  Variant approaches, either based on the Fermi surface nesting or started from
the proximity to a Mott-insulator, were proposed to elucidate the physics in
iron pnictides, but no consensus has been reached. A fundamental problem
concerns the nature of their 3d electrons. Here we report the
magnetoresistivity (\rho_xx) and the Hall resistivity (\rho_xy) of
Ba(Fe1-xCox)2As2 (x=0 and 0.05) in a magnetic field of up to 55T. The magnetic
transition is extremely robust against magnetic field, giving strong evidence
that the magnetic ordering is formed by local moments. The magnetic state is
featured with a huge magnetoresistance and a distinguished Hall resistivity,
\rho_xy(H), which shows a pronounced parabolic field dependence, while the
paramagnetic state shows little magnetoresistance and follows a simple linear
magnetic field dependence on the Hall resistivity. Analyses of our data, based
on a two-carrier model, demonstrate that the electron carriers in the magnetic
state rapidly increase upon applying a magnetic field, partially compensating
the loss of electron carriers at T_M. We argue that the 3d-electrons in
Ba(Fe1-xCox)2As2 are divided into those who are close to forming localized
moments controlling the magnetic transition and the others giving rise to
complex transport properties through their interaction with the former.
