Study On The Microstructure And Electromagnetic Properties Of Rare-Earth Substituted Hexagonal Ferrites

With the development of radar, microwave communication technology and especially the need for anti-electromagnetic interference coatings, self-concealing technology and microwave darkrooms, the study of electromagnetic wave absorbing materials has increased in recent years. Hexaferrites with planar magnetic anisotropy, high natural resonance frequency, high thermal stability, are greatly used as electromagnetic wave absorbers in GHz range. Research shows that, the dielectric and magnetic properties such as resistivity, permeability and saturation magnetization can be controlled by substitution of divalent or trivalent ions. Rare-earth elements (RE) ions have unpaired 4f electrons and the strong spin-orbit coupling of the angular momentum. Doping rare earth ions into barium ferrites, the unpaired electrons in 4f orbit might improve the electrical and magnetic properties. At present, a number of investigations have been reported for studying the effect of Cu2+,Co2+, Zn2+,Ni2+,Mg2+,Sr2+ on magnetic properties of ferrite. Systematic study for the effects of rare-earth ions on the microstructure and electromagnetic properties of hexaferrite has been scarcely reported. We aim to seek and provide valuable information about microstructure and electromagnetic properties of RE substituted hexaferrites. In the present work, the La, Nd, Sm-substitution W-type hexaferrites, Tb, Er, Yb-substitution W-type hexaferrites, La, Nd, Sm-substitution Z-type hexaferrites, were synthesized and the effect of the substitution of RE3+ for Ba2+ on the microstructures and electromagnetic properties of the hexaferrites was investigated. In addition, the relationship between electromagnetic properties and microstructure for samples was investigated. X-ray diffraction (XRD), scanning electron microscope (SEM), vibrating sample magnetometer (VSM), and vector network analyzer (VNA) were used to analyze the structures, electromagnetic and microwave absorption properties.RE-substituted barium hexaferrites, Ba1-xRExCo2Fe16O27 (RE=La3+,Nd3+,Sm3+, Ba1-xRExCo2Fe16O27 (RE=Tb3+, Er3,Yb3+) and Ba3-xRExCo2Fe24O41 (RE=La3,Nd3,Sm3+), were prepared by conventional ceramic sintering method. Optimum heat treatment conditions of W-type and Z-type hexaferrites are 1250℃/3h和1250℃/5h, respectively. X-ray diffraction diagrams confirmed that all the samples exhibit a well-defined hexagonal W-type phase. Value of lattice parameters a remains almost constant, while that of c slightly decreases of the W-type hexaferrites after rare-earth is doped. This suggests that the substitution ions, which have been placed on the crystal sites of barium ferrite, are expected to affect the electromagnetic properties of this material. The a and c of the hexaferrites Z-type increase with increasing La, Nd, Sm content, the crystal structure expands. SEM micrograph of the samples is evident that the grains are typical of platelet morphology. This is useful for absorbing materials.The static magnetic properties of the samples were studied using vibrating sample magnetometer (VSM). The results reveal that, by introducing a relatively small amount of RE instead of Ba2+, an important modification of static magnetic properties for W-type and Z-type hexaferiites can be obtained. The right amount of La, Nd|, Sm, Tb, Er and Yb substitution can increase saturation magnetization in the range of 10.1～28.6%, respectively, and La, Nd|, Sm substitution made saturation magnetization for Z-type hexaferrite increased by 26.0,29.4 and 31.3%, respectively. The saturation magnetization and coercivity for W-type hexaferiites increase the after RE3+ is doped. The saturation magnetization for Z-type hexaferiites increases and the coercivity goes down with the RE content.The complex permittivity and complex permeability of the samples were studied using vector network analyzer (VNA). The results reveal that, the complex permittivity and complex permeability for RE-substitution W-type hexaferrites is a clear variation. It is of reliable theoretical foundation and experimental basis in the future study of rare-earth substituted hexaferrites. However, variation of these parameters for RE-substitution Z-type hexaferrites is not clear. It is caused by the rare-earth ions random replacement of Ba+ in the complex crystal structure of Z-type ferrite complex. The analysis of the electromagnetic parameters showed that, the microwave absorbency of the W-type ferrite increases with more addition of RE ions. When the substitution amount of La, Nd and Sm are 0.25,0.25 and 0.20, the microwave absorbency of samples is up to maximum, which increase 1.69,2.12 and 1.85 times compared with samples that is not substituted, respectively.