Low temperature magnetic hysteresis in Mn₁₂-acetate single crystals

We report precise magnetic hysteresis measurements down to 0.4 K of spin-10 magnetic clusters in small single crystals of Mn₁₂-acetate. At temperatures above 1.6 K, steplike changes in magnetization are observed at regularly spaced magnetic field intervals, as found in earlier measurements. The steps gradually shift to higher magnetic fields as the temperature is decreased to 1.2 K, consistent with the presence of a second-order uniaxial magnetic anisotropy. An abrupt shift in step position is observed at still lower temperature which may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunneling suggested by recent theory.