Design of Single-Molecule Magnets: Insufficiency of the Anisotropy Barrier as the Sole Criterion
Résumé
Determination of the electronic energy spectrum of a trigonalsymmetry mononuclear Yb3+ single-molecule magnet (SMM) by highresolution
absorption and luminescence spectroscopies reveals that the first excited electronic doublet is placed nearly 500 cm−1 above the ground one.
Fitting of the paramagnetic relaxation times of this SMM to a thermally activated (Orbach) model {τ = τ0 × exp[ΔOrbach/(kBT)]} affords an activation
barrier, ΔOrbach, of only 38 cm−1. This result is incompatible with the spectroscopic observations. Thus, we unambiguously demonstrate, solely on
the basis of experimental data, that Orbach relaxation cannot a priori be considered as the main mechanism determining the spin dynamics of SMMs.
This study highlights the fact that the general synthetic approach of optimizing SMM behavior by maximization of the anisotropy barrier, intimately linked to
the ligand field, as the sole parameter to be tuned, is insufficient because of the complete neglect of the interaction of the magnetic moment of the molecule with its environment. The Orbach mechanism is expected dominant only in the cases in which the energy of the excited ligand
field state is below the Debye temperature, which is typically low for molecular crystals and, thus, prevents the use of the
anisotropy barrier as a design criterion for the realization of high-temperature SMMs. Therefore, consideration of additional
design criteria that address the presence of alternative relaxation processes beyond the traditional double-well picture is required.