Preparation and Properties of BiFeO3 Doped with Rare Earth Metals and Transition Metals

Author:Shu Hua Zhong

Supervisor:song rong fang

Database:Doctor

Degree Year:2018

Download:15

Pages:112

Size:5290K

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BiFeO3(BFO)is the most widely studied multiferroic functional material.It has great application prospects in new magnetoelectronic sensors,multi-state memory and spintronics.This kind of material has attracted more and more attention because of its unique intrinsic magnetic properties-electromagnetic coupling performance.This kind of material will be magnetized as well as polarized under the action of electric field,so it has magnetism.Under the action of magnetic field,the material will be magnetized as well as polarized,so it also has ferroelectricity.The ferroelectricity and ferromagnetism of the material itself can be regulated and controlled mutually.The structure of BFO crystal is twisted rhombohedrite(ABO3)structure,its space group belongs to R3c,the lattice constant measured is a=b=5.582 nm,C=13.878 nm.It is generally believed that the ferroelectricity of BFO is caused by the non-centrosymmetry of 6s2 lone pair electrons in Bi3+ions at A site,while the magnetism is caused by the net magnetic moment of 3d electrons in Fe3+ions.Although BFO is a multiferroic material,it has both ferroelectricity and ferromagnetism,but it has two obvious shortcomings.Firstly,the high temperature calcination will cause the volatilization of Bi3+ions,the fluctuation of valence state of Fe3+/Fe2+and oxygen vacancy,which will increase the conductivity of BFO and cause the leakage current density of BFO to be very high,thus affecting its ferroelectric properties.Secondly,due to the alternating inversion of the spin magnetic moments of the adjacent transition metal Fe3+in BFO,a space helical self-locking structure with a wavelength of 62 nm is formed,which makes the ferromagnetism of the material itself too weak,and there is a big gap between the practical application and the application.In order to improve the ferroelectric and ferromagnetic properties of BFO materials,the bond length and bond angle of BFO materials are regulated by doping rare earth metal Nd at A site and transition metal elements Cr,Zn and Mn at B site respectively.The slight change of crystal structure breaks the space periodic helical self-locking structure and greatly improves the multiferroic properties of BFO materials.The effects of doping on the structure,morphology,optical properties,ferroelectric properties and magnetic properties of BFO materials were studied in detail.Firstly,pure BFO and(Nd,Cr)co-doped Bi1-xNdxFe0.975Cr0.025O3(x=0.025,0.05 and 0.075)samples with A/B are synthesized by sol-gel technology.The structure,morphology and multiferroic properties of the samples are studied systematically.The results show that the ferroelectric and ferromagnetic properties of BFO are greatly improved by A/B(Nd,Cr)co-doping.The leakage current density of BNFCO samples is about two orders of magnitude lower than that of pure BFO samples,and the residual polarization intensity(Pr)and coercivity field(Ec)are found to be as high as 13.508μC/cm2 and 6.702 kV/cm,respectively,according to the hysteresis loops.The results show that the ferroelectric and ferromagnetic properties of BFO are greatly improved by A/B Nd and Cr.When the doping concentration is 0.075,the residual magnetization intensity(Mr)of the sample reaches 0.157 emu/g,and coercivity(Hc)is up to 4.999 kOe.This shows that the(Nd,Cr)co-doping is an effective way to improve the multiferroic performance of BFO.Secondly,pure BFO and(Nd,Zn)co-doped BFO nanoparticles are prepared by a sol-gel method.The crystal structure,optical and multiferroic properties of the samples are systematically investigated.The structural study is carried out using X-ray diffraction(XRD)pattern and Rietveld refinement shows that all the samples crystallized in rhombhohedral R3c structure.Surface morphology is examined by scanning electron microscope(SEM).An apparent blue shift can be observed in UV-Visible diffuse absorption spectra.The Eg value of the co-doping samples indicated that BFO is a fascinating optical material,which could be applied in photocatalyst and photoconductive devices.The leakage current property can be improved effectively by(Nd,Zn)co-doping.Compared with pure BFO sample,the leakage current density of the x=0.05 sample decreases about three orders of magnitude.The remanent magnetization(Mr)of the x=0.10 sample reaches 0.105 emu/g while the coercive field(Hc)is as high as 7.0 kOe.The(Nd,Zn)co-doping into BFO nanoparticles has been proved to be an effective way to improve the optical and multiferroic properties.Thirdly,pure BFO and(Nd,Mn)co-doped Bi1-xNdx Fe0.975Mn0.025O3(x=0.025,0.05,0.075and 0.10)nanoparticles are synthesized by a sol-gel method.The effects of(Nd,Mn)co-doped samples on the structural and multiferroic properties are systematically investigated.The results showed that the co-doping concentration x had significant influence on the multiferroic properties of the synthesized nanoparticles.Rietveld refinement showed that the samples crystallized in rhombhohedral R3c structure when the concentration x is lower than 0.075.However,when the concentration x increased to 0.10,the structure became dual-phase model of rhombohedral(R3c)and cubic(Pm-3m).Compared with pure BFO sample,the leakage current density of all the(Nd,Mn)co-doped Bi1-xNdxFe0.975Mn0.025O3 samples is much smaller than that of pure BFO.The best Magnetic properties are obtained in the x=0.10 sample where the remanent magnetization(Mr)is as high as 0.116 emu/g and the coercive field(Hc)is up to 8.4 kOe.The co-doping approach provides an effective method to enhance the multiferroic properties of BFO.Fourth,The pure BFO,A-site Y single-doped Bi0.95Y0.05FeO3(BYFO),B-site Co single-doped BiFe0.95Co0.05O3(BFCO)and A/B(Y,Co)co-doped Bi0.95Y0.05Fe0.95Co0.05O3(BYFCO)nanoparticles are prepared by a sol-gel method.The crystal structure and multiferroic properties of the samples are systematically investigated.Results show that all the A/B-site single-doped and co-doped BFO samples can greatly improve ferroelectric and ferromagnetic,among of them A/B-site co-doped BYFCO sample can achieved the best ferroelectric and ferromagnetic properties.In comparison to pure BFO sample,the leakage current density of the BYFCO sample decreases about two orders of magnitude.The remanent magnetization(Mr)of the BYFCO sample reaches 0.085emu/g.The(Y,Co)co-doping into BFO nanoparticles has been proved to be an effective way to improve the multiferroic properties.