Preparation And Multiferroic Properties Of Rare-earth Doped BiFeO₃ Nanoparticles

Multiferroic is a term coined by Schmid for crystals "in which two or all three of the properties of ferroelectricity, ferromagnetism and ferroelasticity occur simultaneously in the same phase". At present, attentions have mostly been paid on magnetoelectric multiferroics. Magnetoelectric multiferroics not only exhibit the performances of both ferromagnetism (antiferromagnetism) and ferroelectricity (anti ferroelectricity), but also the magnetoelectric coupling effect. Strong coupling between the polarization and magnetization makes multiferroics possess of potiential applications in information data storage, four-state memory, spintronic devices, transducer, sensor and so on.Multiferroic materials have been widely investigated recently. Among them, BiFeO3 (BFO) is well known to be the only prototypical multiferroic material of single phase that possesses both high ferroelectric Curie temperature (Tc=1143K) and high antiferromagnetic Neel temperature (TN=643K). Structurally, BiFeO3 crystallizes in a rhombohedrally distorted perovskite structure with R3C space group at room temperature, which allows spin canting away from perfect antiferromagnetic ordering. However, the direction of the resulting small moment rotates, superimposing a spiral spin arrangement with a wavelength of 62nm, thereby producing a helimagntic order and vanishing magnetization in the bulk. Thus far, incorporation of bismuth ferrite into practical devices has been hindered by leakage problems that lead to low resistivity, presumably due to defect and nonstoichiometry related issues. Hence, there has been a pressing need to generate high-quality samples.Recently, enhanced magnetization in substrate-free nanostructures of BiFeO3 has been reported and attributed to surface-induced magnetization and ferromagnetism caused by apparent oxygen deficiency. The Neel temperature (TN) of BiFeO3 has been shown to decrease with decreasing particle size. Moreover, the critical size of the ferroelectric phase of BiFeO3 was concluded to be 9±1nm by extrapolation from the size-dependent empirical model. Namely, BiFeO3 nanoparticles with size above 9±1nm are of multiferroic properties and show remarkable magnetization. Some rare earth ions'substitution at the Bi site and transitional metal substitution at the Fe site in bulk BiFeO3 were found to reduce leakage current and Curie temperature, to improve dielectric and magnetic properties etc. In this thesis, a combination of cation ionic effect of doping ions and size effect of nanocrystals was tried to improve multiferroic properties of BiFeO3. Rare earth ions'doped and both rare earth ions and transitional metal ions codoped substrate-free BiFeO3 nanocrystals were prepared by the sol-gel methods. The structure, magnetic and dielectric properties, as well as magnetodielectric coefficient of the doped BiFeO3 were investigated. The following is the investigative content and the main conclusions.1. 20mol% La doped nanocrystalline BiFeO3 were prepared by three different techniques based on a sol-gel method. The effect of techniques and additives used in the preparation processes on structure, grain size, magnetic and dielectric properties of the prepared samples was investigated. It was found that the techniques affected the grain size of the nanocrystalline as follows:The grain size of the sample prepared by ethylene glycol (EG) based sol-gel method was 39nm and the addition of glycin made the reduction of the grain size of the sample to 28nm. When the temperature of the evaporation and drying of the sol was raised to 250℃, the grain size of the samples was 55nm.The grain size of the sample prepared by the sol-gel combustion method was 28nm. In addition, the magntic and dielectric properties of the samples prepared with similar techniques (using same solvent) were strongly affected by the grain size. But the magnetic and dielectric properties of the two samples prepared with different techniques (using different solvents) with same grain size of 28nm were different.2. Dysprosium (Dy) doped nanocrystalline BiFeO3 has been prepared by an ethylene glycol (EG) based sol-gel method. Partial substitution of Dy (0-20%) at the Bi site results in a structural change of BFO nanocrystals from the rhombohedral structure(x=0) to the orthorhombic structure (x=0.10, 0.20). The average grain's size varies from 65nm to 24nm when Dy concentration increases from x=0 to x=0.20. Magnetization M at 1T of Dy doped BiFeO3 (BDFO) increases with increasing Dy concentration and decreasing grain size. Moreover, dielectric properties were measured up to high frequency-95MHz and Dy dopant was found to be helpful to improve dielectric ordering and reduce loss. Surprisingly, the large magneto-dielectric coefficient was found to be 4.7% in the as-prepared BiFeO3 nanoparticles, and -6.3% in Bi0.8Dy0.2FeO3 at H=100Oe and f=75MHz.3. Bi0.8Dy0.2-xLaxFeO3 (BDLFOx) (x=0, 0.10 and 0.20 respectively) nanoparticles were prepared by the EG based sol-gel method. The influence of the average radius and the effective magnetic moment of Bi-site ions on the structure, magnetic and dielectric properties, and the effective magnetic susceptibility of BDLFOx nanoparticles were investigated. All the samples were indexed to the orthorhombic structure based on results of X-ray diffraction (XRD). The grain size of the samples increased with the increase of Bi-site