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Abstract

The molecular magnet Fe8 has total spin S = 10, and Ising anisotropy because of the crystal in which it sits. It can tunnel between degenerate easy directions (with an extremely low tunnel splitting of 10 nK), indicating that all 40 or so valence electrons in the molecule simultaneously flip from orbiting in one direction to the opposite direction. Remarkably, this tunneling is topologically quenched at specific values of the external magnetic field. I will discuss this phenomenon and its experimental confirmation within the framework of spin-coherent-state path integrals.

In the solid, different Fe8 molecules interact via dipolar fields. This coupling is of order 100 mK, so it is huge compared to 10 nK. This results in very slow collective dynamics of the magnetization of the solid, and gives rise to a very challenging classical problem. We analyze this relaxational dynamics via rate equations and Monte Carlo simulations of various experimental protocols, and discuss our successes and unsolved questions.