  Phase diagrams of as-grown and O2-annealed FeTe1-xSex determined from
magnetic susceptibility measurement were obtained. For as-grown samples, the
antiferromagnetic order was fully suppressed in the range region x \geq 0.15,
and weak superconductivity appeared when x \geq 0.1. Beginning at x = 0.5, weak
superconductivity was found to evolve into bulk superconductivity.
Interestingly, for O2-annealed samples, complete suppression of magnetic order
and the occurrence of bulk superconductivity were observed when x \geq 0.1. We
found that O2-annealing induces bulk superconductivity for FeTe1-xSex. Oxygen
probably plays a key role in the suppression of the magnetic order and the
appearance of bulk superconductivity.
