Low-energy spin dynamics in the giant keplerate molecule Mo72Fe30: A muon spin relaxation and 1H NMR investigation

We present muon spin relaxation and 1H NMR results on the magnetically frustrated keplerate molecule Mo72Fe30, 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 t. Experiments and theory show that t 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.