Inverse spin switch effects in Ferromagnet / Superconductor hybrids with strong ferromagnets

TL;DR

This study demonstrates that in F/S/F trilayers with strong ferromagnets, the antiparallel magnetic configuration suppresses superconductivity more than the parallel configuration, opposite to the behavior seen with weak ferromagnets.

Contribution

It reveals the inverse spin switch effect in strong ferromagnet/superconductor hybrids, contrasting previous findings with weak ferromagnets, and explains the underlying spin-polarized quasiparticle reflection mechanism.

Findings

Antiparallel configuration increases resistance in superconducting transition.

Strong ferromagnets cause a stronger suppression of superconductivity in antiparallel state.

The effect is linked to enhanced spin-polarized quasiparticle reflection at interfaces.

Abstract

In 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 configuration. This is the so-called spin switch, which also experimentally has been found 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 = Ni_0.80Fe_0.20 (Permalloy, Py) and S = 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.