Suppression of charge ordering and evolution of ferromagnetism in Ru substituted Bi 0.5 Ca 0.5 MnO 3

In this work, we report the effect of systematic Ru doping (at Mn-site) on the magnetic, electrical and thermoelectrical properties of charge-ordered manganite $Bi_{0.5} Ca_{0.5} MnO_{3}$. It is found that Ru doping in $Bi_{0.5} Ca_{0.5} Mn_{1-x} Ru_{x} O_{3} (0.0 \le x \le 0.07)$, unlike other magnetic ions, weakens charge ordering and transforms the charge-ordered antiferromagnetic insulator (x = 0) into a ferromagnetic metal (x = 0.07). A negative magnetoresistance (MR) of about ˜82% is obtained for x = 0.07 sample. Thermoelectric measurements show that charge carriers are predominantly electrons and a high value of thermopower is observed for pure sample, which goes on decreasing with increasing x, attributed to the increased carrier density in Ru doped samples. The magnetic moment at 7 T and 10 K for x = 0.07 is much lower than the expected value. It is proposed that short-range ferromagnetic clusters form for small Ru content which increase size with increasing Ru content and percolate in weak antiferromagnetic background that leads to metallic like resistivity in x = 0.07 sample. Unlike other magnetic transition ions, the greater ability of Ru to induce ferromagnetism and insulator-metal transition in otherwise robust charge-ordered composition $Bi_{0.5} Ca_{0.5} MnO_{3}$ is interesting from the point of view of fundamental physics and applications.