Impulsive excitation of coherent magnons and phonons by subpicosecond laser pulses in the weak ferromagnetFeBO3

Abstract

Coherent magnons and phonons are excited by subpicosecond laser pulses in the weak ferromagnet ${\text{FeBO}}_3$. Impulsive stimulated Raman scattering (ISRS) 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 THz) 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 ${\text{FeBO}}_3$. 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.