No abstract

We demonstrate that in a wide class of multilayered superconductor-ferromagnet structures (e.g., S/F, S/F/N, and S/F/F') the vanishing Meissner effect signals the appearance of the in-plane Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulated superconducting phase. In contrast to the bulk superconductors the FFLO instability in these systems can emerge at temperatures close to the critical one and is effectively controlled by the S layer thickness and the angle between magnetization vectors in the F/F' bilayers. The predicted FFLO state is revealed through the critical temperature oscillations vs the perpendicular magnetic field component.

No abstract

We study peculiarities of proximity effect in clean superconductor -ferromagnet structures caused by either spatial or momentum dependence of the exchange field. Even a small modulation of the exchange field along the quasiparticle trajectories is shown to provide a long range contribution to the supercurrent due to the specific interference of particle-and hole-like wave functions. The momentum dependence of the exchange field caused by the spin -orbit interaction results in the long -range superconducting correlations even in the absence of ferromagnetic domain structure and can explain the recent experiments on ferromagnetic nanowires.The exchange field h in ferromagnetic (F) metals is well known to destroy Cooper pairs resulting, thus, in a strong decay of superconducting (S) correlations in the F material and suppression of Josephson current in SFS junctions (see Refs. 1, 2 for review). Considering the quantum mechanics of quasiparticle excitations this destructive effect of the exchange field can be viewed as a consequence of a phase difference γ ∼ L/ξ h = 2Lh/ V F gained between the electron-and hole-like parts of the total wave function at the path of the length L. Both in the clean and dirty limits the measurable quantities should be calculated as superpositions of fast oscillating contributions e iγ from different trajectories and, thus, rapidly vanish with the increasing distance from the SF boundary.This textbook physical picture appears to be in sharp contrast with a number of recent experiments [3-8] which point to an anomalously large length of decay of superconducting correlations inside the F metal. As we can judge from the observation [8] of a noticeable supercurrent through a Co nanowire, this decay length can be of the order of half a micrometer which well exceeds typical coherence lengths in ferromagnets both in the clean and dirty limits. In the dirty limit such strong proximity effect can hardly be explained even taking account of long-range triplet correlations [2] induced by the exchange field inhomogeneity.Naturally, the inhomogeneity of the field h caused by the ferromagnetic domain structure can improve the conditions of Cooper pair survival in the clean limit as well. To suppress the destructive trajectory interference mentioned above the domain structure should cancel the phase gain γ for a certain group of quasiparticle trajectories. A simple example of such phase gain compensation can be realized in a clean junction consisting of two F layers with opposite orientations of magnetic moment [9,10]. On the other hand in the diffusive limit this compensation effect vanishes [11]. Note, that the exchange field inhomogeneity along the quasiclassical trajectory experiencing multiple reflections from the ferromagnet surface can appear even in the absence of the spatial domain structure. Indeed, the exchange field acting on band electrons in a solid with a finite spin -orbit interaction should obviously depend on the quasiparticle momentum [12]: h = h(k). The normal quasiparticle reflection...

The spread of the Cooper pairs into the ferromagnet in proximity coupled superconductor -ferromagnet (SF) structures is shown to cause a strong inverse electromagnetic phenomenon, namely, the long-range transfer of the magnetic field from the ferromagnet to the superconductor. Contrary to the previously investigated inverse proximity effect resulting from the spin polarization of superconducting surface layer, the characteristic length of the above inverse electrodynamic effect is of the order of the London penetration depth, which usually much larger than the superconducting coherence length. The corresponding spontaneous currents appear even in the absence of the stray field of the ferromagnet and are generated by the vector-potential of magnetization near the S/F interface and they should be taken into account at the design of the nanoscale S/F devices. Similarly to the well-known Aharonov-Bohm effect, the discussed phenomenon can be viewed as a manifestation of the role of vector potential in quantum physics.