High Frequency EPR Spectra of [Fe8O2(OH)12(tacn)6]Br8. A Critical   Appraisal of the Barrier for the Reorientation of the Magnetization in Single   Molecule Magnets
  A detailed multifrequency high field - high frequency EPR (95-285 GHz) study has been performed on the single-molecule magnet of formula   [Fe8O2(OH)12(tacn)6]Br8 9H2O, where tacn=1,4,7-triazacyclononane. Polycrystalline powder spectra have allowed estimating the zero field splitting parameters up to the fourth order terms.   The single crystal spectra have provided the principal directions of the magnetic anisotropy of the cluster. These results have been compared to an evaluation of the intra-cluster dipolar contribution to the magnetic anisotropy, suggesting that single ion anisotropy is mainly contributing to the magnetic anisotropy.   The role of the transverse magnetic anisotropy in determining the height of the barrier for the reversal of the magnetization is also discussed. 
Anne Laure Barra, Dante Gatteschi, Roberta Sessoli