Exchange-biased hybrid γ-Fe2O3/NiO core–shell nanostructures: three-step synthesis, microstructure, and magnetic properties

Abstract

A 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 influence of microstructures on magnetic properties has been analyzed in detail. Microstructural characterizations reveal that the NiO shells consisted of many irregular nanosheets with disordered orientations and monocrystalline structures, packed on the surface of the γ-Fe₂O₃ microspheres. Both the grain size and NiO content of nanostructures increase with the increasing calcination temperature from 300 °C to 400 °C, accompanied by an enhancement of the compactness of NiO shells. Magnetic studies indicate that their magnetic properties are determined by four factors: the size effect, NiO phase content, interface microstructure, i.e. contact mode, area, roughness and compactness, and 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 (H_E = 60 Oe) and enhanced coercivity (H_C = 213 Oe).