THERMALLY ACTIVATED RESONANT MAGNETIZATION TUNNELING IN MOLECULAR MAGNETS - MN12AC AND OTHERS

The dynamical theory of thermally activated resonant magnetization tun neling in uniaxially anisotropic magnetic molecules such as Mn12Ac (S= 10) is developed. The observed slow dynamics of the system is describe d by master equations for the populations of spin levels. The latter a n obtained by the adiabatic elimination of fast degrees of freedom fro m the density matrix equation with the help of the perturbation theory developed earlier for tunneling level splitting [D. A. Garanin, a. Ph ys. A 24, L61 (1991)]. There exists a temperature range (thermally act ivated tunneling) where the escape rate follows the Arrhenius law, but has a nonmonotonic dependence on the bias field due to tunneling at t he top of the barrier. At lower temperatures this regime crosses over to the non-Arrhenius law (thermally assisted tunneling). The transitio n between the two regimes can be first or second order, depending on t he transverse field, which can be tested in experiments. In both regim es the resonant maxima of the rate occur when spin levels in the two p otential wells match at certain field values. In the thermally activat ed regime at low dissipation each resonance has a multitower self-simi lar structure with progressively narrowing peaks mounting on top of ea ch other. [S0163-1829(97)00141-0].