Terahertz magneto-optical response in ferromagnetic Fe-Co-Al alloys

We study the magneto-optical properties of Fe-Co-Al ordered alloys in the terahertz range of frequencies. Using the standard Kubo-based approach to compute intrinsic part of the $\sigma_{xy}(\omega)$ we find a strong dependence of $\sigma_{xy}$ on $\omega$ in the terahertz range. For example, we find that below 10~THz Co$_3$Al has nearly constant $\sigma_{xy}$ and that above 10~THz it is reduced by about 50 times. Furthermore, we find a strong dependence of $\sigma_{xy}$ on the chemical composition. For example, we find that the addition of Al to Fe changes the sign of $\sigma_{xy}$, while the addition of Co to Fe leads to a nonmonotonic dependence of $\sigma_{xy}$ on Co concentration.