Hall resistivity and thermopower have been measured for the colossal magnetoresistance material ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ over wide temperature and magnetic-field ranges. These measurements revealed that a small number of free-electron-like carriers are responsible for the magnetotransport properties. In contrast to perovskite colossal-magnetoresistance (CMR) materials, the anomalous Hall effect is negligibly small even in the ferromagnetic state. An important property of ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ is that the carrier density changes with temperature and magnetic field. The carrier density increases near $T_C$ as the temperature is 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.

• Received 15 June 2000

DOI:https://doi.org/10.1103/PhysRevB.62.12190