NMR Relaxation times of 3He adsorbed on graphite below 4.2 K

Pulsed NMR measurements were performed at 10 and 20 MHz on thin ³He films on graphitic surfaces in the temperature range between 0.35 and 4.2 K. Most of the measurements reported here have been obtained with basal-plane oriented graphite (Grafoil) outgassed at 130 C, but in some experiments graphitized carbon black powder (Sterling FT), vacuum-baked at 1100 C, was used for comparison. The ³He coverages examined range from 0.1 to 80 monolayers on Grafoil and from 0.3 to 1.0 monolayers on Sterling FT. Measurements have also been made on ³He-⁴He films on Grafoil with one-monolayer quantity of ³He mixed with various amounts of ⁴He. The measured free-induction decay time T₂, the spin-echo decay time T₂, and the spin-lattice relaxation time T₁suggest that the observed relaxation phenomena in ³He are largely governed by the ³He-substrate interaction. In the case of ³He on Grafoil, T₂ is much longer than T₂, evidently as a result of large local magnetic field gradients and significant lateral mobility of ³He on this substrate. These results, in conjunction with a simple model of spin diffusion in restricted geometries, lead to values of the diffusion coefficient D_sof about 2.1 × 10^–5 cm²/sec at 4.2 K and 7.0 × 10^–6 cm²/sec at 1.2 K for one-monolayer coverages ; these values of D_sare indicative of a nonlocalized ³He system. To verify the model used, the restricted diffusion analysis was applied to room-temperature measurements on C₆F₆ in Grafoil ; the values of the diffusion coefficient obtained in this way are in good agreement with diffusion rates measured directly in bulk liquid C₆F₆ at room temperature. In the case of ³He on Sterling FT, the restricted diffusion analysis of the T₂ data gives D_st`t2 × 10^–8 cm²/sec for 0.6 monolayer of ³He; this value of D_ssuggests a relatively localized system. The measured T₂ is found to decrease monotonically with coverage and there is no evidence of any phase transition in ³He films for coverages up to one monolayer.Work supported in part by the National Science Foundation and a Navy Equipment Loan contract.