title: 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.
authors.affiliation: †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.|‡CNRS, CRPP, UPR 8641, F-33600 Pessac, France.|§CNRS, ICMCB, UPR 9014, F-33600 Pessac, France.
authors.forename: Kasper S
authors.initials: KS
authors.lastname: Pedersen
authors.affiliation: ∥Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.|⊥Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.
authors.forename: Jan
authors.initials: J
authors.lastname: Dreiser
authors.affiliation: †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Høgni
authors.initials: H
authors.lastname: Weihe
authors.affiliation: @Laboratory for Developments and Methods, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.
authors.forename: Romain
authors.initials: R
authors.lastname: Sibille
authors.affiliation: †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Heini V
authors.initials: HV
authors.lastname: Johannesen
authors.affiliation: †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Mikkel A
authors.initials: MA
authors.lastname: Sørensen
authors.affiliation: †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Bjarne E
authors.initials: BE
authors.lastname: Nielsen
authors.affiliation: †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.|#Institut Laue-Langevin, Grenoble, France.
authors.forename: Marc
authors.initials: M
authors.lastname: Sigrist
authors.affiliation: #Institut Laue-Langevin, Grenoble, France.
authors.forename: Hannu
authors.initials: H
authors.lastname: Mutka
authors.affiliation: #Institut Laue-Langevin, Grenoble, France.
authors.forename: Stephane
authors.initials: S
authors.lastname: Rols
authors.affiliation: †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Jesper
authors.initials: J
authors.lastname: Bendix
authors.affiliation: †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Stergios
authors.initials: S
authors.lastname: Piligkos