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Distinguishing the ultrafast dynamics of spin and orbital moments in solids

Abstract

For an isolated quantum particle, such as an electron, the orbital (L) and spin (S) magnetic moments can change provided that the total angular momentum of the particle is conserved. In condensed matter, an efficient transfer between L and S can occur owing to the spin–orbit interaction, which originates in the relativistic motion of electrons1. Disentangling the absolute contributions of the orbital and spin angular momenta is challenging, however, as any transfer between the two occurs on femtosecond timescales. Here we investigate such phenomena by using ultrashort optical laser pulses to change the magnetization of a ferromagnetic film2,3,4,5,6,7 and then probe its dynamics with circularly polarized femtosecond X-ray pulses8. Our measurements enable us to disentangle the spin and orbital components of the magnetic moment, revealing different dynamics for L and S. We highlight the important role played by the spin–orbit interaction in the ultrafast laser-induced demagnetization of ferromagnetic films, and show also that the magneto-crystalline anisotropy energy is an important quantity to consider in such processes. Our study provides insights into the dynamics in magnetic systems9 as well as perspectives for the ultrafast control of information in magnetic recording media10.

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Figure 1: Geometry of the pump–probe experiment.
Figure 2: Static energy resolved X-ray absorption spectra of CoPd film using circularly polarized light.
Figure 3: Femtosecond evolution of the magnetic and electronic states.
Figure 4: Femtosecond evolution of the magnetic spin and orbital moments.

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Acknowledgements

We thank T. Quast, T. Kachel, K. Holldack and R. Mitzner for help and support during the femtoslicing experiments and J. Arabski, M. Acosta, M. Albrecht and V. Da Costa for sample elaboration and characterization. This work was supported by the CNRS–PICS, by Université de Strasbourg and by the EU Contract Integrated Infrastructure Initiative I3 in FP6-Project No. R II 3 CT-2004-5060008, BESSY IA-SFS Access Program. J.-Y.B. acknowledges a grant from the European Research Council in the final stage of this work (ERC-2009-AdG-20090325 #247452). H.A.D. acknowledges support through the PULSE Institute at SLAC by the US Department of Energy, Office of Basic Energy Sciences.

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C.B., E.B., V.H., V.L.-F. and J.-Y.B designed, carried out the experiments and performed the data analysis. C.S., N.P. and H.A.D. developed the femtoslicing experiment. C.B. and J.-Y.B. wrote the paper. All the authors read and improved the manuscript.

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Correspondence to C. Boeglin.

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The authors declare no competing financial interests.

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Boeglin, C., Beaurepaire, E., Halté, V. et al. Distinguishing the ultrafast dynamics of spin and orbital moments in solids. Nature 465, 458–461 (2010). https://doi.org/10.1038/nature09070

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