COLOSSAL MAGNETORESISTANCE WITHOUT MN3+ MN4+ DOUBLE EXCHANGE IN THE STOICHIOMETRIC PYROCHLORE TL2MN2O7/

Structural analysis from powder neutron and single-crystal x-ray diffr action data for a sample of the Tl2Mn2O7 pyrochlore, which exhibits co lossal magnetoresistance (CMR), shows no deviations from ideal stoichi ometry. This analysis gives an Mn-O distance of 1.90 angstroms, which is significantly shorter than the Mn-O distances (1.94 to 2.00 angstro ms) observed in phases based on LaMnO3 perovskites that exhibit CMR, B oth results in Tl2Mn2O7 indicate oxidation states very close to Tl-2 Mn24+O7. Thus, Tl2Mn2O7 has neither mixed valence for a double-exchan ge magnetic interaction nor a Jahn-Teller cation such as Mn3+, both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl2Mn2O7 based on magnetic ordering dr iven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.