Microstructural and Magnetic Investigations of Wüstite-Spinel Core-Shell Cubic-Shaped Nanoparticles

TLDR

Core‑shell nanoparticles are being engineered for novel magnetic properties in spintronics and recording media, and while spherical particles adopt a spinel structure, cubic particles consist of a wüstite core surrounded by a spinel shell. The study investigates the structure, morphology, and composition of cubic‑shaped nanoparticles and compares them to spherical ones. The authors employed XRD, TEM, STEM‑HAADF imaging, electron tomography, and holography to characterize the nanoparticles. Stresses at the core‑shell interface and within the spinel shell, arising from epitaxial growth and oxidation, together with oxidation‑time‑dependent evolution, produce exchange‑bias coupling between the antiferromagnetic core and ferrimagnetic shell, thereby affecting the magnetic properties.

Abstract

Most studies on the synthesis of nanoparticles are currently focused on the controlled synthesis of new morphologies, including core-shell structures, which are expected to exhibit new magnetic properties for uses in spintronics and recording media applications. In this study, the structure, morphology, and composition of cubic-shaped nanoparticles are carefully investigated and compared to those of spherically shaped nanoparticles through the use of a combination of techniques: X-ray diffraction (XRD) and transmission electronic microscopy (TEM) combined with more sensitive techniques such as scanning transmission electron microscopy-high-angle annular dark field (STEM-HAADF) imaging, electron tomography, and holography. While spherically shaped nanoparticles (NPs) crystallize with the spinel structure, cubic-shaped NPs can be described as a cubic core of wüstite surrounded by a spinel shell. Stresses are observed at the core-shell interface and within the spinel shell due to the epitaxial growth and oxidation mechanisms of the wüstite phase. Furthermore, magnetic measurements displayed an exchange bias coupling between the antiferromagnetic (AFM) core and the ferrimagnetic (FIM) shell structure of cubic-shaped nanoparticles. It is shown that the magnetic properties are influenced by stresses generated by the oxidation of wüstite and, also exhibit variations depending upon the evolution of this core-shell structure as a function of the oxidation time.