The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner-Wohlfarth nanoparticles

TL;DR

This paper investigates how interparticle interactions affect the magnetization reversal and coercivity in ensembles of oriented Stoner-Wohlfarth nanoparticles, revealing the formation of a superferromagnetic state and its impact on magnetic properties.

Contribution

The study introduces a kinetic model incorporating interparticle interactions and demonstrates their effects on coercivity and superferromagnetic state formation in nanoparticle ensembles.

Findings

Interparticle interactions influence the temperature dependence of coercive field.

A superferromagnetic state can form above the blocking temperature.

Coercivity in the superferromagnetic state is independent of measuring time.

Abstract

We consider the influence of interparticle interaction on the magnetization reversal in the oriented Stoner-Wohlfarth nanoparticles ensemble. To do so, we solve a kinetic equation for the relaxation of the overall ensemble magnetization to its equilibrium value in some effective mean field. Latter field consists of external magnetic field and interaction mean field proportional to the instantaneous value of above magnetization. We show that the interparticle interaction influences the temperature dependence of a coercive field. This influence manifests itself in the noticeable coercivity at $T>T_{b}$ ($T_{b}$ is so-called blocking temperature). The above interaction can also lead to a formation of the "superferromagnetic" state with correlated directions of particle magnetic moments at $T>T_{b}$. This state possesses coercivity if the overall magnetization has a component directed along the easy axis of each particle. We have shown that the coercive field in the "superferromagnetic" state does not depend on measuring time. This time influences both $T_{b}$ and the temperature dependence of coercive field at $T<T_{b}$. We corroborate our theoretical results by measurements on nanogranular films (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with concentration of ferromagnetic particles close, but below percolation threshold.