Relaxation time of the nanomagnet Fe4

The magnetic behaviour of the molecular nanomagnet Fe4 is very well simulated by a single spin model Hamiltonian in a crystal field with S=5. The crystal field parameters were determined from the inelastic neutron scattering (INS) spectra. Here we show that the quantum effects are crucial to understand the saturation of the relaxation time observed at very low temperature at variance with the standard master equation result that leads to an Arrhenius law at any temperature. Very deep downward spikes in correspondence of the anticrossing fields are found in the relaxation time versus field at low temperature. We compare our results with those obtained by previous approaches worked out to fit experimental data on Mn12.