Colossal Magnetoresistance Without Mn 3+ /Mn 4+ Double Exchange in the Stoichiometric Pyrochlore Tl 2 Mn 2 O 7

Abstract

Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl₂Mn₂O₇ pyrochlore, which exhibits colossal magnetoresistance (CMR), shows no deviations from ideal stoichiometry. 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 angstroms) observed in phases based on LaMnO₃ perovskites that exhibit CMR. Both results in Tl₂Mn₂O₇ indicate oxidation states very close to Tl³⁺₂Mn⁴⁺₂O₇. Thus, Tl₂Mn₂O₇ has neither mixed valence for a double-exchange 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 driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.