Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization

Abstract

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...

Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current

“…S2). This observation confirms theoretical predictions that the superdiffusive spin current saturates at high excitation powers 12,13 . Conversely, when we reduce the pump fluence to F≈1.3 mJ cm − 2 , we again observe an increase or decrease of magnetization in Fe due to superdiffusive spin transfer, though the effect is weaker than the response shown in Fig.…”

“…This first observation of an ultrafast magnetization enhancement in the Fe layer can be explained by superdiffusive spin transport 12,13 (see also Supplementary Discussion), taking place on timescales comparable to the demagnetization processes explored in earlier works [1][2][3]5 . The mechanism we propose for enhancement of the magnetization is based on filling of majority spin states above the Fermi energy in the Fe layer by majority spins coming from Ni.…”

“…Remarkably, we find that the femtosecond spin dynamics of the individual Ni and Fe layers depends on the relative alignment of their initial magnetization directions: ultrafast excitation and demagnetization of the Ni layer transiently enhances the magnetization of the buried Fe layer when their magnetizations are initially aligned parallel to each other, whereas the magnetization of the Fe layer decreases if its initial magnetization is antiparallel to that of Ni. These findings can be explained consistently by large laser-generated superdiffusive spin currents 12,13 . In this physical picture, superdiffusion of excited spin majority electrons from the Ni layer, through the Ru layer and into the buried Fe layer can transiently increase or decrease the magnetization in Fe depending on its relative spin alignment with respect to Ni (which can be controlled by the external magnetic field and the thickness of the Ru spacer layer).…”

“…Several competing processes have been proposed for magnetization dynamics on ultrafast timescales [5][6][7][8][9][10][11][12][13] . For the trilayer systems explored here, we find that the observed magnetization dynamics in the Ni and buried Fe layer is consistent with superdiffusion of Fig.…”

“…Most of these models are based on phonon-, electronand magnon-mediated spin-flip processes [5][6][7][8][9] , direct laser-induced spin-flips 10 or relativistic spin-light interaction 11 . Most recently, a new model based on superdiffusive spin transport has been proposed 12,13 .…”