  In Fe pnictide (Pn) superconducting materials, neither Mn- nor Cr- doping to
the Fe site induces superconductivity, even though hole carriers are generated.
This is in strong contrast with the superconductivity appearing when holes are
introduced by alkali metal substitution on the insulating blocking layers. We
investigate in detail the effects of Mn doping on magneto-transport properties
in Ba(Fe$_{1-x}$Mn$_x$As)$_2$ for elucidating the intrinsic reason. The
negative Hall coefficient for $x$ = 0 estimated in the low magnetic field ($B$)
regime gradually increases as $x$ increases, and its sign changes to a positive
one at $x$ = 0.020. Hall resistivities as well as simultaneous interpretation
using the magnetoconductivity tensor including both longitudinal and transverse
transport components clarify that minority holes with high mobility are
generated by the Mn doping via spin density wave (SDW) transition at low
temperatures, while original majority electrons and holes residing in the
parabolic-like Fermi surfaces (FSs) of the semimetallic Ba(FeAs)$_2$ are
negligibly affected. Present results indicate that the mechanism of hole doping
in Ba(Fe$_{1-x}$Mn$_x$As)$_2$ is greatly different from that of the other
superconducting FePns family.
