Growth and characterizations of Ba(2)Ti(2)Fe(2)As(4)O single crystals

Single crystals of a new iron-based superconductor Ba(2)Ti(2)Fe(2)As(4)O have been grown successfully via a Ba(2)As(3)-flux method in a sealed evacuated quartz tube. Bulk superconductivity with T(c) ∼ 21.5 K was demonstrated in resistivity and magnetic susceptibility measurements after the as-grown crystals were annealed at 500 °C in vacuum for a week. X-ray diffraction patterns confirm that the annealed and the as-grown crystals possess the identical crystallographic structure of Ba(2)Ti(2)Fe(2)As(4)O. Energy-dispersive x-ray spectra indicate that partial Ti/Fe substitution exists in the [Fe(2)As(2)] layers and the annealing process redistributes the Ti within the Fe-plane. The ordered Fe-plane stabilized by annealing exhibits superconductivity with magnetic vortex pinned by Ti.