Nikolic, Subir Sachdev We describe influence of gapless nodal quasiparticles on vortex dynamics in clean two-dimensional d-wave superconductors. At zero temperature, the guasiparticles give rise to a finite renormalization of vortex mass, as well as a universal sub-Ohmic damping of vortex motion. Slow vortex motion is dissipated only at finite temperatures, or when some perturbation, such as disorder, creates a finite quasiparticle density of states at the gap nodes. These results are obtained by a non-perturbative derivation of the effective vortex action, where the quasiparticles are integrated out exactly in a continuum functional formalism. Fortunately, an uncontrolled perturbative analysis reaches the same conclusions, and all findings are reflected in a simple scaling argument where the gapless Dirac quasiparticles are regarded as a quantum-critical system. Our results appear to differ from those of the semiclassical theory, which obtains singular corrections to a vortex mass appearing in transport equations. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W38.00011: Flux pinning and Critical current density in La$_{2-x}$Sr$_{x}$CuO$_{4+d }$ Hashini Mohottala, B. O. Wells, J. I. Budnick, W. A. Hines, A. R. Moodenbaugh, F.C. Chou We have studied the magnetic characteristics of the critical states in a series of samples of the type La$_{2-x}$Sr$_{x}$CuO$_{4+d}$ that is doped with both Sr and excess O incorporated using electrochemistry. These samples spontaneously phase separate and show both a superconducting phase with T$_{C}$ near 40 K and a magnetic phase with T$_{M}$ near 40 K. Our previous studies established that the superconducting phase is similar to an optimally doped sample while the magnetic phase is consistent with the static spin density wave reported for x=1/8 Sr or Ba doped samples. Magnetization data at various temperatures showed large reversibility in all the samples. The critical current densities J$_{C}$(0) values were at least an order of magnitude smaller than that of the reported values for YBa$_{2}$Cu$_{3}$O$_{7-d }$and La$_{2-x}$Sr$_{x}$CuO$_{4}$. At higher fields J$_{C}$(H) was smaller indicating the existence of weak flux pinning in the system. Based on our magnetization data we conclude that the vortex lattice pinning is different from non-phase separated cuprates. This work was partially supported by the US-DOE through contract DE-FG02-00ER45801 and the Cottrell Scholar Program of the Research Corporation. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W38.00012: Magnetic Pinning in Nb and YBCO Thin Films by Co/Pt Multilayers with Perpendicular Magnetic Anisotropy X.M. Cheng, L.Y. Zhu, C.L. Chien, Marta Z. Cieplak, Z. Adamus, A. Abal'oshev, M. Berkowski Magnetic pinning of vortices has the advantage over intrinsic pinning in that the superconducting critical current can be reversibly tuned by the magnetic field (H). Magnetic pinning by Co/Pt multilayers with perpendicular magnetic anisotropy has been studied in two ferromagnetic/superconducting bilayers of Nb and YBCO with different superconducting properties (e.g. penetration depth $\lambda )$. Magnetic force microscopy reveals similar magnetization (M) reversal process in the two cases, both exhibiting a large density of narrow residual domains but with different domain width w at the final reversal stage. However, the magnetic pinning, revealed by the M-H loop shape in the superconducting state, is different. The Nb film exhibits an enhancement of M with the strongest effect during the final reversal stage, while the YBCO film shows a suppression of M in the vicinity of central M peak and an enhancement of M in large magnetic fields. These different behaviors are related to the different $\lambda $/w ratio in the two cases. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W38.00013: Flux penetration in a ferromagnetic/superconducting bilayer utilizing perpendicular magnetic anisotropy Marta Z. Cieplak, Z. Adamus, A. Abal'oshev, M. Berkowski, M. Konczykowski, X. M. Cheng, L. Y. Zhu, C. L. Chien The Hall sensor array is a useful tool for measuring local magnetic fields. An array of miniature Hall sensors has been used to study the flux penetration in a ferromagnetic/superconducting (F/S) bilayer consisting of Nb as the S layer and Co/Pt multilayer with perpendicular magnetic anisotropy as the F layer, separated by an amorphous Si layer to avoid proximity effect. The F layer is first premagnetized to different magnetization reversal stages to obtain various magnetic domain patterns. The effect of these domain patterns on the flux behavior in the S layer is then studied at various temperatures in the superconducting state. We have observed that, in addition to the vortex pinning enhancement, some domain patterns strongly increase the first penetration field and induce large thermomagnetic instabilities (flux jumps), which are not detectable by magnetometry. We also discuss the profiles of the flux distribution across these F/S bilayers. [Preview Abstract] |
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