Mn-doping induced ferromagnetism and enhanced superconductivity in Bi4-x Mnx O4 S3 (0.075 < = x < = 0.15)
Abstract
We demonstrate that Mn-doping in the layered sulfides Bi4O4S3 leads to stable Bi4-x Mnx O4 S3 compounds that exhibit both long-range ferromagnetism and enhanced superconductivity for 0.075 < = x < = 0.15, with a possible record superconducting transition temperature (Tc) = 15 K among all BiS2-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 BiS2 layers, whereas the enhancement of Tc 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 Bi4-x Ax O4 S3 compounds (A = Co, Ni; x = 0.1, 0.125), where the Tc values remain comparable to that of the undoped Bi4O4S3 system (= 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 Tc values of BiS2-based superconductors.