Direct Observation of a Nuclear Spin Diffusion Barrier

Experiments are reported which measure the relaxation properties of nuclei near a paramagnetic impurity. For Y(C₂H₅SO₄)₃·{}9H₂O: Yb³⁺, nuclei as close as 3 {} 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.