Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W10: Focus Session: Hybrid Magnetic-Superconducting Systems III |
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Sponsoring Units: DMP Chair: Ilya Vekhter, Louisiana State University Room: Morial Convention Center RO8 |
Thursday, March 13, 2008 2:30PM - 3:06PM |
W10.00001: Scanning Tunneling Spectroscopy of Reentrant Superconductivity in the Ferromagnetic Superconductor ErRh$_{4}$B$_{4}$ Invited Speaker: ErRh$_{4}$B$_{4}$ is a reentrant superconductor. As temperature is lowered it transits to the superconducting state at 8.5 K, there is coexistence with magnetic ordering between 1.2K and 0.7K, and then it is a normal ferromagnetic material. We have used a scanning tunneling microscope, STM, to investigate the superconducting and magnetic properties of a ErRh$_{4}$B$_{4}$ single crystal from 300 mK up to 8.5K. We used a superconducting tip as the probe of the STM in order to enhance the different features present in the tunneling spectra corresponding to the ErRh$_{4}$B$_{4}$ density of states. The use of the superconducting probe allowed a detailed study of the destruction of superconductivity as ferromagnetic order develops. The perturbation of the ErRh$_{4}$B$_{4}$ superconducting density of states in the coexistence region is followed as function of temperature and external magnetic field. Spatial variations of the electronic density of states were detected by scanning tunneling spectroscopy maps at different temperatures and fields, and its correlation with the topographic features of the sample is investigated. These STM measurements allowed also the determination of a magnetic field vs. temperature phase diagram of the studied single crystal. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W10.00002: Vortex manipulation in S/F hybrid nanosystems Victor V. Moshchalkov, Alejandro Silhanek, Werner Gillijns, Alexei Aladyshkin, Niels Verellen Tunable field-induced superconductivity has been observed in superconducting films with a periodic array of magnetic dots with magnetization which is varied by using different hysteresis cycles to magnetize the dots. As a result, the shift of the Tc(H) curve to higher fields of a certain polarity (``magnetic bias'') can be controlled by tuning the strength of the field emanating from the dots and compensating the applied field. Static vortex patterns and dynamic effects, such as guided vortex motion and vortex ratchet effects, were studied in the superconductor/ferromagnet hybrids consisting from the superconducting film covered by magnetic dots, bars, loops with the in-plane magnetization. In-plane magnetic dipoles create asymmetric pinning sites responsible for the appearance of the magnetic dipole vortex ratchets theoretically predicted by Carneiro and now found experimentally. Switchable flux pinning landscape has been created by tuning the magnetic states of the in-plane magnetized loops (Phys.Rev.Lett. \textbf{98}, 117005 (2007), Appl. Phys. Lett. \textbf{90}, 182501 (2007), Phys. Rev. B (R) \textbf{76}, 60503 (2007)) [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W10.00003: Switchable pinning landscapes for flux quanta using arrays of ``magnetic vortices'' Javier E. Villegas, Kevin D. Smith, Lei Huang, Yimei Zhu, Rafael Morales, Ivan K. Schuller We constructed a superconducting/ferromagnetic hybrid system in which the \textit{ordering} of the pinning potential landscape for flux quanta can be manipulated. Flux pinning is induced by an array of magnetic nanodots in the ``magnetic vortex'' state, and controlled by the magnetic history. This allows switching \textit{on} and \textit{off} the collective pinning of the flux-lattice. In addition, we observed field-induced superconductivity that originates from the annihilation of flux quanta induced by the stray fields from the ``magnetic vortices.'' [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W10.00004: Vortex lattice matching effects in superconducting Mo$_{3}$Si films with magnetic pinning centers. David Perez de Lara, Alejandro Alija, Alejandro Junquera, Jose M. Colino, Jose I. Martin, Elena Navarro, Maria Velez, Jose V. Anguita, Jose L. Vicent Electron Beam Lithography and sputtering techniques have been used to fabricate arrays of Ni nanodots on Si (100) substrate. Nb films and amorphous Mo$_{3}$Si films were grown on top of the magnetic array by magnetron sputtering. The same arrays were used in both superconducting systems; therefore the same periodic pinning potential is induced in both systems. Magnetotransport and (I, V) curves were measured close to the films critical temperatures. Matching effects are observed between the periodic array of pinning potentials and the driving vortex lattice. The different vortex dynamics will be discussed in both systems. We want to thank Spanish Ministerio Educacion y Ciencia grants NAN2004-09087, FIS2005-07392. