  The NMR results in iron pnictides BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 are
analyzed based on the self-consistent renormalization (SCR) spin fluctuation
theory. The temperature dependence of the NMR relaxation rate (T_1)^{-1} as
well as the electrical resistivity is well reproduced by a SCR model where
two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The
successful description of the crossover feature from non-Fermi liquid to Fermi
liquid behavior strongly suggests that low-lying spin fluctuations in
BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 possess an itinerant AF nature, and that
chemical substitution in the two compounds tunes the distance of these systems
to an AF quantum critical point. The close relationship between spin
fluctuations and superconductivity is discussed compared with the other
unconventional superconductors, cuprate and heavy fermion superconductors. In
addition, it is suggested that magnetism and lattice instability in these
pnictides are strongly linked via orbital degrees of freedom.
