Direct Observation of a Nuclear Spin Diffusion Barrier

Experiments are reported which measure the relaxation properties of nuclei near a paramagnetic impurity. For ${\mathrm{Y}\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{S}{\mathrm{O}}_4\right)}_3$·9${\mathrm{H}}_2$O: ${\mathrm{Yb}}^{3+}$, nuclei as close as 3 Å to the Yb ion are in strong thermal contact with the bulk spins, contradicting the traditional notions of the diffusion barrier. I present evidence that impurity-induced diffusion and interaction with the electron spin-spin reservoir are important processes which act to reduce the barrier dimension.