A comparative EPR study of high- and low-spin Mn-6 single-molecule magnets

We report detailed numerical and spectroscopic studies of two complexes from a family of recently discovered Mn-6(III) single-molecule magnets (SMMs) with large barriers to magnetization reversal. These complexes consist of a pair of Mn-3(III) triangles with a ferromagnetic interaction between the triangles. Recent studies have shown that the exchange interactions within the triangular Mn-3(III) units can be switched from antiferromagnetic to ferromagnetic, resulting in a switching of the spin from S = 4 to 12. This strategy to "increase S" has resulted in the highest magnetic energy barrier and blocking temperature for any known SMM to date. Extensive frequency, temperature and field-orientation dependent single-crystal high-frequency electron paramagnetic resonance measurements have been performed to determine the spin-Hamiltonian parameters associated with the lowest-lying spin multiplet for each complex. We compare the experimental findings with numerical calculations, where the total anisotropy for a complex is determined in terms of single-ion anisotropies using both projection operator techniques and exact matrix diagonalization methods. In particular, we find that the product of the molecular anisotropy, D, and spin, S, does not change significantly upon switching from S = 4 to 12, i.e. D goes down as S goes up. These studies provide important insights concerning strategies for designing SMMs with higher blocking temperatures, particularly for complexes containing manganese in its +3 oxidation state. (C) 2008 Elsevier Ltd. All rights reserved.