Molecular Magnets/Antiferromagnets

   Though the research field "molecular magnets" is rather new unique physical properties were discovered. Our Bielefeld group was able to synthesize a rather large number of related compounds based on polyoxometalate chemistry (see e.g., D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, Oxford University Press, Oxford 2006). Those on the following cover picture (P. Kögerler, B. Tsukerblat, A. Müller, first issue of Dalton Transactions 2010) are especially interesting, e.g. because of the frustration effects.


Two of those will be shortly described

1. Spin ball FeIII30MoVI72 and related M'30M72 (M' = V, Cr, Fe; M = Mo, W) clusters

The FeIII30MoVI72 is a spherical antiferromagnet. Especially interesting is the geometry of the 30 Fe(III) ions as they define the icosidodecahedron, a unique Archimedean solid having all edges equal (Figure). The cluster is an example of a zerodimensional system that shows magnetic ordering at low T. The linking of FeIII triangles causes fascinating frustrations comparable to those of the well known Kagomé lattices. This is of interest for materials scientists as the cluster can be considered as a model for the mentioned lattices (see also I. Rousochatzakis et al., "Highly frustrated magnetic clusters: The kagomé on a sphere", Phys. Rev. B 77, 2008, 094420:1-22 and S. K. Pati, C. N. R. Rao, Chem. Commun. 2008, 4683). Fe30Mo72 has three spin sublattices (Figure). Quite a number of papers were published about the magnetic properties of the cluster which is considered in the literature as seminal (see publications on the topic) Corresponding clusters with CrIII, VO2+ have also been investigated.

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The 30 FeIII centers describe the shown icosidodecahedron. There are three groups ("sublattices") of 10 spins (colors: red, blue, green), with all spins of a sublattice pointing in the same direction, while nearest neighbor spin vectors (three are highlighted) differ in angular orientation by 120.

  2. VIV15

Though the cluster shows a rather high symmetric distribution of 15 magnetic VIV ions (ground state = 1/2), it has layers of different magnetization and generally speaking interesting related properties (see Figure taken from the above mentioned book of Gatteschi et al.). Because of the rather large size and low spin state (decreasing dipolar interactions) it was considered in the literature as a candidate for the observation of Quantum Oscillations (V. V. Dobrovitski et al., "Mechanisms of decoherence in weakly anisotropic molecular magnets" Phys. Rev. Lett. 2000, 84, 3458). A related paper was published by us showing the option for the use of the cluster for the construction of a quantum computer (S. Bertaina, S. Gambarelli, T. Mitra, B. Tsukerblat, A. Müller, B. Barbara, "Quantum oscillations in a molecular magnet", Nature, 2008, 453, 203). Ca. 60 papers appeared worldwide about the magnetic properties of the cluster (see publications on the topic).


Simplified geometry and showing the layers of different magnetization