Preparation And Magnetic Properties Of CoFe₂O₄ And Its Composite Nanofibers

With the progress of science and technology,the miniaturization and lightweight of equipment become the mainstream of development,further promoting the development of nanofibers.Electrospinning technology has become one of the main ways to effectively prepare nanofibers because of its advantages such as simple equipment,low spinning cost and controllable process.CoFe₂O₄ and CoFe₂O₄/ZnFe₂O₄ nanofibers and particles were prepared by electrospinning and hydrothermal methods.In electrospinning,metal nitrates were used as raw materials,ethanol,deionized water and dimethylformamide（DMF）as solvent,and Polyvinylpyrrolidone（PVP）as viscosity agent.The structure,morphology and magnetic properties of CoFe₂O₄ and CoFe₂O₄/ZnFe₂O₄ nanofibers and nanoparticles were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and vibration sample magnetometer（VSM）.（1）Electrospinning technology has been widely used in the preparation of nanofibers.Therefore,nanofibers with different morphology,diameter and orientation can be prepared by adjusting the rotating speed,electric field distance,structure and width of the collector.This is because by controlling the electric field distance,the splitting and stretching time of the precursor can be changed to obtain nanofibers with different diameters.The rotating speed of the drum will change the force of the precursor on the collection surface,forming two kinds of CoFe₂O₄ nanofibers and nanoribbons with different morphologies.The M_s and H_c of nanofibers and nanoribbons are 83.77 emu/g,616.11 Oe and 75.18 emu/g,1145.39 Oe,respectively.The research in this field is of great significance for the production and application of nanofibers.（2）CoFe₂O₄ nanoparticles and CoFe₂O₄ aligned and unaligned nanofibers were prepared by hydrothermal method and electrospinning method respectively.At the same time,the nanofibers with high orientation are collected by roller collecting device.After annealing at the same temperature of 700℃,it was found that its diameter was similar.By studying the hysteresis loops of CoFe₂O₄ nanoarrays,it is found that they have strong anisotropy.The easy axis is parallel to the fiber.The H_c and M_r of easy axis and hard axis are 1330.5 Oe,32.39 emu/g,857.2 Oe and 24.8emu/g,respectively.The M_r and H_c of CoFe₂O₄ nanoparticles are 32.33 emu/g and979.3 Oe,respectively.The change of hysteresis loop and magnetic moment during demagnetization of CoFe₂O₄ nanofiber array was simulated by micro magnetic software.The simulated H_c is 1480 Oe,which is close to the experimental value.Therefore,it will be a low-cost method to prepare nanofiber arrays.（3）The CoFe₂O₄/ZnFe₂O₄ nanofibers and nanoparticles with a molar ratio of1:1 and different annealing temperature were prepared by electrospinning and hydrothermal methods.By analyzing the hysteresis loop,it is found that M_s and H_c increase with the increase of annealing temperature when the sample size is smaller than the critical size of single domain.At the same time,it was found that nanofibers have higher M_s,H_c and M_r than nanoparticles,which is caused by the shape anisotropy of nanofibers.The molar ratios of CoFe₂O₄/ZnFe₂O₄ were 1:0.25,1:0.5 and1:0.75 respectively.It is found that the M_s is higher when the ratio is 1:0.25 than that of a single CoFe₂O₄ fiber,and it is found that the soft and hard magnetic materials with the ratio have the strongest exchange coupling.Through the analysis of the distribution of the switching field and theδm curve,it is found that there are Exchange Spring phases between the soft magnetic and hard magnetic,and the exchange effect of nanofibers is stronger than that of nanoparticles.