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W10.00005: Confined vortex phase in superconductor-ferromagnet nanocomposites Milorad Milosevic, Mauro Doria, Francois Peeters In the fifties, Abrikosov found the coexistence of superconductivity with an external magnetic field in the form of vortices, quantized filaments that cross the material. The puzzling properties of the recently discovered ferromagnetic superconductors are indicative of vortices, but stemming from the \textit{internal} magnetic field. Here we analyze this latter phenomenon and propose a new superconducting phase, made of \textit{confined vortex lines}, prior to the onset of experimentally visible spontaneous vortex phase. Similarly to exotic superconductors, internal complexes of vortex loops may also arise around embedded nanomagnets in artificial superconducting hybrids, where high density of magnetic particles can lead to a disordered vortex phase resembling a \textit{vortex glass}. Our simulations in the Ginzburg-Landau framework also show the remarkable, three-dimensional dynamic effects of the Lorentz force on vortex loops in applied dc current. This provides unique method for the experimental detection of the confined phase through transport measurements, both for bulk and mesoscopic samples. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W10.00006: Interplay of Superconducting with Magnetic Vortices A. Hoffmann, J.E. Pearson, G. Mihajlovi\'c, V. Metlushko, L. Fumagalli, J.C. Sautner, N. Jahedi Periodic arrays of magnetic structures are well known to give rise to commensurate pinning of superconducting vortices in adjacent superconducting films. We compared the pinning effects due to magnetic dots with either single domain or vortex magnetization configuration. There is a clear correlation between the magnetoresistance in the superconductor and the magnetization configuration of the magnetic dots, indicating that the pinning of the superconducting vortices is strongly enhanced for the magnetic vortex state. The origin of this enhanced pinning is due to the locally larger magnetic stray fields produced by the magnetic vortex cores. The absence of an asymmetry for parallel and anti-parallel orientation between the superconducting vortex flux and the magnetic vortex cores suggests that the enhanced pinning is not due to magnetostatic interactions but is rather due to the local suppression of superconductivity by highly localized, large perpendicular stray magnetic fields generated by the magnetic vortex cores. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W10.00007: Enhancement of superconductivity by parallel magnetic field in ultrathin $a$-Pb films. Ashwani Kumar, H. Jeffrey Gardner, Peng Xiong A modified dilution refrigerator equipped with \textit{in situ} film growth and a rotating sample stage is used to study the effect of magnetic field on ultrathin homogeneous Pb films. An insulating layer of Sb ($\sim $ 1 nm) is first deposited to ensure electrical and possibly structural uniformity down to a single atomic layer for the subsequently deposited Pb. Through incremental deposition of Pb, a film with increasing thickness (thus decreasing R and increasing T$_{C})$ is obtained and transport measurements in perpendicular and parallel magnetic field are performed \textit{in situ} at different film thicknesses. Any \textit{perpendicular} field is found to suppress superconductivity, and at sufficient strength induces an electrically inhomogeneous insulating state.$^{1}$ In contrast, the same film is at least two orders of magnitude less sensitive to a \textit{parallel} field. More strikingly, a moderate parallel magnetic field actually \textit{increases} the T$_{C}$ of the film, resulting in large negative magnetoresistance in the transition region in parallel fields as large as 3T. The dependence of this effect on the film thickness, impurities and temperature will be presented and discussed. $^{1}$J.S. Parker et al., Europhys. Lett. 75, 950 (2006). [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W10.00008: An Effective Action approach to inhomogeneous superconductors: Application to F/S/F spin valves Adnan Rebei The interplay between magnetism and superconductivity can be studied in simple systems such as ferromagnetic-superconductor-ferromagnetic tri-layers. Many recent experiments [1] measured a critical temperature dependence of the superconductor as a function of the relative orientations of the surrounding ferromagnetic layers. This layered system gives rise to an in-homogenous gap equation for the superconductor due to the polarization at the ferromagnetic-superconductor interfaces irrespective of the orientation of the magnetization in-plane or out-of-plane. Real space formulations are therefore better suited to study this kind of systems. We show that an Effective Action formulation similar to the one proposed by Weinberg [2] is very advantageous in this respect as was