Room temperature magnetoresistance and cluster-glass behavior in the Tl2-xBixMn2O7 (0 <= x <= 0.5) pyrochlore series

In a novel Tl2-xBixMn2O7 (0 less than or equal to x less than or equal to 0 .5) pyrochlore series magnetoresistance (MR) dramatically increases with Bi substitution: A value of MR at 9 T of 10(4)% is observed for Tl1.8Bi0.2Mn2 O7 at 110 K; for Tl1.9Bi0.1Mn2O7 the room temperature MR (9 T) is 10(2)%. T hose values are unprecedented in pyrochlorelike manganese oxides, and open up the possibility of technological applications. A cluster-glass state dev elops, competing with ferromagnetism for x less than or equal to 0.1 and pr eventing long range ferromagnetic ordering for x greater than or equal to 0 .2. A new mechanism for MR at room temperature, associated with polaronic e ffects, is proposed. [S0031-9007(98)08158-7].