About twenty years ago, E. Beaurepaire and coworkers observed that femtosecond laser pulses trigger the ultrafast loss of spin polarization in a ferromagnetic metal . From that time to present, this phenomenon has sparked many attempts to reveal the underlying mechanism of the fastest spin dynamics ever seen. The outcomes have been several controversial hypotheses such as superdiffusive transport , phonon-assisted Elliot-Yafet spin flip , electronmagnon scattering , dynamic exchange splitting , etc. Yet none of them can thoroughly explain all experimental observations.
In this framework, we investigated the ultrafast demagnetization of Fe using time-resolved broad-band ellipsometry with different laser pulse regimes (sub 15 fs and 100 fs duration). To elucidate the genuine spin dynamic, we disentangled the electronic and spin responses from the extrapolated conductivity tensor. Our experimental evidence suggests that ultrafast demagnetization is essentially a consequence of collective excitations.
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