Preparation And Magnetic Exchange Bias Effects Of Cobalt-based Nanoparticles

With miniaturizing spintronic devices,sizes of magnetic nanoparticles also decreases futher.When the size of ferromagnetic nanoparticles is lower than critical size of superparamagnetism,spintronic devices will lose the ability of magnetic storage.Exchange bias effect has been attracted wide attention because it can be used to increase magnetic anisotropy of ferromagnetic nanoparticles and break through the limitation of superparamagnetic critical size.The exchange bias effect of ferromagnetic/antiferromagnetic composite nanoparticles is not only affected by ferromagnetic layer thickness,antiferromagnetic layer thickness and interfacial roughness,but also by the microstructures such as defects in antiferromagnetic materials.At present,various magnetic nanoparticles with exchange bias effect have been reported by scientists.Cobalt-based nanoparticles have attracted much attention due to low superparamagnetic critical size of cobalt and high magnetic anisotropy of cobalt oxide.Defects in CoO can induce ferromagnetism,but the effect of defects on the exchange bias effect is seldom studied.The effect of defects in antiferromagnetic materials on the exchange bias effect of ferromagnetic/antiferromagnetic nanoparticles system needs to be explored.Based on the above problems,this paper focuses on the influence of defects on the exchange bias effect of cobalt-based nanoparticles.The as-obtained results are as follows:1.CoO nanoparticles with good dispersion were synthesized by a simple solvothermal method.CoO nanoparticles had good crystallinity and free of impurities,but there were lots of oxygen vacancy in the as-obtained samples.The magnetic studies showed samples both exhibit weak ferromagnetism at room temperature and exchange bias effect existed at 5 K.CoO nanoparticles had free of impurities but a large number of oxygen vacancy,so ferromagnetism at room temperature was their intrinsic property.The counterpart mechanism was that oxygen vacancy leaded to ferromagnetic pinning.On the other hand,oxygen vacancy was the main reason of exchange bias effect,in other words,coupling from the antiferromagnetism of bulk phase and the ferromagnetic of oxygen vacancy leaded to exchange bias of CoO.The higher the oxygen vacancy concentration,the greater the exchange bias effect.2.Co/CoO core-shell nanoparticles were prepared by simple solvothermal method.The as-obtained results showed that Co/CoO nanoparticles appeared ferromagnetism at room temperature.The saturation magnetization and coercivity were about 73 emu/g and 290.5 Oe,respectively.FC hysteresis loop of Co/CoO nanoparticles was offset at 5 K,where coercivity and exchange bias fields were 1321.5 Oe and631.7 Oe,respectively.Because of the low concentration of oxygen vacancy in the sample,the exchange bias effect is mainly determined by the coupling effect of ferromagnetic/antiferromagnetic interface,and we used the spin configuration diagram to explain the process of exchange bias effect in Co/CoO nanoparticles.3.The precursor Co（OH）₂ was synthesized by solvothermal method and then annealed to synthesize hexagonal porous Co/Co₃O₄ nanosheets.The magnetic properties of Co/Co₃O₄ nanosheets were studied and analyzed in detail.Co/Co₃O₄ nanosheets exhibited paramagnetism at room temperature because blocking temperature and irreversibility temperature of Co/Co₃O₄ nanosheets were about 3.5 K and 280 K,respectively.While FC hysteresis loop was tested at 2 K,Co/Co₃O₄ nanosheets existed exchange bias effect and the exchange bias field was 567.6 Oe.Because of the large number of oxygen vacancy in Co₃O₄,the exchange bias field of samples is much higher than that of Co/Co₃O₄ nanoparticles in relevant literature.