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Abstract
The cyclisation of a short chain into a ring provides fascinating scenarios in terms of transforming a finite array of spins into a quasi-infinite structure. If frustration is present, theory predicts interesting quantum critical points, where the ground state and thus low-temperature properties of a material change drastically upon even a small variation of appropriate external parameters. This can be visualised as achieving a very high and pointed summit where the way down has an infinity of possibilities, which by any parameter change will be rapidly chosen, in order to reach the final ground state. Here we report a mixed 3d/4f cyclic coordination cluster that turns out to be very near or even at such a quantum critical point. It has a ground state spin of S = 60, the largest ever observed for a molecule (120 times that of a single electron). [Fe10Gd10(Me-tea)10(Me-teaH)10(NO3)10]·20MeCN forms a nano-torus with alternating gadolinium and iron ions with a nearest neighbour Fe–Gd coupling and a frustrating next-nearest neighbour Fe–Fe coupling. Such a spin arrangement corresponds to a cyclic delta or saw-tooth chain, which can exhibit unusual frustration effects. In the present case, the quantum critical point bears a ‘flatland’ of tens of thousands of energetically degenerate states between which transitions are possible at no energy costs with profound caloric consequences. Entropy-wise the energy flatland translates into the pointed summit overlooking the entropy landscape. Going downhill several target states can be reached depending on the applied physical procedure which offers new prospects for addressability.
Highlights
Coordination clusters (CCs) constructed from aggregations of paramagnetic metal ions which are cooperatively coupled may exhibit molecular-based slow relaxation of magnetisation leading to bistability, hysteresis and quantum tunnelling effects characteristic of so-called single molecule magnets.[1]
This cyclic coordination cluster system is synthesised from racemic N,N-bis-(2-hydroxyethyl)amino-2-propanol (Me-teaH3), Fe(NO3)[3] and Gd(NO3)[3]
If one considers the exchange between adjacent Gd ions, it turns out to be virtually zero
Summary
Coordination clusters (CCs) constructed from aggregations of paramagnetic metal ions which are cooperatively coupled may exhibit molecular-based slow relaxation of magnetisation leading to bistability, hysteresis and quantum tunnelling effects characteristic of so-called single molecule magnets.[1] The intention is that such systems can be developed to provide information storage at a single molecule level giving a significantly enhanced capability for miniaturisation.[2] One goal along this line is to create large ground state spins that are stabilised by an easy-axis anisotropy and whose magnetic multiplet is well separated from higher-lying levels. Nature usually does not favour 'giant spins' since electrons are fermions that prefer to pair into nonmagnetic singlets. Amongst other reasons this explains why highspin molecules remain notoriously rare.
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