Multi-fields modulation on transport properties in perovskite manganites

Abstract

Mixed-valence manganites have been studied extensively in the last 20 years. They exhibit interesting properties, such as metal-insulator transition, colossal magnetoresistance, phase separation, and charge ordering. The properties of manganites are not only susceptible to internal factors, such as doping, lattice distortion, but also very sensitive to external perturbations, including electric field, magnetic field, and light illumination. The half doped manganites are particularly interesting due to the presentence of multiple phases. The Pr0.5Sr0.5MnO3 (PSMO) is one representative of half doped manganites. The bulk PSMO has a PMI-FMM transition at around 270 K and a FMM-AFI transition at around 140 K. This thesis presents a systematical investigation on PSMO thin films. More specifically, the growth conditions, strain effect, and light response of PSMO thin films grown on different substrates were studied in detail. PSMO films freshly grown on the LaAlO3(001) substrate exhibit tensile strain in the out-of-plane direction and compressive strain in the in-plane direction without annealing. After annealing at 900°C in the air, strain of the film is significantly relaxed. The transition temperature from paramagnetic to ferromagnetic increases from 200 K to 220 K, while the antiferromagnetic insulating state is suppressed at low temperature. In addition, the magnetoresistance increases above the ferromagnetic transition temperature, whereas it decreases at low temperatures. To investigate the photoconductivity in the manganites, the temperature dependent resistance is measured under the light illumination for PSMO films on both the LaAlO3(001) (LAO) and SrTiO3(001) (STO) substrates. Without light illumination, the films show both PMI-FMM and FMM-AFI transitions. With light illumination, the two transitions still exist; however, the transition temperatures decreases and they decrease with increasing the light intensity. At low temperature, i.e. in the AFI region, the resistance decreases much more significantly compared to other temperature regions. This is explained in terms of photo-induced charge transfer in the FMM clusters, resulting in the expansion of FM clusters. The photo-induced ionization of localized carriers in the AFI state leads to more pronounced decrease at lower temperatures. To study the combined effects of strain and light illumination, the PSMO films with different thicknesses are grown on the STO, LAO, and (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. We found that lattice strain may play an important role in determining whether a PSMO thin film can exhibit the AFI state, whereas the type of strain turns out to be not critical. The films with tensile or compressive strains could exhibit similar resistance change under light illumination. When the light is applied, the resistance of the PSMO film decreases in both PMI and AFI regions, but increases in the FMM region. No significant change in the resistance is observed around the two transition temperatures.