Preparation And Transport Properties Of Half Metallic La_0.7Sr_0.3MnO₃、Fe₃O₄and Co₂FeAl Particles

Organic spintronics is a new and promising research field where organic materials are used to mediate or control a spin polarized signal. It is hence a fusion of organic electronics and spin electronics (or spintronics). Organic materials, on the one hand, open the way to cheap, low-weight, mechanically flexible, chemically interactive, and bottom-up fabricated electronics. The application of the electron’s spin (instead of or in addition to its charge), on the other hand, allows for non-volatile devices. Spintronic devices are also potentially faster and consume less electrical power, since the relevant energy scale for spin dynamics is consider ably smaller than that for manipulating charges.La0.7Sr0.3MnO3, Fe3O4and Co2FeAl are the spintronics materials with theoretically half-metallic properties, which have promising application prospect at the field of low-field magnetoresistance (LFMR). In this thesis, La0.7Sr0.3MnO3nanoparticles were synthesized using coprecipitation way. Then, LSMO/oleic acid molecules nanoparticles were fabricated with organic molecule oleic acid chemically absorbed. Fe3O4particles with different morphologys and particle sizes were synthesized by hydrothermal method. Co2FeAl particles were synthesized by coprecipitation and thermal deoxidization method. The crystal structure and microstructure of the above samples were characterized by powder X-ray diffraction (XRD, Rigaku D/Max-2400) using Cu K a radiation and transmission electron microscopy (TEM, FEI G2F30S), respectively. The morphology of the above sample was determined by scanning electron microscopy (SEM, Joel6610). The static magnetic properties were characterized by using a vibrating sample magnetometer (VSM, Lakeshore7304). At last, a direct current four-terminal method was used to messure the transport properties of the obtained nanoparticles at various temperatures, applied fields and biases. The main content are showing as fellows:(1)The resistivity of the oleic acid coated La0.7Sr0.3MnO3nanoparticles varies at different temperatures and voltages. The tunnel barrier height around1.3±0.15eV was obtained by fitting the Ⅰ-Ⅴ curve according to the Simmons equation. The LFMR value reaches-18%with applied current of0.1μA at10K and has a strong bias voltage dependence. Our results show that oleic acid coated LSMO nanoparticles has larger LFMR than that of bare LSMO, indicating the enhanced spin-dependent tunneling and weak spin scattering in oleic acid molecule barrier.(2) Through study of octahedral and sphere Fe3O4particle system, we can get that the room temperature LFMR of Fe3O4changes with the size of the particles. In the sphere system, the LFMR is about-3.5%,-9%and-13%for the samples with particle size around1μm,750nm and200nm, respectively. In the octahedral system, the LFMR is about-2.5%and-8%for the samples with particle size around3-5μm and50-100nm, respectively. The LFMR of these two systems are both voltage-dependent with a strong thermal effect. The nonlinearity of the sample’s Ⅰ-Ⅴ curves were both enhanced with the decrease of the particle size. It indicates that the interparticles’ barrier height and thickness were both increased with the decrease of the particle size.(3) Half-matallic Heusler alloy Co2FeA1nano-scaled particles was synthesized by coprecipitation and thermal deoxidization method. The microstructre, magnetism and transport properties were studied.