Session L34: Focus Session: Hybrid Magnetic-Superconducting Systems

Geometry and magnetic-state induced phenomena in S/F nanohybrids: unusual flux pinning effects and bistable superconductivity

Superconducting/Ferromagnetic (S/F) hybrids exhibit a plethora of induced effects and novel physical properties, due to the interplay between the competing S and F orders. We will show a few examples of those, in a series of experiments on a simple hybrid system: a S thin film with an array of F nanodots. Changing the array geometry, the nanodots size or their magnetic-state allows to investigate a large variety of physical phenomena. We will focus on two of them: flux pinning effects and stray-magnetic-field induced manipulation of superconductivity. We will firstly consider \textit{geometry} induced effects; in particular, we will compare the pinning properties of periodic, quasiperiodic, and fractal arrays [1]. Secondly, we will discuss the effects induced by particular nanodot magnetic-states. We will show experiments on the interaction between flux quanta and nanodot magnetic vortices, which can be used to obtain switchable flux pinning potentials [2]. Finally, we will describe an experiment in which the magnetic reversal events of the nanodot magnetic vortices are imprinted into the transport properties of a superconducting thin film [3]. This yields a very unusual hysteretic magnetoresistance. This effect is induced by the stray magnetic fields from the nanodots, which drive the superconducting-to-normal transition of the hybrid depending on the magnetic history. \\[4pt] [1] J.E. Villegas \textit{et al}., Phys. Rev. Lett. 97, 027002 (2006). \\[0pt] [2] J.E. Villegas \textit{et al., }Phys. Rev. B 77\textbf{,} 134510 (2008). \\[0pt] [3] J.E. Villegas \textit{et al.}, Phys. Rev. Lett. 99, 227001 (2007).   

Vortex ratchet effect induced by different magnetic configurations in magnetic superconducting hybrids.

We have used E-Beam Lithography to prepare hybrid systems consisting of arrays of nanometric Ni rings (elliptical and circular)covered by a superconducting Nb film. These nanometric rings were characterized by a First Order Reversal Curve method to realize the onion and vortex magnetic state at remanence. The transport properties of the superconducting Nb film were measured in the mixed state by applying a magnetic field(H) perpendicular to the sample. Classical pinning matching effects of very high order were observed in resistance vs H, which vary with the magnetic state of Ni rings. Interestingly, a ratchet effect characterized by a dc output voltage produced by an applied ac current is found. Moreover, the ratchet effect is drastically modified by the remanent magnetic state of the Ni rings. The systematic and origin of the ratchet effect will be discussed.   

Antivortex complexes and intrinsic ratchet dynamics in superconductors with progressive magnetic topology

Theoretically and experimentally, we analyze characteristic properties of a superconducting (Sc) film deposited on parallel arrays of ferromagnetic (Fm) dots with gradually increasing diameter in a periodic saw-tooth manner. Due to their perpendicular magnetization, dots induce vortex-antivortex molecules in the sample, with number of constituent (anti) vortices growing with magnet size. Resulting gradient of antivortex density between the dots predetermines local nucleation of superconductivity in the sample as a function of applied external field and temperature. In applied drive however, antivortices act collectively in an asymmetric potential of the dots and pinned vortices, and exhibit unique ratchet dynamics intrinsic to the Sc-Fm hybrids.   

Vortex dynamics and vortex lattice reconfiguration in superconducting-magnetic hybrids.

Amorphous superconducting films (Mo$_{3}$Si) have been grown on top of array of nanometric magnets. These periodic magnetic centers have been fabricated on Si substrates by Electron Beam Lithography and sputtering techniques. In the mixed state the competition between the intrinsic and random pinning potential of the superconducting film and the artificial induced periodic pinning potential governs the vortex lattice behavior. Close to critical temperature, the periodic potentials could overcome the random potentials, then the vortex lattice dynamics shows effects which are related with the array dimension and symmetry. We will show in these hybrid systems enhancements of matching effects between the vortex lattice and the array unit cell, and different vortex lattice configurations.   

Scanning Tunneling Spectroscopy Study of Proximity Effect in Bilayer Manganite/Cuprate Thin Films

Recent work has suggested novel proximity and spin diffusion effects in ferromagnet/superconductor heterostructures composed of transition-metal perovskites. We have performed scanning tunneling spectroscopy (STS) on La$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO) bilayer thin films. Films were epitaxially grown on $<$001$>$ SrTiO$_{3}$ substrates using pulsed laser deposition with either the LCMO or YBCO layer on top. The STS data taken at 4.2 K were analyzed for spectral signatures of a pairing gap on the LCMO layer and spin diffusion in the YBCO layer, and to determine the length scale of the proximity effect and the role played by magnetic domain walls.   

