Preparation And Cd Doping Of La_0.67Sr_0.33MnO₃ Film By Sol-gel Process And Study On The Electric And Magnetic Properties

With the development of the information, the consumer demand for hard disk drives is rapidly growing. At the same time, the sensitiveness of the head has been growing. Specially, the discovery of the giant manganetoresistance(GMR) and tunnel manganetoresistance(TMR) effects in the multilayer films of the manganese pervoeskite oxides. Most of these have been centered until now on the La2/3Sr1/3MnO3 compound, as it presents a ferromagnetic–paramagnetic first-order transition above room temperature that is related to exceptionally high values of magnetoresistance, the well-known colossal magnetoresistance (CMR). Unfortunately, the CMR phenomenon becomes observable only on a magnetic field of several Teslas, which is far away of applications. Towards this end, another property of perovskite manganites, extrinsic MR effect, can be utilized in the boundary of the grains. In this thesis, we fabricated polycrystalline La0.67 (Sr1-xCdx) 0.33MnO3 thin films on Si (100) substrates by sol-gel and the rapid annealing process. We have study the low field manganetoresistance(LEMR) of the films effected by the grains boundary which were annealed at different temperature. We also study the MR of the films by increase of the doping amount x in La_0.67(Sr_1-xCd_x)_0.33MnO₃. The low-field magnetoresistance values under 0.3 T are observed to be 2.48% (x=0) at 300K and 37.5% (x=4/8) at 85K, respectively. The main investigation of my thesis as the following:In chapter one, a introduction of the development about the manganese pervoeskite oxides RE1-xAExMnO3, the double exchange (DE) mechanism proposed by Zener, and the Jahn–Teller (JT) polarization which have been used to interpret the properties of the La1-xAxMnO3 system, the application of the manganetoresistance and tunnel manganetoresistance effects in the multilayer films. We also discussed the effect in the grains boundary and the research intention of my thesis.In chapter two of this thesis, we succeeded in studying the best process of the sol–gel to fabricate polycrystalline films on Si (100) single crystal substrates, and the rapid annealing process at pretreatment and last sintering.In chapter three, La0.67Sr0.33MnO3 films were fabricated at different pretreatment and last sintering temperature. By XRD and MR measure, we find the best process. Comparing annealing process with general process, we find that the MR of the film can be improved markedly by annealing process. Small grains are obtained at low anneal temperature, and the average grain boundary can be increased. The MR of the film can be improved markedly, which can be explained by grain boundary theory.In chapter four, A series of La0.67(CdxSr1-x)0.33MnO3 films were fabricated by doping Cd and fixing the ratio of La at 0.67. It is shown that with increase of the doping amount x in La_0.67(Sr_1-xCd_x)_0.33MnO₃, the average A-site cation radius decreases, the temperature of metal-insulator transition (TMI) decreases monotonically,ρand magnetoresistance (MR) values increase dramatically, which can be explained by the lattice effects. The enhanced MR effect at low temperature for the LSCMO film is attributed to the suppression of the spin dependent scattering of polarized electrons at the grain boundaries. The low-field magnetoresistance values under 0.3 T are observed to be 2.48% (x=0) at 300K and 37.5% (x=4/8) at 85K, respectively.The last chapter is the conclusion.