Exchange-Biased Hybrid γ-Fe2O3/NiO Core-Shell Nanostructures: Three-Step Synthesis, Microstructure, and Magnetic Properties
Two-step solvothermal method combining a calcination process was conducted to synthesize γ-Fe2O3/NiO core-shell nanostructures with controlled microstructure. The formation mechanism of this binary system has been discussed, and the influences of microstructures on magnetic properties have been analyzed in detail. The microstructural characterizations reveal that the NiO shells consisted of many irregular nanosheets with disordered orientations and monocrystalline structures, packed on the surface of γ-Fe2O3 microspheres. Both grain size and NiO content of nanostructures increase with calcination temperature rising from 300°C to 400°C, accompanied by an enhancement of the compactness of NiO shells. Magnetic investigations indicate that their magnetic properties are determined by four factors: size effect, NiO phase content, interface microstructure, i.e. contact mode, area, roughness and compactness, as well as FM-AFM (where FM and AFM denote the ferromagnetic γ-Fe2O3 and the antiferromagnetic NiO components, respectively) coupling effect. At 5 K, the γ-Fe2O3/NiO core-shell nanostructures display certain exchange bias (HE = 60 Oe) and enhanced coercivity (HC = 213 Oe).