Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted β-emitter 8Li

We report measurements of the Meissner screening profile in a Nb(300 nm)/Al2O3 thin film using Li-8 beta-detected nuclear magnetic resonance (beta-NMR). The NMR probe Li-8 was ion-implanted into the Nb film at energies <= 20 keV, corresponding to mean stopping depths comparable to Nb's magnetic penetration depth lambda. Li-8's strong dipole-dipole coupling with the host Nb-93 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 Li-8(+) 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 Li-8 beta-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.

Automated summary

In this study, we used Li-8 beta-detected nuclear magnetic resonance (beta-NMR) to measure the Meissner screening profile in a Nb/Al2O3 thin film. By fitting the data to a model considering temperature, magnetic field, and ion implantation energy, we obtained the magnetic penetration depth and the mean free path of the carrier for the Nb film.