A first-principles linear response theory for open quantum systems and its application to Orbach and direct magnetic relaxation in Ln-based coordination polymers

TL;DR

This paper develops a first-principles linear response theory for open quantum systems to accurately model magnetic relaxation in lanthanide-based single-molecule magnets, linking microscopic theory with experimental observables.

Contribution

It introduces a novel ab initio linear-response formalism combined with electronic structure methods for magnetic relaxation analysis in SMMs.

Findings

Successfully reproduces low-temperature relaxation processes.

Accurately models high-temperature Orbach relaxation.

Demonstrates feasibility of ab initio simulations for magnetic susceptibility.

Abstract

Single-Molecule Magnets (SMMs) exhibit slow magnetic relaxation as a result of axial magnetic anisotropy inhibiting spin-phonon transitions. In order to establish a direct link between physical observables and the microscopic theory of magnetic relaxation, we here develop and numerically implement a first-principles linear-response theory for open quantum systems that provides access to the complex a.c. magnetic susceptibility in the presence of an oscillating a.c. magnetic field. Once combined with density functional theory and multiconfigurational electronic structure simulations, this formalism is applied in a fully first-principles fashion to three cyanido-bridged Ln/Y-based coordination polymers with general formula {Ln$^{III}_x$ Y$^{III}_{1-x}$ [Co(CN)$_6$]}, where Ln = Yb (1), Tb (2), and Dy (3). The method is able to reproduce the low-temperature direct relaxation process and its field dependence, as well as the high-temperature Orbach relaxation regime for all the investigated compounds. These results demonstrate the feasibility of ab initio simulations of magnetic a.c.susceptibility in lanthanide-based SMMs and support the potential of further development of ab initio open quantum systems methods towards the completion of a magnetization dynamics theory.