Non-exponential relaxation in a resonant quantum tunneling system of magnetic molecules

We have studied the low temperature relaxation behavior of the octanulcear iron molecular magnet, Fe8. For temperatures below 400mK we observed a crossover to a quantum tunneling regime. In this regime the relaxation is non-exponential and we argue that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows a square-root time dependence. A simple model is presented for a longer time range that is in good agreement with the data over 4 decades in time. Numerical calculations of the local field distribution indicate unusual correlation effects.