Mn-doping induced ferromagnetism and enhanced superconductivity in Bi_(4−x)Mn_xO_4S_3 (0.075 ≤ x ≤ 0.15)

Abstract

We demonstrate that Mn doping in the layered sulfides Bi_4O_4S_3 leads to stable Bi_(4−x)Mn_x O_4S_3 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 ) ∼15 K among all BiS_2-based superconductors. We conjecture that the coexistence of superconductivity and ferromagnetism may be attributed to Mn doping in the spacer Bi_2O_2 layers away from the superconducting BiS_2 layers, whereas the enhancement of T_c may be due to excess electron transfer to BiS_2 from the Mn^(4+)/Mn^(3+) substitutions in Bi_2O_2. This notion is empirically corroborated by the increased electron-carrier densities upon Mn doping, and by further studies of the Bi_(4−x)A_xO_4S_3 compounds (A = Co, Ni; x=0.1 , 0.125), where the T_c values remain comparable to that of the undoped Bi_4O_4S_3 system (∼4.5 K) due to lack of 4+ valences in either Co or Ni ions for excess electron transfer to the BiS_2 layers. These findings therefore shed new light on feasible pathways to enhance the T_c values of BiS_2-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.