Evidence for resonant tunneling of magnetization in Mn-12 acetate complex

We have measured the de magnetization at low temperatures of tetragonal crystals of Mn-12 acetate complex [Mn12O12(CH3COO)(16)(H2O)(4)], a material composed of a large (Avogadro's) number of identical magnetic molecules, each of spin 10. Exchange coupling between Mn ions within each molecule is very strong, while the interaction between molecules is negligible. A large, uniaxial anisotropy (similar to 60 K) gives rise to a doubly degenerate ground state corresponding to spin projections of +/-10 along the easy axis (c axis); hysteretic behavior is found below a blocking temperature T-b similar to 3 K. Based on measurements of oriented crystallites at temperatures between 1.7 and 3.2 K, we report strong evidence for resonant tunneling of the magnetization: periodic steps in the hysteresis loop, and periodic marked increases in the magnetic relaxation rate at the magnetic fields corresponding to these steps. A total of seven increases in the relaxation rate were found within the temperature range of our experiments with a period of 0.46 T; we suggest that many more such steps would be found at lower temperatures. We attribute these observations to thermally assisted resonant tunneling of the magnetization and propose a detailed model to account for our results.