F19 NMR and defect spins in vacuum-annealed LaO0.5F0.5

We report results of magnetization and $^{19}\mathrm{F}$ NMR measurements in the normal state of as-grown vacuum-annealed ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution ${M}_{\ensuremath{\mu}}(H,T)$ from a $\ensuremath{\sim}1000$ ppm concentration of local moments, an order of magnitude higher than can be accounted for by measured rare-earth impurity concentrations. ${M}_{\ensuremath{\mu}}(H,T)$ 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 $x={\ensuremath{\mu}}_{\mathrm{eff}}H/(T+{T}_{W}), {T}_{W}\ensuremath{\approx}1.7$ K. There is no evidence for magnetic order down to 2 K, and the origin of ${T}_{W}$ is not clear. $^{19}\mathrm{F}$ frequency shifts, linewidths, and spin-lattice relaxation rates are consistent with purely dipolar $^{19}\mathrm{F}$/defect-spin interactions. The defect-spin correlation time ${\ensuremath{\tau}}_{c}(T)$ obtained from $^{19}\mathrm{F}$ spin-lattice relaxation rates obeys the Korringa relation ${\ensuremath{\tau}}_{c}T=\text{const}$, indicating the relaxation is dominated by conduction-band fluctuations.