Magneto-optical wave mixing in Garnets M. Barthelemy1, M. Sanches Piaia1, H. Vonesh1, M. Vomir1, P. Molho2, B. Barbara2 and J.-Y. Bigot1 1. Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg CNRS, 23 rue du Loess, BP 43, 67034, Strasbourg, France. 2. Institut Louis Néel, CNRS - Université Joseph Fourrier, 25 avenue des Martyrs, BP 166, 38042 Grenoble, France.
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Abstract We report on magnetic field dependent four wave mixing signals emitted from an iron bismuth Garnet film. The coherent and population contributions to the magneto optical response are separately measured in a three beams configuration. Advantages of this technique over the commonly used pump probe magneto-optical configurations are discussed.
Introduction Since ultrafast demagnetization was observed [1], several approaches are used to explain the loss of angular momentum induced by ultrashort pulses in magnetic materials. One of them considers the coupling between photon and spin momenta giving rise to coherent magnetism. Previous experiments based on Kerr and Faraday pump probe experiments show that the coherent contribution plays a nonnegligible role in the demagnetization process [2]. On the other hand, the transfer of angular momentum between the orbital and spin momenta has been recently unraveled using femtosecond X-ray pulses [3]. In this work, we report on the magneto optical coherent emission measurement based on four wave mixing configuration.
Coherent magnetism The physical origin of ultrafast coherent magnetism can be understood by taking into account the spin photon interaction in the framework of short pulse excitation of a magnetic system. The following interaction Hamiltonian considers a single electron of mass m, charge e, momentum p and spin s:
Ó Springer International Publishing Switzerland 2015 J.-Y. Bigot et al. (eds.), Ultrafast Magnetism I, Springer Proceedings in Physics 159 DOI 10.1007/978-3-319-07743-7_67
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strong magneto optical response and its transparency at 800nm makes it a good candidate for degenerating four wave mixing measurements at this wavelength.
Fig. 2.. a) Population magneto optical contribution as a function of delay T for =0. Full symbols: =30° with a corresponding precession period Tp=162ps. Empty symbols: =150° , Tp=146ps. b) Coherent Magneto Optical contribution measured by varying the delay and setting T=500fs.
As shown in Figure 2, this experimental configuration allows a selective measurement of coherent and population contributions involved in the magnetization dynamics. Figure 2.a) illustrates the MOFWM signal S k1k2k3(,T) measured as a function of the delay T. The oscillations correspond to the precession of the magnetization around the effective magnetic field. The period of the oscillations depends on the magnetic field orientation. The double exponential decay is related to the population of spins involving the spin-lattice interaction. In contrast, the dynamical signals Sk1,k2,k3 measured as a function of with a fixed T=500fs for two opposite directions of the magnetic field =90° and =-90° gives information on the coherent spins dynamics. The difference Sk1,k2,k3(,T=500fs)= Sk1,k2,k3(,T=500fs)|H+ - Sk1,k2,k3(,T=500fs)|H- corresponds to the magneto optical coherent contribution as shown in Figure 2b). Details concerning the respective role of the coherent and population dynamics of the spins in the magneto-optical response will be reviewed and discussed.
Acknowledgments: This work was supported by the European Research Council, project ERC Advanced Grant ATOMAG (ERC-2009-AdG-20090325 247452).
References [1] E. Beaurepaire, J.-C. Merle, A. Daunois, & J.-Y. Bigot, “Ultrafast Spin Dynamics in Ferromagnetic Nickel” Phys. Rev. Lett. 76, 4250-4253 (1996). [2] J.-Y. Bigot, M. Vomir, & E. Beaurepaire, “Coherent ultrafast magnetism induced by femtosecond laser pulses” Nature Phys. 5, 515-520 (2009).
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[3] C. Boeglin, E. Beaurepaire, V. Halté, V. López-Flores, C. Stamm, N. Pontius, H. A. Dürr and J.-Y. Bigot, “Distinguishing the ultrafast dynamics of spin and orbital moments in solids” Nature 465, 458-461 (2010). [4] H. Vonesh and J.-Y. Bigot, “Ultrafast spin-photon interaction investigated with coherent magneto-optics” Phys. Rev. B 85, 180407(R) (2012).
