Phonoemissive Spin Tunneling in Molecular Nanomagnets

TL;DR

This paper proposes a phonon-assisted spin tunneling mechanism for magnetization reversal in molecular nanomagnets like Fe8, but finds the calculated rates are too low to explain experimental relaxation, leaving the problem open.

Contribution

It introduces a new phonon-emissive spin tunneling mechanism for magnetic relaxation in nanomagnets under large magnetic fields.

Findings

Calculated spin-flip rates are several orders of magnitude too low.

The proposed mechanism cannot fully explain experimental relaxation rates.

The understanding of magnetic relaxation remains an open problem.

Abstract

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-St\"uckelberg 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.