Electron spin resonance in Eu based Fe pnictides

Abstract

The phase diagrams of EuFe${}_{2-x}$Co${}_x$As${}_2$ $(0\le x\le 0.4)$ and EuFe${}_2$As${}_{2-y}$P${}_y$ $(0\le y\le 0.43)$ 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) $(T<T_{\mathrm{SDW}})$ and the normal metallic regime $(T>T_{\mathrm{SDW}})$ are clearly distinguished. At $T>T_{\mathrm{SDW}}$ 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 $T<T_{\mathrm{SDW}}$ the anisotropy probes the local ligand field, while the coupling to the conduction electrons disappears. With increasing substitution $x$ or $y$ the transition temperature $T_{\mathrm{SDW}}$ 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 $T_{\mathrm{c}}=21$ K, which is in agreement with the expected complex anisotropic SC gap structure.

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Cited by in corpus (6)

• Topological magnetic order and superconductivity in EuRbFe${}_4$As${}_4$
• Anisotropy in the Magnetoresistance Scaling of BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$
• Optical properties of superconducting EuFe2(As1-xPx)2
• EPR study of superconductors
• Electronic Scattering Effects in Europium-Based Iron Pnictides
• Re-entrance of resistivity due to the interplay of superconductivity and magnetism in $\ce{Eu_{0.73}Ca_{0.27}(Fe_{0.87}Co_{0.13})2As2}$