Determination of the single-ion anisotropy energy in a S = 5/2 kagome   antiferromagnet using X-ray absorption spectroscopy
  We report x-ray absorption and x-ray linear dichroism measurements at the Fe L2,3 edges of the geometrically frustrated systems of potassium and hydronium iron jarosite. Comparison with simulated spectra, involving ligand-field multiplet calculations modelling the 3d-2p hybridization between the iron ion and the oxygen ligands, has yielded accurate estimates for the ligand metal-ion hybridization and the resulting single-ion crystal field anisotropy energy. Using this method we provide an experimentally verified scenario for the appearance of a single-ion anisotropy in this nominally high-spin 3d5 orbital singlet 6S system, which accounts for features of the spin-wave dispersion in the long-range ordered ground state of potassium iron jarosite. 
M. A. de Vries, T. K. Johal, A. Mirone, J. S. Claydon, G. J. Nilsen,   H. M. Ronnow, G. van der Laan and A. Harrison