STRUCTURAL AND MAGNETOTRANSPORT PROPERTIES OF THE COLOSSAL MAGNETORESISTANCE MATERIAL TL2MN2O7

The crystal structure and its variation with temperature of the spin-c harge coupled material Tl2Mn2O7 pyrochlore is investigated. Structure refinement from the neutron powder-diffraction data reveals that Tl2Mn 2O7 has nearly stoichiometric composition and is crystallized in the c ubic-pyrochlore-type structure with a characteristic Mn-O-Mn bond angl e of 133 degrees. There is no structural anomaly associated with the c hange in magnetotransport properties at 142 K, suggesting weak spin-la ttice and charge-lattice correlations. Despite the similarity of magne totransport properties observed in the perovskite and the pyrochlore c olossal magnetoresistance manganese oxides, the double-exchange mechan ism, which is usually applied to the perovskite materials, does not se em to explain the behavior in the pyrochlore Tl2Mn2O7 compound. A supe rexchange-type interaction may stabilize the ferromagnetic ordering of Mn spins and cause the metallic conduction at low temperatures in Tl2 Mn2O7.