Unravelling the contributions to spin-lattice relaxation in Kramers single-molecule magnets

TL;DR

This paper uses ab initio spin dynamics to analyze spin relaxation mechanisms in single-molecule magnets, clarifying the roles of zero-field splitting and phonons in limiting spin lifetime.

Contribution

It introduces a comprehensive ab initio approach to distinguish and understand the contributions of different relaxation mechanisms in single-molecule magnets.

Findings

Orbach relaxation rate depends mainly on zero-field splitting.

Raman relaxation is influenced by low-energy phonons.

Provides a clear interpretation of spin relaxation mechanisms.

Abstract

The study of how spin interacts with lattice vibrations and relaxes to equilibrium provides unique insights on its chemical environment and the relation between electronic structure and molecular composition. Despite its importance for several disciplines, ranging from magnetic resonance to quantum technologies, a convincing interpretation of spin dynamics in crystals of magnetic molecules is still lacking due to the challenging experimental determination of the correct spin relaxation mechanism. We apply ab initio spin dynamics to a series of twelve coordination complexes of Co(II) and Dy(III) ions selected among $\sim$240 compounds that largely cover the literature on single-molecule magnets and well represent different regimes of spin relaxation. Simulations reveal that the Orbach spin relaxation rate of known compounds mostly depends on the ions' zero-field splitting and little on the details of molecular vibrations. Raman relaxation is instead found to be also significantly affected by the features of low-energy phonons. These results provide a complete understanding of the factors limiting spin lifetime in single-molecule magnets and revisit years of experimental investigations by making it possible to transparently distinguish Orbach and Raman relaxation mechanisms.