  Unusually large low field magetoresistance (LFMR), ~ 10 %, at 300 K has been
observed with the sample of mono-dispersed Fe3O4 magnetite nanospheres, ~ 200
nm, compactly cold-pressed and sintered at 800 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 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.
