  Knowledge of the doping (x)-temperature (T) phase diagram of Fe-based
superconductors is crucial in understanding the mechanism of high transition
temperature superconductivity. Here, we measured the Se doping dependence of
the magnetic susceptibility for Te-annealed FeTe$_{1-x}$Se$_x$. Two
characteristic temperatures $T^*_{\chi}$ and $T^{**}_{\chi}$ were observed in
the temperature dependence of magnetic susceptibility, where the magnetic
susceptibility decreased below T$^{**}_{\chi}$ and tended to increase with
further decreasing temperature below T$^*_{\chi}$. The decrease in magnetic
susceptibility below T$^{**}_{\chi}$ is considered to be due to the opening of
the pseudogap. Whereas the increase in magnetic susceptibility below
T$^*_{\chi}$ is attributed to the increase in the carrier concentration of
electrons. Furthermore, we found the large superconducting fluctuation, which
may be related to BCS-BEC crossover.
