Interference effects of the superconducting pairing wavefunction due to the Fulde–Ferrell–Larkin–Ovchinnikov like state in ferromagnet/superconductor bilayers

Abstract

The theoretical description of the Fulde-Ferrell-Larkin-Ovchinnikov like state establishing in nanostructered bilayers of ferromagnetic (F) and superconducting (S) material leads to critical temperature oscillations and reentrant superconductivity as the F-layer thickness gradually increases. The experimental realization of these phenomena is an important prerequisite for the fabrication of the Ferromagnet/Superconductor/Ferromagnet core structure of the superconducting spin-valve. A switching of the spin-valve is only expected if such nonmonotonic critical temperature behavior is observed in F/S bilayers as well as in the S/F bilayers, a combination of which the spin-valve core structure can be regarded to consist of. In our former investigations we could demonstrate the required non-monotonic behavior of the critical temperature in S/F bilayers. In this study we succeeded in the preparation of F/S bilayers, where the superconducting material is now grown on top of the ferromagnetic metal, which show deep critical temperature oscillations as a function of the ferromagnetic layer thickness as well as an extinction and recovery, i.e. a reentrant behavior, of superconductivity.Especially, the latter is necessary to obtain a spin-valve with a large critical temperature shift between the parallel and antiparallel configurations of magnetizations in the F layers.

Reentrant superconductivity and superconducting critical temperature oscillations in F/S/F trilayers of Cu41Ni59/Nb/Cu41Ni59 grown on cobalt oxide

“…[3] where it appears at 13.6 nm. Also the resulting value for the thickness per layer at T c minimum, d CuNi, AV = 23 nm/2 = 11.5 nm, is significantly larger than observed for F/S and S/F bilayers [8,9]. This observation will be further discussed at the end of Section.…”

“…[9], this indicates an increase of ξ F0 , i.e. a decrease of E ex , which may be caused by a degradation of the Cu 41 Ni 59 alloy (possibly by converting Ni to NiO x, which is antiferromagnetic [42]).…”

“…The sample was tilted by 7° in order to avoid channeling effects of the helium atoms in the mono-crystalline Si-substrate. Since the energy loss of the helium atoms when scattered elastically with copper, nickel and cobalt is very similar, the signals overlap in the spectrum, as shown for sample #2 of the AF-FSF5a series in Therefore, it is not possible to determine the thickness of the individual layers definitely, as it can be done in the case of S/F and F/S bilayers [8,9] and F/S/F trilayers [3]. We began the fitting from the topmost layer, the silicon cap layer.…”

Evidence for aπjunction inNb/Ni0.96V0.04/Nbtrilayers revealed by superfluid

“…Interest in the proximity effect between superconductors (S) and ferromagnets (F) has existed for some time, e.g., [1][2][3][4][5][6][7], and it continues as new measurements and ever more complex structures deepen understanding, e.g., [8][9][10][11][12]. Due to the spin-singlet state for the Cooper pairs of the BCS theory, ferromagnetism would be expected to realign the electron spins and destroy the Cooper pairing.…”

“…Thus, when ferromagnetism and superconductivity coexist, such as when Cooper pairs propagate into an adjacent ferromagnet in the proximity effect, competition between the two states creates several new possibilities for superconductivity. These include the famed oscillation of the amplitude of the order parameter known as the Fulde-Ferrell-Larkin-Ovchinnikov state (FFLO) [8,10,13,14], gapless superconductivity, [15] and π-junctions where the order parameter changes phase across the junction [2,16,17]. Such rich possibilities not otherwise found in superconductors or ferromagnets alone have led to both fundamental and practical interest in ferromagnetic/superconducting heterostructures, including as components of superconducting computational circuits [18].…”