Relaxation of the magnetization ofMn12acetate

The magnetization of a ${\mathrm{Mn}}_{12}$-acetate single crystal was measured with a cantilever magnetometer to temperatures below 60 mK. Contrary to expectations we did not observe steps in the hysteretic magnetization with indices higher than 11. The data suggest a significant degree of higher-order anisotropy. The fourth-order term appears to be about $-3\times {10}^{-4}{k}_B.$ Both hysteresis in magnetization at different ramping rates of the field, and relaxation at different fixed values of the magnetic field were studied. The relaxation is found to be logarithmic below 1 K, and not single exponential. We contend that the process of magnetization reversal is initiated by tunneling across the anisotropy barrier from a weak population in excited levels from which they can easily tunnel across the barrier, and that the consequent emission and reabsorption of phonons helps to promote a significant fraction of the spins to these excited levels.