Cao Wan-qiang

Study on Conductive and Magnetic Properties of Ag-Doped La 0.67 Sr 0.33 MnO 3

Rare-earth doped manganese oxide La 0.67 Sr 0.33 MnO 3 (LSMO) has broad application prospects in the field of spin electronics, which has the paramagnetic-ferromagnetic transition temperature (T c ) above room temperature. Recent research shows that Ag-doping has effective improvements of grain size and conductivity of LSMO for improving the colossal magnetoresistance effect (CMR). In this paper, sol-gel method was used to prepare La 0.67 Sr 0.33 MnO 3 : Agx（x=0, 0.5 mol%） polycrystalline powder. By using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and X-ray photoelectron spectroscopy (XPS) technologies, the microstructure, crystalline phase structure, resistivity, magnetic properties, Ag and Mn valence states of LSMO were tested before and after Ag doping, respectively. The analysis shows that some Ag atoms exist at the grain boundary of LSMO in the form of element to enhance the conductivity; other Ag atoms go into the crystal lattice replacing A site La 3+ in the perovskite structure ABO3, which obtains a small amount of La 2 O 3 hybrid phase in LSMO and decreases the ratio of Mn 3+ /Mn 4+ . Ag doping weakens the double exchange effect and the magnetic property of LSMO. The substitution of La with Ag also makes minority spinon band of LSMO slightly reduced, which can weakly decrease the conductivity of LSMO.

Field induced effect of relaxor ferroelectrics at high electric field

Relaxor ferroelectrics have been widely used in dielectric, pyroelectric, electric field modulation microwave devices and electrostriction device. These applications refer to effect of electric field, increase in strength of polarization from ferroelectric phase stretching to paraelectric phase successively, to give rise to an effect similar to field induced effect. A result of the peak temperature of dielectric permittivity changing with the electric field 2/3 power was obtained by using approximation method, which is coincident with the experimental result of dielectric peak. The numerical simulation results found that there exist a thermal dynamics related polarization-temperature coefficient. Increasing the coefficient will decrease shift effect of dielectric peak, or will decrease effect of field induced phase transition, but, will increase pyroelectric effect. On the contrary, the stronger shift effect of dielectric peak will increase dielectric tunability and its temperature stability for electric field modulated microwave devices. The above theoretic results have certain reference implications for dielectric tunability of microwave device and pyroelectric property by doping modification method.

Effect of electric field induced strain for double-well ferroelectrics

Based on the Devonshire-Laudon phenomenological theory, the effects of polarization due to dipole stretching and electric hysteresis loop and strain due to dipole’s 90° turning in tetrogonal phase by appling electric field are investigated with the statistic method. The numerical simulation method is used to exam the influences of material parameters, coupling coefficient of dipole, and temperature on hysteresis loop and electrostriction. The significative results are obtained: the lareger the ratio of the two material parameters, α 0/ β , the height of electric hysteresis loop is, and the broader the plane bottom of electrostriction is; the larger the coupling coefficient is, the wider the butterfly shaped strain loop is; when temperature is close to the Curie temperature, the separation of two electrostriction loops is farther at the intersection point. The expirical law is confirmed that the product of electrostrictive coefficient and the Curie-Weiss constant C is a constant. The above principles can explain explicitly the relevant experimental results of electrostriction.