  The longitudinal in-plane magnetoresistance (LMR) has been measured in
different Ba(Fe_(1-x)Co_x)2As2 single crystals and in LiFeAs. For all these
compounds, we find a negative LMR in the paramagnetic phase whose magnitude
increases as H^2. We show that this negative LMR can be readily explained in
terms of suppression of the spin fluctuations by the magnetic field. In the
Co-doped samples, the absolute value of the LMR coefficient is found to
decrease with doping content in the paramagnetic phase. The analysis of its T
dependence in an itinerant nearly antiferromagnetic Fermi liquid model
evidences that the LMR displays a qualitative change of T variation with
increasing Co content. The latter occurs at optimal doping for which the
antiferromagnetic ground state is suppressed. The same type of analysis for the
negative LMR measured in LiFeAs suggests that this compound is on the verge of
magnetism.
