The origin of second-order transverse magnetic anisotropy in Mn12-acetate

The problem of the role of transverse fields in Mn12-acetate, a molecular nanomagnet, is still open. We present structural evidences that the disorder of the acetic acid of crystallization indices sizeable distortion of the Mn(III) sites, giving rise to six different isomers, four of them with symmetry lower than tetragonal. Using a ligand field approach the effect of the structure modifications on the second order transverse magnetic anisotropy, forbidden in tetragonal symmetry, has been evaluated. The order of magnitude of the quadratic transverse anisotropies well agree with the values derived by the analysis of the field sweep dependence of the hysteresis loops performed by Mertes et al. (Phys. Rev. Lett 87, 227205 (2001)) and allows to better simulate the EPR spectra.