Investigation of the Collective Properties in Monolayers of Exchange-Biased Fe3−δO4@CoO Core–Shell Nanoparticles

Owing to its potential to enhance the effective magnetic anisotropy energy of nanoparticles, exchange bias has been extensively studied during the last few years. Although most of studies have been conducted in the powder state, the effect of the dimensionality of nanoparticle assemblies has been scarcely investigated. In this context, we report on the study of the collective properties of monolayers of exchange-biased nanoparticles with different core–shell structures. Nanoparticles consist in a ferrimagnetic Fe3−δO4 core coated with an antiferromagnetic CoO shell. They were assembled in monolayers with local order by using the Langmuir–Blodgett technique. Exchange bias is significantly altered in 2D assemblies in comparison to powder samples which can be assimilated to 3D random assemblies. We show that exchange bias in Fe3−δO4@CoO nanoparticles is directly influenced by dipolar interactions between Fe3−δO4 cores, which are enhanced by shape anisotropy of 2D assemblies. Besides the dramatic influence of the core–shell structure, the systematic variation of the core size and the shell thickness showed that above a critical value, dipolar interactions compete exchange bias.