Nonlinear spin relaxation in strongly nonequilibrium magnets

TL;DR

This paper develops a comprehensive theory for nonlinear spin relaxation in strongly nonequilibrium magnets, emphasizing the role of resonator coupling, cross correlations, and self-organization in transition coherence.

Contribution

It introduces a new theoretical framework based on nonlinear stochastic differential equations for describing complex spin relaxation processes from highly nonequilibrium states.

Findings

Identification of local spin fluctuations as triggers for relaxation

Analysis of the role of resonator coupling in relaxation dynamics

Description of self-organization of transition coherence

Abstract

A general theory is developed for describing the nonlinear relaxation of spin systems from a strongly nonequilibrium initial state, when, in addition, the sample is coupled to a resonator. Such processes are characterized by nonlinear stochastic differential equations. This makes these strongly nonequilibrium processes principally different from the spin relaxation close to an equilibrium state, which is represented by linear differential equations. The consideration is based on a realistic microscopic Hamiltonian including the Zeeman terms, dipole interactions, exchange interactions, and a single-site anisotropy. The influence of cross correlations between several spin species is investigated. The critically important function of coupling between the spin system and a resonant electric circuit is emphasized. The role of all main relaxation rates is analyzed. The phenomenon of self-organization of transition coherence in spin motion, from the quantum chaotic stage of incoherent fluctuations, is thoroughly described. Local spin fluctuations are found to be the triggering cause for starting the spin relaxation from an incoherent nonequilibrium state. The basic regimes of collective coherent spin relaxation are studied.