Ru-doping-induced ferromagnetism in charge-ordered La0.4Ca0.6MnO3

Impurity effects on the stability of a charge-ordered antiferromagnetic state in electron-doped manganite La0.4Ca0.6MnO3 are investigated by doping Ru for Mn. Our results show that Ru doping at a tiny level of similar to 2% is sufficient to suppress significantly the charge-ordered insulated state and generate a ferromagnetic metallic one. The blocked metastable state and the first-order metal-insulator transitions observed in the slightly doped samples can be attributed to the phase-separated state where ferromagnetic domains are embedded in the charge-ordered matrix. The doping effect is further investigated theoretically based on the two-orbital model. The calculation suggests that two factors are vitally important for the Ru-doping driven strong ferromagnetic tendency: (1) the ferromagnetic coupling between Ru and Mn spins and (2) the enhancement of e(g) electron density while the topological defects always contribute to the instability of charge-ordered state.