Fabrication,Micro-Structure And Magnetic Properties Of Spinel Ferrite Nanofibers

Spinel ferrite（MFe₂O₄）is widely used in biomedicine,microwave absorbing materials,lithium battery anode,high density storage and microwave communication devices because of its unique and tunable magnetic-electrical properties.In spinel ferrite structure,superexchange interaction between cations and oxygen ions have a crucial influence on magnetic properties.Therefore,the magnetism of spinel ferrite is tunable via the control of cation distributions.In this thesis,the atom occupations of zinc ferrite single-particle-chain nanofibers were imaged by aberration-corrected scanning transmission electron microscope（Cs corrected TEM）.The relationship between the microstructure and the macro magnetism was emplored on the basis of the experimental data including general TEM and macroscale physical property measurement.Effect of non-magneic zinc ions and magnetic rare earth doping on microstructure and macro-magnetism was studied too.This work should be significant for the basic study on the microstructure of spinel ferrite and the control of its properties.The main contents are as follows:1.Fabrication,characterization by Cs corrected TEM and magnetic properties of single-particle-chain zinc ferriteZnFe₂O₄ nanofibers with single-particle-chain morphology were prepared and the morphology,microstructure and element distribution were characterized.The results show that the structure of nanofibers is continuous,the elements are evenly distributed,and the nanofibers are in the shape of single-particle-chain.Then,the atomic images from six orientations were obtained by Cs corrected TEM,indicated normal spinel structure of zinc ferrite.Simulation software was used to simulate the atomic images,and the simulation results were in good agreement with the experimental results.Magnetic characterization shows that the sample is paramagnetic at room temperature,and shows hysteresis behavior with high H_c at 2K.The 1/χ-T curve agrees with Curie-Weiss law.T_irr is 25 K,T_B is 17 K and effective anisotropy constant K=123 J/m³.The superparamagnetic critical size is 45 nm,which means both superparamagnetic and antiferromagnetic structures exist.The low magnetization may be related to the antiferromagnetic phase of zinc ferrite,and the high coercive force may be caused by the pinning of surface defects.2.Effects of Non-magnetic Zn ion doping on the microstructure and magnetic properties of NiFe₂O₄ nanofibersNi_1-xZn_xFe₂O₄ were prepared and effects of Zn doping on the morphology,crystal structure and element distribution were characterized.The results showed that the morphology of sample remain nanofibers after zinc doping,and the structure remain face centered cubic（fcc）.The lattice constant increase while the grain size first increases and then decreases with the increase of x.In FTIR spectrum,the peak related with tetrahedral metal-oxygen bond is shifted to lower wave number,which shows that the A sites lattice increases during doping,the reason is nonmagnetic zinc ions tend to occupy A sites,replacing iron ions with smaller ion radius.Then the results captured by Cs corrected TEM show that zinc ions do not only occupied the A sites,but also occupied part of the B sites.Finally,Magnetic characterization show that with the increament of zinc ions,zinc ions occupied A sites and reduce the magnetic moment of A sites,cause increment of n_B.After reaching the critical value,zinc ions doping weakens superexchange interaction between A sites and B sites thereby reduces the molecular magnetic moment.Zinc ions at B sites result in smaller increament in molecular magnetic moment.This corresponds with the microstructure characterization of Cs-corrected TEM.The H_c tends to decrease.Calculation proves that nanofibers are all single domain structure and the magnetization process is governed by coherent rotation.3.Effects of magnetic rare earth ion doping on the microstructure and magnetic properties of NiFe₂O₄Effects of three kinds of magnetic rare earth ion doping on the NiFe₂O₄ were studied.The results show that rare earth doping has little effect on the morphology,however crystal size and lattice constant decrease because of rare earth doping.The results of EDX shows that the solubility of rare earth elements in nickel ferrite is different,in which Nd is the highest.Magnetic characterization show that the saturation magnetization of all the doped samples is reduced because of weakening of A-B superexchange interaction and the redistribution of cations.NiSm_0.1Fe_1.9O₄ which was annealed at 800℃produced superparamagnetism caused by the size effect,while the coercivities of other specimens increases because of the large single ion anisotropy of rare earth element and the symmetry breaking of unit cell caused by rare earth doping.At the end,we discussed the effect of two factors in the doping process on magnetic properties.