Magnetization tunneling in high-symmetry single-molecule magnets: Limitations of the giant spin approximation

Electron paramagnetic resonance (EPR) studies of a Ni-4 single-molecule magnet (SMM) yield the zero-field-splitting (ZFS) parameters D, B-4(0), and B-4(4), based on the giant spin approximation (GSA) with S=4; B-4(4) is responsible for the magnetization tunneling in this SMM. Experiments on an isostructural Ni-doped Zn-4 crystal establish the Ni-II ion ZFS parameters. The fourth-order ZFS parameters in the GSA arise from the interplay between the Heisenberg interaction Js(1)center dot s(2) and the second-order single-ion anisotropy, giving rise to mixing of higher-lying S not equal 4 states into the S=4 state. Consequently, J directly influences the ZFS in the ground state, enabling its determination by EPR.