Explaining the paradoxical diversity of ultrafast laser-induced demagnetization

Abstract

Pulsed-laser-induced quenching of ferromagnetic order has intrigued researchers since pioneering works in the 1990s. It was reported that demagnetization in gadolinium proceeds within 100 ps, but three orders of magnitude faster in ferromagnetic transition metals such as nickel. Here we show that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing. Our interpretation is supported by ab initio estimates of the spin-flip scattering probability, and experimental fluence dependencies are shown to agree perfectly with predictions. A phase diagram is constructed in which two classes of laser-induced magnetization dynamics can be distinguished, where the ratio of the Curie temperature to the atomic magnetic moment turns out to have a crucial role. We conclude that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.

Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current

“…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.…”

“…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.…”

“…A number of distinct models have been proposed to describe how laser excitation can couple so quickly to the spins, given that the light itself does not directly exert a significant torque on the electron spins. 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 .…”

Laser-induced ultrafast demagnetization in the presence of a nanoscale magnetic domain network

“…3b). The thermalization time can also be characterized 11 by the delay time τ M at which the drop in magnetization reaches 63% of its maximum. The values of τ th and τ M obtained for the different infrared pump fluence levels are summarized in Table 1.…”

“…Despite some controversy about the validity of MOKE as a probe of the magnetization of an excited system on the femtosecond timescale [4][5][6][7] , significant progress has been made: the importance of subtle effects, such as direct transfer of spin angular momentum 8 or coherent magneto-optical processes 9 , have been revealed. Phenomenological and microscopic models have been proposed and further refined [10][11][12][13] .…”