Josephson effect in superconductor/ferromagnet-normal/superconductor structures

The critical current IC of superconductor/ferromagnet-normal/superconductor (S/FN/S) Josephson junctions is calculated in the framework of linearized Usadel equations. The dependence of IC on the distance L between superconductors and thicknesses dF,N of ferromagnetic and normal layers is analyzed. It is shown that IC(L,dF) may exhibit damping oscillations as a function of both arguments. The conditions have been determined under which the decay length and period of oscillation of IC(L) at fixed dF are on the order of decay length of superconducting correlations in the N metal, ξN, that is much larger than in F film. We demonstrate also that the positions of the points L=Ln, at which IC=0 exhibit damping oscillations as a function of dF. The number of transitions from 0 to π states in IC(L,dF) increases under L→Ln. Outside these narrow intervals of L around Ln sign and value of IC are independent on dF for dF≳ξF. This fact is important for possible applications of S/FN/S Josephson junctions and S/FNF/S spin valve Josephson devices

Josephson φ\varphi-junctions based on structures with complex normal/ferromagnet bilayer

We demonstrate that Josephson devices with nontrivial phase difference $0<\varphi_g <\pi$ in the ground state can be realized in structures composed from longitudinally oriented normal metal (N) and ferromagnet (F) films in the weak link region. Oscillatory coupling across F-layer makes the first harmonic in the current-phase relation relatively small, while coupling across N-layer provides negative sign of the second harmonic. To derive quantitative criteria for a $\varphi$-junction, we have solved two-dimensional boundary-value problem in the frame of Usadel equations for overlap and ramp geometries of S-NF-S structures. Our numerical estimates show that $\varphi$-junctions can be fabricated using up-to-date technology.Comment: 14 pages, 9 figure

Double proximity effect in hybrid planar Superconductor-(Normal metal/Ferromagnet)-Superconductor structures

We have investigated the differential resistance of hybrid planar Al-(Cu/Fe)-Al submicron bridges at low temperatures and in weak magnetic fields. The structure consists of Cu/Fe-bilayer forming a bridge between two superconducting Al-electrodes. In superconducting state of Al-electrodes, we have observed a double-peak peculiarity in differential resistance of the S-(N/F)-S structures at a bias voltage corresponding to the minigap. We claim that this effect (the doubling of the minigap) is due to an electron spin polarization in the normal metal which is induced by the ferromagnet. We have demonstrated that the double-peak peculiarity is converted to a single peak at a coercive applied field corresponding to zero magnetization of the Fe-layer

Observation of the Triplet Spin-Valve Effect in a Superconductor-Ferromagnet Heterostructure

The theory of superconductor-ferromagnet (S-F) heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing has been detected in a Nb/Cu41Ni59/nc-Nb/Co/CoOx spin-valve type proximity effect heterostructure, in which a very thin Nb film between the F-layers serves as a normal conducting (nc) spacer. The resistance of the sample as a function of an external magnetic field shows that for not too high fields the system is superconducting at a collinear alignment of the Cu41Ni59 and Co layer magnetic moments, but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature Tc for NCA is lower than the fixed measuring temperature. The existence of a minimum Tc, at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of a singlet superconductivity suppression by the long-range triplet pairing generation.Comment: 7 pages, 4 fgures, Submitted to Physical Review Letter

Spin-polarized supercurrents for spintronics: a review of current progress

During the past 15 years a new field has emerged, which combines superconductivity and spintronics, with the goal to pave a way for new types of devices for applications combining the virtues of both by offering the possibility of long-range spin-polarized supercurrents. Such supercurrents constitute a fruitful basis for the study of fundamental physics as they combine macroscopic quantum coherence with microscopic exchange interactions, spin selectivity, and spin transport. This report follows recent developments in the controlled creation of long-range equal-spin triplet supercurrents in ferromagnets and its contribution to spintronics. The mutual proximity-induced modification of order in superconductor-ferromagnet hybrid structures introduces in a natural way such evasive phenomena as triplet superconductivity, odd-frequency pairing, Fulde-Ferrell-Larkin-Ovchinnikov pairing, long-range equal-spin supercurrents, $\pi$-Josephson junctions, as well as long-range magnetic proximity effects. All these effects were rather exotic before 2000, when improvements in nanofabrication and materials control allowed for a new quality of hybrid structures. Guided by pioneering theoretical studies, experimental progress evolved rapidly, and since 2010 triplet supercurrents are routinely produced and observed. We have entered a new stage of studying new phases of matter previously out of our reach, and of merging the hitherto disparate fields of superconductivity and spintronics to a new research direction: super-spintronics.Comment: 95 pages, 23 Figures; published version with minor typos corrected and few references adde

Conductance spectroscopy in ferromagnet-superconductor hybrids

We present a theoretical model for the proximity effect in F–SFF–F structures (where F is a ferromagnet and S is a superconductor) with non-collinear magnetization vectors in the F-layers and with arbitrary magnitudes of exchange fields. The electrical conductance of these structures is analyzed within the Keldysh–Usadel formalism in the diffusive regime as a function of the misorientation angle between magnetizations of the F-layers and transparencies of the SF and FF interfaces. We show that long-range triplet superconducting correlations manifest themselves either as a zero-bias peak in the case of perfect transparency of the FF interface, or as a two-peak structure in the case of finite transparency. The predicted features may serve as a diagnostic tool for the characterization of interfaces in superconducting hybrid structures