Mn-doping induced ferromagnetism and enhanced superconductivity in Bi4-xMnxO4S3 (0.075 ≤ x ≤ 0.15)

We demonstrate that Mn doping in the layered sulfides Bi4O4S3 leads to stable Bi4-xMnxO4S3 compounds that exhibit both long-range ferromagnetism and enhanced superconductivity for 0.075 <= x <= 0.15, with a possible record superconducting transition temperature (T-c) similar to 15 K among all Bi2O2-based superconductors. We conjecture that the coexistence of superconductivity and ferromagnetism may be attributed to Mn doping in the spacer Bi2O2 layers away from the superconducting Bi2S2 layers, whereas the enhancement of T-c may be due to excess electron transfer to BiS2 from the Mn4+/Mn3+ substitutions in Bi2O2. This notion is empirically corroborated by the increased electron-carrier densities upon Mn doping, and by further studies of the Bi(4-x)A(x) O4S3 compounds (A = Co, Ni; x = 0.1, 0.125), where the T-c values remain comparable to that of the undoped Bi4O4S3 system (similar to 4.5 K) due to lack of 4+ valences in either Co or Ni ions for excess electron transfer to the BiS2 layers. These findings therefore shed new light on feasible pathways to enhance the T-c values of BiS2-based superconductors, although complete elucidation of the interplay between superconductivity and ferromagnetism in these anisotropic layered compounds awaits the development of single crystalline materials for further investigation.