Enhancement of the superconducting transition temperature in Nb/Permalloy bilayers by controlling the domain state of the ferromagnet

TL;DR

This study demonstrates that domain walls in a ferromagnetic layer can enhance superconductivity in Nb/Permalloy bilayers by allowing Cooper pairs to sample multiple magnetization directions, especially in larger samples.

Contribution

It reveals that domain walls in a single ferromagnetic layer can mimic the spin switch effect in F/S/F trilayers, enhancing superconductivity by sampling diverse magnetization orientations.

Findings

Superconductivity is enhanced near domain walls in large samples.

The effect is absent in microstructured samples with smaller dimensions.

Domain wall width matches the superconducting coherence length, enabling sampling of multiple magnetization directions.

Abstract

In (S/F) hybrids the suppression of superconductivity by the exchange field h_ex of the ferromagnet can be partially lifted when different directions of h_ex are sampled simultaneously by the Cooper pair. In F/S/F trilayer geometries where the magnetization directions of the two F-layers can be controlled separately, this leads to the so-called spin switch. Here we show that domain walls in a single F-layer yield a similar effect. We study the transport properties of Ni_0.8Fe_0.2/Nb bilayers structured in strips of different sizes. For large samples a clear enhancement of superconductivity takes place in the resistive transition, in the very narrow field range (order of 0.5 mT) where the magnetization of the Py layer switches and many domains are present. This effect is absent in microstructured samples. Comparison of domain wall width \delta_w to the temperature dependent superconductor coherence length \xi_S(T) shows that \delta_w ~ \xi_S(T), which means that the Cooper pairs sample a large range of different magnetization directions.