High spin cycles: topping the spin record for a single molecule verging on quantum criticality

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. A single molecule is synthesised with a very large spin state. Molecules that have large spin states have several potential applications such as providing contrast agents for magnetic resonance imaging, however, high-spin molecules are fairly rare because spins have a tendency of pairing up into non-magnetic singlets. An international team of researchers led by Jürgen Schnack from Bielefeld University and Annie Powell from Karlsruhe Institute of Technology now demonstrates a type of molecular magnet that has a ground state spin of S=60, which is 120 times that of a single electron. The mixed 3d/4f cyclic coordination cluster forms a complex nano-torus, whose arrangement gives rise to unusual forms of magnetic frustration, pushing this system towards a quantum critical point that has tens of thousands of energetically degenerate states.