Magnetic Propetries And M（o|¨）ssbauer Studies Of Ferrite/SiO₂Nanocomposites

M（o|¨）ssbauer effect is the nucleus of recoilless γ-ray emission and absorption.M（o|¨）ssbauer spectrum will be affected by the outside environment in the nucleus for thehyperfine energy levels of the nucleus having a relation with outside environment. So,we can get informations of atomic hyperfine field, valence state, and symmetrythrough researching M（o|¨）ssbauer spectrum parameters. It makes M（o|¨）ssbauer effect hasbeen widely used in Physics, Chemistry, Materials Science and Biology. Among them,the study of magnetic materials has become one of the most frequently used areas inM（o|¨）ssbauer spectroscopy.Nanocomposites being a kind of new composite materials are booming, whichinvolves multi-disciplinary professional basic knowledge, such as MaterialsChemistry, Material Physics, Organic Materials, Solid State Physics, Chemistry andPhysics of Polymers and Functional Material. New nanocomposites has become aresearch hot spot in those areas for transforming traditional polymeric materials instructure, the applicability and potential.Cobalt ferrite （CoFe₂O₄） is broad applied in magnetic recording, magneto-opticalrecording, electronics and other fields for having the good electromagnetic properties. Nickel ferrite （NiFe₂O₄） is a common soft magnetic material. Being as the materialsof magnetic head, moment of magnetic, microwave absorbing and magnetic hysteresisshrinkage, is widely used in electronic industry. In addition, nano NiFe₂O₄ferrite isapplied to form a complete set of small micro lithium battery or thin film lithiumbattery for having good chemical properties. Scholars discovered that it can be metthe application demand of ferrite in different fields through replacing cation. Becausethe magnetism of ferrite strongly is depend on its chemical components. Even smallamounts of cation substitution can make the magnetism of the ferrite changeobviously. Substituting non-magnetic cation (Al³⁺) for magnetic ions (Fe³⁺) can helpus study the magnetic interactions in ferrite. Some researches have been reported forCo-Al and Ni-Al ferrite nano materials, but there were still some problems. Forexample, the appearances of the aggregate of the nano particles and the non-collinearstructure in ferrimagnetisms can not be avoided. Then their correspondingnanocomposites can solve those problems. In this paper, we used X-ray diffraction（XRD）, vibrating sample magnetometer （VSM） and M（o|¨）ssbauer spectroscopy （MS） tostudy the structural and magnetic properties of CoAl_xFe_2-xO₄/SiO₂andNiAl_xFe_2-xO₄/SiO₂nanocomposites prepared by sol-gel method.In chapter1, we first give a brief introduction to the theory of M（o|¨）ssbauer effectand its application in the magnetic materials. Then, the definition, properties,preparation method of nanocomposites and the struction of spinel ferrite are given. Atlast, we point the basis and research contents of the paper.In chapter2, we give a description on the experiment reagent，experimentalapparatus and experimental measurement during the process of preparingnanocomposites.In chapter3, we study the structural, magnetic properties and cation distributionof CoAl_xFe_2-xO₄/SiO₂nanocomposites. The cations distribution determinedcombining the results of MS and XRD indicate that Al³⁺besides preferring octahedral（B） sites, have also a small tendency to occupy tetragonal （A） sites. And thepreference of Al³⁺to occupy B-site increases with increasing Al³⁺content. MS showthat the samples of CoAl_0.2Fe_1.8O₄/SiO₂nanocomposites transfer from mixed state of superparamagnetic and magnetic order to the completely magnetic order withincreasing of annealing temperature, and the relative intensity of magnetic superfinesplit sextet corresponds to Fe3+at the surface decreases with increasing grain size.In chapter4, we study the structural and magnetic properties ofNiAl_xFe_2-xO₄/SiO₂nanocomposites. MS and VSM reveal that the samples transferfrom the completely magnetic order to superparamagnetism as Al³⁺content increasesfrom0to1.0. In addition, the samples of NiAl_xFe_2-xO₄/SiO₂（x=0.2,0.4,0.6）is Néelcollinear structure and the sample of x=0is an inverse spinel exhibiting non-collinearspin structure.In chapter5, we research the structural and magnetic propertiesCoAl_xFe_2-xO₄/SiO₂nanocomposites thin films and the effect of annealing temperatureon the magnetic properties of CoAl_0.2Fe_1.8O₄/SiO₂nanocomposites thin film. VSMreveal that the thin films have perpendicular magnetic anisotropy and it is distinct forthe samples of x=0.2and0.6. In addition, the coercivity of sampleCoAl_0.2Fe_1.8O₄/SiO₂thin film tends to increase firstly then decrease with increasingannealing temperature and the perpendicular magnetic anisotropy is obvious for thesamples annealed at1000and1100oC.In conclusion, we obtain the fowling results: SiO₂matrix was amorphous in thenanocomposites （the mass ratio of ferrite and SiO₂was70:30）. SiO₂is a space of theferrite nanoparticles nucleation, but also effectively restrain the agglomeration andgrow up of nanoparticles. In addition, it can minimize the spin disorder in theferromagnetic materials. Non-magnetic cation Al³⁺have a preference to occupy B-site,and substituting Al³⁺for Fe³⁺will reduce the ferromagnetic and magnetic coupling inferrite.