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.
Categories: cond-mat.mtrl-sci cond-mat.supr-con