Preparation And Magnetic Properties Of Hard @ Soft Ferrite Composite Nanofibers

Compared with rare earth permanent magnet materials with better magnetic properties,permanent magnet ferrites have better temperature stability and chemical stability,and are still ideal permanent magnet materials in many fields.To improve the comprehensive magnetic properties of permanent magnet ferrite and expand its application has become the primary research problem.With the introduction of exchange coupling,it has become a hot topic in the field of scientific research to prepare hard and soft magnetic composites with high comprehensive magnetic properties by using the exchange coupling effect combined with the characteristics of high coercivity of hard magnetic materials and the characteristics of high saturation magnetization of soft magnetic materials.The fibrosis of the composite material can improve the anisotropy of the material morphology,and the core-shell structure can make the two phases orderly coupling and the magnetization inversion interface stable,which is conducive to the improvement of magnetic properties.In this paper,core-shell composite nanofibers such as Sr Fe₁₂O₁₉@Zn Fe₂O₄（SFO@ZFO）were prepared by coaxial electrostatic spinning,and the magnetic properties and exchange coupling effects of the composites were studied under different conditions.（1）SFO@ZFO core-shell nanofibers with different heat treatment temperatures were obtained by keeping the same core-shell molar ratio and changing the heat treatment temperature.The sample is composed of two phases,and its structure changes from fiber-coated particles to core-shell fibers,and finally forms a dispersed sheet structure.The core-shell structure nanofibers have obvious coupling interface and high aspect ratio.The magnetic properties of the composites reach the highest Ms（43 emu/g）and Hc（4524 Oe）at 900℃,which is superior to other laminar and granular materials.The influence of heat treatment temperature on magnetic properties and exchange coupling effect of core-shell composite permanent magnet was investigated（2）Core-shell nanofibers with different hard and soft magnetic molar ratios were obtained by keeping the heat treatment temperature unchanged and changing the core-shell molar ratios.The relationship between different core-shell molar ratios of SFO@ZFO nanocomposites and their structural characteristics,magnetic properties and exchange coupling was investigated.All samples show that Sr Fe₁₂O₁₉and Zn Fe₂O₄phases coexist in different proportions.The magnetic properties of composites increase first and then decrease.When the hardness ratio is 2,the magnetic properties reach the highest Ms（55 emu/g）and Hc（4745 Oe）,which are superior to other morphs.By adjusting the ratio of soft and hard phases,the exchange coupling effect is enhanced and the magnetic properties of the composites are improved.（3）The core-shell nanofibers of SFO@ZFO,Sr Fe₁₂O₁₉@Co Fe₂O₄（SFO@CFO）and Sr Fe₁₂O₁₉@Ni Fe₂O₄（SFO@NFO）were prepared by changing the types of soft magnetic phases in the shell.All the composites are composed of pure Sr Fe₁₂O₁₉and different spinel phases.SFO@ZFO and SFO@NFO have a distinct core-shell structure,while SFO@CFO fibers are laminated and granular.The hysteresis loops of several composites show single-phase characteristics,indicating the existence of exchange coupling between hard and soft magnetic phases.SFO@CFO showed the highest Ms（69.43 emu/g）and Mr（33.82 emu/g）,and SFO@ZFO had the highest Hc（2924 Oe）.The magnetic properties of composites are explained from the aspects of nanofiber diameter,exchange coupling between two phases and the nature of soft magnetic phase.It shows that the exchange coupling effect of composite permanent magnet material is closely related to the type of composite material.