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Coherent ultrafast magnetism induced by femtosecond laser pulses

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Abstract

The quest for ultrafast magnetic processes has triggered a new field of research—femtomagnetism: using femtosecond laser pulses to demagnetize ferromagnetic metallic thin films. Despite being the subject of intense research for over a decade, the underlying mechanisms that govern the demagnetization remain unclear. Here, we investigate how an ultrashort laser pulse couples to the spin of electrons in ferromagnetic metals. It is shown that a single 50-fs laser pulse couples efficiently to a ferromagnetic film during its own propagation. This result indicates that the material polarization induced by the photon field interacts coherently with the spins. The corresponding mechanism has its origin in relativistic quantum electrodynamics, beyond the spin–orbit interaction involving the ionic potential. In addition, this coherent interaction is clearly distinguished from the incoherent ultrafast demagnetization associated with the thermalization of the spins. We forecast that the corresponding coherent self-induced processes are the dawn of a new era for future research in magnetism.

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Figure 1: Sequence of mechanisms in ultrafast magnetization dynamics.
Figure 2: Single-femtosecond-pulse Faraday experiment.
Figure 3: Coherent electronic and magnetic responses of ferromagnetic films.
Figure 4: Distinction between the coherent and incoherent magnetic processes.

Change history

  • 16 June 2009

    In the version of this article originally published online, corrections were needed to: Fig. 1, right-hand Gel label should read Gsl; Fig. 2b,c, the 0 label on the x axes should read 1; Fig. 3a,c, the legend for the red line should read 0°. These corrections have now been made in all versions of the article.

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Acknowledgements

We thank I. Perakis for helpful discussions on coherent magnetic effects in semiconductors, and J. Arabski and M. Albrecht for their technical support. This project was carried on thanks to the financial support of the Centre National de la Recherche Scientifique in France.

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J.Y.B. and M.V. designed and carried out the experiments, analysed the data and wrote the paper. E.B. supervised the samples elaboration and characterization.

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Correspondence to Jean-Yves Bigot.

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Bigot, JY., Vomir, M. & Beaurepaire, E. Coherent ultrafast magnetism induced by femtosecond laser pulses. Nature Phys 5, 515–520 (2009). https://doi.org/10.1038/nphys1285

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