  I present a brief overview on the interplay between magnetism and
superconductivity in one of the Fe-based superconductor systems, \fts, where
the research of our group has centered. The parent compound Fe$_{1+y}$Te is an
antiferromagnet; with Se doping, antiferromagnetic order is suppressed,
followed by the appearance of superconductivity; optimal superconductivity is
achieved when $x\sim50\%$, with a superconducting temperature \tc of
$\sim$15~K. The parent compound has an in-plane magnetic ordering wave vector
around (0.5,\,0) (using the tetragonal notation with two Fe atoms per cell).
When Se concentration increases, the spectral weight appears to be shifted to
the wave vector around (0.5,\,0.5), accompanying the optimization of
superconductivity. A neutron-spin resonance has been observed around
(0.5,\,0.5) below \tc, and is suppressed, along with superconductivity, by an
external magnetic field. Taking these evidences into account, it is concluded
that magnetism and superconductivity in this system couple to each other
closely---while the static magnetic order around (0.5,\,0) competes with
superconductivity, the spin excitations around (0.5,\,0.5) may be an important
ingredient for it. I will also discuss the nature of magnetism and substitution
effects of 3$d$ transition metals.
