Effect of substituting Ru4+ and other tetravalent ions in the B-site of rare earth manganates on the magneto-transport properties and charge-ordering

Substitution of Ti and Zr in the B-site of La_0.7Ca_0.3MnO₃ lowers the T_C and the temperature of insulator—metal transition. Substitution of Ru in the B-site of La_0.7Ca_0.3MnO₃, however, does not alter the T_C or T_IM significantly. More importantly, the effect of substitution of Ti and Zr in the B-site of the charge-ordered Nd_0.5Ca_0.5MnO₃ progressively destroys the charge-ordering in this insulating material. In contrast, substitution by Ru in the B-site renders the material ferromagnetic with the T_C increasing with the Ru content and the material also shows insulator-metal transition. This marked effect of Ru substitution on the charge-ordering is also seen in Nd_0.5Sr_0.5MnO₃ where the T_C increases with Ru content while charge-ordering gets destroyed. The extraordinary effect of Ru in favouring the ferromagnetic metallic state and destroying the charge-ordering is attributed mainly to its unique electronic configuration (t_2g⁴e_g⁰.