Transition rates for a S \ge 1 model coupled to a phonon bath

TL;DR

This paper derives phonon-assisted transition rates for S ≥ 1 spins coupled to a phonon bath, revealing how higher-order processes enable relaxation pathways and affect magnetization dynamics in magnetic systems.

Contribution

It introduces a derivation of transition rates for S ≥ 1 spins coupled to phonons, accounting for forbidden transitions and higher-order processes, enhancing understanding of magnetic relaxation.

Findings

Forbidden transitions occur in 2D and 3D phonon baths without higher-order processes.

Higher-order phonon processes enable access to previously inaccessible states.

Relaxation times are affected by the availability of relaxation paths and energy barriers.

Abstract

We investigate transition rates between different spin configurations for $S \ge 1$ spins weakly coupled to a $d$-dimensional phonon bath. This study is motivated by understanding observed magnetization relaxation as a function of temperature in diverse magnetic systems such as arrays of magnetic nanoparticles and magnetic molecules. We assume that the magnetization of the spin system relaxes through consecutive emission or absorption of a single phonon. From a weak, linear spin-phonon coupling Hamiltonian, we derive transition rates that would be used to examine dynamic properties of the system in kinetic Monte Carlo simulations. Although the derived phonon-assisted transition rates satisfy detailed balance, in the case of two and three dimensional phonon baths, transitions between degenerate states are not allowed. Thus, if there are no alternative paths along which the spin system can relax, the relaxation time diverges. Otherwise, the system finds other paths, which leads to an increase in the relaxation time and energy barrier. However, when higher-order phonon processes are included in the transition rates, it is found that the system can reach the states which were inaccessible due to the forbidden transitions. As a result, the system recovers some of the dynamic properties obtained using the Glauber transition rate.