The temperature dependence of electron spin resonance (ESR) was studied in ${\text{EuFe}}_{2-x}{\text{Co}}_x{\text{As}}_2$ system. The ESR spectrum of all the samples indicates that the linewidth strongly depends on the temperature. Moreover, the linewidth shows the Korringa behavior, indicating an exchange coupling between the conduction electrons and the ${\text{Eu}}^{2+}$ ions. The linewidth, $g$ factor and the integrate ESR intensity show anomalies at the temperature of the spin-density-wave (SDW) ordering. The linewidth below the SDW transition does not rely on the temperature. This gives the evidence for the gap opening at the $T_{\text{SDW}}$. The slope of the linewidth is closely associated to $T_{\text{SDW}}$ and $T_c$. Such exotic behavior should be related to the nesting of the Fermi surface.

• Received 31 July 2009

DOI:https://doi.org/10.1103/PhysRevB.81.052503