The temperature evolution of electron spin resonance is studied upon cooling the crystal samples of Cs${}_2$CuCl${}_4$ through the Néel 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 spectrum.

• Received 6 February 2012

DOI:https://doi.org/10.1103/PhysRevB.85.184423