Large structure-dependent room temperature exchange bias in self-assembled BiFeO3 nanoparticles

TL;DR

This study demonstrates that the magnetic exchange bias in BiFeO3 nanoparticle aggregates at room temperature can be significantly tuned by controlling their self-assembled structures and surface functionalization, revealing structure-dependent magnetic properties.

Contribution

It introduces a novel approach to tuning exchange bias in BiFeO3 nanoparticles through self-assembly and surface functionalization, highlighting the role of aggregate structure and surface spins.

Findings

Large coercivity and exchange bias observed in nanoparticle aggregates.

Exchange bias switches from negative to positive depending on aggregate structure.

Surface spin pinning influences the sign and magnitude of exchange bias.

Abstract

We studied the magnetic properties of self-assembled aggregates of BiFeO3 nanoparticles (~ 20-40 nm). The aggregates formed two different structures - one with limited and another with massive cross-linking - via `drying-mediated self-assembly' process following dispersion of the nanoparticles within different organic solvents. They exhibit large coercivity H_C (>1000 Oe) and exchange bias field H_E (~ 350-900 Oe) in comparison to what is observed in isolated nanoparticles (H_C ~ 250 Oe; H_E ~ 0). The H_E turns out to be switching from negative to positive depending on the structure of the aggregates with |H_E| being larger. The magnetic force microscopy reveals the magnetic domains (extending across 7-10 nanoparticles) as well as the domain switching characteristics and corroborate the results of magnetic measurements. Numerical simulation of the `drying-mediated-self-assembly' process shows that the nanoparticle-solvent interaction plays an important role in forming the `nanoparticle aggregate structures' observed experimentally. Numerical simulation of the magnetic hysteresis loops, on the other hand, points out the importance of spin pinning at the surface of nanoparticles as a result of surface functionalization of the particles in different suspension media. Depending on the concentration of pinned spins at the surface pointing preferably along the easy-axis direction - from greater than 50\% to less than 50% - H_E switches from negative to positive. Quite aside from bulk sample and isolated nanoparticle, nanoparticle aggregates - resulting from surface functionalization - therefore, offer remarkable tunability of properties depending on structures.