Preparation And Multiferroic Properties Of A Site Doped BiFeO₃ Nanoparticles

Multiferroic materials are a term coined by Schmid for crystals "in which two or all three of the basic properties occur simultaneously in the same phase, which are ferroelectricity, ferromagnetism and ferroelasticity". Multiferroic materials not only exhibit the performances of ferroelectricity, ferromagnetism (antiferromagnetism) and ferroelectricity (anti ferroelectricity), but also the strong coupling effect, which makes multiferroics possess of potiential properties that is different from the basic properties mentioned above. So, multiferroic materials become a time honored research subject in materials subject.With the development of the technological, the miniaturization and diversification of electronic devices, it is both scientifically interesting and technologically challenging to combine the function of multiferroic materials and finite-size effect of nanoparticles to further meet the needs of development of science, which makes the multiferroic nano-materials become the hot research subject. However, there are very few materials that can exhibit both magnetization and electric polarization. Among the rare multiferroic materials, BiFeO3 (BFO) is the most extensively studied compound. It is one of the well-known single-phase multiferroic materials with a distorted perovskite (ABO3) structure and its ferroelectric transition temperature (Tc～1103 K) and antiferromagnetic Neel temperature (TN～643 K) are well above the room-temperature (RT). But the major problem is that the magnetic ordering of BFO does not allow net magnetization and the second phase always exists. The alkaline earth ions'substitution at the Bi site in bulk BiFeO3 is found to improve dielectric and magnetic properties etc. So, we combinate the cation ionic effect of doping ions and size effect of nanoparticles, to prepare samples of Ba doped BiFeO3 nanoparticles, Ba and Ca codoped BiFeO3 nanoparticles, and La,Ca codoped BiFeO3 nanoparticles by the sol-gel methods, in order to improve multiferroic properties of BiFeO3. In our work, we made a systematic study the influence of the substituting element on structure, magnetic and dielectric properties of the doped BiFeO3. The following is the investigative content and the main conclusions.1. Ba doped nanocrystalline BiFeO3 has been prepared by a sol-gel method based ethylene glycol (EG). Partial substitution of Ba (5%-30%) at the Bi site results in a structural change of BFO nanocrystals from the rhombohedral structure(x=0.05,0.10.0.15) to tetragonal structure (x=0.20, 0.25.0.30) with Ba substitution increased. These results are in agreement with those of the RT Mossbauer spectra of the samples. Accompanying structure transformation, the dispersion of dielectric constant and loss of samples were bracket into two trends, which indicates that dielectric properties of the samples have some connection with the crystalline structure of samples. Moreover, all samples presented the high ratios of Mr/M and large coercivities. This suggests that the BBFOx nanocrystalline is promising for applications in data-storage media.2. In order to further study the influence of the properties of substitutional ion in Bi-site ions on the structure, magnetic and dielectric properties, we studied the Bi0.8Ca0.2-xBaxFeO3 (BCBFOx) (x=0.07,0.10,0.13,0.19 and 0.20 respectively) nanoparticles. The lattice structure of the nanoparticles transformed from orthorhombic (x=0.07,0.10,0.13 and 0.19) to tetragonal (x=0.20) with x increased. Moreover, magnetic properties of the samples were greatly improved for all samples. The abnormal phenomenon of the value of Mr and magnetization of sample for x=0.20 may be caused by the common action of the average ion radii of the substituting element, the crystal structure of the samples and the average grain size of sample.3. In order to obtain the singe-phase samples of Ca doped BiFeO3, La ions used to stablize perovskite structure were selected to modify the magnetic and dielectric properties of BiFeO3 together with Ca and La ions. Bi0.9-xLa0.1CaxFeO3 nanocrystals (BLCFOOx, x=0,0.01,0.13,0.17,0.20 respectively) were prepared by a sol-gel method. X-ray diffraction analysis showed that the average grain size of the prepared samples lay in the range of 24-19 nm. The lattice structure of the nanoparticles transformed from orthorhombic to cubic with Ca substitution increased. Dielectric properties were measured up to high frequency～80 MHz. It was found that the La and Ca codopant were helpful to improve the dielectric properties of the samples. Magnetic properties of the samples were improved and all the samples presented a wasp-waisted hysteresis loop, which was very rarely reported in BFO. Characteristics of wasp-waisted hysteresis loops show low hysteresis loss in low magnetic fields, and is helpful to design some multiferroic equipments.