Size-dependent magnetic properties of Mn-Co-NiO based heterostructured nanoparticles

Abstract

In this work, we investigate the synthesis, along with the structural and magnetic properties, of novel Mn-Co-NiO-based heterostructured nanocrystals (HNCs). The objective is to develop novel, well-structurally ordered inverted antiferromagnetic (AFM) NiO–ferrimagnetic (FiM) spinel phase overgrowth HNCs. Inverted HNCs are particularly promising for magnetic device applications because their magnetic properties are more easily controlled by having well-ordered AFM cores, which can result in magnetic structures having large coercivities, tunable blocking temperatures, and other enhanced magnetic effects. The synthesis of the HNCs is accomplished using a two-step process: In the first step, NiO nanoparticles are synthesized using a thermal decomposition method. Subsequently, Mn-Co overgrowth phases are grown on the NiO nanoparticles via hydrothermal nanophase epitaxy, using a fixed pH level (∼5.3) of the aqueous medium. This pH level was selected based on previous work in our laboratory showing that NiO/Mn3O4 HNCs of constant size have optimal coercivity and exchange bias when synthesized at a pH of 5.0. The crystalline structure and gross morphology of the Mn-Co-NiO-based HNCs have been analyzed using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques, respectively. Analysis using these techniques shows that the HNCs are composed of a NiO core and a CoMn2O4 overgrowth phase. Rietveld refinement of XRD data shows that the NiO core has the rocksalt (Fm3̄m) cubic crystal structure and the CoMn2O4 overgrowth has the spinel (I41/amd) crystal structure. Moreover, an increased relative amount of the CoMn2O4 overgrowth phase is deposited with decreasing NiO core particle size during the synthesis of the HNCs. The results from PPMS magnetization and high-resolution transmission electron microscopy (HRTEM) characterization of the Mn-Co-NiO-based HNCs are discussed herein.