Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q34: Focus Session: Superconductivity: Magnetic Properties |
Hide Abstracts |
Sponsoring Units: DMP GMAG Chair: Carmen Almasan, Kent State University Room: 404 |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q34.00001: Influence of Magnetic Domain Structure on Abrikosov Vortex Dynamics in Superconductor-Ferromagnet Hybrids Invited Speaker: We will review the experimental and theoretical aspects of transport properties and vortex static and dynamic characteristics in magnetically coupled superconductor-ferromagnet hybrid structures. Magnetotransport characteristics and scanning tunneling microscopy (STM) images of vortex structures reveal rich superconducting phase diagram in these systems. Focusing on particular combination of a Permalloy ferromagnet with a well ordered rotatable periodic stripe-like magnetic domain structure with alternating out-of-plane component of magnetization, and a small coherence length superconductor, we find directed nucleation of superconductivity above domain wall boundaries. We show that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels. Changing the in-plane direction of magnetic stripe domains it is possible to re-configure the direction of the superconducting nano-channels and controllably rotate the direction of the in-plane anisotropy axis in the superconductor. Deeper into the superconducting state we observe strongly anisotropic vortex pinning effect due to the presence of the ordered magnetic domain structure. We show that the hybrid exhibits commensurability features that are related to the matching periodicities of the Abrikosov vortex lattice and the magnetic stripe domains. Using STM vortex imaging we show that the periodic magnetic induction in the superconductor creating a series of (anti)pinning channels for externally added magnetic flux quanta forcing confinement of the Abrikosov vortices and formation of quasi-1D vortex arrays. We will also discuss potential for electronic applications of ferromagnet-superconductor hybrid systems. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q34.00002: The influence of magnetic domain landscape on the flux pinning in ferromagnetic/superconducting bilayers Marta Z. Cieplak, Z. Adamus, M. Konczykowski, L. Y. Zhu, C. L. Chien A line of miniature Hall sensors has been used to study the influence of the disorder in the magnetic domain landscape on flux pinning in the ferromagnetic/superconducting (F/S) bilayers. The bilayers consist of Nb as the S layer and Co/Pt multilayer with perpendicular magnetic anisotropy as the F layer, separated by a Si buffer layer to avoid the proximity effect. By changing of the Pt layer thickness, the magnetic domain landscape with different degree of disorder, ranging from uniformly distributed narrow domains (quasi-ordered landscape) to highly disordered landscape with domains of different sizes, can be predefined in the F layer. The flux behavior is then measured in the superconducting state using the Hall sensors. It is found that the quasi-ordered landscape with domains width comparable to the magnetic penetration depth produces large enhancement of the vortex pinning and smooth flux penetration. The more disordered magnetic domain patterns cause less pinning and create large edge barrier for vortex entry followed by strongly inhomogeneous flux penetration. The possible origins of this behavior will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q34.00003: Soft magnetic lithography and giant magnetoresistance in superconducting/ferromagnetic hybrids. V. Vlasko-Vlasov, U. Welp, A. Imre, D. Rosenmann, J. Pearson, W. Kwok We report on direct visualization confirmed by the transport measurements of strong interactions between superconducting vortices and ferromagnetic domains in bilayers of type-II SC lead films and FM permalloy films with perpendicular magnetic anisotropy. Domains in permalloy formed a submicron stripe lattice that could be easily aligned in the film plane. We show that domain walls yield a robust magnetic pinning potential providing preferential vortex motion along the stripe domains. The effect is observed in a wide temperature range and results in a noticeable anisotropy of critical currents. The anisotropy increases near Tc when the core pinning becomes inefficient and the anisotropy direction is changed by reorienting the stripe domains. Such a tunable magnetic lithography is a convenient way of varying transport properties of superconductors and developing new cryotronic devices such as microscale superconducting switches and modulators. In our samples we found an unusually high magnetoresitance of 10$^{6 }${\%} in the fields of $\sim $10 Oe for the currents perpendicular to the domain walls. It can be referred to the granular structure of the lead films assisting the formation of easy flux flow channels along the stripe domains. --- The work was supported by the U.S. DOE Office of Science under Contract No. DEAC02-06CH11357. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q34.00004: Experimental approach to search magnetic pinning in YBCO films grown by chemical deposition method Carlos Monton, Anna Palau, Jone Zabaleta, Narcis Mestres, Teresa Puig, Xavier Obradors In the last 10 years we have developed experience in generating YBa$_{2}$Cu$_{3}$O$_{7}$ (YBCO) films and coated conductors by chemical solution deposition (CSD) capable to carry current densities in the range of 3 to 4 MA/cm$^{2}$ (at 77K and H=0). To improve these performances specific defects were grown by chemical nanostructurated routes. Interfacial pinning was obtained by the growth of nanostructurated templates generated by strain induced or assisted self-assembled processes [1]. On the other hand isotropic defect pinning contribution was increased by adding nanocomposites with second phase within the YBCO matrix. These samples were grown by modified solution precursors [2] reaching the maximum value of the critical current density, Jc (1T, 77K) = 2.2MA/cm$^{2}$, reported so far [2]. However recent theoretical works suggest that vortex pinning can be improved even more by using hybrids superconductor/ferromagnetic (Sc/FM) materials. In this work we explore the interaction of FM random distributed CSD La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ nanoislands with the YBCO film and their effect on vortex pinning. