Micro-Hall magnetometry studies of thermally assisted and pure quantum tunneling in single molecule magnet Mn-12-acetate

We have studied the crossover between thermally assisted and pure quantum t unneling in single crystals of high spin (S = 10) uniaxial single molecule magnet Mn-12 using micro-Hall effect magnetometry. Magnetic hysteresis expe riments have been used to investigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. The se experiments demonstrate that the crossover occurs in a narrow (similar t o 0.1 K) or broad (similar to 1 K) temperature interval depending on the ma gnitude and direction of the applied field. For low external fields applied parallel to the easy axis, the energy levels that dominate the tunneling s hift abruptly with temperature. In the presence of a transverse field and/o r large longitudinal field these energy levels change with temperature more gradually. A comparison of our experimental results with model calculation s of this crossover suggests that there are additional mechanisms that enha nce the tunneling rate of low-lying energy levels and broaden the crossover for small transverse fields. (C) 2001 Elsevier Science Ltd. All rights res erved.