Particle size and cooling field dependence of exchange bias in core/shell magnetic nanoparticles

TL;DR

This numerical simulation study investigates how particle size and cooling field influence the exchange bias effect in core/shell magnetic nanoparticles, revealing size-dependent behavior and the impact of cooling conditions on magnetic properties.

Contribution

The paper introduces a detailed Monte Carlo simulation model that captures the microscopic origins of exchange bias in core/shell nanoparticles, highlighting size oscillations and cooling field effects.

Findings

Exchange bias decreases with increasing particle size.

Oscillations in exchange bias are linked to interfacial magnetization.

Cooling field affects the magnitude and sign of exchange bias.

Abstract

We present a numerical simulation study of the exchange bias (EB) effect in nanoparticles with core/shell structure aimed to unveil the microscopic origin of some of the experimental phenomenology associated to this effect. In particular, we have focused our study on the particle size and field cooling dependence of the hysteresis loop shifts. To this end, hysteresis loops after a field cooling process have been computed by means of Monte Carlo simulations based on a model that takes into account the peculiar properties of the core, shell and interfacial regions of the particle and the EB and coercive fields have been extracted from them. The results show that, as a general trend, the EB field $h_{EB}$ decreases with increasing particle size, in agreement with some experimental observations. However, closer inspection reveals notable oscillations of $h_{EB}$ as a function of the particle radius which we show to be closely related to the net magnetization established after field cooling at the interfacial shell spins. For a particle with ferromagnetic interface coupling, we show that the magnitude and sign of $h_{EB}$ can be varied with the magnetic field applied during the cooling process.