Pulsed NMR of submonolayer 3He adsorbed on grafoil

The nuclear spin relaxation times T I and T2 of submonolayer and multilayer 3He films adsorbed on Grafoil have been measured at temperatures between 1.2 and 4.2 K by a pulsed NMR technique. The T1 data for high-coverage films (solid and dense fluid phases) and the substrate registered phase are interpreted in terms of 3thermally activated vacancies. In solids the quantum exchange inherent in He is shown to be important at low temperatures. The data for multilayer films are discussed in the light of the particle exchange between layers and the relaxation time of each layer. The dynamical behavior of adatoms in the solid, fluid, and substrate registered phases as well as the nature of phase transitions between them are discussed on the basis of information obtained from the analysis of T1 and T 2 data. The present results as a whole seem to support the phase diagram determined by specific heat measurements. In addition, the nuclear susceptibility in submonolayer films has been measured by the same technique. The effect of Fermi degeneracy was not seen in the temperature range between 1.2 and 4.2 K.

Section: Nuclear Susceptibilitysupporting

confidence: 82%

“…The geometry of sample cell is illustrated in Fig. 2, which is almost the same as that described by Hickernell et al 7 The sample cell was prepared by the following procedure. First, a strip of Grafoil sheet with a thickness of 0.013 cm was baked at 1100~ in vacuo for 75 h to eliminate most of the magnetic impurities.…”

Section: The Sample Cellmentioning

confidence: 93%

Pulsed NMR study of submonolayer and multilayer 3He films adsorbed on Grafoil

Satoh

Sugawara

1980

“…It is interesting to compare the T1 data obtained here with those reported by Hickernell et al 10 For all coverages they found T1 to be less than 1 sec at 4.2 K, with somewhat higher values of T~ at lower temperatures. The temperature dependence, in qualitative terms, is similar to the one observed here, but their T1 values are appreciably shorter at all temperatures.…”

Section: He On Grafoilmentioning

confidence: 54%

“…Recently, several different NMR experiments on 3He-graphite systems have been reported. [7][8][9][10][11][12] The main objective of the NMR experiments described in the present paper was to determine the behavior of relaxation times and nuclear magnetic susceptibility for 3He adsorbed on graphite, with a view to establishing the degree of localization of 3He and its dependence on temperature and coverage. Results are presented for two different graphite substrates: Grafoil,* basal-plane-oriented graphite foil, and Sterling FT, t a graphitized carbon black powder.…”

Section: Introductionmentioning

confidence: 99%

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

Grimmer

Luszczynski

1978

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-planeoriented graphite (Grafoil) outgassed izt 130~ but in some experiments graphitized carbon black powder (Sterling b-T), vacuum-baked at 1100~ was used for comparison. The aHe coverages examined range from O. 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 T~, the spin-echo decay time T+2, and the spin-lattice relaxation time T1 suggest that the observed relaxation phenomena in 3He are largely governed by the 3He-substrate interaction. In 3 "~ r the case of He 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 con]unction with a simple model of spin diffusion in restricted ~eometries, lead to values of the diffusion coefficient Ds of about 2.1 xlO-Scm /sec at 4.2K and 7.0xlO-6cm2/sec at 1.2K for one-monolayer coverages; these values of Ds are 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 3 t the case of He on Sterling FT, the restricted diffusion analysis of the T2 data gives Ds-~2 • lO-S cmZ/sec for 0.6 monolayer of 3He; this value of Ds suggests a relatively localized system. The measured T'2 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.

“…In more typical circumstances, as first pointed out by Bloembergen et al 16 in the very early days of magnetic resonance, the nonequilibrium distribution of the spins that are farther away from the walls diffuse to the surface in order to relax and hence a single relaxation rate is observed. [17][18][19][20][21][22][23] In liquid and solid 3He the nuclear spin diffusion is extremely rapid, 14A5'24"25 with the rate many orders of magnitude greater than that 9 26 27 seen for other materials. " Indeed, the diffusion is so fast that it drops out of the problem, 1"2 and the dominant "bottleneck" for relaxing spins becomes the rate at which the 3He spins nearest the walls can relax.…”

Section: Motivating Experimentsmentioning

confidence: 99%

Spin relaxation of 3He to paramagnetic centers in surfaces at very low temperatures

Albers

Wilkins

1979

The memory function formalism is used to derive a generalized golden rule expression for the spin-lattice relaxation rate l/T1 for 3He to paramagnetic centers embedded in or residing on surfaces in contact with the 3He. This expression is applied to several simple models of relaxation to paramagnetic 3 spins which do not in teract among themselves, both for He in the Fermi-liquid phase and for a solid surface layer of 3He on the substrate. The magnitude of 1/T1 is calculated for each case as well as the temperature and magnetic field dependence. Finally, the relationship between the spin-lattice relaxation rate and the Kapitza conductance across surfaces via magnetic interactions is determined.