Study On Magnetoresistance Properties And EPR Spectra Of La_2/3(Sr_1-xCe_x)_1/3MnO₃ Under Room Temperature And Low Magnetic Field

La_2/3Sr_1/3MnO₃ film has the half gold properties of nearly 100% spin polarization s and soft magnetic properties of small coercive force. Under low magnetic field, magnetic resistance in the grain boundaries is more apparent than intrinsic magnetic resistance. And magnetic resistance is very stable in a wide range of temperature. So it is very necessary and important that we study magnetic resistance effect and the relevant physical mechanisms of these samples under the near room temperature and low magnetic field. This leads to some new important practical application based on magnetic resistance effect. In this paper, lanthanum strontium manganese La_2/3Sr_1/3MnO₃ oxide which owns perovskite structure is regarded as matrix material. Based on this, we study the magnetic resistance effect under near room temperature and low magnetic field via doping optimizedly rare earth cerium ion.Ce doped perovskite lanthanum strontium manganese oxide samples were successfully prepared by improved sol-gel method. Ce-doped samples were studied and analyzed via X-ray diffraction, electron paramagnetic resonance and others. A set of simple and effective magnetic resistance measurement system was established by ourselves. And Ce-doped samples were measured via the measurement system. The main research results are as follows:（1） Preparation method and process of samples were improved. A series of Ce-doped lanthanum strontium manganese oxide was prepared by improved sol-gel method in the experiment. Powder materials were prepared via drying 8 hours at constant temperature in drum wind drying oven, instead of placing 72 hours at room temperature before; films were attached to the substrate via drying sol solution with a heat gun. The cycle of sample materials preparation was greatly reduced by these methods.（2） XRD results show that: Ce ions are doped to six-party perovskite structure La_2/3Sr_1/3MnO₃ lattice structure instead of Sr ions under low doping concentration. While under high doping concentrations, excessive Ce ions will separate out in the form of CeO₂, attach on the surface of Ce-doped grain and hinder the growth of grain further. Ce-doping will transform six-party perovskite structure and distort to some extent, which leads to transformation of samples crystal phase. The average grain maximum size of samples is 35.9nm when doping concentration x is 0.3. Grain size increases, which is helpful to generate boundary, thus its extrinsic magnetoresistance will also increase.（3） Electron paramagnetic resonance spectrum results show that: Doped Ce ions suppress generation of paramagnetic phase in the sample. Absorption spectrum on the form transformed Gaussian type into Lorentz type gradually, which shows that doped Ce ions prompts change of molecular motion characteristic in the samples. The width peak-peak increases and symmetry of the spectral shape decreases with calcination temperature increasing. When the optimal doping concentration is 0.2 or 0.3, the width peak-peak reaches the maximum and resonance absorption is also the maximum. At the same time, coupling is the strongest in the sample and thus magnetic resistance will reach the maximum.（4） A set of simple and effective magnetic resistance measurement system was set up. We measured the prepared Ce-doped samples under room temperature and low magnetic field via using the system. Results show that optimal doping concentration x is 0.2 or 0.3, and Ce-doped lanthanum strontium manganese oxide has the largest magnetic resistance, which is about-4.6%. This is because that average A-site cation radius decreases with doped Ce ions increasing, therefore Mn-O octahedral structure distorted and Curie temperature of the sample is close to the room temperature. Thus it is because of the two combined effects that the magnetic resistance of the sample reaches the maximum.