Thermally assisted macroscopic quantum resonance on a single-crystal of Mn12-acetate

Magnetization measurements have been performed on a single monocrystal of the molecule Mn12-acetate (Mn12Ac). Steps were observed in the hysteresis loop for values of the applied field at which level crossings of the collective spin states of each manganese clusters take place. At these fields, the magnetization relaxes at short time scales, being otherwise essentially blocked. This novel behavior is interpreted in terms of resonant quantum tunneling of the magnetization from thermally activated energy levels. Hysteresis loop measurements performed for different field orientations and alternating current (ac) susceptibility experiments confirm this picture.