Quantum Tunneling of the Magnetization in Mn12-ac

We present new experiments on the zero-field magnetic relaxation in a single crystal of Mn12-acetate. This study shows the important role played by the interactions between the spins and their environment. It suggests the existence of two relaxation regimes. Above 2.0 K, the relaxation time follows the Arrhenius law. The time decay is not strictly exponential due to dipolar interactions. At lower temperatures, the relaxation time depends weakly on the temperature. A square root decay is observed, as recently predicted by a theoretical model in which the tunneling is assisted by dipolar and hyperfine interactions.