Low-Temperature Quantum Relaxation in a System of Magnetic Nanomolecules

Abstract

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. We show the low-$T$, short-time relaxation will then have a $\sqrt{t/\tau }$ 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 of internal fields in the sample, which depends very strongly on sample shape. The results are directly applicable to the $F{e}_8$ system. We also give some results for the long-time relaxation.

4 pages, 3 PostScript figures, LaTex

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