Strong coupling of antiferromagnetic resonance with sub-THz cavity fields

Abstract

We study coupling of electromagnetic waves to an antiferromagnetic resonance at THz frequencies [1,2]. In ferromagnetic materials, the Purcell effect and magnon-polaritons were shown [3], allowing single magnon detection [4] or observation of level attraction due to dissipative coupling [5]. Antiferromagnetic materials, in contrast to ferromagnets, are interesting due to their high-frequency dynamics, while they are still characterized by low coupling of spins to the lattice. For applicational reasons their abundance, huge number of existing materials and lack of stray fields are also advantages. However, in the case of antiferromagnets, there are only a few examples of an indirect strong coupling of spin waves with electromagnetic waves [6, 7] or at GHz frequencies [8]. Here, we report a direct and strong magnon-phonon coupling in high-temperature antiferromagnet hematite α-Fe2O3.A cube of hematite (0.2 mm edge length) was placed inside a gold-plated tube (2 mm long, 0.58 mm internal diameter). This cavity has the lowest mode at about 0.24 THz, as predicted by electrodynamic simulations. The cavity was placed inside a double-cone holder that allowed focusing THz beam into the cavity and collecting the transmitted THz beam form it. We measured transmission spectra in the 0.2-0.3 THz frequency band using a continuous wave spectrometer based on extenders to a vector network analyser. This system is characterized by very high dynamic range and very high frequency resolution.We used temperature as a tuning parameter, because the frequency of the antiferromagnetic resonance in hematite rises monotically with it from room temperature until about 700 K. Our data (Fig. 1) show very clear avoided crossing of the cavity mode and the antiferromagnetic resonance with the minimum splitting of about 6 GHz and the cooperativity factor of about 40. The splitting predicted by a microscopic model [3] is about 15 GHz. The difference is caused by imperfect matching of the cavity magnetic field with the magnetic moment of the antiferromagnetic resonance in the cube, that is confirmed by elecrodynamic simulations. We notice that we can tune the coupling between the experimental setup and the cavity that results in different lineshapes of the cavity mode. We fitted observed spectra with a model based on the input-output theory [3]. We also observed coupling with higher order modes of the cavity that is not as clear as with the lowest mode.We acknowledged support by the Sino-Swiss Science and Technology Cooperation (SSSTC) grant no. EG-CN_02_032019, EPFL and the SNF R'Equip under Grant No. 206021_144983. **