We present the first materials specific ab initio theory of the magnetization induced by circularly polarized laser light in metals. Our calculations are based on non-linear density matrix theory and include the effect of absorption. We show that the induced magnetization, commonly referred to as inverse Faraday effect, is strongly materials and frequency dependent, and demonstrate the existence of both spin and orbital induced magnetizations which exhibit a surprisingly different behavior. We show that for nonmagnetic metals (as Cu, Au, Pd, Pt) and antiferromagnetic metals the induced magnetization is antisymmetric in the light's helicity, whereas for ferromagnetic metals (Fe, Co, Ni, FePt) the imparted magnetization is only asymmetric in the helicity. We compute effective optomagnetic fields that correspond to the induced magnetizations and provide guidelines for achieving all-optical helicity-dependent switching.PACS numbers: 78.20. Ls, 75.70.Tj, 75.60.Jk All-optical helicity-dependent magnetization switching has recently emerged as a promising way to manipulate and ultimately control the magnetization in a magnetic material using ultrashort optical laser pulses [1][2][3][4][5][6][7]. As femtosecond optical laser pulses are the shortest stimuli known to mankind, all-optical helicity-dependent switching offers novel options to achieve magnetization reversal at a hitherto unprecedented speed. The action of a circularly polarized laser pulse on the magnetization of a material was at first observed for an antiferromagnetic 3d-metal oxide [1] and later magnetization reversal was demonstrated in a ferrimagnetic rare-earth transitionmetal alloy [2,8]. Importantly, recent work demonstrated that all-optical helicity-dependent switching is not limited to a special class of materials, but can be achieved in a broader variety of material classes, including metallic multilayers, synthetic ferrimagnets [6], and even ferromagnets such as FePt [7], which is the prime candidate material for future ultradense magnetic recording [9].While these discoveries exemplify that ultrafast magnetization reversal driven by circularly polarized laser pulses could soon revolutionize magnetic recording its underlying physical mechanism is poorly understood. The influence of the circularly polarized laser pulse has been attributed to the inverse Faraday effect (IFE) [1][2][3]7], which was discovered fifty years ago [10]. The IFE is an optomagnetic counterpart of the magneto-optical Faraday effect, that is, the circularly polarized laser light imparts a magnetization in the material which exerts a torque on the pre-existing magnetization and assists the magnetization switching. However, although various models for the IFE have been proposed [11][12][13][14][15][16][17] there does as yet not exist any knowledge as to how the induced magnetization, or optomagnetic field arises, and even less is known about the materials dependence of the IFE. As materials specific theory is lacking it is neither known for which materials large effects are pr...

Starting from the Dirac-Kohn-Sham equation we derive the relativistic equation of motion of spin angular momentum in a magnetic solid under an external electromagnetic field. This equation of motion can be rewritten in the form of the well-known Landau-Lifshitz-Gilbert equation for a harmonic external magnetic field, and leads to a more general magnetization dynamics equation for a general time-dependent magnetic field. In both cases with an electronic spin-relaxation term which stems from the spin-orbit interaction. We thus rigorously derive, from fundamental principles, a general expression for the anisotropic damping tensor which is shown to contain an isotropic Gilbert contribution as well as an anisotropic Ising-like and a chiral, Dzyaloshinskii-Moriya-like contribution. The expression for the spin relaxation tensor comprises furthermore both electronic interband and intraband transitions. We also show that when the externally applied electromagnetic field possesses spin angular momentum, this will lead to an optical spin torque exerted on the spin moment.

Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

The influence of possible magnetic inertia effects has recently drawn attention in ultrafast magnetization dynamics and switching. Here we derive rigorously a description of inertia in the Landau-Lifshitz-Gilbert equation on the basis of the Dirac-Kohn-Sham framework. Using the FoldyWouthuysen transformation up to the order of 1/c 4 gives the intrinsic inertia of a pure system through the 2 nd order time-derivative of magnetization in the dynamical equation of motion. Thus, the inertial damping I is a higher order spin-orbit coupling effect, ∼ 1/c 4 , as compared to the Gilbert damping Γ that is of order 1/c 2 . Inertia is therefore expected to play a role only on ultrashort timescales (sub-picoseconds). We also show that the Gilbert damping and inertial damping are related to one another through the imaginary and real parts of the magnetic susceptibility tensor respectively.

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