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 pertu rbation of the form AS(z)2 results from spin-orbit coupling and produc es an energy barrier As2 which separates positive and negative values of S(z). Spin-phonon interactions are treated by second-order perturba tion theory. An upper limit of the pre-exponential factor in the Arrhe nius law is obtained, which is consistent with the very large experime ntal value.