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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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