Coexistence of superconductivity and ferromagnetism in Sr(0.5)Ce(0.5)FBiS(2-x)Se(x) (x = 0.5 and 1.0), a non-U material with T(c) < T(FM)

We have carried out detailed magnetic and transport studies of the new Sr(0.5)Ce(0.5)FBiS(2-x)Se(x) (0.0 ≤ x ≤ 1.0) superconductors derived by doping Se in Sr(0.5)Ce(0.5)FBiS(2). Se–doping produces several effects: it suppresses semiconducting–like behavior observed in the undoped Sr(0.5)Ce(0.5)FBiS(2), the ferromagnetic ordering temperature, T(FM), decreases considerably from 7.5 K (in Sr(0.5)Ce(0.5)FBiS(2)) to 3.5 K and the superconducting transition temperature, T(c), gets enhanced slightly to 2.9–3.3 K. Thus in these Se–doped materials, T(FM) is marginally higher than T(c). Magnetization studies provide evidence of bulk superconductivity in Sr(0.5)Ce(0.5)FBiS(2-x)Se(x) at x ≥ 0.5 in contrast to the undoped Sr(0.5)Ce(0.5)FBiS(2) (x = 0) where magnetization measurements indicate a small superconducting volume fraction. Quite remarkably, as compared with the effective paramagnetic Ce–moment (~2.2 μ(B)), the ferromagnetically ordered Ce–moment in the superconducting state is rather small (~0.1 μ(B)) suggesting itinerant ferromagnetism. To the best of our knowledge, Sr(0.5)Ce(0.5)FBiS(2-x) Se(x) ((x) = 0.5 and 1.0) are distinctive Ce–based bulk superconducting itinerant ferromagnetic materials with T(c) < T(FM). Furthermore, a novel feature of these materials is that they exhibit a dual and quite unusual hysteresis loop corresponding to both the ferromagnetism and the coexisting bulk superconductivity.