Preparation And Magnetic Properties Of Spinel_ferrites Ni_xZn_1-xFe₂O₄: Nanoparticles, Bulk And Thin Films

Spinel ferrite is a kind of widely used soft magnetic material which can be applied in varied fields such as magnetic devices and switching devices.The performance of spinel ferrite is found to be related to their stoichiometric compositions and microstructures,which are predestined by the preparation processes. As the result,the research on the relationship between the properties of spinel ferrites and the corresponding preparation processes is quite important.The traditional bulk ZnFe₂O₄ belongs to the normal spinel type with antiferromagnetic properties below the N(?)el temperature(T_N) at about 10.5 K and behaves paramagnetic at room temperature.However,in recent years,it was found that T_N will increase when the particle size decreases to nanoscale.Sometimes, superparamagnetism or weak ferromagnetism can even be observed at room temperature.It is considered to be due to uncompensated spins in the two sublattices when the particle size is around nanoscale.Meanwhile,the cation occupancy is simpler compared with other kinds of spinel ferrites since all the Zn²⁺ ions reside in the tetrahedral sites(A sites) and the Fe³⁺ ions are totally in the octahedral sites(B sites).Therefore the change of the cation occupancy can be much easier to be observed.This makes ZnFe₂O₄ a good candidate to be studied on the relationship between the cation occupancies and the preparation processes.NiZn ferrite is one of the most widely used spinel ferrites.It is the best soft magnetic material in the frequency range from 1 MHz to several hundred MHz because of the high resistivity and high permeability.In the same time,researchers pay much attentions on the substitutions based on NiZn ferrite due to the request of improving the performance or lowering down the sintering temperature.With the development of integrated circuits,some of the electronic devices need to be miniaturized and fabricated as planar shape,such as planar resistor,capacitor, inductors,et al.One of the key steps of designing planar electronic devices is to fabricate films with excellent electro-magnetic performance.Due to the diversities of the demagnetic field and microstructure from the bulk material,films behave quite different on a lot of properties.The study on the similarities and differences between the spinel ferrite film and the bulk may help designing spinel ferrite materials with better performance.The main contents of this thesis are as follows:In Chapter 1,the up-to-date development of the nano-materials and some special effects are summarized.Some of the popular fabrication methods of fabricating nano-size materials are introduced.At last,the research purpose of the NiZn nanoparticles,bulk and nanofilms are explained.Chapter 2 describes the crystalline structure and magnetic properties of spinel ferrite.To help better understand the discussions in the following chapters,the principle of M(o|¨)ssbauer effect,hyperfine interactions and the theory of high-frequency magnetic dissipation are explained.The fabrication methods,including PVA sol-gel method,Low-Temperature Solid-State Reaction(LTSSR),NaOH co-precipitation and alternative sputtering, which are employed in the thesis,are introduced in Chapter 3.Chapter 4 introduces the characterization methods and equipments employed in the thesis.The basic principles of them are discussed as well.The magnetic properties of ZnFe₂O₄ nanoparticles fabricated by LTSSR method is discussed in Chapter 5.The relevent results of ZnFe₂O₄ nanoparticles prepared by PVA sol-gel and NaOH co-precipitation methods are also given as comparisons.In Chapter 6,Ni_xZn_1-xFe₂O₄(0.1≤x≤0.9) is fabricated by a NaOH co-precipitation method.The crystalline structures,magnetic properties and permeability are systematically studied.The effect of stoichiometric composition on the performances is discussed.The permeability spectra of the sample with x=0.3 are fitted to estimate the resonant principle.In Chapter 7,Ni_0.15Cu_0.2Zn_0.65Fe₂O₄ ferrite is fabricated by NaOH co-precipitation method.The relationships among the calcining temperatures, sintering temperatures and the crystalline structure as well as magnetic properties are studied.The effect of sintering temperatures on the high-frequency resistivity, magnetic and dielectric properties is discussed in detail to offer a reference on the research of high-frequency impedance matching.Ni_xZn_1-xFe₂O₄(0.2≤x≤0.8) nanofilms are prepared by alternative sputtering with the following heat treatment.The dependence of crystalline structures, morphologies,magnetic properties as well as high-frequency magnetic properties on varied fabrication conditions(stoichiometric composition,thickness,heat-treatment condition,e.g.) are discussed in Chapter 8.The distributions of the magnetic moments in the film are studied by CEMS.The researches in this thesis are summarized and conclusions are given in Chapter 9.The probable future work is discussed.In general,ZnFe₂O₄ nanoparticles are synthesized by different methods.The relationship among the macro-magnetic,micro-magnetic properties and the preparation routes are studied.Ni_xZn_1-xFe₂O₄(0.1≤x≤0.9) and Cu substituted NiZn ferrite(Ni_0.15Cu_0.2Zn_0.65Fe₂O₄) are fabricated by NaOH co-precipitation method.The high-frequency magnetic properties,dielectric properties and the microstructures are studied.Ni_xZn_1-xFe₂O₄(0.2≤x≤0.8) films are prepared by an alternative sputtering method.The effects of sputtering and heat-treatment conditions on the properties of the films are discussed.The high-frequency permeability of NiZn ferrite films is compared with that of sintered NiZn ferrite prepared by NaOH co-precipitation method.These systematic studies can contribute to the study of the applications of NiZn ferrite.Meanwhile,the research methods employed in this thesis can also be applied to other kind of similar ferrites and even other ceramics.