Mixed Quantum-Thermal Relaxation inMn12Acetate Molecules

We present a theoretical and experimental study of the magnetic field dependence of the relaxation of the ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}$ acetate compound. For $T\ge 2\mathrm{K}$ the molecule which starts from the $|-S〉$ state climbs to the excited states by means of thermal mechanism, while the remaining barrier is crossed by tunneling. This mixed quantum-thermal mechanism shows a critical dependence of the relaxation time with respect to the external magnetic field.