In this work, we report x-ray powder diffraction, elemental analysis, electrical resistivity, magnetic susceptibility, specific heat, and electron spin resonance (ESR) in single crystals of ${\mathrm{Sr}}_{1\text{−}x}{\mathrm{Eu}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. We observed a breakdown of the previously reported scaling between the ${\mathrm{Eu}}^{2+}$ Korringa relaxation rate obtained from ESR and the spin density wave temperature evolution for Sr-rich samples. This result suggests a distinct evolution of the orbital differentiation of the Fe $3d$ bands along the Sr-based series when compared to the Ba counterpart. We argue that this difference is related to a larger splitting between the structural (tetragonal-to-orthorhombic) and the Fe-driven spin density wave transitions induced by Eu doping in this series. In fact, our results indicate that the two transitions follow an opposite $x$-Eu dependence for Sr-concentrated samples. Our work shows that ${\mathrm{Sr}}_{1\text{−}x}{\mathrm{Eu}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ series and the comparison with their Ba-based counterparts are exciting platforms to be explored for understanding the interplay among orbital differentiation, magnetism, and structural distortions in the iron pnictides.

• Received 6 October 2022
• Revised 1 February 2023
• Accepted 29 March 2023

DOI:https://doi.org/10.1103/PhysRevB.107.134512