Finite-size effects in the nuclear magnetic resonance of epitaxial palladium thin films

Abstract

We have measured the NMR of ${}^{8}$Li${}^{+}$ implanted in a set of thin epitaxial films of Pd. We find a large, negative, strongly temperature-dependent Knight shift $K$ consistent with previous measurements on polycrystalline films. The temperature dependence of the shift exhibits a characteristic deviation from the susceptibility $\ensuremath{\chi}(T)$. In particular, at low temperature, $K(T)$ continues to follow a simple Curie-Weiss dependence. This result provides important insight into the origin of the low-temperature behavior of $\ensuremath{\chi}(T)$ in strongly paramagnetic metals. In addition, we find the room temperature shift depends on film thickness, with changes on the order of 20% between films 100 nm and 30 nm thick. We also observe a surface-related resonance in both Au-capped and uncapped films with a small positive shift. These features bear a striking similarity to the Pt NMR line shapes in much smaller Pt particles. However, they seem to originate, not from adsorbed species, but rather in confinement effects on the highly exhange-enhanced Pd $d$ band.