Investigation of electronic spin dynamics in the giant Keplerat molecule Fe30Mo72 by 57Fe Moessbauer spectroscopy and magnetisation measurements

In the frustrated polyoxomolybdate nanomolecule Fe30Mo72, 30 Fe(III) ions (S=5/2) are located on the vertices of an icosidodecahedron coupled via nearest neighbor antiferromagnetic interactions J0. However, recent ac susceptibility measurements by Schroeder et al. show that the interactions are non-isotropic and can be described by a random distribution around J0. We performed 57Fe Moessbauer spectroscopy on this system down to 2 K. Dynamic magnetic hyperfine spectra with magnetically inequivalent sites appear below 6 K. We conclude slow paramagnetic relaxation in agreement with dc magnetisation measurements. Furthermore, the magnetic field dependence at low temperatures (T=50 mK) has been studied by ac susceptibility measurements. No magnetisation steps could be observed as predicted by the quantum rotational band model. The implications of our results on this low temperature quantum model are discussed.