Magnon-photon coupling in orthoferrites

We study the magnon-photon coupling at THz frequencies [1]. The Purcell effect and magnon-polaritons were shown in ferromagnetic materials [2, 3]. It is interesting to observe these interactions in antiferromagnetic materials due to their high-frequency magnetic resonances. We report on an magnon-phonon coupling in high-temperature antiferromagnets, bismuth ferrite BiFeO₃ (BFO) and dysprosium ferrite DyFeO₃ (DFO). We investigated polycrystaline samples of different thicknesses, thus having different sets of Fabry-Perrot-type cavity modes. We measured transmission spectra in 200-350 GHz frequency band at 300-600 K temperature range using frequency extenders to a vector network analyzer. Magnetic-resonance frequencies soften with rising temperature and cross with cavity modes. In BFO samples, we found that the width of the resonant line broadens at these crossings, consistent with the Purcell effect. In DFO samples, our data show avoided crossings of resonances with cavity modes, which is an indication of magnon-polaritons.

[1] D. L. Mills et al,, Rep. Prog. Phys. 37, 817 (1974).
[2] H. Huebl et al., Phys. Rev. Lett. 111, 127003 (2013).
[3] L. Bai et al., Phys. Rev. Lett. 114, 227201 (2015).