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

Electron paramagnetic resonance (EPR) studies of a Ni₄ single-molecule magnet (SMM) yield the zero-field-splitting (ZFS) parameters D, B₄⁰, and B₄⁴, based on the giant spin approximation (GSA) with $S=4$; B₄⁴ is responsible for the magnetization tunneling in this SMM. Experiments on an isostructural Ni-doped Zn₄ crystal establish the NiII ion ZFS parameters. The fourth-order ZFS parameters in the GSA arise from the interplay between the Heisenberg interaction J ̂ \^s₁∙ ̂ \^s₂ and the second-order single-ion anisotropy, giving rise to mixing of higher-lying S≠4 states into the $S=4$ state. Consequently, J directly influences the ZFS in the ground state, enabling its determination by EPR.