Property Modulation And Microstructures Of Multiferroic(Double-) Perovskite Oxides

Multiferroic materials possess two or more ferroic orders simultaneously,such as ferroelectricity,ferromagnetism or ferroelasticity,and they exhibit the coupling between them.The unique feature of magnetoelectric(ME)multiferroic materials allows the switching of electric polarization by external magnetic field or vice versa,and hence leads to very promising applications in memories,sensors,spintronic devices,and so on.However,the single-phase ME multiferroic materials are very rare,among them BiFeO3 with perovskite structure has attracted much attention due to its coexistence of ferroelectricity and antferromagnetism at room temperature.Its ferroelectricity originates from the ionic displacement induced by 6s2 lone-pair electrons of Bi3+ion,the antiferromagnetism is resulted from BiFeO3 spiral spin structure with a periodicity of 62-64 nm,which cancels its net magnetism in bulk and leads to a very weak magnetism,restricting the magnetoelectric coupling effect.Moreover,the high leakage current induced by the volatilization of Bi element,valance fluctuation of Fe,and the formation of oxygen vacancies during synthesis process,also hinders its practical applications.To solve the above issues,in this thesis we utilize two kinds of technical routes to improve the magnetism of BiFeO3 and reduce its leakage current.First,spatial confinement such as synthesizing nanoparticles,is used to suppress the spiral spin structure of BiFeO3 and thereby enhance its magnetism.Then,element doping which could introduce chemical pressure,is also utilized to suppress the spiral spin structure,and consequently enhance its magnetism.In addition,element doping could also stabilize the crystal structure to reduce the oxygen vacancies and to decrease its leakage current.Finally,based on the substitution by Cr with exact half of B-site cations,double perovskite Bi2FeCrO6 ceramics are synthesized by solid state reaction under high pressure and temperature,and their crystal structures,magnetic and dielectric properties are also investigated.The main conclusions in this thesis are listed as followings:1.Pure phase BiFeO3 nanoparticles with relatively small sizes were synthesized via sol-gel process.The size distribution of the nanoparticles is in the range of 30-200 um.Due to the existence of nanoparticles with sizes smaller than 62 nm,the BiFeO3 nanoparticles exhibit weak ferromagnetism.In addition,the coexistence of ferromagnetic and antiferromagnetic moments leads to the appearance of exchange bias effect.Moreover,the FC/ZFC curves reveal the existences of ferromagnetism in the BiFeO3 nanoparticles and the superparamagnetic behavior due to the small particle sizes.2.Element doping effects on the physical properties and microstructures of BiFeO3 nanoparticles by La,Pr and Ba doping at A site and Ba/Cr co-doping at both A and B sites are particularly investigated The results showed that with either La,Pr or Ba as dopant,the BiFeO3 nanoparticles undergo a structural distortion with a certain degree,and thereby the spiral spin structure is suppressed,enhancing the magnetism at room temperature.In particular,when the Ba doping concentration was 20mol%,the BiFeCh nanoparticles had the largest remanent magnetization of 0.51 emu/g among the Ba-doped nanoparticles,which is two orders of magnitude larger than the pristine BiFeO3 nanoparticles.In addition,Ba as a dopant could help to stabilize the crystal structure of BiFeO3 and to hinder the formation of oxygen concentration,and thus reduce the leakage currents.We also introduce Cr as a second dopant based on the Ba-doped BiFeO3 nanoparticles.However,when increasing the Cr doping concentration,the impurity phases show a tendency to emerge,and the magnetism is also decreased.3.Based on the element doping,Cr with exactly half of the B-site cations is doped to BiFeO3,and Bi2FeCr06 ceramics with double perovskite structure were synthesized by solid-state reaction under high pressure and temperature.The Rietveld refinements of the XRD data indicated that the Bi2FeCrO6 ceramics crystallized in a rhombohedral structure with R3c space group,and the lattice parameters were determined to be a=5.5560 A and c=13.7329 A,respectively.SEM images demonstrate that the ceramic grains have a polyhedron morphology and contact closely with each other,and the average grain size is measured to be 2.80 ?m.Core level XPS peaks of Cr 2p3/2 and Fe 2p3/2 reveal that dual oxidation states of Fe ions(Fe2+and Fe3+)and Cr ions(Cr3+and Cr6+)exist in the ceramics.The O Is XPS peak confirms the existence of oxygen in the forms of crystal lattice oxygen(from[FeO6]and[CrO6]octahedra)and the adsorbed oxygen.Magnetic measurements show that the Bi2FeCr06 ceramics exhibit a weak ferromagnetic behavior under a strong antiferromagnetic background.Due to the collaboration between the ferromagnetic and antiferromagnetic behaviors,the M-H loops exhibit an exchange bias effect.Bi2FeCrO6 ceramics possess relatively large dielectric constant and small dielectric loss,which shows potential applications in the dielectric devices.