  We have carried out detailed magnetic and transport studies of the new
Sr0.5Ce0.5FBiS2-xSex (x = 0.5, 1) superconductors derived by doping Se in
Sr0.5Ce0.5FBiS2. Se-doping produces several effects: it suppresses
semiconducting like behavior observed in the undoped Sr0.5Ce0.5FBiS2,
ferromagnetic ordering temperature, TFM, decreases considerably from 7.5 K (in
Sr0.5Ce0.5FBiS2) to 3.5 K and superconducting transition temperature, Tc, gets
enhanced slightly to 2.9 - 3.3 K. Thus in these Se-doped materials, TFM is just
marginally higher than Tc. Magnetization studies provide an evidence of bulk
superconductivity in Sr0.5Ce0.5FBiS2-xSex. Quite remarkably, as compared with
the effective paramagnetic Ce-moment (~ 2.2 muB), the ferromagnetically ordered
Ce-moment in the superconducting state is rather small (~ 0.1 muB). To the best
of our knowledge, the title compounds are the first Ce-based superconducting
itinerant ferromagnetic materials (Tc < TFM). We stress that Ce-4f electrons
are responsible for both superconductivity and ferromagnetism just as U-5f
electrons are in UCoGe. Furthermore, a novel feature of these materials is a
dual hysteresis loop corresponding to both the ferromagnetism and the
coexisting superconductivity. Such features of Sr0.5Ce0.5FBiS2-xSex put these
materials apart from the well known U-containing superconducting ferromagnets
reported so far.

Superconductivity