Electronic spin dynamics and static order in coupled magnetic keplerates Fe30Mo72

In the polyoxomolybdate nanomolecule Fe30Mo7230 S=5/2 FeIII ions are located on the vertices of an icosidodecahedron coupled via nearest-neighbor antiferromagnetic interactions J. In the paramagnetic molecules the electronic ground state and excitations are governed by the strong magnetic frustration on the Kagome-like surface. A modified system with much stronger inter-molecular exchange interaction shows several anomalies in the magnetic susceptibility below 5 K. We performed 57Fe-Moessbauer spectroscopy on the isolated and magnetically interacting molecules. Around 4.45 K (broad maximum in χ(T)) dynamic magnetic hyperfine spectra with magnetically inequivalent sites appear. Around 2.45 K a drop in χ(T) is accompanied by a slowing down of hyperfine dynamics and indicates antiferromagnetic order