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We study coupling of electromagnetic waves to magnetization waves at THz frequencies [1]. The Purcell effect and magnon-polaritons are intensively studied in ferromagnets at microwave frequencies [2]. Antiferromagnets are interesting due to their high-frequency magnetization dynamics, however, there are only a few examples of weak [3] or indirect [4] magnon-photon coupling in that materials at THz frequencies. We report strong magnon-phonon coupling in the high-temperature antiferromagnet hematite α-Fe₂O₃. A cube of hematite was placed inside a 3-dimensional cavity of a mode at about 0.24 THz. We measured transmission though the cavity in 0.2-0.3 THz band at above room temperature. Frequency of the antiferromagnetic resonance of hematite rises with temperature and interacts with the cavity mode, showing a very clear avoided crossing of about 6 GHz splitting and the cooperativity factor of about 50.

[1] D. L. Mills et al, Rep. Prog. Phys. 37, 817 (1974).
[2] Y. S. Gui et al, Phys. Rev. Lett. 95, 056807 (2005).
[3] M. Bialek et al, Phys. Rev. B 101, 024405 (2020).
[4] X. Li et al, Science 361, 794 (2018).