We propose a semi-classical model for femtosecond-laser induced demagnetization due to spinpolarized excited electron diffusion in the super-diffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200 fs in Ni, without the need to invoke any angular momentum dissipation channel.Excitation with femtosecond laser pulses is known for more than a decade to cause an ultrafast quenching of the magnetization in metallic ferromagnets [1]. The achieved demagnetization times are typically 100-300 fs for ferromagnets such as Ni [1,2]. Hence, laser-induced demagnetization opens up new, interesting routes for magnetic recording with hitherto unprecedented speeds [3]. However, in spite of the technological importance the mechanism underlying the femtosecond demagnetization remains highly controversial. A common belief is that there should exist an ultrafast channel for the dissipation of spin angular momentum [4][5][6][7][8]. Several such mechanisms through which an excited electron can undergo a spinflip in a ferromagnetic metal are currently being debated. The main proposed mechanisms for a fast spin-flip process are a Stoner excitation, an inelastic magnon scattering, an Elliott-Yafet-type of phonon scattering [4,5], spin-flip Coulomb scattering [6], laser-induced spin-flips [7], or relativistic quantum electrodynamic processes [8]. An effect that, until recently [9], has been regarded to play only a marginal role is the spin-polarized transport of laser-excited hot electrons.In this Letter we show that spin-dependent transport of laser-excited electrons provides a considerable contribution to the ultrafast demagnetization process and can even completely explain it. We demonstrate this by developing a transport equation for the super-diffusive flow of spin-polarized electrons. A few approaches to describe the electron motion have been attempted previously [10,11]. In our theory, however, we take into account the whole process of multiple, spin-conserving electron scattering events and electron cascades created by inelastic electron scattering. Also the presence of different metallic films in the probed material is treated. We solve the developed theory numerically for ferromagnetic Ni, for which the femtosecond demagnetization is well documented [1,2,12], and show that a large demagnetization in a few hundred femtoseconds is generated.The typical geometry for a femtosecond laser experiment is depicted in Fig. 1. The intense laser beam creates excited hot electrons in the ferromagnetic film, which will start to move in a random direction. Our goal is to compute the time-dependent magnetization resulting from the super-diffusive motion in the laser spot. Due to the fact that the electronic mean-free-path (up to a few tens of nm) is much smaller tha...

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The spin-flip (SF) Eliashberg function is calculated from first principles for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization. This is used to compute the SF probability and demagnetization rate for laser-created thermalized as well as nonequilibrium electron distributions. Increased SF probabilities are found for thermalized electrons, but the induced demagnetization rate is extremely small. A larger demagnetization rate is obtained for nonequilibrium electron distributions, but its contribution is too small to account for femtosecond demagnetization.

The switching of rare-earth-based ferrimagnets triggered by thermal excitation is investigated on the basis of an atomistic spin model beyond the rigid-spin approximation, distinguishing magnetic moments due to electrons in d and f orbitals of the rare earth. It is shown that after excitation of the conduction electrons a transient ferromagneticlike state follows from a dissipationless spin dynamics where energy and angular momentum are distributed between the two sublattices. The final relaxation can then lead to a new state with the magnetization switched with respect to the initial state. The time scale of the switching event is to a large extent determined by the exchange interaction between the two sublattices.

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...

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The Heisenberg–Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale.

Ultrafast laser excitation of a metal causes correlated, highly nonequilibrium dynamics of electronic and ionic degrees of freedom, which are however only poorly captured by the widely-used twotemperature model. Here we develop an out-of-equilibrium theory that captures the full dynamic evolution of the electronic and phononic populations and provides a microscopic description of the transfer of energy delivered optically into electrons to the lattice. All essential nonequilibrium energy processes, such as electron-phonon and phonon-phonon interactions are taken into account. Moreover, as all required quantities are obtained from first-principles calculations, the model gives an exact description of the relaxation dynamics without the need for fitted parameters. We apply the model to FePt and show that the detailed relaxation is out-of-equilibrium for picoseconds.