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

Abstract

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 $x = \mu_\mathrm{eff}H/(T + T_W)$, $T_W \approx 1.7$ 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 $\tau_cT = \text{const.}$, indicating the relaxation is dominated by conduction-band fluctuations.