Microscopic Theory for the Markovian Decay of Magnetization Fluctuations in Nanomagnets

TL;DR

This paper develops a microscopic theory explaining the exponential decay of magnetization fluctuations in nanomagnets, linking decay rates to energy spectrum discreteness and conservation laws, and provides an analytical expression for the decay rate.

Contribution

It introduces a microscopic framework using the Zwanzig-Mori formalism to identify a single decay rate governing magnetization fluctuations in nanomagnets, connecting physical mechanisms to decay behavior.

Findings

Decay rate $oldsymbol{ extomega_c}$ derived analytically

Decay governed by conservation laws and energy spectrum discreteness

Explains exponential decay observed experimentally in nanomagnets

Abstract

We present a microscopic theory for the phonon-driven decay of the magnetization fluctuations in a wide class of nanomagnets where the dominant energy is set by isotropic exchange and/or uniaxial anisotropy. Based on the Zwanzig-Mori projection formalism, the theory reveals that the magnetization fluctuations are governed by a single decay rate $\omega_c$, which we further identify with the zero-frequency portion of the associated self-energy. This dynamical decoupling from the remaining slow degrees of freedom is attributed to a conservation law and the discreteness of the energy spectrum, and explains the omnipresent mono-exponential decay of the magnetization over several decades in time, as observed experimentally. A physically transparent analytical expression for $\omega_c$ is derived which highlights the three specific mechanisms of the slowing down effect which are known so far in nanomagnets.