Including nanoparticle shape into macrospin models

TL;DR

This study assesses the macrospin approximation's ability to model realistically shaped magnetic nanoparticles, linking particle shape to effective magnetic parameters through extended theoretical modeling and micromagnetic simulations.

Contribution

It introduces a generalized Stoner-Wohlfarth model that incorporates nanoparticle shape effects, validated against micromagnetic simulations for various geometries.

Findings

Macrospin approximation valid for particles 10-60 nm in size with certain elongations.

Extended SW model accurately reproduces hysteresis loops of realistically shaped MNPs.

Results connect nanoparticle morphology with effective magnetic parameters.

Abstract

We investigate the feasibility of the macrospin approximation to account for the actual shape of soft magnetic nanoparticles (MNPs) with realistic geometries. Specifically focusing on magnetite, we use the superellipsoidal parametrisation to account for a variety of shapes, with a continuous interpolation from spherical to cubic morphologies, as well as different elongations. Our procedure consists of the direct comparison between angular-dependent hysteresis loops obtained by full micromagnetic simulations, with those produced by an extended Stoner-Wohlfarth (SW) model that incorporates both the intrinsic cubic magnetocrystalline anisotropy, and an effective uniaxial contribution arising from the particle elongation. The limits of validity of the macrospin description are approximately 10-60 nm for axial ratios r>1.5, and 20-60 nm for 1.0<r<1.5. These results establish a direct connection between nanoparticle morphology and effective macrospin parameters, demonstrating the suitability of the generalized SW model for describing the magnetic response of realistically shaped MNPs.