Magnetic resonance in the ordered phases of the 2D frustrated quantum   magnet Cs2CuCl4
  The temperature evolution of the electron spin resonance is studied at cooling the crystal samples of Cs2CuCl4 through the Neel point 0.62 K. A coexistence of the high-frequency spinon type resonance developed in the spin-liquid phase and of the low-frequency antiferromagnetic resonance was found in the ordered phase. The low-frequency magnetic resonance spectrum in the low field range has two gapped branches and corresponds well to the spectrum of spin excitations of a planar spiral spin structure with two axes of the anisotropy. The field induced phase transitions result in a more complicated low-frequency spectra. 
A. I. Smirnov, K. Yu. Povarov, S. V. Petrov, A. Ya. Shapiro
cond-mat.str-el