${}^{19}$F NMR and defect spins in vacuum-annealed LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$

We report results of magnetization and ${}^{19}$F NMR measurements in the normal state of as-grown LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution $M_{\mu }(H,T)$ from a $\sim$1000~ppm concentration of local moments, an order of magnitude higher than can be accounted for by measured rare-earth impurity concentrations. $M_{\mu }(H,T)$ can be fit by the Brillouin function $B_J(x)$ or, perhaps more realistically, a two-level $\tanh (x)$ model for magnetic Bi $6p$ ions in defect crystal fields. Both fits require a phenomenological Curie-Weiss argument $x={\mu }_{\mathrm{e}\mathrm{f}\mathrm{f}}H\mathrm{/}(T+T_W)$, $T_W\approx 1.7$ K. There is no evidence for magnetic order down to 2 K, and the origin of $T_W$ is not clear. ${}^{19}$F frequency shifts, linewidths, and spin-lattice relaxation rates are consistent with purely dipolar ${}^{19}$F/defect-spin interactions. The defect-spin correlation time ${\tau }_c(T)$ obtained from ${}^{19}$F spin-lattice relaxation rates obeys the Korringa relation ${\tau }_{c}T=\text{const.}$, indicating the relaxation is dominated by conduction-band fluctuations.