ゾルゲル法によって作製したLa2CuO4およびSrTiO3の電気的および磁気的特性

Investigation of magnetism and electrical properties of nano-sized particles has been a subject of interest in past years owing that their properties are deviated compared to their bulk. Perovskite oxide is one of the best materials to study such effect, since lattice distortion and non-stoichiometric form is allowed in the structure, depending on various stimuli, thus the material is potentially to appear with extraordinary variety of properties. For example, modulating electrical properties from insulating to metallic is possible. Therefore, investigation of a perovskite oxide in nanoscale order is become important. La2CuO4 (LCO) and SrTiO3 (STO) nanoparticles (NPs) were selected in this research work. LCO is basically a frontier of strong correlated materials of an antiferromagnetic (AF) Mott insulator. The strong electron-electron interactions makes the material to exhibit superconductivity, by introducing charge carriers to the system. As an AF and metal oxide, the LCO also potentially to have various applications such as spin-valve device and gas sensors, respectively. Moreover, STO also holds important electronic properties owing to its wide band gap, and, since it is an excellent substrate for epitaxial growth, physics of oxide heterostructure based on STO material is also of interest to be studied. To date, there are no details report on the magnetic and electric properties on free-standing STO and LCO. By controlling the properties of such nano-sized system, the results obtained will become fundamental for further research work and very potential to be developed for future electronic and magnetic device applications. In the present work, free-standing LCO and STO NPs were synthesized by sol-gel method and its magnetic and/or electric properties were reported. Various method have been applied to synthesize a perovskite, such solid-state, co-precipitation, sol-gel, hydrothermal and spray technique. Sol-gel was selected considering the products are finer, higher in purity, more controllable and easy to conduct. Morphological observation by field emission scanning electron microscopy (FESEM) showed that the NPs has spherical structure and their size could be controlled depending on the annealing condition during synthesis, the size product is proportional with annealing temperature/time of synthesis. X-ray diffractometry (XRD) results indicating the minimum annealing condition to obtain LCO and STO phase at least at 600℃ for 45 min and 500℃ for 5 h, respectively. Additionally, the obtained smallest NPs have 69 nm and 21 nm size, for LCO and STO, respectively. Size dependent magnetic properties of LCO based compound was investigated by superconducting quantum interference device (SQUID) magnetometer, as a function of temperature and magnetic field. Magnetization enhancement was observed with reducing size. Additionally, the magnetism of LCO also was assessed in microscopic view by muon spin rotation (μSR) and the results showed drastically reduction of Néel temperature (TN) with decreasing size. The LCO NPs with 69 nm size has TN at about 60 K, compared to their bulk TN ≈ 300 K. Size effect also studied in the doped sample, the LCO is doped by Sr to exhibit superconductivity. Cancellation of superconductivity was observed in small particle size. Electric properties of the fabricated LCO and STO NPs were also investigated. Firstly, electrodes separated with nanoscale gap were fabricated by electron beam lithography (EBL). The synthesized STO and LCO NPs were aligned at the electrodes by dielectrophoresis (DEP) technique. DEP is a technique to manipulate the movement of particles by applying non-uniform electric field, therefore the NPs bridging electrodes and electrical characteristics can be measured at various temperature. The results showed that the conductance of the samples increases with raising temperature, for both LCO and STO NPs samples. Two conduction mechanisms originating from hopping and tunneling appeared in the Arrhenius plot.九州工業大学博士学位論文 学位記番号：生工博甲第330号 学位授与年月日：平成30年12月26日1 Introduction and Literature Review|2 Methodology|3 Size Controlled Synthesis of La2CuO4 and SrTiO3 Nanoparticles by Sol-Gel Technique|4 Size Dependent Magnetic Properties of La2CuO4 Nanoparticles Synthesized by Sol-Gel Method|5 Assembly and Electrical Properties of La2CuO4 and SrTiO3 Nanoparticles Based on Dielectrophoresis for Future Electronic Device Applications|6 The Effect of Particle Size on Magnetic Properties of La1.80Sr0.2CuO4 Nanoparticles Fabricated by Sol-Gel Method|7 Weak Ferromagnetism in La2CuO4-y Nanoparticles|8 Conclusions and Suggestions九州工業大学平成30年