Resonant quantum tunneling of the magnetization beyond the giant spin approximation

We have measured the field dependence of the AC magnetic susceptibility of a single crystal of the molecular nanomagnet Fe-8. Accelerations in the magnetic relaxation are observed at the field values corresponding to levels crossings. Other anomalies are observed around 1.5 and 3.5 kOe. We propose that they are due to a resonant tunnel mechanism involving the first excited spin state. With a simplified two spins model we find that a sizeable tunnel splitting is calculated at anti-crossing between states belonging to S = 10 and 9 multiplets.