The spin-phonon relaxation mechanism of single-molecule magnets in the presence of strong exchange coupling

TL;DR

This paper provides a detailed first-principles analysis of spin-phonon relaxation mechanisms in single-molecule magnets, highlighting how exchange coupling and cluster size influence relaxation pathways and suppression.

Contribution

It offers the first comprehensive microscopic understanding of spin-phonon relaxation in polynuclear complexes, specifically a Co(II) dimer, and predicts relaxation suppression with increased cluster size.

Findings

Simulations accurately reproduce experimental relaxation data.

Exchange coupling and zero-field splitting significantly affect relaxation pathways.

Increasing cluster size to four cobalt units could fully suppress Raman relaxation.

Abstract

Magnetic relaxation in coordination compounds is largely dominated by the interaction of the spin with phonons. Large zero-field splitting and exchange coupling values have been empirically found to strongly suppress spin relaxation and have been used as the main guideline for designing new molecular compounds. Although a comprehensive understanding of spin-phonon relaxation has been achieved for mononuclear complexes, only a qualitative picture is available for polynuclear compounds. Here we fill this critical knowledge gap by providing a full first-principle description of spin-phonon relaxation in an air-stable Co(II) dimer with both large single-ion anisotropy and exchange coupling. Simulations reproduce the experimental relaxation data with excellent accuracy and provide a microscopic understanding of Orbach and Raman relaxation pathways and their dependency on exchange coupling, zero-field splitting, and molecular vibrations. Theory and numerical simulations show that increasing cluster nuclearity to just four cobalt units would lead to a complete suppression of Raman relaxation. These results hold a general validity for single-molecule magnets, providing a deeper understanding of their relaxation and revised strategies for their improvement.