Study Of Physical Properties In Highly-polarized Granular Systems

In recent years, high-polarized granular systems have attracted much attention for its scientific and technologic significance. Among them two materials, electrorheological fluids (ER fluids) and half-metallic granular systems, have become focus of extensive research. The former is composed of small dielectric particles suspended in the base fluids. These particles will aggregate into a certain structure when the applied field yields a critical value. The other is fabricated from "half-metallic" oxides, i.e., ferromagnetic oxides which are predicted theoretically to be completely spin-polarized and really found experimentally to have very high spin-polarization. When a magnetic field is applied to half-metallic granular systems, the resistance will drop rapid and dramatically, which is called the extrinsic magnetoresistance effects in half-metallic granular systems. Both of the two materials have important fundamental values and extensive technological applications.The purpose of this work is to study properties of ER fluids and half-metallic granular films. The main results of our study are listed as follows.I. The study of ground states in monodisperse and polydisperse electrorheological fluidsUpon the application of an external field, the suspended particles in an ER fluid aggregate in a certain structure. The study of the microcrystalline structure in ER fluids is not only helpful in understanding further the ER response to the external field, but also offers a new technique to form mesocrystals with unique photonic properties.However, most of computer simulations and theoretical studies of complex cases such as polydisperse ER fluids are based on the point-dipole (PD) approximation, i.e., treating the dielectric spheres as point dipoles interacting with each other. Obvious deviation from experimental observations is caused since many-body and multipolar effects are neglected in this approximation. We proposed a dipole-induced-dipole (DID) model based on amultiple image method to deal with multipolar interaction in ER fluids. The model is applied to the study of ground states in monodisperse and polydisperse ER fluids. The effects of dielectric constants on the structure formation in both systems are studied in a wide range of dielectric contrasts between the particles and the fluids. Our calculation shows different behaviors of structure formation in positive- and negative-polarized systems and a critical dielectric constant at which a ground state transition happens is reported. Furthermore, we obtain the interaction between two chains containing particles of various dielectric constants and apply it to polydisperse ER fluids where the particles have the same size but different dielectric constants. We find the ground state may become unstable due to a polydispersity in the particle dielectric constants. And when the dielectric constants of particles differ greatly from each other, identical particles tend to aggregate into uniform chains. Because of its simplicity and effectiveness, our DID model can offer a good basis for computer simulations in polydisperse ER fluids.II. The study of magnetotransport properties in half-metallic granular systemsHalf-metallic granular systems have become one of the candidates for applications of magnetoresistance elements due to their dramatic magnetoresisance effects at low fields. However, a thorough understanding of the properties and mechanisms of these materials is still lacking. In this paper, we discuss the properties observed in two kinds of half-metallic granular systems, powders and polycrystalline materials and propose two models to explain the experimental results.(a) Spin-polarized tunneling between ferromagnetic granules is usually regarded as the main transport mechanism in half-metallic granular systems. However, the rapid decay of magnetoresistance in these systems is still a puzzling problem till today. Here we compare two kinds of magnetotransport mechanisms for granular systems and conclude that some factors which will affect...