ionic average radius. The magnetization of the samples was found to decrease with the effective magnetic moment of Bi-site ion decreasing. The dielectric constant and loss were improved when Bi-site ionic average radius decreased and the effective magnetic moment of Bi-site ions increased. The enhanced magnetization M and the polarization P in the nanoparticles made the effective magnetic susceptibility of the samples enhanced greatly.4. In order to further improve the multiferroic properties of BiFeO3, Co ions were further selected to substitute Fe ions based on the substitution of Dy ions for Bi ions. During the optimizational preparation of Bi0.8Dy0.2Fe0.99Co0.01O3, it was found that it is helpful to obtain Bi0.8Dy0.2Fe0.99Co0.01O3 only with the minor impurity when the temperature of evaporation and drying was raised to 250℃. Based on the above techniques, glycine was added to further reduce the impurity. Compared to the addition of glycine, polyvinyl alcohol was more helpful to obtain the pure Bi0.8Dy0.2Fe0.99Co0.01O3 nanoparticles. Thus, Bi0.8Dy0.2Fe1-xCoxO3 nanoparticles of single phase (x=0, 0.01,0.02, respectively) were prepared by a sol-gel method using polyvinyl alcohol as a surfactant. It was found that Co-substitution at Fe-site further enhanced the magnetization. Especially, the remanences and coercivities of the nanoparticles were enhanced by an order magnitude when the tiny amount of Co content(x=0.02) was substituted for Fe ion. The enhanced coercivities of the nanoparticles were proportion to Co content. Co co-doped Bi0.8Dy0.2FeO3 nanoparticles also presented the improved dielectric properties in low electric field. Moreover, dielectric constants of the as-prepared nanoparticles were so sensitive to magnetic field that their large magneto-dielectric coefficient was obtained in low field of H=100Oe. The sign of magneto-dielectric coefficient changed from negative to positive with Co content increasing.5. In order to obtain the samples of high Co content, La ions used to stablize perovskite structure were selected to modify the magnetic and dielectric properties of BiFeO3 together with Co ions. Bi0.8La0.2Fe1-xCoxO3 nanocrystals (BLFCOx, x=0,0.005,0.01,0.02,0.05,0.10,0.20 respectively) were prepared by a sol-gel method using polyvinyl alcohol as a surfactant. Partial substitution of Co (0-20%) at the Bi site resulted in a change from the orthorhombic structure(x=0,0.005,0.01 0.02 and 0.05) to the tetragonal structure (x=0.10,0.20) of Bi0.8La0.2Fe1-xCoxO3 nanocrystals. The average grain's size varies from 55nm to 34nm when Co concentration increases from x=0 to x=0.20. The substitution of less than 2mol% Co improved further dielectric properties of Bi0.8La0.2FeO3 nanoparticles in the frequency range of below 25MHz at room temperature, while the samples of high Co content indicated higher dielectric loss. The magnetization at 10KOe, the coercivities and remanences of BLFCOx nanocrystals increased monotonicly when Co content was not more than 5mol%. The magnetization at 10KOe, the coercivities and remanence of the sample of x=0.05 was 2.20emu/g, 0.83emu/g and 1420Oe respectively, which is the maximal values among the reported values of magnetic parameters of rare earth doped BiFeO3. It was interesting that the hysteresis loop of the BLFCOx nanocrystals of x≤0.02 presented a wasp-waisted shape. The shape of M-H loop disappeared when the annealing time during the preparation was prolonged to more than 8h. This showed that the loops were sensitive to the composition of samples and annealing time during the preparation. Based on the property, it could be possible to explore a new way to design new multiferroic applications of low hysteresis loss in low magnetic fields. A large and positive magneto-dielectric coefficient was obtained in the sample of Bi0.8La0.2Fe0.9Co0.1O3 at the applied field of H=100Oe.6. In order to further investigate the effect of transition metal ions'substitution for Fe ions on magnetic and dielectric properties, Cr ions were selected to improve the multiferroic properties of BiFeO3 together with La ions of 20mol%. Bio.8Lao.2Fe1-xCrxO3nanocrystals of nearly single phase (BLFCrOx, x=0,0.005,0.01,0.02, respectively) were prepared by a sol-gel method using polyvinyl alcohol as a surfactant. The effects of Cr substitution on the structure and the electrical and ferroelectric properties of BLFCrOx samples have been studied by performing XRD, TEM, Magnetic Measurements Variable Field Translation Balance (MMVFTB) and a LCR HiTester. The results indicated that the structure of Bi0.8La0.2FeO3 was sensitive to the Cr substitution. A small amount of Cr doping led to structure transformation dramatically. The samples of x=0 and x=0.005 were index to the orthorhombic structure. When Cr content was increased from x=0.005 to x=0.01, the crystal structure of the sample transformated from the orthorhombic structure to the tetragonal structure. The doping of 0.5mol% Cr led to the enhancement of the magnetization at 1T of Bi0.8La0.2FeO3 nanocrystals. But when Cr content was further increased, the magnetization at 1T of the nanocrystals decreased. Cr doping didn't improve dielectric loss of Bio.8Lao.2Fe03 due to the existence of the minor impurity. Under the conditions of the optimal preparation, the grain size of Bi0.8La0.2Fe0.995Cr0.005O3 can be decreased from 52nm to 38nm. The magnetization at 1T of the sample was 1.23emu/g. Dielectric loss of the sample can be improved.