$^{19}$F NMR and defect spins in vacuum-annealed LaO$_{0.5}$F$_{0.5}$BiS$_2$
/ Authors
S. Yadav, S. Delgado, O. O. Bernal, D. MacLaughlin, Y. Liu, D. Jiang, O. Santana, A. Mushammel, L. Shu, K. Huang
and 20 more authors
D. Yazici, M. B. M. N. M. I. O. Mining, Technology, NM Socorro, Caltech, CA Pasadena, California Polytechnic State University, Los Angeles, Ca, U. California, Riverside, F. University, Shanghai, China., Lawrence Berkeley National Lab, CA Livermore, Gda'nsk University of Technology, Gda'nsk, P. U. O. California, San Diego
/ 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.