Large Enhancement of Magnetoresistance in Tl ₂ Mn ₂ O ₇ : Pyrochlore Versus Perovskite

The synthesis of and magneto-transport in the “colossal magnetoresistance” (CMR) pyrochlore-structure compounds Tl 2 − x Sc x Mn 2 O 7 for 0 ≤ x ≤ 0.5 are reported. The maximum resistivity (ρ) increases by 10 5 while the Curie temperature ( T c ) decreases by only 25% between x = 0 and 0.4. For x = 0.2 to 0.3, the magnetoresistance at both high field (6 tesla) and low field (<0.05 tesla) are significantly enhanced over the undoped ( x = 0) compound. Structural analysis indicates that all of the manganese ions are near the +4 oxidation state. The behavior of ρ and T c suggests that scandium substitution affects mainly electronic conduction on the thallium-oxygen sublattice while leaving the magnetism essentially unchanged. These results demonstrate a distinct way of manipulating magneto-transport properties in CMR materials.