Room Temperature Magnetoresistance and Cluster-Glass Behavior in theTl2−xBixMn2O7(0≤x≤0.5)Pyrochlore Series

In a novel ${\mathrm{Tl}}_{2-x}{\mathrm{Bi}}_x{\mathrm{Mn}}_2{\mathrm{O}}_7$ ( $0\le x\le 0.5$) pyrochlore series magnetoresistance (MR) dramatically increases with Bi substitution: A value of MR at 9 T of ${10}^4\%$ is observed for ${\mathrm{Tl}}_{1.8}{\mathrm{Bi}}_{0.2}{\mathrm{Mn}}_2{\mathrm{O}}_7$ at 110 K; for ${\mathrm{Tl}}_{1.9}{\mathrm{Bi}}_{0.1}{\mathrm{Mn}}_2{\mathrm{O}}_7$ the room temperature MR (9 T) is ${10}^2\%$. Those values are unprecedented in pyrochlorelike manganese oxides, and open up the possibility of technological applications. A cluster-glass state develops, competing with ferromagnetism for $x\le 0.1$ and preventing long range ferromagnetic ordering for $x\ge 0.2$. A new mechanism for MR at room temperature, associated with polaronic effects, is proposed.