Spin-Polarisation measurement using NbN-Insulator-Ferromagnet Tunnel Junction with oxidized barrier

TL;DR

This paper presents a simple fabrication method for NbN-based superconductor-insulator-normal metal tunnel junctions that effectively measure ferromagnetic spin polarization at low temperatures using the Meservey-Tedrow technique.

Contribution

It introduces a two-step process utilizing NbN surface oxide as an insulator for spin polarization measurements, demonstrating its effectiveness at 1.6 K.

Findings

NbN film thickness below 10 nm shows Zeeman splitting under magnetic field

Devices with Co electrodes reliably measure spin polarization at 1.6 K

Simple fabrication process suitable for low-temperature spin measurements

Abstract

We report a two-step process for the fabrication of superconductor-insulator-normal metal tunnel junctions using NbN as the superconducting electrode and its surface oxide as the insulating tunnel barrier, and investigate its efficacy in measuring spin-polarisation of ferromagnets using the Meservey-Tedrow technique. We observe that for NbN film thickness below 10 nm, under the application of parallel magnetic field, the superconducting density of states show clear "Zeeman" splitting into spin-up and spin-down sub-bands. Tunnelling measurements on devices where ferromagnetic Co is used as the normal electrode shows that these devices can be used to reliably measure spin polarisation of a ferromagnet at temperatures up to 1.6 K. The simplicity of our fabrication process, and the ability to perform spin-polarisation measurements at 4He temperatures makes NbN a very attractive candidate for spin polarisation measurements.