Study On The Preparation And Laser-Irradiated Effect Of Colossal Magnetoresistive Manganites

The phenomenon of colossal magnetoresistance (CMR) in doped perovskite manganites with the formula R1-xAxMnO3 (R=La, Sm, Pr ; A=Ca, Sr, Ba, and Pb etc.) has recently attracted considerable attention because of their value in fundamental physics and their potential applications such as magnetic storage, magnetic sensors, and IR detecting systems. These compounds in which the electrons are strongly correlated display a novel spin-based electrical transport, including interplay among charge carriers, magnetic coupling, orbital ordering and structural distortion, and they promise much to condensed matter physics. In this thesis, CMR thin films, La1-xBaxMnO3 films, were prepared by pulsed laser deposition (PLD), and the effects of the preparation conditions and film thickness on the properties of these materials were studied systematically and detailedly. In addition, the effect of CO2 laser irradiation on CMR thin films was explored initially. The main content of this thesis is summarized as follows.In the first chapter, we present a brief review for the history and the future for CMR manganites. We first review the structure, spin-charge-orbital ordering, magnetotransport, and the electrical phase of the CMR manganites, and then describe the underlying physics mechanism and the external factors of CMR effect and the technological applications. The aim of this thesis concludes with an outlook in both applications and theoretical research on the CMR manganites.In the chapter 2, we concisely introduce the growth technique and measurement methods used in the following work. We first describe the conventional solid reaction and pulsed laser deposition in detail. After the introduction to the devices measuring the magnetotransport and structure such as X-ray diffraction, atomic force microscope, and PPMS, we focus our attention on the resistance-temperature measurement system made by us.In the chapter 3, we mainly studied the effect of the growth conditions such as deposition temperature, oxygen partial pressure, and annealing temperature and time, on the properties of CMR manganites. We found that LBMO films show a good magnetic and electrical transport under the optimized growth parameters. We also studied the strain effect of LBMO films with different thickness. The experimental data show that the Curie temperature and the insulator-metal transition temperature decrease with an increase in the compressive strain, which is very different from the usual results that compressive strain enhances the ferromagnetism and then TC. The anomalous magnetotransport of La0.67Ba0.33MnO3 thin films was explained by strain effect on the stability of eg orbital. In addition, we prepared LCMO thin films with (00l)-oriented direction using pulsed laser deposition.In the chapter 4, we studied the magnetic properties and electrical transport in the LBMO thin films directly grown on the single crystal (001) Si substrate. It is found that LBMO/Si films exhibit polycrystalline and preferential growth in (00l) direction. The transport in the high temperature is explained by small-polaron variable-range hopping model, and the"upturn"of the resistivity-temperature curves in the low temperature zone possess the characteristic of semiconducting transport. LFMR value above 17% in the temperature range 4-300K under 2T was found in LBMO thin films. The effect of film thickness on LBMO/Si films was further investigated. The results indicate that, as the film thickness is increasing, an increase in the grain size is accompanied by a decrease in the resistivity and an increase in TP. This behavior results from the grain boundaries (GBs) effect in the polycrystalline CMR manganites. However, the GBs'effect fails to explain the magnetotransport in the LBMO/Si films with different deposition temperature. Higher-order tunneling effect, which dominates when the second-order tunneling process of eg electron are broken in the case of the enlarged grain boundary, was utilized to explain the nonlinear magnetotransport dependence of grain size.In the chapter 5, we investigated the effect of CO₂ laser irradiation on La-(Ba, Ca)-Mn-O thin films. The crystal structure, the surface morphology, and the magnetic and electrical properties for the as-grown and the CO₂-laser-irradiated LBMO films were measured and systematically analyzed by XRD, AFM, SQUID, PPMS, and R-T measurement system, respectively. The experimental results indicate that, after irradiation, the improvement of the crystallinity, the microstructure, and oxygen content, is accompanied by a sharp decrease in the resistivity and a remarkable increase in Tp and TC. In particular, the TCR value as high as 8.8%K-1 at 307K of the CO2-laser-irradiated films increase by more than two times, enabling its better promising for bolometric application at the room temperature. Similar results are obtained from the CO₂-laser-irradiated LCMO films. These results indicate that the quality of CMR manganite films can be improved by CO₂ laser irradiation, which is believed to be a new and effective method to optimize CMR materials. Furthermore, the growth of CMR films in low deposition temperature using PLD assisted by CO2 laser irradiation was also studied. It is found that CO₂ laser irradiation can improve the structure defects and oxygen deficiencies in the film prepared in low deposition temperature, and then enhance the magnetic properties and electrical transport.A summary of all our work is given in the final chapter.