Nmr behavior of tl metal at very low temperatures

Bulk samples of polycrystalline Tl were cooled in a two stage nuclear demagnetization cryostat to temperatures below 70 μK to 150 μK in fields ranging from 33 mT to 0.17 T. A change in the NMR signal was observed for spin polarisations above 40% where the single line split into two lines with a nearly field and temperature independent splitting of 17^ŷ3 kHz. This is attributed to the onset of nuclear spin ordering. The intensity of the NMR line above the transition temperature T_c[μK]=10³(B[T]+0.03) exceeded the intensity expected from the Curie susceptibility by roughly a factor of 2.5. Towards higher temperatures (蝤1 mK) this factor decreased monotonically to 1. Heat pulse experiments indicated irregularities in the specific heat in the temperature range 0.1 to 1 mK. Nuclear ordering and a possible change in the low temperature behavior of the conduction electrons are considered as the origin of these effects.