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We have performed detailed $^{57}$Fe M\"ossbauer spectroscopy measurements onBa$_{0.78}$K$_{0.22}$Fe$_2$As$_2$ and BaFe$_{2-x}$Ni$_x$As$_2$ single crystalmosaics showing antiferromagnetic ordering below $T_N$ with superconductivitybelow $T_C$. Analysis of the M\"ossbauer spectra shows a decrease in themagnetic hyperfine (hf) field but no change in the magnetic volume fractionbelow $T_C$. This clearly indicates the coexistence of magnetism andsuperconductivity in these compounds. The decrease in the magnetic hf fieldbelow $T_C$ depends on the difference between $T_N$ and $T_C$, being thelargest for $T_N$ close to $T_C$. Two different explanations for thisobservation are given. We also find that the non-magnetic volume fraction below$T_N$ correlates with the Ni doping $x$, being large for high $T_C$ and smallfor high $T_N$.

Interplay between superconductivity and antiferromagnetism in some iron-pnictide single crystals studied by $^{57}$Fe M\"ossbauer spectroscopy