The spin chirality induced anomalous Hall effect in pyrochlore ferromagnets

Temperature and magnetic field dependences of the anomalous Hall effect have been investigated for single crystals of Mo-based ferromagnets with pyrochlore structure. The Hall resistivity of Nd₂Mo₂O₇ compound shows unconventional temperature dependence whereas Gd₂Mo₂O₇ exhibits rather normal behaviour. The Berry phase model can explain the difference well; it is attributed to the difference in nature of the anisotropy of the rare-earth moment and the resultant Mo spin state. The Hall resistivity of Nd₂Mo₂O₇ changes its sign with increasing field applied along the [111] direction, while it monotonically approaches zero with the field applied along the [100] or [110] direction. This behaviour is also in accord with the prediction of the Berry phase theory.