Possible Observation of Electromagnons in BiFeO3 Crystal

We show the first optical observation via inelastic light scattering of sharp low energy modes in multiferroic BiFeO$_{3}$. We have detected two different modes ($\Psi $ and $\Phi $ modes) and interpreted them as the optical fingerprint of the cycloidal magnetic order. The energies of the $\Phi $ modes are equally spaced and exhibit a linear dependence as a function of the mode index (n) down to zero while the energy sequence of the $\Psi $ modes is not equally spaced and exhibit a gap at low energy. Our experimental data are in remarkable agreement with the theory of de Sousa and Moore who pointed out that these modes could possibly be observed due to coupling between spins and lattice degrees of freedom. These experimental findings offer a unique opportunity for optical probing of the electromagnetic excitations of multiferroics. [1] Cazayous et al., Phys. Rev. Lett. 101, 097003 (2008).