Low temperature magnetic hysteresis in Mn-12-acetate single crystals

We report precise magnetic hysteresis measurements down to 0.4 K of spin-10 magnetic clusters in small single crystals of Mn-12-acetate. At temperatur es above 1.6 K, steplike changes in magnetization are observed at regularly spaced magnetic field intervals, as found in earlier measurements. The ste ps gradually shift to higher magnetic fields as the temperature is decrease d to 1.2 K, consistent with the presence of a second-order uniaxial magneti c anisotropy. An abrupt shift in step position is observed at still lower t emperature which may be the first evidence of an abrupt, or first-order, tr ansition between thermally assisted and pure quantum tunneling suggested by recent theory. (C) 2000 American Institute of Physics. [S0021-8979(00)3180 8-4].