Functional (Supra)Molecular Nanostructures - ruben-group

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Habilitation Dr. Mario Ruben ULP Strasbourg 6.2. Supramolecular Metamagnets (in collaboration with Prof. P. Müller, University of Erlangen, Germany) In view of their structure, antiferro- or ferromagnetically coupled ring-like transition metal complexes are widely regarded as finite equivalents for 1D infinite magnetic chains, implying that physical concepts found for these chains should describe them too. Even-numbered, AF- coupled wheels are characterized by an S=0 spin ground state and previous magnetic studies revealed that [Co II 4L 1 4] with L= E or F (see Figure 17), which can be also considered ring- like molecules with n=4, behave exactly as an intramolecular antiferromagnetic (AF) exchange coupled molecular system with a S=0 ground state. [26] However, theoretical work has shown that in such systems excited magnetic states seem to be easily accessible. [27] Additionally, the even-numbered ring-like AF-coupled molecular complexes might show large quantum effects, such as tunnelling of the Néel vector [28] or quantum coherence phenomena. [29] Furthermore, very similar ring-like systems form a suitable series of objects to investigate the origins of magnetic anisotropy, partly because of their high symmetry. Figure 20. Molecular structure of the grid-like [2x2] [Co II 4 E4] complex (b) and the field dependence of the magnetic moment of a single crystal at 1.9 K for magnetic fields along the main axes showing the anisotropy in the magnetic saturation (a). The inset displays the magnetization curve of the powder sample. Further magnetic studies of the Co II 4L4 systems as function of temperature confirmed the general presence of intramolecular antiferromagnetic (AF) exchange interaction 31
Habilitation Dr. Mario Ruben ULP Strasbourg independent of the substitution pattern of the involved ligands L. Subsequently recorded magnetisation curves on single crystals of [Co II 4E4](BF4)8 at 1.9 K revealed the following anisotropic situation: For magnetic fields perpendicular to the grid plane, the magnetization shows a behaviour reminiscent of a thermally broadened magnetization step due to a ground- state level crossing, as it is often observed in antiferromagnetic clusters. [30] In contrast, the magnetic moments for applied fields within the plane of the Co II 4-wheel, increase linearly with equal slope as function of the magnetic field (Figure 20). No long-range ordering develops as intermolecular interactions are negligibly small preventing the system from undergoing a metamagnetic phase transition. However, the analysis of its magnetism can be completely recast in the language of metamagnetism, and in this sense it was proposed to call them examples for single-molecule metamagnets. [31] 6.3. Single Ion Molecular Magnets (SIMMs) (in collaboration with Prof. P. Müller, University of Erlangen, Germany) An exciting development in nanoscale magnetic materials occurred in 1993 when [Mn12O12(O2CMe)16(H2O)4] (hereafter Mn12) was identified as a nanoscale magnet, [32] the first to comprise discrete, magnetically non-interacting molecular units rather than a 3D extended lattice (as in metals and metal oxides, for example). The discovery initiated the field of molecular nanomagnetism and such molecules have since been termed single-molecule magnets (SMMs). They derive their special magnetic properties from the combination of a large spin (S) and an Ising (easy-axis) magneto-anisotropy exhibiting a negative zero-field splitting parameter (D), which gives rise to the superparamagnetic–like property of a barrier to magnetization relaxation (Figure 21). This class of compounds does not only display magnetisation hysteresis, but also quantum tunnelling of magnetisation (QTM) [33] and quantum phase interference. [34] Usually, SMMs, like Mn12 or Fe8, consist of several magnetic ions coupled by strong intramolecular exchange interactions, resulting in a high spin ground state. [35] Although considerable progress has been made in the synthesis of high-spin systems involving d-metal ions, restricted anisotropy terms have prevented, at least so far, from higher blocking temperatures. [36] Since 4f-lanthanide ions posses, in comparison to 3d-metal ions, a much larger intrinsic anisotropy, they are candidates for achieving larger barrier heights. However, large transverse anisotropy terms increase the tunnel splittings and shortcut the barrier resulting in reduced effective heights. [37] Recently, Ishikawa et. al. reported on AC- 32
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