MAGNETIC-RELAXATION IN BIG MAGNETIC MOLECULES

The relaxation time of the magnetization of a big magnetic molecule is evaluated. Isotropic exchange is assumed to be dominant, resulting in a fixed spin modulus s (equal to 20 in the case of interest). A perturbation of the form AS(z)2 results from spin-orbit coupling and produces an energy barrier As2 which separates positive and negative values of S(z). Spin-phonon interactions are treated by second-order perturbation theory. An upper limit of the pre-exponential factor in the Arrhenius law is obtained, which is consistent with the very large experimental value.