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].