Chemical order and size effects on the magnetic anisotropy of FePt and CoPt nanoparticles

TL;DR

This study explores how size and chemical order influence the magnetic anisotropy energy in FePt and CoPt nanoparticles, revealing significant effects due to finite size and surface symmetry breaking.

Contribution

It extends the Nél model to nanoparticles, demonstrating that finite size and surface effects can significantly alter magnetic anisotropy energies.

Findings

Disordered clusters can have MAE up to 0.5×10^6 J/m³, exceeding bulk values.

Surface effects reduce MAE in ordered clusters due to symmetry breaking.

Finite size effects cause notable variations in magnetic properties.

Abstract

We investigate the consequence of the dimension reduction on the magnetic anisotropy of FePt and CoPt nanoparticles. Using an extension of the magnetic anisotropy model of N\'eel, we show that, due to a statistical finite size effect, chemically disordered clusters can display a magnetic anisotropy energy (MAE) as high as 0.5\times10^6 J/m3, more than one order of magnitude higher than the bulk MAE. Concerning L10 ordered clusters, we show that the surface induces a reduction of the MAE as compared to the bulk, due to the symmetry breaking at the cluster surface, which modifies the chemical order.