Ab initio theory of magnetization induced by light absorption in ferromagnets

Abstract

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. To face the continuing demand for large density and energy efficient data storage devices, the possibilities of magnetiza-tion manipulation without using a magnetic field are widely being investigated [1]. One of the promising candidates is the all-optical helicity-dependent switching (AO-HDS), as it allows the control of the magnetization state by only using circularly left (σ +) or right (σ −) polarized light pulses of a few tens of femtoseconds. AO-HDS has been observed in a wide range of materials such as GdFeCo ferrimagnetic alloys [2], rare earth-transition metal alloys and multilayers [3,4], and FePt L1 0 granular media [5] which is considered promising for ultrahigh-density storage devices. This outstanding diversity of materials suggests a common underlying mechanism, although it remains debated. To explain the AO-HDS, the two theoretical explanations usually invoked in the literature are the inverse Faraday effect (IFE) [2,4-6] and a difference of absorption induced by the magnetic circular dichroism (MCD) [7,8]. While the IFE was first introduced to describe the influence of the presence of a circularly polarized light on the magnetic state of transparent media [9], Battiato et al. showed that, without any assumption on the nature of the material, light generates a static contribution at the second-order perturbation in the density matrix [10,11], which they hold responsible for the IFE in lossy media. However, in this approach [10] the repopulation at the origin of the IFE does not grow linearly with time, as it would be the case for an absorption-related phenomenon, and it fades away after the perturbation has been switched off. This fact leaves a gap between the IFE and the mechanism involved in the irreversible change of magnetization leading to the AO-HDS phenomenon [12]. Then, using this formulation and density functional theory, Berritta et al. [13] computed the value of this contribution for different types of materials. Conversely , the second effect, due to MCD, relies on a difference of absorption inducing a different temperature depending on * [email protected] the relative orientation of the magnetization and the helicity of the light. Through this thermal effect, the switching probability depends on the magnetization orientation, as shown by several parametrized models [7,8]. Moreover, its probabilistic and absorption-based nature is in agreement with the fact that the AO-HDS phenomenon is cumulative, i.e., it requires multiple pulses [4], as well as a large absorptivity of the compound.