Structural, magnetic and dielectric properties in (Bi, Ln) FeO3 ( Ln = La, Dy) bulks and nanoparticles

[[abstract]]Multiferroic bismuth ferrite (BiFeO3) has gained a considerable attention from the aspects of technology and fundamental science due to the coexistence of (anti)ferromagnetic and ferroelectric properties (so called multiferroics), which allows an additional degree of freedom for the design of spintronic devices. Moreover, the high transition temperature of BiFeO3 is the most important advantage for application in practice. In this master thesis, we prepared series of Bi1-xLnxFeO3 (Ln = La and Dy, x = 0 ~ 0.40) polycrystals by solid state reaction method. Moreover, sol-gel method is used to synthesize pure Bi1-xDyxFeO3 (x = 0 ~ 0.40). Their structural, magnetic and dielectric properties are systematically investigated. The crystal structures start transforming from rhombohedral to cubic ones near x = 0.15 for La-series and to orthorhombic ones near x = 0.10 for Dy-series. For samples prepared by solid state method, it is interesting to find that the room temperature magnetization at 20 kOe (Meff ) for Dy-doped samples are one-order larger compared with that of La-doped samples, which is ascribed to the suppression of cycloidal spin structure. Furthermore, the dielectric constant is enhanced greatly from 68 to 300 with Dy = 0.10 to 0.30, which should be associated with the structural instability during the transformation. Finally, the grain size of Dy- series prepared by sol-gel method is in nanoscale. Those nanoparticles show the possibility of superparamagnetic behavior due to the finite coercive field (Hc) and exchange bias (He). At low temperature (10 K), the anharmonic cycloidal spin structure is attributed to the larg Meff . The restrained leakage problem in nanoscaled Dy- series proves the ability of sol-gel method to control the leakage problem.