Title: Temperature-dependent optical and magneto-optical spectra of ferromagnetic BCC Fe

Abstract: Optical and magneto-optical properties of magnetic materials have been widely exploited to characterize magnetic structures and phenomena, however, their temperature dependence is not well understood. This study implements the supercell approach with thermal lattice and magnetic disorders to obtain optical and magneto-optical spectra at finite temperatures based on Williams-Lax theory. Our results show that large optical spectrum signals are generated at photon energies below 1 eV, originating from the phonon- and magnon-assisted intraband transitions as lattice and magnetic temperatures increase. In addition, the prominent peak near 2.7 eV is redshifted proportionally to magnetic temperature but depends much less on lattice temperature. By analyzing unfolded bands, we show that the reduction of exchange splitting due to the thermal demagnetization causes this redshift. Our unfolded electronic band structure with magnetic disorder shows band kinks, which are characteristic evidence of the coupling between electrons and magnetic excitations. First-order magneto-optical spectra at finite temperature are also predicted, but due to their small magnitude suffer more from sampling errors. We discuss the effect of zero-point vibrations and the connection of these simulations to the Drude model for intraband transitions.