Low-temperature magnetic hysteresis in Mn ₁₂ acetate single crystals

Abstract

Precise magnetic hysteresis measurements of small single crystals of\nMn$_{12}$ acetate of spin 10 have been conducted down to 0.4 K using a high\nsensitivity Hall magnetometer. At higher temperature (>1.6K) step-like changes\nin magnetization are observed at regularly spaced magnetic field intervals, as\npreviously reported. However, on lowering the temperature the steps in\nmagnetization shift to higher magnetic fields, initially gradually. These\nresults are consistent with the presence of a second order uniaxial magnetic\nanisotropy, first observed by EPR spectroscopy, and thermally assisted\ntunnelling with tunnelling relaxation occurring from levels of progressively\nlower energy as the temperature is reduced. At lower temperature an abrupt\nshift in step positions is found. We suggest that this shift may be the first\nevidence of an abrupt, or first-order, transition between thermally assisted\nand pure quantum tunnelling, suggested by recent theory.\n