Vlasov, S. M.; Bessarab, P.F.; Uzdin, V.M.; Jonsson, Hannes Calculations of the onset temperature for tunneling in multispin systems Transitions between magnetic states of a system coupled to a heat bath can occur by exceeding the energy barrier, but as temperature is lowered quantum mechanical tunneling through the barrier becomes the dominant transition mechanism. A method is presented for estimating the onset temperature for tunneling in a system with an arbitrary number of spins using the

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... cond derivatives of the energy with respect to the orientation of the magnetic vectors at the first order saddle point on the energy surface characterizing the over-the-barrier mechanism. An application to a monomer and a dimer of molecular magnets containing a Mn 4 group is presented and the result found to be in excellent agreement with reported experimental measurements.