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Phonoemissive Spin Tunneling in Molecular Nanomagnets

A new mechanism is proposed for the magnetization reversal of molecular nanomagnets such as \Fe8. In this process the spin tunnels from the lowest state near one easy direction to the first excited state near the opposite easy direction, and subsequently decays to the second easy direction with the emission of a phonon, or it first emits a phonon and then tunnels to the final state. This mechanism is the simplest imaginable one that allows magnetization relaxation in the presence of a longitudinal magnetic field that is so large that the nuclear spin environment cannot absorb the energy required for energy conservation to hold. It is proposed as a way of understanding both magnetization realaxation and Landau-Zener-Stückelberg experiments. The requisite Fermi golden rule rate, and the spin-flip rates are calculated, and it is found that these rates are much too low by several orders of magnitude. Thus the understanding of magnetic relaxation in the experiments remains an open question.