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q34.00005: Interplay between ferromagnetism and superconductivity at interfaces of La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$/ La$_{0.7}$Ca$_{0.3}$MnO$_3$ trilayers Felio Perez, Eval Bacca, Maria E. Gomez, Hongtao Shi, David Lederman We report studies of La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3 $O$_{7-\delta}$/ La$_{0.7}$Ca$_{0.3}$MnO$_3$ trilayers onto (001) SrTiO$_3$ substrates by high-pressure dc sputtering. We have grown heterostructures with a constant thickness of the ferromagnetic layers of 287 unit cells (110~nm) and ranging the thickness of the superconducting-middle layer between 5 (6~nm) and 15 unit cells (17~nm). The transport measurements show a strong suppression of the superconducting properties when the thickness of superconducting layer is reduced below 10 unit cells. However, the magnetic response out of plane shows the presence of the superconductor until 5 unit cells. The difference between the electrical characterization and the onset of the diamagnetic transition might to be related of presence of the spontaneous vortex phase in this temperature interval. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q34.00006: Probing the magnetic states in a ferromagnet using a superconductor Leyi Zhu, Tingyong Chen, Chia-Ling Chien In a superconductor (S)/ferromagnet (F) bilayer, the superconducting properties of the S layer are sensitive to the domain pattern in the adjacent F layer.[1-2] We exploit this effect to investigate Ni films, which instead of retaining in-plane anisotropy as usual, unexpectedly acquire perpendicular anisotropy when the thickness is above a critical value. Using Ni/Nb bilayers, the perpendicular magnetization component can be sensitively probed by the measurements of the superconducting transition of Nb in a magnetic field, which alters the domain pattern in Ni. Above the critical Ni thickness, an in-plane magnetic field can manipulate the stripe domains in Ni between parallel stripes and random labyrinth states resulting in as much as 90 mK in the transition temperature of the Nb layer. This clearly demonstrates that superconductor is a sensitive probe of the magnetic domain state of a ferromagnet. In turn, the results also show that along the parallel stripe domains, superconductivity is less detrimental. [1]. A. Yu. Rusanov, M. Hesselberth, J. Aarts, and A. I. Buzdin, Phys. Rev. Lett. 93, 057002 (2004). [2]. L. Y. Zhu, T. Y. Chen, and C. L. Chien, Phys. Rev. Lett. 101, 017004 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q34.00007: Evidence for Induced Magnetization in Superconductor/Ferromagnet Bilayers Itay Asulin, Ofer Yuli, Gad Koren, Oded Millo The inverse proximity effect in superconductor/ferromagnet heterostructures has been the focus of recent theoretical studies. The different approaches share the basic conclusion that a sizable magnetic moment should penetrate into the superconductor side. The sign of this moment, its spatial behavior and the actual mechanism of this effect are still controversial. Very few experimental works provided evidence for the existence of such a magnetic moment inside the superconductor. However, the effects of such an induced magnetization on the density of states of the superconductor was not observed so far and needs further theoretical and experimental clarification. We have performed scanning tunneling spectroscopy of (001)YBa$_{2}$Cu$_{3}$O$_{7-\delta }$/SrRuO$_{3}$ and (110)YBa$_{2}$Cu$_{3}$O$_{7-\delta }$/SrRuO$_{3}$ bilayers, where the SrRuO$_{3}$ is an itinerant ferromagnet, and found an anomalous splitting of both the gap and zero bias conductance peak features that may provide evidence for the existence of such an induced magnetic moment inside the superconductor. The relevant length and energy scales of the effect will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q34.00008: Interaction between magnetism and superconductivity in La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ multilayers T. Hu, H. Xiao, C. C. Almasan, C. Visani, Z. Sefrioui, J. Santamaria Angular and field dependent magnetoresistance measurements were
performed on
La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$
(LCMO/YBCO) heterostructures below and above the superconducting
transition temperature $T_{c} \approx$ 28 K of the YBCO layer, in
order to address the origin of the long range proximity effect
found in these heterostructures. The proximity-induced
