3He-19F interaction at the surface of fluorocarbon polymers

The interaction between 19F nuclear spins in Teflon and 3He spins on its surface has been studied in magnetic fields up to 6 Tesla. At temperatures below 1 K, a monolayer of 3He is sufficient to increase the NMR relaxation rate of 19F more than an order of magnitude over the value with vacuum or with a coverage of pure 4He. With a DLX 6000 substrate, the coupling between the two nuclear species weakens towards 6 T. With an Aldrich substrate, the coupling is still active, but it was not observed that the magnetization of a 3He film follows that of 19F substrate spins. In monolayer experiments, the field dependence of the fluorine nuclear relaxation time T1F is stronger than a quadratic law above 2.5 T, while with liquid 3He this field dependence is reduced to a linear law. Most of the data are discussed in terms of a coupling model, in which the 19F-3He nuclear dipolar interaction is modulated by motion of 3He atoms. The temperature dependence for T1F points to mechanisms involving vacancies, mass fluctuations or exchange inside the 3He surface layer or, in one case, paramagnetic centers, in the 3He-19F coupling. The relaxation time of the fluorine dipolar reservoir in the Aldrich substrate is of the order of minutes at 3 K and 2.5 T, even in the presence of 3He