Low-temperature quantum relaxation in a system of magnetic nanomolecules

We argue that to explain recent resonant tunneling experiments on crystals of Mn-12 and Fe-8, particularly in the low-T limit, one must invoke dynamic nuclear spin and dipolar interactions. The low-T, short-time relaxation will then have a root t/tau form, where tau depends on the nuclear T-2, on the tunneling matrix element Delta(10) between the two lowest levels, and on the initial distribution N(xi) of internal fields in the sample, which depends strongly on sample shape. We also give results for the long-time relaxation. [S0031-9007(98)06470-9].