EVIDENCE FOR RESONANT-TUNNELING OF MAGNETIZATION IN MN-12 ACETATE COMPLEX

We have measured the de magnetization at low temperatures of tetragona l crystals of Mn-12 acetate complex [Mn12O12(CH3COO)(16)(H2O)(4)], a m aterial composed of a large (Avogadro's) number of identical magnetic molecules, each of spin 10. Exchange coupling between Mn ions within e ach molecule is very strong, while the interaction between molecules i s negligible. A large, uniaxial anisotropy (similar to 60 K) gives ris e to a doubly degenerate ground state corresponding to spin projection s 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 re port strong evidence for resonant tunneling of the magnetization: peri odic steps in the hysteresis loop, and periodic marked increases in th e magnetic relaxation rate at the magnetic fields corresponding to the se 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 tempe ratures. We attribute these observations to thermally assisted resonan t tunneling of the magnetization and propose a detailed model to accou nt for our results.