NMR-investigations on thallium-samples of high spin-polarization

Abstract

The big indirect exchange interaction and the huge nuclear magnetic moments of the two Tl-isotopes makes Tl useful at low temperatures for non linear pulsed NMR studies of inter- and intra-spin interactions mediated by the conduction electrons. We extended earlier experiments to higher fields (0.1 T < B_o < 1 T) on a cylindrically shaped sample of 6N purity and found the proposed spin waves trapped in the skin depth of the sample which manifest themselves in line splitting with increasing tipping angles. We also found frequency shifts in partial agreement with the expectations. However, the spin susceptibility was enhanced above the paramagnetic value at temperatures below 2 mK, whereas the line width decreased by a factor of two between 5 and 1 mK. Thus, while solving the high spin-polarization puzzle of Tl (line splitting of already merged lines) we were left with two other unexplained observations.