Magnetoresistance (MR) of a nanostructured material, monodisperse Fe3O4 nanospheres of about 200nm coated with thin SiO2 and compactly cold pressed and sintered, has been investigated. A high MR, up to 10.8% at 1T and 17% at 8T, has been observed at 100K. This enhanced MR is likely arising from the spin-polarized tunneling of conducting electrons through the insulating SiO2 boundaries. The decrease of the MR with the temperature increasing was attributed to the existence of the local spins in the grain boundaries. In addition, the MR ratio decreases with the SiO2 thickness increasing and sintering temperature decreasing. It suggests that the magnetic property of the insulating barrier is crucial for enhancing the low-field MR at a high temperature.

Unusually large low field magetoresistance (LFMR), ~ 10 %, at 300 K has been observed with the sample of mono-dispersed Fe 3 O 4 magnetite nanospheres, ~ 200 nm, compactly cold-pressed and sintered at 800 o C. A detailed analysis on the transport and magnetic measurements indicates that the electron conduction is dominated by the spin-dependent scattering or tunneling at the grain boundaries. At low temperature, 140 K and 100 K near the Verwey transition, ~ 115 K, the LFMR (below 2 kOe) does a e-mail : cpchen@pku.edu.cn, Phone : +86-10-62751751, Fax : 86-10-62751615 2 not show any sign of dependence on the transition, and does not follow the variation of magnetization to reach the saturation region either. On the other hand, at 300 K, the MR saturates fast with the magnetization below 2 kOe. This temperature dependent property in LFMR is very likely attributed to the scattering or tunneling of the conduction electron passing through the grain boundary layer with spin disordered state.3

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High quality La 0.67 Ca 0.33 MnO 3 (LCMO) film was deposited via a novel pulsed electron deposition technique on SrTiO3(100) single crystal substrate. The micro-bridge with different widths was fabricated by using electron beam lithography (EBL) technique and their transport properties were studied. For the micro-bridges with width of 2 and 1.5 m, the insulator-to-metal transition temperature (T P ) keeps unchanged compared with the film. For the micro-bridges with width of 1 m, the T P shifts towards the lower temperature by 50 K. When the width decreases down to 500 nm, the insulator-to-metal transition disappears. The magnetoresistance behavior of these micro-bridges was studied, and the results show that the low field magnetoresistance (LFMR) decreases and the high field magnetoresistances (HFMR) keep almost unchanged as the width of micro-bridge is reduced. colossal magnetoresistance, pulsed electron deposition, electron beam lithography, electrical transport properties With the development of nanotechnology, the properties of materials on micro/nano scale attract more and more attention because many materials on such a scale show some different properties. Many new phenomena are anticipated to exhibit for further understanding of the underlying mechanism of the materials. Many multi-component oxides having interesting properties, such as superconductivity [1,2] , colossal magnetoresistivity (CMR) [3,4] and ferroelectricity, will play important roles in potential device applications of electronic devices, such as the high sensitive

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