LARGE ENHANCEMENT OF MAGNETORESISTANCE IN TL2MN2O7 - PYROCHLORE VERSUS PEROVSKITE

The synthesis of and magneto-transport in the ''colossal magnetoresist ance'' (CMR) pyrochlore-structure compounds Tl2-xScxMn2O7 for 0 less t han or equal to less than or equal to 0.5 are reported. The maximum re sistivity (rho) increases by 10(5) while the Curie temperature (T-c) d ecreases by only 25% between x = 0 and 0.4. For x = 0.2 to 0.3, the ma gnetoresistance at both high field (6 tesla) and low field (<0.05 tesl a) are significantly enhanced over the undoped (x = 0) compound. Struc tural analysis indicates that all of the manganese ions are near the 4 oxidation state. The behavior of rho and T-c suggests that scandium substitution affects mainly electronic conduction on the thallium-oxyg en sublattice while leaving the magnetism essentially unchanged. These results demonstrate a distinct way of manipulating magneto-transport properties in CMR materials.