Electron spin resonance study of anisotropic interactions in a   two-dimensional spin gap magnet PHCC
  Fine details of the excitation spectrum of the two-dimensional spin-gap magnet PHCC are revealed by electron spin resonance investigations. The values of anisotropy parameters and the orientations of the anisotropy axes are determined by accurate measurements of the angular, frequency-field and temperature dependences of the resonance absorption. The properties of a spin-gap magnet in the vicinity of critical field are discussed in terms of sublevel splittings and g-factor anisotropy. 
V. N. Glazkov, T. S. Yankova, J. Sichelschmidt, D. H\"uvonen, A.   Zheludev
cond-mat.str-el