Upper critical field in trilayer ferromagnet-superconductor-ferromagnet (FSF) structures

Abstract

Upper critical magnetic field Hc2 in geometries parallel and perpendicular to the heterostructure surface in thin film ferromagnet–superconductor–ferromagnet trilayer spin-valve cores is studied theoretically and experimentally. A wedge deposition technique is used for single-run preparation of a set of samples with thickness dF1 of the bottom and dF2 of the top ferromagnet (F) Cu41Ni59. The critical field Hc2 is measured in a temperature range of 0.4–8 K and magnetic fields of up to 9 T. A transition from an oscillatory to reentrant behavior of the superconducting transition temperature versus F-layer thickness induced by an external magnetic field is observed for the first time. To correctly interpret the experimental data, we develop a quasiclassical theory, which makes it possible to estimate the temperature dependence of the critical field and the superconducting transition temperature for an arbitrary set of system parameters. A fairly good agreement between our theoretical predictions and experimental data is demonstrated for all samples, using a single set of fit parameters. This confirms the adequacy of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) physics in determining the unusual superconducting properties of the studied Cu41Ni59/Nb/ Cu41Ni59 spin-valve core trilayers.