Low-energy spin dynamics in the giant keplerate molecule{Mo72Fe30}: A muon spin relaxation andH1NMR investigation

Abstract

We present muon spin relaxation and $^{1}H$ NMR results on the magnetically frustrated keplerate molecule ${{\mathrm{Mo}}_{72}{\mathrm{Fe}}_{30}}$, 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 $\ensuremath{\tau}$. Experiments and theory show that $\ensuremath{\tau}$ 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 $1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, 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.