Low-energy spin dynamics in the giant keplerate molecule { Mo72 Fe30 }: A muon spin relaxation and H1 NMR investigation

We present muon spin relaxation and H1 NMR results on the magnetically frustrated keplerate molecule { Mo72 Fe30 }, aimed at studying the local spin dynamics as a function of temperature. We find that, in common with other molecular magnets, the relaxation spectrum of this material is characterized by a single dominating electronic correlation time τ. Experiments and theory show that τ has a thermally activated behavior with an activation gap of the order of the energy difference between the lowest-lying rotational bands in this material. This shows that, in the intermediate temperature range just above 1 K, the relaxation of the electronic spin system occurs via Orbach processes involving energy levels belonging to the two lowest-lying rotational bands. Our data thus provide an experimental estimate of the first interband gap.