Magnetic flux dynamics and structural features in fluorinated HgBa2CuO4 asprobed by F-19 NMR

F-19 NMR spectra and spin-lattice relaxation rates have been measured on tw o powder HgBa2CuO4Fx samples. The results combined with our neutron powder- diffraction data [A. M. Abakumov rt al., Phys. Rev. Lett. 80, 385 (1998)] c onfirm that fluorine atoms can be successfully incorporated in the lattice of superconducting Hg-1201 phase in the nonstoichiometric O3 site. Partial occupation of another oxygen position by fluorine atoms reveals to be possi ble with the increase of F content. The temperature dependence of the F-19 NMR linewidth reveals the crossover at 35 K in the magnetic-flux line struc ture between the low-temperature latticelike regime and high-temperature "m elted" regime. For the 1/T-1 temperature dependence a maximum has been foun d at 65 K indicating the strong influence of magnetic-flux line motion on t he F-19 spin-lattice relaxation rate.