Lattice Distortion Mediated Paramagnetic Relaxation in High-Spin High-Symmetry Molecular Magnets

TL;DR

This paper explains the field-dependent maxima in magnetic relaxation rates of certain molecules as resulting from lattice phonon-induced distortions, rather than resonant tunneling, supported by a nonperturbative model calculation.

Contribution

It introduces a phonon-mediated mechanism involving Jahn-Teller-like distortions to explain relaxation maxima in high-spin molecular magnets.

Findings

Relaxation maxima are due to lattice phonon coupling causing Jahn-Teller-like distortions.

The model's approximate results align with experimental observations.

Lattice distortions lower the energy barrier, increasing relaxation rates.

Abstract

Field-dependent maxima in the relaxation rate of the magnetic molecules Mn_12-Ac and Fe_8-tacn have commonly been ascribed to some resonant tunneling phenomenon. We argue instead that the relaxation can be understood as purely due to phonons. The rate maxima arise because of a Jahn-Teller-like distortion caused by the coupling of lattice phonons to degenerate Zeeman levels of the molecule at the top of the barrier. The binding energy of the distorted intermediate states lowers the barrier height and increases the relaxation rate. A nonperturbative calculation of this effect is carried out for a model system. An approximate result for the field variation near a maximum is found to agree reasonably with experiment.