  Transport and magnetic properties have been systematically investigated for
SmMnAsO_{1-x} with controlled electron-doping. As the electron band-filling is
increased with the increase of oxygen deficiency (x), the resistivity
monotonically decreases and the transition from Mott-insulator to metal occurs
between x=0.17 and 0.2. Seebeck coefficient at room temperature abruptly
changes around the critical doping level from a large value (\sim -300 \mu V/K)
to a small one (\sim -50 \mu V/K) both with negative sign. In the metallic
compounds with x=0.2 and x=0.3, Mn spins order antiferromagnetically around 30
K, and the Hall coefficient with the negative sign shows a reduction in
magnitude upon the magnetic transition, indicating the change in the multiple
Fermi surfaces. Gigantic positive magnetoresistance effect is observed in a
wide range of temperature, reaching up to 60 % at 2 K for the x=0.3 compound.
The effect is attributed to the field-induced change of the complex Fermi
surfaces in this multi-orbital correlated electron system.
