Coexistence of Superconductivity and Ferromagnetism in Dilute Co-doped   $La_{1.89} Ce_{0.11} Cu O_{4\pm\d}$ System
  Thin films of the optimally electron-doped $T'$-phase superconductor La$_{1.89}$Ce$_{0.11}$CuO$_{4\pm \d}$ are investigated by dilute Co doping, formed as La$_{1.89}$Ce$_{0.11}$(Cu$_{1-x}$Co$_{x}$)O$_{4\pm\d}$ (LCCCO) with $x$ = 0.01 -- 0.05. The following results are obtained for the first time: for the whole dilute Co doping range, LCCCO thin films show long-range ferromagnetic ordering at the temperature range from 5 K to 300 K, which is likely due to the RKKY interaction; in the very dilute Co doping, $x$ = 0.01 and 0.02, the superconductivity is maitained, the system shows the coexistence of superconductivity and ferromagnetism in the CuO$_{2}$ plane. This may be based on the nature of the charge carriers in electron-doped high-$T_{c}$ cuprate superconductors. 
K. Jin, J. Yuan, L. Zhao, H. Wu, X.Y. Qi, B.Y. Zhu, L.X. Cao, X.G.   Qiu, B. Xu, X.F. Duan, and B.R. Zhao