Effect of exchange interaction on superparamagnetic relaxation

TL;DR

This paper investigates how exchange interaction influences superparamagnetic relaxation by calculating reaction rates for coupled spins, revealing a critical exchange coupling value that separates different switching regimes.

Contribution

It introduces a theoretical analysis of exchange interaction effects on superparamagnetic relaxation using Langer's approach, identifying a critical coupling value for switching behavior.

Findings

Identifies a critical exchange coupling value j_c = 1 - h^2 separating switching regimes.

Shows that for large exchange coupling, the system behaves like a single spin (Néel-Brown model).

Provides a quantitative framework for understanding exchange effects on magnetic relaxation.

Abstract

We use Langer's approach to calculate the reaction rate of a system of two (classical) spins interacting via the exchange coupling $J$ in a magnetic field $H$, with uniaxial anisotropy of constant $K$. We find a particular value of the exchange coupling, that is $j\equiv J/K = j_c\equiv 1-h^2$, where $h\equiv H/2K$, which separates two regimes corresponding to a two-stage and one-stage switching. For $j\gg j_c$ the N\'eel-Brown result for the one-spin problem is recovered.