Inelastic Neutron Scattering and Magnetic Susceptibilities of the Single-Molecule Magnets [Mn₄O₃X(OAc)₃(dbm)₃] (X = Br, Cl, OAc, and F):  Variation of the Anisotropy along the Series

Abstract

The single-molecule magnets (SMMs) [Mn4O3X(OAc)3(dbm)3] (X = Br, Cl, OAc, and F) were investigated by a detailed inelastic neutron scattering (INS) study. Up to four magnetic excitations between the zero-field split levels of the lowest S = 9/2 cluster ground-state have been resolved. From the determined energy-level diagrams and the relative INS intensities we can show that the inclusion of a rhombic term in the zero-field splitting (ZFS) Hamiltonian is essential in these compounds. On the basis of the Hamiltonian: Ĥ ZFS = D[ − 1/3 S(S + 1)] + E( − ) + , the following sets of parameters are derived: For X = Cl: D = −0.529 cm-1, |E| = 0.022 cm-1, and = −6.5 × 10-5 cm-1; for X = Br: D = −0.502 cm-1, |E| = 0.017 cm-1, and = −5.1 × 10-5 cm-1; for X = OAc: D = −0.469 cm-1, |E| = 0.017 cm-1, and = −7.9 × 10-5 cm-1; and for X = F: D = −0.379 cm-1 and = −11.1 × 10-5 cm-1. The wave functions derived from the energy analysis are in excellent agreement with the relative intensities of the observed INS transitions. The observed temperature maxima of the out-of-phase component of the variable frequency AC magnetic susceptibility T max[χ‘ ‘] correlate very well with the energy splittings determined by INS. Direct information about the rate of quantum tunneling is contained in the cluster wave functions derived in this study. The difference in the quantum tunneling between X = Cl and Br is shown to be directly related to differences in the rhombic anisotropy parameter |E|.

Correlation effects in the electronic structure of theMn4molecular magnet

“…Also, we calculated the exchange parameters J ij in the Heisenberg spin Hamiltonian H = i,j J ij S i · S j (where S i are the spins of Mn ions), assuming spin configuration where Mn 3+ spins are directed up, and the Mn 4+ spin is directed down. The calculated couplings between Mn 4+ and Mn 3+ are -29, -14, and -12 K, and the couplings within the Mn 3+ triangle are about 75 K, which is in qualitative agreement with the previously reported exchanges [19,20]. The exact diagonalization of the Heisenberg exchange Hamiltonian yields the correct S = 9/2 ground state of Mn 4 , but the excited spin states are not reproduced correctly.…”

“…The present work is, to our knowledge, the first example of applying the LDA+CDMFT method to large systems with very low molecular symmetry, containing 80 atoms per unit cell, establishing feasibility of such calculations and reliability of results. Moreover, the LDA+CDMFT method allows studies of the electronic structure of Mn 4 at finite temperatures.The family of Mn 4 SMMs [3,19,20,21] includes different compounds which have structurally similar cores made of four Mn ions located in the corners of distorted tetrahedron, but possess different ligands, and …”

“…The family of Mn 4 SMMs [3,19,20,21] includes different compounds which have structurally similar cores made of four Mn ions located in the corners of distorted tetrahedron, but possess different ligands, and…”

A Zig‐Zag [MnII4] Cluster from a Novel Bis(β‐diketonate) Ligand

“…Interestingly, each chelating pocket binds a metal ion disposed in cis configuration with respect to its counterpart from the other ligand. This is presumably because the ionic radius of Mn II is too large to Tetranuclear manganese clusters are interesting from the magnetic point of view, [26] or as synthetic models of the dioxygen-evolving complex of photosystem II. [27] Of 4+ cation, [28,29] the [Mn 2 IIIMn 2 II (Me-hmp) 6 Cl 4 ] cluster [Me-hmpH = 2-(hydroxymethyl)-6-methylpyridine], [30] [HL = a (2-hydroxyphenyl)bipyridine or -phenanthroline ligand].…”

Diffusion inélastique des neutrons sur les aimants moléculaires

“…Les propriétés intrinsèques comme la valeur du spin S des ions, leurs connectivités et nucléarités y sont parfaitement contrôlées. 6 Si les interactions dipolaires à longue portée sont suffisamment faibles, on peut observer le comportement d'un seul de ces agrégats moléculaires magnétique indépendamment des molécules voisines. 7 Les stratégies de synthèse consistent à fournir des degrés de liberté de coordination mais aussi des éléments limitateurs qui vont "borner" les possibilités de réaction dans des limites raisonnables et souhaitées.…”