Optical probe for anomalous Hall resonance in ferromagnets with spin chirality.

We have investigated the infrared optical Hall conductivity, sigma(xy)(omega) for band-filling-controlled ferromagnetic crystals of Nd2Mo2O7, revealing the dynamical properties of their anomalous Hall effect (AHE). A resonant structure and its systematic filling dependence were observed in the Hall conductivity spectra in the midinfrared region (typically at 0.1 eV), while similar effects were not discerned in the diagonal (longitudinal or ordinary) conductivity spectra. This property of sigma(xy)(omega) provides crucial and essential information to understand the microscopic mechanism of AHE including its dc limit. Specifically, the interband transition at the magnetic-monopole-like band-anticrossing point, which is split by spin chirality, is the dominant source in AHE.