Coexistence of superconductivity and ferromagnetism in Sr_0.5Ce_0.5FBiS_2-xSe_x (x = 0.5 and 1.0), a non-U material with T_c < T_FM

Abstract

We have carried out detailed magnetic and transport studies of the new Sr_0.5Ce_0.5FBiS_2-xSe_x (0.0 ≤ x ≤ 1.0) superconductors derived by doping Se in Sr_0.5Ce_0.5FBiS₂. Se–doping produces several effects: it suppresses semiconducting–like behavior observed in the undoped Sr_0.5Ce_0.5FBiS₂, the ferromagnetic ordering temperature, T_FM, decreases considerably from 7.5 K (in Sr_0.5Ce_0.5FBiS₂) 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_0.5Ce_0.5FBiS_2-xSe_x at x ≥ 0.5 in contrast to the undoped Sr_0.5Ce_0.5FBiS₂ (x = 0) where magnetization measurements indicate a small superconducting volume fraction. Quite remarkably, as compared with the effective paramagnetic Ce–moment (~2.2 μB), the ferromagnetically ordered Ce–moment in the superconducting state is rather small (~0.1 μB) suggesting itinerant ferromagnetism. To the best of our knowledge, Sr_0.5Ce_0.5FBiS_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.