The magnetotransport properties of the ${\mathrm{Tl}}_2{\mathrm{Mn}}_{2-x}{\mathrm{Ru}}_x{\mathrm{O}}_7$ pyrochlore are studied. It is shown that ${\mathrm{Ru}}^{4+}$ substitution in the ${\mathrm{Mn}}^{4+}$ network provides a convenient way to modify the charge density in the Tl-O band without disrupting it with foreign cations. The experimental data reveal that the magnetoresistance has a quadratic dependence on the magnetization and increases when the charge density in the Tl-O conduction band is reduced. Above the Curie temperature, activated resistivity is found, signaling carrier localization and suggesting the existence of spin polarons. These results provide a solid support to recent theoretical predictions for magnetoresistance in low carrier density ferromagnets.

• Received 21 December 1998

DOI:https://doi.org/10.1103/PhysRevLett.83.2022