In this work, we investigate the synthesis, along with the structural, magnetic, and surface chemical environment 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 5.0.1 The crystalline structure, surface/interface chemical environment and gross morphology of the Mn-Co-NiO-based HNCs have been analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) 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 (Fm-3m) 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. Analysis of the O 1s, Ni 2p, Co 2p, and Mn 2p regions of the XPS spectra measured from our samples is consistent with well-structured core/overgrowth phases having M-OH (M: Ni, Co, Mn) bonding at their surface regions. The results from PPMS magnetization and high-resolution transmission electron microscopy (HR-TEM) characterization of the Mn-Co-NiO-based HNCs will be discussed.
(1) Shafe et al., ACS Applied Materials & Interfaces, 13, 24013−24023 (2021)