Spin Chirality Fluctuation and Anomalous Hall Effect in Itinerant Ferromagnets

The anomalous Hall effect due to the spin chirality order and fluctuation is studied theoretically in a Kondo lattice model without the relativistic spin-orbit interaction. Even without the correlations of the localized spins, $\sigma_{xy}$ can emerge depending on the lattice structure and the spin anisotropy. We reveal the condition for this chirality-fluctuation driven mechanism for $\sigma_{xy}$. Our semiquantitative estimates for a pyrochlore oxide Nd$_2$Mo$_2$O$_7$ give a finite $\sigma_{xy} \sim 10 \Ohm^{-1} \cm^{-1}$ together with a high resistivity $\rho_{xx} \sim 10^{-4}-10^{-3} \Ohm \cm$, in agreement with experiments.