MAGNETIC-RELAXATION OF MESOSCOPIC MOLECULES

The relaxation dynamics of nanoscale molecules such as Mn12Ac arises f rom spin-lattice coupling and interaction with nuclear spins. Using a resolvent method in terms of the energy eigenstates and the first Born approximation with respect to phonon scattering, and averaging over t he hyperfine field, we obtain a controlled approximation for the non-e quilibrium magnetic relaxation behaviour and, in particular, for the c orresponding rate. The rate is finite at T = 0, then increases linearl y with T, and shows Arrhenius behaviour at higher temperature; for zer o magnetic held B there are two different activation energies. The res onances as a function of B are shown to be slightly asymmetric about B = 0. Taking account of a quartic crystal field gives rise to a temper ature-dependent shift of the resonant values of B. We find that, contr ary to previous results, the rate is independent of the magnetic field at low but finite temperatures; for T --> 0 it is linear in B. Finall y we compare our findings with various experimental data.