Evolution of Eu2 ⁺ spin dynamics in Ba _1-xEu _xFe ₂As ₂

Single crystals of Ba _1-xEu _xFe ₂As ₂ were studied by magnetic susceptibility, heat capacity, resistivity, and electron spin resonance (ESR) measurements. Spin-density wave (at T _SDW) and antiferromagnetic (at T _N) phase transitions were mapped as a function of x. For 0.2, we found a single Eu2 ⁺ ESR Dysonian line that presents an isotropic linear increase (Korringa) of its linewidth (ΔH) above T _SDW which systematically decreases with decreasing x. In contrast, for a critical concentration x _c (0.10<x _c<0.20), ΔH decreases with increasing T, suggesting a distinct relaxation process that we associate with a Eu2 ⁺ Kondo single impurity regime. The Korringa rate suppression towards the Ba-rich compounds is claimed to be due to the reduction of the q-dependent exchange interaction between the Eu2 ⁺ f electrons and the conduction electrons, which is likely associated with an increasing of localization of Fe d electrons. This result may help the understanding of the SDW phase suppression (that can lead to superconductivity) in this class of materials.