Proximity effect in [Nb(1.5nm)/Fe(x)]$_{10}$/Nb(50nm) superconducting/ferromagnet heterostructures

TL;DR

This study explores how the proximity effect influences superconductivity in Nb/Fe superlattices, revealing a transition from an intermediate mixed phase to a homogeneous state as Fe layer thickness increases.

Contribution

It provides new insights into the structural, magnetic, and superconducting properties of Nb/Fe superlattices, especially the formation of a weak ferromagnetic alloy and the associated proximity-induced superconducting phases.

Findings

Formation of a depth-modulated FeNb alloy with weak ferromagnetism.

Existence of an intermediate superconducting-normal phase at certain Fe thicknesses.

Disappearance of the intermediate phase when Fe layer exceeds 4 nm.

Abstract

We have investigated the structural, magnetic and superconducting properties of [Nb(1.5nm)/Fe(x)]$_{10}$ superlattices deposited on a thick Nb(50nm) layer. Our investigation showed that the Nb(50nm) layer grows epitaxially at 800$^\circ$C on Al$_2$O$_3$(1$\bar{1}$02) substrate. Samples grown at this condition posses a high residual resistivity ratio of 15-20. By using neutron reflectometry we show that Fe/Nb superlattices with $x<$ 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has properties of a weak ferromagnet. Proximity of this weak ferromagnetic layer to a thick superconductor leads to an intermediate phase that is characterized by co-existing superconducting and normal-state domains. By increasing the thickness of the Fe layer to $x$ = 4 nm the intermediate phase disappears. We attribute the intermediate state to proximity induced non-homogeneous superconductivity in the periodic Fe/Nb structure.