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

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.

Slow Magnetic Relaxation in Chiral Helicene-Based Coordination Complex of Dysprosium

“…The room temperature value is 13.96 cm 3 •K•mol −1 in good agreement with the expected value of 14.17 cm 3 •K•mol −1 for an isolated Dy III ion [42]. Upon cooling, χMT decreases monotonically down to 11.20 cm 3 •K•mol −1 due to the thermal depopulation of the MJ states. Below 5 K, the more rapid decrease could be attributed to the presence of weak dipolar antiferromagnetic interactions as determined by quantum calculations on the analogue complex involving hfac − ancillary ligands [27].…”

“…The room temperature value is 13.96 cm 3 •K•mol −1 in good agreement with the expected value of 14.17 cm 3 •K•mol −1 for an isolated Dy III ion [42]. Upon cooling, χMT decreases monotonically down to 11.20 …”

“…Relaxation times on the order of few seconds is slow enough to observe the opening of the hysteresis loop at 0.50 K (Figure 5) which remains opened at higher temperatures ( Figure S6). One must mention that the hysteresis loop of the racemic form of [Dy(L)(hfac) 3 (Figure 4c). It must be mentioned that at moderate fields at least two relaxation processes coexist which merge into one at higher fields than 800 Oe (Figure 4b).…”

“…The design of single molecule magnet (SMM), with the aim of enhancing its peculiar magnetic properties, has been a prolific field in the scientific community for decades [1][2][3]. Indeed, SMMs can pave the way towards a new generation of materials as, for example, molecular qubits for quantum computing [4], memory storage devices [5] or spin valves [6].…”

“…A first example of the coupling between a Dy III -based SMM and a chiral antenna has been reported recently with the complex [Dy(L)(hfac) 3 ] with L = 3-(2-pyridyl)-4-aza [6]-helicene and hfac = 1,1,1,5,5,5-hexafluoroacetylacetone [27]. The Dy III ion, with its 6 H 15/2 ground state, easily leads to Ising type of magnetic anisotropy in coordination spheres like N 2 O 6 and this is achieved with the common bidentate 2,2 -bipyridine (bpy) ligand and three hfac − ligands [13,[28][29][30].…”