We report measurements of the Meissner screening profile in a Nb(300
nm)/Al2​O3​ thin film using 8Li β-detected nuclear magnetic
resonance (β-NMR). The NMR probe 8Li was ion-implanted into the Nb
film at energies ≤ 20 keV, corresponding to mean stopping depths
comparable to Nb's magnetic penetration depth λ. 8Li's strong
dipole-dipole coupling with the host 93Nb 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/T1​. From a fit of the 1/T1​ data to a model
accounting for its dependence on temperature, magnetic field, and
8Li+ implantation energy, we obtained a magnetic penetration depth
λ0​ = 51.5(22) nm, consistent with a relatively short carrier
mean-free-path â„“ = 18.7(29) nm typical of similarly prepared Nb films. The
results presented here constitute an important step towards using 8Li
β-NMR to characterize bulk Nb samples with engineered surfaces, which are
often used in the fabrication of particle accelerators.Comment: 16 pages, 4 figure