Carrier density change in the colossal-magnetoresistance pyrochlore Tl2Mn2O7

Hall resistivity and thermopower have been measured for the colossal magnet oresistance material Tl2Mn2O7 over wide temperature and magnetic-field rang es. These measurements revealed that a small number of free-electron-like c arriers are responsible for the magnetotransport properties. In contrast to perovskite colassal-magnetoresistance (CMR) materials, the anomalous Hall effect is negligibly small even in the ferromagnetic state. An important pr operty of Tl2Mn2O7 is that the carrier density changes with temperature and magnetic field. The carrier density increases near Te as the temperature i s lowered or as the magnetic field is increased, which explains the CMR of this material. The conduction-band edge shift caused by the development of magnetization is a possible mechanism causing the carrier density change.