Preparation And The Exchange Bias Effect Of CoFe₂O₄@NiO Magnetic Nanofibers

With the rapid development of spintronic devices,the storage density of magnetic memory devices is gradually increasing,which makes the magnetic electronic devices tend to be more miniaturized.However,with the decrease of the size of magnetic devices,the particle size of magnetic materials is also further decreasing.When the size of ferromagnetic particles is lower than its critical value,it will show superparamagnetic effect,which makes it lose the function of magnetic storage.The exchange bias effect that can be used to break the super-paramagnetic limit of magnetic recording media and to enhance the coercivity of permanent magnets begins to come into focus.At present,exchange bias systems with ferromagnetic and antiferromagnetic materials have been extensively studied,while ferrimagnetic spinel CoFe₂O₄has high anisotropy,high Curie Temperature(Tc=793K),and moderate Hc.The antiferromagnetic NiO exhibits finite size effect and superparamagnetic effect at a certain temperature.In the external magnetic field,when the temperature of NiO is cooled,the exchange bias effect will be generated at the interface of the two phases to overcome the superparamagnetic effect.However,the traditional composite nanofibers are greatly limited because of the lack of obvious coupling interface and uneven particle distribution.Therefore,in this paper,ferromagnetic CoFe₂O₄and antiferromagnetic NiO are used as the research system,the CoFe₂O₄@NiO nanofibers with core shell structure are obtained through the coaxial electrospinning method,and under different conditions,the magnetic properties and exchange bias effect of the two phases have been researched.(1)At the same heat treatment temperature,by controlling the molar ratio of core CoFe₂O₄to shell NiO,the core-shell nanofibers with different ratios are obtained,respectively.Due to the interaction of the two phases,an effective heterogeneous coupling interface is formed.The core layer and shell layer are evenly distributed,the structure is complete,and it has a high aspect ratio.The diameter is about 100 nm,and the inner diameter is about 35 nm.The exchange bias of CoFe₂O₄@NiO The exchange bias is different at different core-shell molar ratios.When the core-shell molar ratio is 1:3,the maximum exchange bias effect is obtained at the measured temperature of 5 K,and the value of H_EBis 763 Oe,Hc is 11890 Oe,and Mr is 47emu/g.The anisotropy and domain structure are also discussed.The increase of Hc will affect H_EB,which has important application value in spintronic devices such as spin valves,sensors and magnetic tunnel junction.(2)The core-shell nanofibers were obtained at different heat treatment temperatures by controlling the temperature during heat treatment when the molar ratio of core and shell was the same.The magnetic properties and exchange bias effects of CoFe₂O₄@NiO nanofibers at different heat treatment temperatures were also investigated.With the increase of heat treatment temperature,the crystallinity of nanofibers becomes better and better.At 500?,the nanofibers are continuous and uniform,the obvious core-shell structure and good heterogeneous coupling interface are obtained.Hc and H_EBfirst increased and then decreased,and reached the maximum value when annealing temperature was 550?,with M_Sof 42.7 emu/g and Hc of 958 Oe(measured temperature was 300 K).Ms was 47.2 emu/g,Hc was 12653Oe,and H_EBwas 758 Oe(measured at 5 K).This is because with the increase of annealing temperature,as the distance between the core and the shell changes,the coupling effect at the interface increases,the coercion force increases and the exchange bias effect increases.The results of this study provide important applications for spintronic and magnetic recording devices.