First-principles study of lattice and magnetic temperature effects on the optical properties of ferromagnetic BCC Fe

Pump-probe magneto-optical spectrum measurements of magnetic materials have been utilized to investigate the (de)magnetization process and spin dynamics via the temperature-dependent optical response. First-principles density functional theory has successfully predicted the frequency-dependent dielectric function at 0K and recently studied the introduction of the lattice temperature using phonon dispersion. In this work, we develop an analogous approach based on atomistic spin dynamics to compute optical spectra at finite magnetic temperatures. We found that the imaginary part of the averaged diagonal component of optical conductivity at a low photon energy range grows drastically as both lattice and spin temperatures increase, which is induced by a huge change in matrix elements of low-energy optical transitions. A peak near 3 eV in the imaginary part of diagonal optical conductivity redshifts as the lattice and spin temperatures rise, explaining the discrepancy of the peak position between 0 K and measurement at 300 K. Spin temperature induced shifts contribute about four times more than shifts due to lattice temperature. Based on the result, this work might be expanded to the optical properties calculation of paramagnets or antiferromagnets.