Intrinsic Electroresistance and Time Dependent Transport in Some Perovskite Manganites

Abstract: This thesis studies the intrinsic disorder effects and the electro- and magnetotransport properties of some perovskite manganites. Recently reported colossal electro-resistance (ER) (a relative change in resistance with electric field /current ) in manganites, analogous to their respective colossal magnetoresistance (CMR) behavior, demonstrates another way of controlling the transport properties of manganites directly by using the electric current. In the last few years, there is considerable debate regarding the origin of this ER. In order to better understand this effect in some manganites, a technique for precisely measuring the intrinsic ER of a material has been developed and the technique has been utilized to measure the ER of polycrystalline Sm0.60Sr0.40MnO3 and Sm0.55Sr0.45MnO3 manganites. In this technique, the contribution of Joule heating to the ER has been successfully eliminated. It is found that the changes of the intrinsic ER with current density are much more significant in Sm0.60Sr0.40MnO3 compared to Sm0.55Sr0.45MnO3. We have also tuned the existing disorders/inhomogeneity in Sm0.60Sr0.40MnO3 (SSMO) by Fe doping in Mn site to probe the possible mechanisms responsible for ER. It is found that ER increases with Fe doping and it is strongly suppressed by an applied dc magnetic field. A discussion of the mechanism behind these phenomena is presented. We have investigated the time dependence of the electrical resistivity in the Sm0.60Sr0.40Mn1-xFexO3, (1-y) Sm0.60Sr0.40 MnO3 +yBaTiO3 and (Sm0.5Gd0.5)0.55 Sr0.45Mn1-zRuzO3 manganites by doping with Fe, BaTiO3 and Ru to understand better the origin of relaxation effects and to learn how to control it. It is found that the relaxation effects strongly depend on the relative fraction of coexisting phases (for instance, ferromagnetic metallic (FMM), antiferromagnetic (AF) insulating with charge-orbital ordered (CO/OO) state, paramagnetic, etc.). We have shown that the relaxation effect in manganites can be controlled by using the doping element. The relaxation effects are enhanced due to both the Fe and BaTiO3 doping in SSMO; however, they are suppressed by Ru substitution in (Sm0.5Gd0.5)0.55Sr0.45MnO3. The mechanism behind these changes in the relaxation effects is discussed. We have also investigated the effect of strain induced disorder on the physical properties of BaTiO3/La0.66Ca0.34MnO3 heterostructure grown on a SrTiO3 substrate where very thin (16 nm) La0.66Ca0.34MnO3 films grown on SrTiO3 are normally subjected to tensile strain. We found that this strain sensitive heterostructure has significant magnetocapacitance effect which is important for technological applications. A discussion of the mechanism behind this magnetocapacitance effect is introduced