  The phase diagrams of EuFe$_{2-x}$Co$_x$As$_2$ $(0 \leq x \leq 0.4)$ and
EuFe$_2$As$_{2-y}$P$_y$ $(0 \leq y \leq 0.43)$ are investigated by Eu$^{2+}$
electron spin resonance (ESR) in single crystals. From the temperature
dependence of the linewidth $\Delta H(T)$ of the exchange narrowed ESR line the
spin-density wave (SDW) $(T < T_{\rm SDW})$ and the normal metallic regime $(T
> T_{\rm SDW})$ are clearly distinguished. At $T > T_{\rm SDW}$ the isotropic
linear increase of the linewidth is driven by the Korringa relaxation which
measures the conduction-electron density of states at the Fermi level. For $T <
T_{\rm SDW}$ the anisotropy probes the local ligand field, while the coupling
to the conduction electrons disappears. With increasing substitution $x$ or $y$
the transition temperature $T_{\rm SDW}$ decreases linearly accompanied by a
linear decrease of the Korringa-relaxation rate from 8 Oe/K at $x=y=0$ down to
3 Oe/K at the onset of superconductivity at $x \approx 0.2$ or at $y \approx
0.3$, above which it remains nearly constant. Comparative ESR measurements on
single crystals of the Eu diluted SDW compound Eu$_{0.2}$Sr$_{0.8}$Fe$_2$As$_2$
and superconducting (SC) Eu$_{0.22}$Sr$_{0.78}$Fe$_{1.72}$Co$_{0.28}$As$_2$
corroborate the leading influence of the ligand field on the Eu$^{2+}$ spin
relaxation in the SDW regime as well as the Korringa relaxation in the normal
metallic regime. Like in Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ a coherence peak is
not detected in the latter compound at $T_{\rm c}=21$ K, which is in agreement
with the expected complex anisotropic SC gap structure.

Superconductivity;Strongly Correlated Electrons