Experimental observation of the optical spin–orbit torque

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Electrical and optical control of magnetization are of central importance in the research and applications of spintronics. Non-relativistic angular momentum transfer or relativistic spin–orbit coupling provide efficient means by which electrical current driven through a ferromagnet can exert a torque on the magnetization. Ferromagnetic semiconductors like (Ga,Mn)As are suitable model systems with which to search for optical counterparts of these phenomena, where photocarriers excited by a laser pulse exert torque upon magnetization. Here, we report the observation of an optical spin–orbit torque (OSOT) in (Ga,Mn)As. The phenomenon originates from spin–orbit coupling of non-equilibrium photocarriers excitated by helicity-independent pump laser pulses, which do not impart angular momentum. In our measurements of the time-dependent magnetization trajectories, the signatures of OSOT are clearly distinct from the competing thermal excitation mechanism, and OSOT can even dominate in (Ga,Mn)As materials with appropriately controlled micromagnetic parameters.

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Figure 1: Schematic illustration and experimental observation of OSOT.
Figure 2: Direct experimental reconstruction of the magnetization trajectory from MO signals.
Figure 3: Characterization of 3% Mn-doped (Ga,Mn)As.
Figure 4: Suppression of the damping and thermal excitation mechanism by magnetic field.
Figure 5: Suppression of the thermal excitation mechanism at higher Mn dopings.


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The authors acknowledge discussions with A.H. MacDonald, J. Sinova and J. Wunderlich, and support from the European Research Council (advanced grant no. 268066), Framework Programme 7 (grant no. 215368), the Engineering and Physical Sciences Research Council (grant no. EP/H029257/1), the Ministry of Education of the Czech Republic (grant no. LM2011026), the Grant Agency of the Czech Republic (grant nos. 202/09/H041 and P204/12/0853), the Charles University in Prague (grant nos. SVV-2012-265306 and 443011) and the Academy of Sciences of the Czech Republic Praemium Academiae.

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V.N. and K.O. prepared the samples. N.T., P.N., E.R., T.Ja., D.B., P.M., K.O. and T.Ju. conducted the experiments and analysed the measured data. P.N. and F.T. performed data modelling. J.Z. and T.Ju. developed the microscopic theory and performed the calculations. P.N. and T.Ju wrote the manuscript.

Correspondence to P. Němec.

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Tesařová, N., Němec, P., Rozkotová, E. et al. Experimental observation of the optical spin–orbit torque. Nature Photon 7, 492–498 (2013) doi:10.1038/nphoton.2013.76

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