Effect of Exchange Interaction on Magnetic Thermal Fluctuation and Spin Susceptibility

TL;DR

This paper derives a quantum-theoretic expression for thermal fluctuations in spin systems, revealing how exchange interactions and lattice structure influence spin susceptibility and resonance broadening.

Contribution

It introduces a fundamental quantum derivation of thermal fluctuations including exchange interactions, highlighting their effects on spin susceptibility and resonance line broadening.

Findings

Thermal fluctuation decreases exponentially with exchange interaction.

Spin susceptibility is affected by exchange strength and lattice structure.

Non-uniform fluctuations cause additional resonance broadening.

Abstract

The expression of the thermal fluctuation parameter in the stochastic Landau-Lifshitz-Gilbert equation has been derived from a fundamental quantum theory of spins and phonons, in which the exchange interaction between nearest atoms has been included. Our studies show that the thermal fluctuation decreases exponentially with increasing exchange interaction. The non-uniform fluctuation of local spins make the spin susceptibility much different from the result derived by the macro-spin model or single spin model. The related spin susceptibility depends not only on the strength of exchange interaction, but also on the lattice structure. The non-uniform fluctuation can lead to an extra broadening of the resonance line width along with the broadening arisen from the Gilbert damping.