A Study of Manganese and Cobalt Incorporated Nickel Oxide Based Core-Shell Magnetic Nanoparticles

The synthesis along with the structural and magnetic properties of manganese (Mn) and cobalt (Co) -incorporated nickel oxide (NiO) inverted core-shell nanoparticles (CSNs) were investigated. The primary objective of this study was to determine the effect of substitution of nickel (Ni) by transition metal ions (Mn2+/Co2+) in affecting the magnetic properties of the resultant CSNs. The core of the CSNs is comprised of NiO and the shell constitutes a Nix(Mn/Co)1-xO phase. The synthesis of the CSNs was accomplished in two steps: first, NiO nanoparticles were synthesized using a thermal decomposition method. In the second step, our hydrothermal nanophase epitaxy method was used to create the core-shell structure. Rietveld refinement of X-ray diffraction (XRD) data show rock salt structure throughout in the Mn/Co incorporated CSNs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show a combination of pseudo-spherical and faceted shapes of CSNs whereas energy dispersive spectroscopy (EDS) indicates transition metal incorporation in the CSNs. The high-resolution (HR) TEM images confirmed the formation of distinct core and shell regions. Magnetic characterization shows that the Mn- and Co-substituted nickel oxide-based CSNs possess an inverted magnetic structure, with an antiferromagnetic core and a ferro- or ferrimagnetic shell. The coercivity and exchange bias properties are of larger magnitude in Mn-incorporated than in Co-incorporated CSNs