Ab initio theory of magnetization induced by light absorption in ferromagnets

We use density functional theory to quantitatively compute the effect of light absorption in ferromagnetic materials. We show that, in the presence of spin-orbit coupling, optically induced transitions do not conserve the magnetization and that a systematic induced demagnetization, whose magnitude depends on both the helicity of the light and the direction of the magnetization, is observed. Very differently from the inverse Faraday effect, this mechanism is due to the absorption of light and depends on the magnetic state of each atom, and therefore cannot be described by an effective optomagnetic field. Then, based on these results, we derive a set of parameters which can be used in micromagnetic simulations in order to account for light transition effects on the magnetization dynamics.