Polarization-dependent excitation of coherent spin precession by 150 fs linearly polarized laser pulses is observed in the easy-plane antiferromagnet FeBO 3 . We show that the mechanism of excitation is impulsive stimulated Raman scattering. This process is shown to be determined not only by the magnetooptical constants of the material, but also by the properties of the spin precession itself. Though carrying no angular momentum, the linearly polarized laser pulses act on the spins as effective fields that can be considered as an ultrafast inverse Cotton-Mouton effect.
The nflection of light from a medium with ordered spin smclure characterized by the breakdown of time-reversal and parity s y " e v y is exgected to be non-reciprocal even if the net magnetic moment of the medium equals Zem. We repoa on the fust experimental observation of sponmwnk non-recipmcal mtarion and circular dichroism in spin magnetoelectric Cr203. Nokrecipmcal effects were obmved M o w the antifemmagnetic tansition temperahlre Tw = M7 U and their temperahre behaviour roughly obrresponds to that of the order parameter. Observed values of (1 -4) x for the magnetoelecuic susceptibility in the optical range are several orden of magnitude higher than predicted earlier. This increase of the susceptibility is presumably atuibutable lo eleclmnic dipole transitions in the optical range. tutroductionAmong different optical phenomena in crystals a particular type of effect may be assigned to the so-cklled non-reciprocal (NR) effects. They are characterized by different phase velocities and/or attenuations for light waves travelling via the same optical path but in opposite directions. The most typical exaniples of these effects are the Faraday rotation observed in transmission and the Ken effect observed in reflection (see e.g. [l]). As far as we know, up until now non-reciprocal optical effects were observed exclusively in media possessing a magnetic moment. This moment can be induced by a magnetic field in dia-and paramagnets or can arise spontaneously, ai in feqoor ferrimagnets. A magnetic moment induced by an electric current may also give rise to non-recipmcal phenomena [2]. In all these cases the timereverd symmetry (1') is broken; that is, the crystal may be in two different states converted into one another by the operation 1' .There exists a special class of magnetically ordered materials in which below the magnetic transition temperature TN there is no net magnetic moment, but in addition to the timereversal symmetry breaking the parity symmetry (7) is also broken. At the same time the combined symmetry operation i' is retained. Magnetoelectrics (ME) are the most well known and widely studied materials [ 3 4 belonging to this class. Soon after the discovery of ME, theoretical analysis of light propagation in magnetoelectric antiferromagnets showed 17-10] that new optical phenomena should be found, when the spatial dispersion is taken into account. These phenomena, though beiig in some manifestations similar to those observed in media with a net magnetic moment, may exhibit some important differences. Some of these effects have been observed iecently in "ission in 1111. Since these new phenomena are related to the spatid~dispepion they have smaller values as compared with
Coherent magnons and phonons are excited by subpicosecond laser pulses in the weak ferromagnet FeBO 3 . Impulsive stimulated Raman scattering ͑ISRS͒ is proven to be the microscopic mechanism of the excitation. It is shown that coherent magnons can be excited by both linearly and circularly polarized laser pulses where the efficiency of the process depends on the mutual orientation of the magnetic and crystallographic axes and the light propagation direction. The strong ellipticity of the ferromagnetic magnon mode is demonstrated, both experimentally and theoretically, to be essential for the excitation and observation of such coherent magnons. Because of this ellipticity, the amplitude of the coherent magnons excited by linearly polarized light may exceed by 2 orders of magnitude the amplitude of those excited by circularly polarized light. The primary difference between the excitation of coherent magnons by linearly polarized pulses via ISRS and via the earlier reported process of photoinduced magnetic anisotropy is discussed. Furthermore, the ISRS process is found to be responsible for the excitation of two optical phonon branches ͑8.4 and 12.1 THz͒ observed in our experiments. A coherent excitation, with a temperature-independent frequency of 0.7 THz, has also been observed in the magnetically ordered phase but could not be assigned to any optical phonon modes known in FeBO 3 . The well-pronounced dependence of the amplitude of this mode on temperature suggests that this mode of nonmagnetic origin becomes Raman active only in the magnetically ordered phase and, therefore, can be excited and observed only below the Néel temperature.
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