Quantum limit of the triplet proximity effect in half-metal - superconductor junctions

We apply the scattering matrix approach to the triplet proximity effect in superconductor-half metal structures. We find that for junctions that do not mix different orbital modes, the zero bias Andreev conductance vanishes, while the zero bias Josephson current is nonzero. We illustrate this finding on a ballistic half-metal/superconductor (HS) and superconductor/half-metal/superconductor (SHS) junction with translation invariance along the interfaces, and on HS and SHS systems where transport through the half-metallic region takes place through a single conducting channel. Our calculations for these physically single mode setups – single mode point contacts and chaotic quantum dots with single mode contacts – illustrate the main strength of the scattering matrix approach: it allows for studying systems in the quantum mechanical limit, which is inaccessible for quasiclassical Green's function methods, the main theoretical tool in previous works on the triplet proximity effect.   

Re-entrant resistance in mesoscopic superconductor-ferromagnet-superconductor structures

We report measurements of the resistance as a function of temperature and magnetization alignment in hybrid structures consisting of superconducting electrodes connected by two ferromagnetic nanowires separated by less than a superconducting coherence length. It has been predicted that such structures could exhibit a supercurrent due to Cooper pair splitting and coherent transport through the ferromagnets. Although we have not observed a zero-voltage supercurrent, we find that as the temperature is lowered below the critical temperature of the superconductor, the resistance of the structure shows a minimum and then rises, suggestive of re-entrant behavior. The resistance of the antiparallel alignment of the magnetization of ferromagnetic wires is found to be lower than in the parallel case just below Tc but becomes distinctly larger than in the parallel case at the lowest temperature. We discuss possible explanations and implications of this result.   

Ferromagnets without inversion symmetry - room for superconductivity?

Motivated by the recent discoveries of ferromagnetic and non-centrosymmetric superconductors, we present a mean-field theory [1] for a superconductor that \textit{both} lacks inversion symmetry and displays ferromagnetism, a scenario which is believed to be realized in UIr under applied pressure [2]. We study the interplay between the order parameters to clarify how superconductivity is affected by the presence of ferromagnetism and spin-orbit coupling. We find that the spin-orbit coupling seems to enhance both ferromagnetism and superconductivity in both singlet and triplet channels. We discuss our results in the context of the heavy fermion superconductor UIr and analyze possible symmetries of the order parameter. \\[3pt] [1] J. Linder, A. H. Nevidomskyy, and A. Sudb{\o}, Phys. Rev. B {\bf 78}, 172502 (2008). \\[0pt] [2] T. Akazawa et al., J. Phys. Cond. Mat. {\bf 16}, L29 (2004); J. Phys. Soc. Jpn. {\bf 73}, 3129 (2004).   

Inhomogeneous superconducting states and Umklapp processes in ferromagnet/superconductor nanostructures

A new boundary-value problem is derived for a ferromagnetic metal/superconductor (F/S) nanostructure assuming that superconductivity in a structure is characterized by a superposition of the BCS pairing with zero total momentum in the S layers and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing with nonzero 3d coherent momentum \textbf{k} in the FM layers. The processes of mutual transformation between the BCS and FFLO pairs at the S/FM boundary are shown to be the Umklapp processes during which the excess coherent pair momentum k becomes confined in the F layer. As a result, significantly new boundary conditions are introduced for Eilenberger equations. The interplay between the BCS and 3D FFLO states is explored in F/S bilayer, F/S/F trilayer, and F/S superlattice. Two $\pi $ phase superconducting states with electron-electron repulsion in the F layers are predicted in F/S/F trilayers. Two $\pi $ phase magnetic states (0$\pi $ and $\pi \pi )$ are also predicted in F/S superlattices. Theoretical results are used to explain the data on measurements of Tc in Gd/La superlattice.   

Splitting of a Cooper pair by a pair of Majorana bound states

Majorana fermions are spatially localized superpositions of electron and hole excitations in the middle of a superconducting energy gap. These unusual particles have been predicted to occur at the interface between a magnetic and a superconducting electrode, in contact with a topological insulator (such as a BiSb crystal or a HgTe quantum well). A single qubit can be encoded nonlocally in a pair of spatially separated Majorana fermions. We have discovered a unique experimental signature of the nonlocality, involving the injection of an electron into one bound state followed by the emission of a hole by the other bound state (equivalent to the splitting of a Cooper pair over the two states).   

