INTERMEDIATE-VALENCE MODEL FOR THE COLOSSAL MAGNETORESISTANCE IN TL2MN2O7

The colossal magnetoresistance exhibited by Tl2Mn2O7 is an interesting phenomenon, as it is very similar to that found in perovskite mangane se oxides, although the compound differs both in its crystalline struc ture and electronic properties from the manganites. At the same time, other pyrochlore compounds, though sharing the same structure with Tl2 Mn2O7, do not exhibit the strong coupling between magnetism and transp ort properties found in this material. Mostly due to the absence of ev idence for significant doping into the Mn-O sublattice and the tendenc y of Tl to form conduction bands, the traditional double-exchange mech anism mentioned in connection with manganites does not seem suitable t o explain the experimental results in this case. We propose a model fo r Tl2Mn2O7 consisting of a lattice of intermediate-valence ions fluctu ating between two magnetic configurations, representing Mn 3d orbitals , hybridized with a conduction band, which we associate with Tl. This model had been proposed originally for the analysis of intermediate-va lence Tm compounds. With a simplified treatment of the model we obtain the electronic structure and transport properties of Tl2Mn2O7, With g ood qualitative agreement with experiments. The presence of a hybridiz ation gap in the density of states seems important to understand the r eported Hall data.