shown in normal-ferromagnetic-normal systems in [3]. \newline \newline [1] Yu. Rusanov, S. Habraken and J. Aarts, Phys. Rev. B 73, 060505(R) (2006); A. Singh, C. Sugers and H. v. Lohneysen, Phys. Rev. B 75, 024513 (2007); T. Ambrose (unpublished). \newline [2] S. Weinberg, Nucl. Phys. B 413, 567 (1994). \newline [3] A. Rebei and M. Simionato, Phys. Rev. B 71, 174415 (2005). [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W10.00009: Multilayered ferromagnet/superconductor nanostructures: proximity effect, decoupled superconductivity, and hierarchy of critical temperatures. Y. N. Proshin, N. G. Fazleev, M. G. Khusainov The four-layered F'/S'/F''/S'' nanostructure consisting of rather dirty superconducting (S) and ferromagnetic (F) metals is studied within the theory of the proximity effect taking detailed account of the boundary conditions. The new pi phase superconducting states are obtained for the F'/S'/F''/S'' nanostructure in addition to the known ``superlattice'' states. The dependence of the critical temperatures versus the F layers thicknesses is investigated. It is shown that the F'/S'/F''/S'' nanostructure can experience decoupled superconductivity. The latter manifests itself through a hierarchy of the critical temperature Tc, which can be different for different S' and S'' layers. An optimal set of parameters is determined, for which the difference between the critical temperatures for different S' and S'' layers becomes significant. The corresponding phase diagrams are constructed and discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W10.00010: Paramagnetic Intrinsic Meissner Effect in Layered Superconductors Andrei Lebed Free energy of a quasi-two-dimensional superconductor with a coherence length perpendicular to the conducting layers being less than an inter-layer distance is calculated. The free energy is shown to differ from that in the textbook Lawerence-Doniach model at high fields, where the Meissner currents are found to create an unexpected positive magnetic moment due to shrinking of the Cooper pairs ``sizes'' by a magnetic field. This unique phenomenon - paramagnetic intrinsic Meissner effect (PIME) in a bulk [1] - is suggested to detect by measuring in-plane magnetization and torque in layered organic and high-Tc superconductors as well as in superconducting superlattices. \newline \newline [1] A.G. Lebed, Physical Review Letters, submitted. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W10.00011: Evidence of spontaneous vortex phase in ferromagnet-superconductor nanocomposites Tatiana Rappoport, Yutao Xing, Hans Micklitz, Milorad Milosevic, Ivan Solorzano-Naranjo, Elisa Baggio-Saitovitch The interplay between superconductivity and magnetism gives rise to many intriguing phenomena. We report a novel manifestation of this interplay: the appearance of a spontaneous vortex phase in superconducting films with embedded magnetic nanoparticles. These systems can be seen as artificial analogues of ferromagnetic superconductors. Unlike traditional vortices in superconductors, this vortex phase appears without any applied magnetic field. The vortices nucleate due exclusively to the stray field of the magnetic nanoparticles, which serve the dual role of providing the internal field while working as pinning centers. Transport measurements reveal that this vortices have a phase transition from a liquid to a disordered solid resembling a vortex glass. The transition is characterized by means of a scaling analysis. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W10.00012: Synchronization by internal cavity mode and resonant electromagnetic emission from intrinsic Josephson junction stacks Alexei Koshelev Intrinsic Josephson-junction stacks realized in mesas fabricated out of high-T$_c$ superconductors may be used as sources of coherent electromagnetic radiation. The major challenge is to synchronize Josephson oscillations in all junctions to get significant radiation. A simple way to solve this problem is to excite the in-phase Fiske mode when the Josephson frequency matches the Fiske-resonance frequency set by the stack lateral size[1]. A finite direct coupling to such mode exists in mesas with lateral modulation of the Josephson critical current identical in all junctions [2]. The powerful almost standing electromagnetic wave is excited inside the crystal in the resonance promoting full synchronization. We evaluate behavior of the I-V characteristics and radiated power near the resonance. We will discuss several relevant issues including (i) stability of the coherent state, (ii) mechanism of damping including external radiation and leaking of radiation into the bulk crystal, and (iii) angular dependence of external radiation. [1]L. Ozyuzer, \emph{et al.} Science \textbf{318}, 1291 (2007) [2]A. E. Koshelev and L. N. Bulaevskii, cond-mat 0708.3269 [Preview Abstract] |
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