CPM Seminar

Collective Relaxational Dynamics in Molecular Nanomagnets

Anupam Garg

Department of Physics and Astronomy
Northwestern University

At low temperatures, the molecular magnet Fe₈ displays spectacular quantum dynamics wherein the spin angular momentum degree of freedom tunnels through 20 units of hbar, with a tunnel splitting of 1peV. This tunneling also engenders other dynamical behaviours that require a many-body description, and constitute a challenging classical physics problem. Examples include relaxation in zero field from a magnetized state, magnetization reversal in a swept field (the Landau-Zener-Stuckelberg protocol), and magnetization in nonzero field starting from a nonmagnetized state. One sees behavior that is square-root-of-time at short time, and highly nonexponential at long time. To understand this, it is found essential to consider decoherence and the dipolar interaction between molecules. We have performed Monte Carlo simulations of a very well justified model, and we have developed and solved rate equations and kinetic equations to compare with the simulations and the experiments. The agreement between simulations, kinetic equation, and experiments is very good in most respects, but not so good for ultra long times and ultra-slow phenomena. The problem of magnetization is particularly interesting, as it entails the relaxation of energy in addition to spin.