Sign reversal of ac Josephson current in a ferromagnetic Josephson junction

It is known that in a superconductor/insulator/superconductor (SIS) junction, when a finite voltage is applied, the Josephson current shows a logarithmic divergence, i.e., the so-called Riedel peak(RP) at the gap voltage, $V$=2$\Delta $/e, ($\Delta $ is a superconducting gap). In a double barrier Josephson junction such as SXS junction, on the other hand, the voltage dependence of $I_{c}$ has not been investigated so far, where X is a normal metal (N) or a ferromagnet (F). We study the voltage dependence of Josephson critical current ($I_{c})$ in a variety of SXS junctions. In a SNS junction, $I_{c }$shows the RP at the gap voltage similar to a SIS junction. On the other hand, in a SFS junction, $I_{c}$ shows a damped oscillation with the alternation of sign as a function of thickness ($d)$ of F due to 0-$\pi $ transition. The RP exhibits a strong dependence on $d$, and changes its sign. It is predicted that the RP disappears at the 0-$\pi $ transition in the SFS junction.   

Josephson junctions with a synthetic antiferromagnetic interlayer

We have measured the critical current of Josephson junctions of the form Nb/Co/Ru/Co/Nb, where the two Co layers are exchange-coupled antiferromagnetically by the thin (0.6 nm) Ru interlayer. The antiferromagnetic coupling causes nearly complete cancellation of the intrinsic magnetic flux produced by the Co domains, and allows us to study large-area junctions with total Co thicknesses ranging from 2 to 20 nm -- four times thicker than in previous studies of Nb/Co/Nb Josephson junctions [1]. The dependence of the critical current on an in-plane external magnetic field results in a nearly perfect Fraunhofer pattern, due to the intrinsic flux cancellation. The junctions were fabricated by sputtering the S/F/N/F/S multilayer onto a Si substrate, followed by subtractive patterning by photolithography and ion milling into circular junctions ranging in diameter from 10-80 microns. The critical current density of the junctions decays exponentially with Co thickness, with a characteristic decay length of $\xi _{F}$ = 2.2 nm. There is no sign of a crossover to a slower decay at large Co thicknesses, which, if observed, might be a signature of the predicted long-range spin triplet state [2]. [1] Robinson et al., Phys. Rev. Lett. 68, 177003, 2006. [2] Bergeret et al., Rev. Mod. Phys. 77, 1321, 2005. [Work Supported by US DOE under grant DE-FG02-06ER4634]   

SFS Josephson Junctions using PdNi alloy

We have studied the variation of critical current in Superconductor/Ferromagnet/Superconductor (S/F/S) Josephson Junctions as a function of ferromagnet thickness (d$_{F})$ using a weakly ferromagnetic alloy, Pd$_{82}$Ni$_{12}$. The critical current density oscillates and decays over five orders of magnitude as d$_{F}$ is increased from 32 to 100 nm. These oscillations are indicative of 0-$\pi $ transitions in S/F/S junctions. We find the characteristic length of oscillation ($\xi _{F2} )$ to be 4.3 $\pm $ 0.1 nm and the characteristic length of decay ($\xi _{F1} )$ to be 7.9 $\pm $ 0.4 nm. Earlier studies [1] using a similar PdNi alloy in S/I/F/S junctions found $\xi _{F1} \approx \xi _{F2} \approx 2.8{\kern 1pt}nm$, however, those measurements were performed for d$_{F}$ between 4.5 and 14 nm. In our experiment, $\xi _{F1} >\xi _{F2} $, indicating that our samples are in the regime $E_{ex} \tau >\hbar $ [2, 3], where E$_{ex}$ is the exchange energy and $\tau $ is the mean free time between electron collisions in the ferromagnet. In spite of covering this wide range, we see no evidence of a crossover to a slower decay, which, if present, would be indicative of long-range spin triplet correlations [4]. [1] T. Kontos et al.,Phys. Rev. Lett. 89, 137007 (2002). [2] F. S. Bergeret, et al., Phys. Rev. B, 64, 134506 (2001) [3] Kashuba, et al., Phys. Rev. B. 75, 132502 (2007). [4] F.S. Bergeret, et al., Rev. Mod. Phys. 77, 1321 (2005). [This work is supported by US-DOE grant, DE-FG02-06ER46341.]