Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted $\beta$-emitter $^{8}$Li

TL;DR

This study uses $^{8}$Li $eta$-NMR to measure the depth-resolved Meissner screening profile in a niobium thin film, providing insights into its magnetic penetration depth and mean-free-path relevant for superconducting applications.

Contribution

It demonstrates a novel application of $^{8}$Li $eta$-NMR for depth-resolved characterization of superconducting thin films, linking spin-lattice relaxation to magnetic screening profiles.

Findings

Magnetic penetration depth $oldsymbol{_{0}}$ = 51.5 nm

Mean-free-path $oldsymbol{}$ = 18.7 nm

Validated $^{8}$Li $eta$-NMR as a tool for superconducting film analysis

Abstract

We report measurements of the Meissner screening profile in a Nb(300 nm)/Al$_{2}$O$_{3}$ thin film using $^{8}$Li $\beta$-detected nuclear magnetic resonance ($\beta$-NMR). The NMR probe $^{8}$Li was ion-implanted into the Nb film at energies $\leq$ 20 keV, corresponding to mean stopping depths comparable to Nb's magnetic penetration depth $\lambda$. $^{8}$Li's strong dipole-dipole coupling with the host $^{93}$Nb nuclei provided a "cross-relaxation" channel that dominated in low magnetic fields, which conferred indirect sensitivity to the local magnetic field via the spin-lattice relaxation (SLR) rate $1/T_{1}$. From a fit of the $1/T_{1}$ data to a model accounting for its dependence on temperature, magnetic field, and $^{8}$Li$^{+}$ implantation energy, we obtained a magnetic penetration depth $\lambda_{0}$ = 51.5(22) nm, consistent with a relatively short carrier mean-free-path $\ell$ = 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step towards using $^{8}$Li $\beta$-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.