  In the iron pnictide superconductors, theoretical calculations have
consistently shown enhancements of the static magnetic susceptibility at both
the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM)
wavevectors. However, the possible existence of FM fluctuations has not yet
been examined from a microscopic point of view. Here, using $^{75}$As NMR data,
we provide clear evidence for the existence of FM spin correlations in both the
hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide
superconductors. These FM fluctuations appear to compete with superconductivity
and are thus a crucial ingredient to understanding the variability of $T_{\rm
c}$ and the shape of the superconducting dome in these and other iron-pnictide
families.