conductance in the LCMO layer at $T |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q34.00009: Directional control of the inverse superconducting spin-switch Cristina Visani, Norbert M. Nemes, Mirko Rocci, Christian Miller, Javier Garcia-Barriocanal, Diego Arias, Zouhair Sefrioui, Carlos Leon, Jacobo Santamaria, Mar Garc\'Ia-Hern\'andez, Suzanne G. E. te Velthuis , Axel Hoffmann, Mike R. Fitzsimmons We report results on the study of spin dependent transport in La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/YBa$_{2}$Cu$_{3}$O$_{7-\delta }$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ trilayers, focusing on the effects induced by magnetic anisotropy of the LCMO. The study of magnetic anisotropy through the combination of transport measurements, magnetometry, x-ray magnetic circular dichroism and polarized neutron reflectometry, allowed us to unravel the origin of the inverse superconducting spin switch. Applying the magnetic field along the easy axis we obtained a plateau-like response reflecting a well defined antiparallel alignment of the LCMO layers over a wide magnetic field range yielding large magnetoresistance that we ascribe unambiguously to the spin dependent transport. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q34.00010: Oxide thin film based inverse superconducting spin switches. Norbert M. Nemes, C. Visani, C. Miller, M. Rocci, F. Bruno, J. Garcia-Barriocanal, Z. Sefrioui, C. Leon, J. Santamaria, M. Iglesias, F. Mompean, M. Garcia-Hernandez, A. Hoffmann, S. G. E. te Velthuis Thin film F/S/F trilayers made of YBa2Cu3O7 (S, YBCO) and La0.7Ca0.3MnO3 (F, LCMO) behave as inverse superconducting spin switches (SSS) as the critical temperature of the superconductor depends on the relative orientation of the magnetization of the F layers in a way that the resistivity is increased in the antiparallel configuration. This is caused by enhanced pair-breaking due to the spin dependent transport of quasiparticles transmitted from the ferromagnetic electrodes into the superconductor. Similar inverse SSS is obtained from exchange biased LCMO/YBCO/Cobalt trilayers with a broad AP field range. Spin diffusion across the superconductor, proximity effect at the F/S interface, stray fields due to domain walls of the ferromagnet and the magnetic anisotropy all play a role. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q34.00011: Direct and Inverse Spin Switch Effect in Superconducting Spin Valves Jian Zhu, Carl Boone, Xiao Cheng, Ilya Krivorotov We report the observation of direct and inverse spin switch effects in ferromagnet/ superconductor/ ferromagnet/ antiferromagnet (FM/SC/FM/AF) spin valves with FM = Ni$_{81}$Fe$_{19}$, AF = Ir$_{25}$Mn$_{75}$ and SC=Nb. In these spin valve structures, the magnetization of the free layer can be switched between parallel (P) and antiparallel (AP) orientations with respect to the orientation of the fixed layer by a small in-plane magnetic field. Near the superconducting transition temperature, the P state has a higher resistance than the AP state. This is the direct spin switch effect expected from the proximity effect in superconducting spin valves. However, when the fixed ferromagnetic layer is brought into a multi-domain state in zero magnetic field, resistance in this state becomes significantly higher than that of the P state and rapidly decreases to the P state value with increasing magnetic field. We explain this inverse spin switch effect by the formation of N\'eel - quasi-N\'eel domain wall pairs in the ferromagnetic bilayer. The N\'eel - quasi-N\'eel domain wall pairs induce stray magnetic fields with a significant component perpendicular to the plane of the sample. This field penetrates the SC and gives rise to vortex flow resistance. Our work shows that the inverse spin switch effect is magnetostatic in origin. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q34.00012: Even- to Odd-frequency Pair Conversion by Magnetic Interfaces in SC/N Junctions Matthias Eschrig, Jacob Linder, Takehito Yokoyama, Asle Sudb{\O} We study the proximity-induced superconducting correlations in a ballistic or diffusive normal metal (N) connected to a superconductor (SC) when the interface between them is spin-active. One of the hallmarks of the proximity effect in a non-magnetic bilayer is a minigap in the density of states of the normal metal. It scales with the Thouless energy of the normal metal and with the transmission probability of the interface. For a spin-active interface, the transmission properties of spin-$\uparrow$ and spin-$\downarrow$ electrons into N are different, giving rise to spin-dependent phase shifts at the interface. This leads to a rather surprizing result. Remarkably, for any interface spin polarization there is a critical interface resistance, above which the conventional singlet proximity component vanishes at the chemical potential, while an odd-frequency triplet component remains finite. At the same time, the minigap is replaced by a low-energy band with enhanced density of states. We propose a way to unambiguously observe the odd-frequency component. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q34.00013: Proximity effect in Exchange-spring/Superconductor hybrid system Jiyeong Gu, Jesse Burgess In ferromagnet/superconductor hybrid systems, it has been well known that the superconducting property is modified when the local magnetic environment changes in the adjacent ferromagnetic layers. In exchange-coupled hard/soft ferromagnet systems, the mutual coupling of the soft and hard magnetic layers creates a spiral magnetic domain structure. We utilized the exchange-coupled hard/soft ferromagnet system, such as NiFe/SmCo and NiFe/SmFe, to vary the magnetic environment near the superconducting layer. The switching behavior of both NiFe/SmCo and NiFe/SmFe systems were studied. In NiFe/SmFe systems, the reversible switching of the soft-layer was sharper, and occurred over a much smaller applied magnetic field range, than for NiFe/SmCo. The magnetic field required to achieve soft-layer saturation was also an order of magnitude smaller, making it more responsive to weaker applied fields. The superconducting property of the hybrid system was investigated as functions of the temperature and magnetic field. [Preview Abstract] |
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