Evolution of the metallic state in LaNiO 3 / LaAlO 3 superlattices measured by Li 8 β -detected NMR

Using ion-implanted $^{8}\mathrm{Li} \ensuremath{\beta}$-detected NMR, we study the evolution of the correlated metallic state of ${\mathrm{LaNiO}}_{3}$ in a series of ${\mathrm{LaNiO}}_{3}/{\mathrm{LaAlO}}_{3}$ superlattices as a function of bilayer thickness. Spin-lattice relaxation measurements in an applied field of $6.55\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ reveal two equal amplitude components: one with metallic ($T$ linear) $1/{T}_{1}$ and a second with a more complex $T$ dependence. The metallic character of the slow relaxing component is only weakly affected by the ${\mathrm{LaNiO}}_{3}$ thickness, while the fast component is much more sensitive, exhibiting the opposite temperature dependence (increasing toward low $T$) in the thinnest, most magnetic samples. The origin of this bipartite relaxation is discussed in terms of electronic phase separation.