We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state established due to the proximity effect in superconducting Nb/Cu 41 Ni 59 bilayers. Using a special wedge-type deposition technique, series of 20-35 samples could be fabricated by magnetron sputtering during one run. The layer thickness of only a few nanometers, the composition of the alloy, and the quality of interfaces were controlled by Rutherford backscattering spectrometry, high resolution transmission electron microscopy, and Auger spectroscopy. The magnetic properties of the ferromagnetic alloy layer were characterized with superconducting quantum interference device (SQUID) magnetometry. These studies yield precise information about the thickness, and demonstrate the homogeneity of the alloy composition and magnetic properties along the sample series. The dependencies of the critical temperature on the Nb and Cu 41 Ni 59 layer thickness, T c (d S ) and T c (d F ), were investigated for constant thickness d F of the magnetic alloy layer and d S of the superconducting layer, respectively. All types of non-monotonic behaviors of T c versus d F predicted by the theory could be realized experimentally: from reentrant superconducting behavior with a broad extinction region to a slight suppression of superconductivity with a shallow minimum. Even a double extinction of superconductivity was observed, giving evidence for the multiple reentrant behavior predicted by theory. All critical temperature curves were fitted with suitable sets of parameters. Then, T c (d F ) diagrams of a hypothetical F/S/F spin-switch core structure were calculated using these parameters. Finally, superconducting spin-switch fabrication issues are discussed in detail in view of the achieved results.

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/Cu 41 Ni 59 /nc-Nb/Co/CoO x 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 Cu 41 Ni 59 and Co layer magnetic moments, but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature T c for NCA is lower than the fixed measuring temperature. The existence of a minimum T c , 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.An odd-in-frequency triplet pairing generation in singlet superconductor/ferromagnet thin-film heterostructures was predicted theoretically [1][2][3]. At least two ferromagnetic layers (F 1 ,F 2 ) with a non-collinear alignment of their magnetizations, are required to couple the conventional opposite-spin singlet s-wave pairing channel with the unconventional, odd-triplet s-wave pairing channel. The latter one is of extraordinary long range in F layers [1,2,4], because the magnetized conduction band of a ferromagnetic metal serves as an eigenmedia supporting the equal-spin pairing.Intense activities followed to formulate optimal conditions and realize experimental schemes for the generation and detection of this odd-triplet pairing utilizing the Josephson effect [5][6][7][8][9][10][11][12][13][14].

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a

Abstract. The upper critical magnetic field H c2 in thin-film FSF trilayer spinvalve cores is studied experimentally and theoretically in geometries perpendicular and parallel to the heterostructure surface. The series of samples with variable thicknesses d F1 of the bottom and d F2 of the top Cu 41 Ni 59 F-layers are prepared in a single run, utilizing a wedge deposition technique. The critical field H c2 is measured in the temperature range 0.4 − 8 K and for magnetic fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of the superconducting transition temperature versus F-layers thickness, induced by an external magnetic field, has been observed for the first time. In order to properly interpret the experimental data, we develop a quasiclassical theory, enabling one to evaluate the temperature dependence of the critical field and the superconducting transition temperature for an arbitrary set of the system parameters. A fairly good agreement between our experimental data and theoretical predictions is demonstrated for all samples, using a single set of fit parameters. This confirms adequacy of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual superconducting properties of the studied Cu 41 Ni 59 /Nb/Cu 41 Ni 59 spin-valve core trilayers.Experimental and theoretical analysis of the upper critical field in FSF trilayers 2

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.