NMR on gallium single crystals: Not yet an absolute thermometer at very low temperatures

A twin crystal of Ga was investigated in the temperature and field ranges of 0.5 to 10 mK and 60 to 250 mT, respectively. In these fields the magnetic interaction is too large to be treated as a perturbation of the quadrupolar interaction. Therefore, the eigenvalues of the Schrödinger equation were calculated numerically, and then used to determine the crystalline axes by pulsed NMR on ⁶⁹Ga and ⁷¹Ga. The temperature dependence of the NMR line intensities was measured using ¹⁹⁵Pt-NMR for thermometry. The observed intensities do not follow the theoretical expectations for a spin system of low nuclear polarization. In addition, a temperature-dependent frequency shift was observed. These results suggest that the effect of interactions between the spins, as well as more complicated spin dynamics, need to be considered in order to use gallium as an absolute thermometer at temperatures below 1 mK.