Magnetic properties of an octanuclear iron(III) cation

Abstract

The magnetic properties of j[(tacn)6Fe8(M3-0)2(M2-0H)12]Br7(H20)}Br'H20, Fe8, tacn = 1,4,7-triazacyclononane, a molecule comprising eight iron(III) ions bridged by oxo and hydroxo groups, are reported. The magnetic susceptibility, both dc and ac, and the magnetization indicate that at low-temperature spin levels with 8 < S < 10 are populated. EPR confirms that levels of high spin multiplicity are populated at 4.2 K. For the first time an attempt is made to calculate the energy of the spin levels of such a large cluster within a spin Hamiltonian formalism. The 1 679 616 states originating from the coupling of eight S = s/2 spins are classified by using total spin and point group symmetry, and the Hamiltonian matrix is calculated using an irreducible tensor operator approach. In this way the susceptibility can be calculated, and satisfactory fits of the experimental magnetic susceptibility are achieved. The values of the required parameters compare well with those previously reported in analogous iron complexes.

Quantum Interference Oscillations of the Superparamagnetic Blocking in anFe8Molecular Nanomagnet

“…A schematic diagram of the magnetic structure of the Fe 8 molecular core [28,29] is shown in Fig. 5 ration, with a ground state S = 10 and a first excited S = 9 multiplet lying about δE 9,10 /k B = 44 K above it [28][29][30][31], follows from the fact that |J 4 | > |J 3 | (all couplings are antiferromagnetic). In addition to symmetric exchange interactions, one has to consider also single-ion magnetic anisotropies, dipolar interactions and antisymmetric Dzyaloshinskii-Moriya (DM) interactions, which mix states of different S [20].…”

“…5. This approxima- tion can be justified by the fact that exchange interactions in Fe 8 as well as in other SMMs have been treated by iteratively coupling spins in pairs [28,33]. The twospin Hamiltonian reads as follows…”

Thermally assisted resonant quantum tunneling of magnetization in Fe8 clusters

“…11 It was suggested that a possible source of tunneling in the Mn 12 system could be a small transverse magnetic field, perhaps of dipolar or hyperfine origin, 9 whereas the Fe 8 system is described by the spin Hamiltonian with the transverse anisotropy, which may require separate theoretical investigation. 10 In this paper we report on the magnetization and ac-susceptibility study on the 13 The materials used in this experiment are pure Fe 8 crystallites with an average size of 0.2ϫ0.1ϫ0.1 mm. Since it is impossible to measure only one crystallite on the Quantum Design superconducting quantum interference device ͑SQUID͒ magnetometer, a sample composed of aligned crystallites embedded in a nonmagnetic matrix was used.…”

La technique classique du rapport de flipping. Application aux aimants moléculaires et aux aimants photo-commutables

“…Il existe une grande variété de comportements magnétiques dans les systèmes où les ions de transition sont couplés par des ponts organiques. Par exemple, les clusters moléculaires magnétiques, qui sont en fait des molécules magnétiques de taille nanométrique, peuvent présenter un comportement classique, tout comme un comportement quantique, car leur taille est intermédiaire entre celle des simples molécules magnétiques et celle du matériau massif [3].…”

“…La délocalisation de spin est comme toujours surestimée par les calculs DFT dans les composés contenant du cuivre, mais l'alternance de spin sur le pont azide, due à la polarisation de spin, est en accord avec l'expérience. O], (avec tacn = 1,4,7-triazacyclono-nane) a attiré un fort intérêt dans ces dernières années en raison de l'observation de l'effet tunnel quantique de l'aimantation [3]. Une autre propriété remarquable est la forte anisotropie magnétique du cluster, bien que chaque ion Fe III (S = 5/2) soit individuellement isotrope.…”