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 L_2,3 edges of the geometrically frustrated systems of potassium and hydronium iron jarosite. Comparison with simulated spectra, involving ligand-field multiplet calculations modeling 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.