Design of Single-Molecule Magnets: Insufficiency of the Anisotropy Barrier as the Sole Criterion

TLDR

The Orbach relaxation mechanism dominates only when the excited ligand‑field state lies below the Debye temperature, a condition rarely met in molecular crystals, which limits the use of the anisotropy barrier as a design criterion for high‑temperature single‑molecule magnets. The study calls for new design criteria that account for alternative relaxation pathways beyond the conventional double‑well model. Experimental spectroscopy and relaxation measurements show that the Orbach activation barrier (≈38 cm⁻¹) is far smaller than the 500 cm⁻¹ energy gap to the first excited doublet, proving that Orbach relaxation is not the dominant mechanism and that optimizing only the anisotropy barrier is insufficient for designing high‑temperature single‑molecule magnets.

Abstract

Determination of the electronic energy spectrum of a trigonal-symmetry mononuclear Yb(3+) single-molecule magnet (SMM) by high-resolution 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.