The Origin of Transverse Anisotropy in Axially Symmetric Single Molecule Magnets

Single-crystal high-frequency electron paramagnetic resonance spectroscopy has been employed on a truly axial single molecule magnet of formula [Mn12O12(tBu-CH2CO2)16(CH3OH)4]·CH3OH to investigate the origin of the transverse magnetic anisotropy, a crucial parameter that rules the quantum tunneling of the magnetization. The crystal structure, including the absolute structure of the crystal used for EPR experiments, has been fully determined and found to belong to I4 tetragonal space group. The angular dependence of the resonance fields in the crystallographic ab plane shows the presence of high-order tetragonal anisotropy and strong dependence on the MS sublevels with the second-highest-field transition being angular independent. This was rationalized including competing fourth- and sixth-order transverse parameters in a giant spin Hamiltonian which describes the magnetic anisotropy in the ground S = 10 spin state of the cluster. To establish the origin of these anisotropy terms, the experimental results...