3He on MgO: Nuclear magnetic relaxation properties

Abstract

Using MgO powder as a substrate, the magnetic relaxation of 3He and adsorption isotherms of 3He and Kr have been measured. Relaxation data as a function of coverage were obtained at temperatures of 2.0, 2. 7, 3.5, and 4.2 K, and as a function of temperature at coverages of x = 0.84 and 1.0 monolayer. The Kr isotherms show steplike features whose size increases following an extended bakeout at 950~ under vacuum. This is interpreted as indicating that the MgO surface consists of a mixture of uniform and nonuniform regions. Extended heating increases the fraction of the total surface area that is uniform. The values of Te vs. coverage at T= 2 and 2.7K are found to change from being roughly coverage independent to decreasing as x -3 at a coverage of x = O. 9, indicating a decrease in film mobility. At 3.5 and 4.2 K an approximately constant T2 is found at all coverages with no evidence of a sudden change of mobility. The low-temperature T 2 data continue to decrease as the second layer forms, implying a low mobility for small second-layer coverages. On the basis of the relatively short values obtained for T1 it is concluded that a relaxation mechanism in addition to dipolar coupling is presen t.