Interfacial roughness and proximity effects in superconductor/ferromagnet CuNi/Nb heterostructures

TL;DR

This study investigates how interfacial roughness in Nb/CuNi superconductor/ferromagnet heterostructures affects their structural and electronic properties, revealing that smoother interfaces on Al2O3 substrates lead to higher superconducting transition temperatures.

Contribution

It provides new insights into the relationship between interfacial roughness and superconducting properties in S/F heterostructures, highlighting substrate effects on interface quality.

Findings

Lower interfacial roughness on Al2O3 substrates correlates with higher T_C.

Superconducting transition temperature is about 4% higher on Al2O3 substrates.

Interfacial roughness has a weak effect on Cooper pair transparency.

Abstract

We report an investigation of the structural and electronic properties of hybrid superconductor/ferromagnet (S/F) bilayers of composition Nb/Cu$_{60}$Ni$_{40}$ prepared by magnetron sputtering. X-ray and neutron reflectometry show that both the overall interfacial roughness and vertical correlations of the roughness of different interfaces are lower for heterostructures deposited on Al$_2$O$_3$(1$\bar{1}$02) substrates than for those deposited on Si(111). Mutual inductance experiments were then used to study the influence of the interfacial roughness on the superconducting transition temperature, $T_C$. These measurements revealed a $\sim$ 4% higher $T_C$ in heterostructures deposited on Al$_2$O$_3$, compared to those on Si. We attribute this effect to a higher mean-free path of electrons in the S layer, caused by a suppression of diffusive scattering at the interfaces. However, the dependence of the $T_C$ on the thickness of the ferromagnetic layer is not significantly different in the two systems, indicating a weak influence of the interfacial roughness on the transparency for Cooper pairs.