Synthesis,Microstructure And Magnetic Properties Of Exchange Biased Composite Nanomaterials

In recent decades,core-shell magnetic nanosystems with exchange bias have attracted wide attention from researchers because of their considerable applications in data storage,biomedicine and electronic devices.In this paper,two kinds of core-shell magnetic nanosystems,?-Fe₂O₃/NiO and Ni/NiO were synthesized and the influences of microstructure on its exchange bias and test conditions on related magnetic properties were studied.The main results are as follows.Two-step solvothermal method combining a calcination process was conducted to synthesize?-Fe₂O₃/NiO core-shell nanostructures with controlled microstructure.The formation mechanism of this binary system has been discussed,and the influences of microstructures on the magnetic properties have been analyzed in detail.The microstructural characterizations reveal that the NiO shells were built of many irregular nanosheets with disordered orientation and monocrystalline structure,and these NiO nanosheets were packed together on the surface of maghemite microspheres.As the calcination temperature increased from 300°C to 400°C,the grain size and NiO content of nanostructures increase,as well as the compactness of NiO shells enhances.Keep the calcination temperature unchanged at 300°C,as the precursor concentration decreases?the NiO content increases?,the thickness of the NiO shell increases significantly,and the grain size of NiO increases.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?and FM-AFM?where FM and AFM denote the ferromagnetic maghemite and the antiferromagnetic NiO components,respectively?coupling effect.At 5 K,the maghemite/NiO core-shell nanostructures display certain exchange bias?60 Oe?and enhanced coercivity?213 Oe?.The value of the exchange bias field is small,mainly because the NiO shell is composed of irregular nanosheets,has a disordered orientation and does not form a complete coating wrap around the iron oxide core.A reflux method combining a calcination process was conducted to synthesize Ni/NiO binary nanoparticles with average size of<10 nm and Ni content of¹.3%.Both the microstructures of Ni/NiO interface and the states of different phases have significant impacts on their magnetic properties.By tuning the test temperature and cooling field during the loop measurement,the change rule of several typical parameters such as coercivity H_C and exchange bias H_E was found to be complicated in nature.Large H_E?482 Oe?and enhanced H_C?1335 Oe?were observed at 5 K,mainly due to the strong coupling interaction between Ni and NiO components.As for the present Ni/NiO binary nanoparticles,the complex magnetic behaviors are ascribed to?i?the ferromagnetic contribution of Ni nanoparticles,?ii?the intrinsic antiferromagnetic role of NiO volume phase,and?iii?the spin-glass like character corresponding to the frozen disordered state at the surface of partial NiO particles.The comprehensive effect of these three magnetic structures promotes the generation of strongly-coupled Ni/NiO particle system,resulting to improve magnetic performance.