Synthesis, microstructure, and magnetic properties of -Fe2O3/NiO core/shell nanoflowers

Abstract

The main purpose of this work is to explore the influences of microstructures on the magnetic properties, as well as the formation mechanism of -Fe2O3/NiO core/shell nanoflowers. The synthesis of nanoflower-like samples includes three processes. Firstly, Fe3O4 nanospheres are synthesized by the solvothermal reaction of FeCl3 dissolved in ethylene glycol and NaAc. Secondly, Fe3O4/Ni(OH)2 core/shell precursor is fabricated by solvothermal method through using the early Fe3O4 spheres and Ni(NO3)26H2O in an ethanol solution. Finally, the precursor Fe3O4/Ni(OH)2 is calcined in air at 300 ℃ for 3-6 h, and therefore resulting in -Fe2O3/NiO core/shell nanoflowers. Their microstructures are characterized by using XRD, XPS, SEM, HRTEM and SAED techniques. The results show that the final powder samples are -Fe2O3/NiO with typical core/shell structure. In this core/shell system, the -Fe2O3 sphere acts as core and the NiO acts as shell, which are comprised of many irregular flake-like nanosheets with monocrystalline structure, and these nanosheets are packed together on the surfaces of -Fe2O3 spheres. The calcination time of Fe3O4/Ni(OH)2 precursor has significant influences on the grain growth, the NiO content and the compactness of NiO shells in the -Fe2O3/NiO core/shell system. VSM and SQUID are used to characterize the magnetic properties of -Fe2O3/NiO core/shell nanoflowers. The results indicate that the 3 h-calcined sample displays better ferromagnetic properties (such as higher ms and smaller HC) because of their high -Fe2O3 content. In addition, as the coupling interaction between the FM -Fe2O3 and AFM NiO components, we observe that the -Fe2O3/NiO samples formed in 3 h and 6 h display certain exchange bias (HE=20 and 46 Oe, respectively). Such a coupling effect allows a variety of reversal paths for the spins upon cycling the applied field, and thereby resulting in the enhancement of coercivity (HC(FC)=252 and 288 Oe, respectively). Further, the values of HE and HC for the former are smaller than those of the latter, this is because of the AFM NiO content in 6 h-calcined sample much higher than that in 3 h-calcined sample. Especially, the temperature dependences of the magnetization M of the two samples under both ZFC and FC conditions indicate that an extra anisotropy is induced. In a word, the size effect, NiO phase content, and FM-AFM (where FM denotes the ferromagnetic -Fe2O3 component, while AFM is the antiferromagnetic NiO component) interface coupling effect have significant influence on the magnetic properties of -Fe2O3/NiO core/shell nanoflowers.