Carrier density dependence of magnetoresistance in Tl2Mn2-xRuxO7 pyrochlores

The magnetotransport properties of the Tl2Mn2-xRuxO7 pyrochlore are studied . It is shown that Ru4+ substitution in the Mn4+ network provides a conveni ent way to modify the charge density in the Tl-O band without disrupting it with foreign cations. The experimental data reveal that the magnetoresista nce has a quadratic dependence on the magnetization and increases when the charge density in the Tl-O conduction band is reduced. Above the Curie temp erature, activated resistivity is found, signaling carrier localization and suggesting the existence of spin polarons. These results provide a solid s upport to recent theoretical predictions for magnetoresistance in low carri er density ferromagnets.