Long-wavelength magnetic and magnetoelectric excitations in the ferroelectric antiferromagnet BiFeO₃

We present a terahertz spectroscopic study of magnetic excitations in ferroelectric antiferromagnet BiFeO₃. We interpret the observed spectrum of long-wavelength magnetic resonance modes in terms of the normal modes of the material's cycloidal antiferromagnetic structure. We find that the modulated Dzyaloshinski-Moriya interaction leads to a splitting of the out-of-plane resonance modes. We also assign one of the observed absorption lines to an electromagnon excitation that results from the magnetoelectric coupling between the ferroelectric polarization and the cycloidal magnetic structure of BiFeO₃.