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 I4̄ 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 have been
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 has been possible to establish magnetostructural correlations with spin Hamiltonian parameters up to the sixth order. Transverse anisotropy in
axial single molecule magnets was found to originate from the multispin nature of the system and from the
breakdown of the strong exchange approximation. The tilting of the single-ion easy axes of magnetization
with respect to the 4-fold molecular axis of the cluster plays the major role in determining the transverse
anisotropy. Counterintuitively, the projections of the single ion easy axes on the ab plane correspond to
hard axes of magnetization.