The Origin of Transverse Anisotropy in Axially Symmetric Single Molecule Magnets

Single-crystal high-frequency ESR spectroscopy was employed on a truly axial single mol. magnet [Mn12O12(tBuCH2CO2)16(CH3OH)4]·CH3OH to study the origin of the transverse magnetic anisotropy, a crucial parameter that rules the quantum tunneling of the magnetization. The crystal structure, including the abs. structure of the crystal used for EPR expts., was fully detd. and found to belong to I-4 tetragonal space group. The angular dependence of the resonance fields in the crystallog. ab plane shows high-order tetragonal anisotropy and strong dependence on the MS sublevels with the 2nd-highest-field transition being angular independent. This was rationalized including competing 4th- and 6th-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 exptl. results were further analyzed using a simplified multispin Hamiltonian which takes into account the exchange interactions and the single ion magnetic anisotropy of the MnIII centers. It was possible to establish magnetostructural correlations with spin Hamiltonian parameters up to the 6th order. Transverse anisotropy in axial single mol. magnets was found to originate from the multispin nature of the system and from the breakdown of the strong exchange approxn. The tilting of the single-ion easy axes of magnetization with respect to the 4-fold mol. axis of the cluster plays the major role in detg. the transverse anisotropy. Counterintuitively, the projections of the single ion easy axes on the ab plane correspond to hard axes of magnetization.