Vibronic effects on the quantum tunnelling of magnetisation in Kramers single-molecule magnets

TL;DR

This paper develops a non-perturbative vibronic model for single-molecule magnets, revealing how spin-phonon coupling via magnetic polarons influences quantum tunnelling and magnetic relaxation at low temperatures.

Contribution

It introduces a novel ab initio vibronic model that quantifies the impact of spin-phonon coupling on quantum tunnelling in single-molecule magnets.

Findings

Magnetic polarons lower tunnelling probability by stabilizing spin states.

Spin-phonon coupling affects magnetic relaxation even at very low temperatures.

Vibronic effects are significant in understanding magnetisation dynamics.

Abstract

Single-molecule magnets are among the most promising platforms for achieving molecular-scale data storage and processing. Their magnetisation dynamics are determined by the interplay between electronic and vibrational degrees of freedom, which can couple coherently, leading to complex vibronic dynamics. Building on an ab initio description of the electronic and vibrational Hamiltonians, we formulate a non-perturbative vibronic model of the low-energy magnetic degrees of freedom in monometallic single-molecule magnets. Describing their low-temperature magnetism in terms of magnetic polarons, we are able to quantify the vibronic contribution to the quantum tunnelling of the magnetisation, a process that is commonly assumed to be independent of spin-phonon coupling. We find that the formation of magnetic polarons lowers the tunnelling probability in both amorphous and crystalline systems by stabilising the low-lying spin states. This work, thus, shows that spin-phonon coupling subtly influences magnetic relaxation in single-molecule magnets even at extremely low temperatures where no vibrational excitations are present.