Modelling exchange bias in core/shell nanoparticles

TL;DR

This paper develops an atomistic Monte Carlo model of core/shell nanoparticles to understand exchange bias phenomena, revealing how interfacial spins contribute to hysteresis loop shifts and asymmetries.

Contribution

It introduces a tunable, geometry-aware atomistic model that links microscopic parameters to macroscopic exchange bias effects in nanoparticles.

Findings

Loop shifts occur within specific parameter ranges.

Pinned interfacial spins cause hysteresis asymmetries.

Uncompensated interfacial spins explain experimental phenomena.

Abstract

We present an atomistic model of a single nanoparticle with core/shell structure that takes into account its lattice strucutre and spherical geometry, and in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and interfacial regions. By means of Monte Carlo simulations of the hysteresis loops based on this model, we have determined the range of microscopic parameters for which loop shifts after field cooling can be observed. The study of the magnetic order of the interfacial spins for different particles sizes and values of the interfacial exchange coupling have allowed us to correlate the appearance of loop asymmetries and vertical displacements to the existence of a fraction of uncompensated spins at the shell interface that remain pinned during field cycling, offering new insight on the microscopic origin of the experimental phenomenology.