Unified decoupling scheme for exchange and anisotropy contributions and temperature-dependent spectral properties of anisotropic spin systems

TL;DR

This paper introduces a unified decoupling scheme for exchange and anisotropy in spin systems, enabling accurate calculation of temperature-dependent spectral properties and bridging quantum and classical approaches.

Contribution

The authors develop a unified decoupling procedure for Green's functions that seamlessly connects quantum and classical methods for anisotropic spin systems.

Findings

Full agreement between classical Green's functions and spectral density methods for critical temperature.

The Green's function approach is more straightforward and flexible than spectral density methods.

Temperature-dependent exchange stiffness correlates with magnetization across different approaches.

Abstract

We compute the temperature-dependent spin-wave spectrum and the magnetization for a spin system using the unified decoupling procedure for the high-order Green's functions for the exchange coupling and anisotropy, both in the classical and quantum case. Our approach allows us to establish a clear crossover between quantum-mechanical and classical methods by developing the classical analog of the quantum Green's function technique. The results are compared with the classical spectral density method and numerical modeling based on the stochastic Landau-Lifshitz equation and the Monte Carlo technique. As far as the critical temperature is concerned, there is a full agreement between the classical Green's functions technique and the classical spectral density method. However, the former method turns out to be more straightforward and more convenient than the latter because it avoids any \emph{a priori} assumptions about the system's spectral density. The temperature-dependent exchange stiffness as a function of magnetization is investigated within different approaches.