Research On Changes Of Microstructure And Magnetic Properties Of W-type Hexaferrite By Doping

Magnetoplumbite-type hexagonal ferrites typified by SrO·6Fe₂O₃,that is,M-type ferrites have hitherto been dominantly used for ferrite sintered magnets.As for such M-type ferrite magnets,efforts have been made to achieve very high performances.Therefore,focusing on that ferrite grain sizes are made closer to the single domain grain sizes,that ferrite grains are aligned in a magnetic anisotropic direction,and that high densities are achieved in that concerned magnets.Furthermore,as a result of such efforts the properties of M-type ferrite magnets have approached to the upper limits,so that it is difficult to expect drastic improvements of the magnetic properties under the present circumstances.The W-type ferrite magnet has been known as a ferrite magnet that has potentiality to surpass in magnetic properties the M-type ferrite magnet.The W-type ferrite magnet is higher by about 10%instaurations magnetization than the M-type ferrite magnet and of the same order in anisotropic magnetic field as the M-type ferrite magnet.W-type ferrite has divalent cation sites,which can be replaced by a trivalent cation or variety of divalent cation,so that W-type ferrite can changed the magnetic properties more widely than M-type ferrite.So we need to find a kind of permanent magnet materials with high performance and stability,and the W-type hexaferrite is one of the important direction.W-type hexaferrites?BaMe2Fei6019?Presents relatively large saturation magnetization,large magnetic crystalline anisotropy,excellent chemical stability and are widely used as microwave device,magnetic optical,magnetic recording medium and permanent magnet.Many attempts have been made to improve magnetic properties of W-type hexagonal ferrites concerning doping in order to fulfill various applications.In this paper,the ions substituted W-type hexaferrites were prepared by the sol-gel and the solid state reaction methodologies.The phase structure of magnetic powder was carried out by X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?.A field emission scanning electron microscope were used to observe the microstructure of the magnet.The magnetic properties of W-type hexaferrites were metric by the vibrating sample magnetometer?VSM?and the permanent magnetic measuring apparatus respectively.The conclusions are as following:1.A series of W-type hexagonal ferrites with the chemical composition Ba_1-xCa_xCo₂Fe₁₆O₂₇?x = 0,0.1,0.3,0.4 and 0.5?were synthesized by using a sol-gel method.The effects of doping on structural and magnetic properties are studied.X-ray diffraction diagrams confirmed that all the samples exhibit a well-defined hexagonal W-type Phase.The values of lattice parameters a remains almost constant,while that of c slightly deceases of the W-type hexaferrites after calcium doping.The crystal cell volume also decreases due to the substitution ions,which have been placed on the crystal sites of barium ferrite.SEM micrograph of the samples is evident that the grain size particles are the hexagonal structures and the particles are evenly distributed.The influence of calcium on magnetic properties of the sintered magnets was studied systematically.With the Ca addition increases gradually as a general trend,and achieved the maximum value 63.6 A m2/kg at x = 0.50.The magnetic properties of the magnets can better by Ca doped strontium magnets.2.W-type hexaferrites with chemical composition of Ba1-xLaxZn2Fe16O27?x ?0-0.25?were synthesized by traditional ceramics process.Effects of the substituted amount x of La³⁺ on the ferrites microstructure and magnetic properties have been investigated.As a result,at x<0.20,all samples are single phase after the La substitution.At x = 0.25,another phase of La₂O₃ is present.The SEM images of the samples show that the grains of the samples are hexagonal structure and the particles are distributed evenly.The lattice constant a and c decreases continuously with the increasing of La³⁺ content,which leads to the decreasing of the unit cell volume V,as well as the increasing of the porosity.The saturation magnetization?Ms?increases first and then decreases.The maximum value of the saturation magnetization 71.38Am2/kg at x =0.10,which increased by 22.1%than un-substituted W-type hexaferrite.The coercivity increases first,then decreases and again increases.The X-ray photoelectron spectroscopy study shows that the valence states of Fe are Fe³⁺trivalent state in un-substituted W-type hexaferrite.It can be seen that there is a valence states of La³⁺,while its change to Fe³⁺ + Fe2+ in La-substituted hexaferrite.It is confirmed that the La³⁺ ions substitution can transform a part of Fe³⁺ ions to Fe2+ions by X-ray photoelectron spectroscopy data.That means magnetic properties of the magnets can better by La doped barium magnets.3.W-type hexaferrites with chemical composition of BaFe²⁺₂Fe³⁺_16-xGd_xO₂₇?x=0-0.20?were synthesized by traditional ceramics process.Effects of drying temperature,sintering temperatures and the substituted amount x of Gd³⁺,on the ferrites microstructure and magnetic properties have been observed.W-type ferrite having chemical formula Dare2+2re³⁺16O27,this compound consisted of multiphases?Fe3O4 and Fe2W?at 170? and the Fe2W single phase of these samples was obtained by drying temperature from 180? to 200?.But the sample drying at 210? consists of mixed phases of ?-Fe₂O₃,BaM and Fe2W phases.For the W-type BaFe2+2Fe³⁺16-xGdxO27?x=0-0.20?,at x ? 0.15,all samples are single phase after the Gd substitution.At x = 0.20,another phase of Gd2O₃ is present.SEM micrograph of the samples is evident that the grains are the hexagonal structures.The lattice constant a and c increases continuously with the increasing of Gd³⁺ content,which leads to the increasing of the unit cell volume.It can be seen that there is a valence states of Gd³⁺in the X-ray photoelectron spectroscopy of Gd-substituted hexaferrite.When the Gd³⁺doping content x from 0 to 0.05,the samples of the saturation magnetization?Ms?,remanence?Br?and the maximum energy product?BH?.increase,after that x increases from 0.05 to 0.15,then it start decreased.The intrinsic coercivity?Hcjj?and the magnetic coercive force?Hcb?of the samples increase with the Gd³⁺ doping content x from 0 to 0.10,and then decreases.That means magnetic properties of the magnets can change by Gd doped barium magnets.