Probing the core and shell contributions to exchange bias in Co/Co$_3$O$_4$ nanoparticles of controlled size

TL;DR

This study investigates how the core and shell sizes in Co/Co$_3$O$_4$ nanoparticles influence exchange bias, combining experimental synthesis and characterization with atomistic simulations to understand the underlying magnetic interactions.

Contribution

It provides new insights into the size-dependent exchange bias in core/shell nanoparticles through combined experimental and simulation approaches.

Findings

Exchange bias and coercivity peak at intermediate shell thickness.

Maximum lattice strain correlates with maximum exchange bias.

Simulations confirm the role of interfacial surface spins in magnetization reversal.

Abstract

Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for the exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenology. In this work, we have prepared core/shell structured nanoparticles with the total averaged diameter around $\sim$ 27 nm and a wide range of shell thicknesses through the controlled oxidation of Co nanoparticles well dispersed in an amorphous silica host. Structural characterizations give compelling evidence of the formation of Co$_3$O$_4$ crystallite phase at the shells surrounding the Co core. Field cooled hysteresis loops display nonmonotonous dependence of the exchange bias $H_E$ and coercive $H_C$ fields, that become maximum for a sample with an intermediate shell thickness, at which lattice strain is also maximum for both the phases. Results of our atomistic Monte Carlo simulations of the particles with the same size and compositions as in experiments are in agreement with the experimental observations and have allowed us to identify a change in the contribution of the interfacial surface spins to the magnetization reversal giving rise to the maximum in $H_E$ and $H_C$.