THz spectroscopy in the pseudo-Kagome system Cu3Bi(SeO3)2O2Br
  Terahertz (THz) transmission spectra have been measured as function of temperature and magnetic field on single crystals of Cu3Bi(SeO3)2O2Br. In the time-domain THz spectra without magnetic field, two resonance absorptions are observed below the magnetic ordering temperature T_N~27.4 K. The corresponding resonance frequencies increase with decreasing temperature and reach energies of 1.28 and 1.23 meV at 3.5 K. Multi-frequency electron spin resonance transmission spectra as a function of applied magnetic field show the field dependence of four magnetic resonance modes, which can be modeled as a ferromagnetic resonance including demagnetization and anisotropy effects. 
Zhe Wang, M. Schmidt, Y. Goncharov, V. Tsurkan, H.-A. Krug von Nidda,   A. Loidl, and J. Deisenhofer
cond-mat.str-el