Inverse spin switch effects in ferromagnet-superconductor-ferromagnet trilayers with strong ferromagnets

Abstract

In ferromagnet-superconductor-ferromagnet $(F∕S∕F)$ trilayers where the magnetization directions of the $F$ layers can be controlled separately, it has theoretically been predicted that the antiparallel (AP) configuration can have a higher superconducting transition temperature ${T}_{c}$ than the parallel $(P)$ configuration. This is the so-called spin switch, which has been found experimentally for the case of weak ferromagnets. Here we show that strong ferromagnets yield the opposite effect. We study the transport properties of $F∕S∕F$ trilayers with $F={\mathrm{Ni}}_{0.80}{\mathrm{Fe}}_{0.20}$ (Permalloy, Py) and $S=\mathrm{Nb}$, structured in strips of different sizes. Using two different thicknesses for the Py layers, we can switch, in a well-defined way, between the AP and $P$ configurations. In the superconducting transition we find a clear increase of the resistance in the AP state. We ascribe this to enhanced reflection of spin-polarized quasiparticles at the $S∕F$ interfaces, which leads to a stronger suppression of superconductivity on the $S$ side.