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.

References in corpus (16)

• Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2
• Superconductivity at 43 K in Samarium-arsenide Oxides $SmFeAs{O}_{1-x}{F}_x$
• Superconductivity at 22 K in Co-doped BaFe2As2 Crystals
• Evidence for an electron nematic phase transition in underdoped iron pnictide superconductors
• Superconductivity induced by phosphorus doping and its coexistence with ferromagnetism in EuFe${}_2$(As${}_{0.7}$P${}_{0.3}$)${}_2$
• Superconductivity in SrFe_(2-x)Co_xAs_2: Internal Doping of the Iron Arsenide Layers
• High-Temperature Superconductivity in Eu0.5K0.5Fe2As2
• Possible re-entrant superconductivity in EuFe2As2 under pressure
• Electrical Resistivity and Specific Heat of EuFe2As2 Single Crystals: Magnetic homologue of SrFe2As2
• Metamagnetic transition in EuFe${}_2$As${}_2$ single crystals
• EuFe${}_2$As${}_2$ under high pressure: an antiferromagnetic bulk superconductor
• Effects of the magnetic orderings on the dynamical conductivity: optical investigations of EuFe${}_2$As${}_2$ single crystals
• Anomalous Compressibility Effects and Superconductivity in EuFe2As2 under High Pressures
• Superconductivity at 34.7 K in the iron arsenide Eu0.7Na0.3Fe2As2
• Suppression of spin-density-wave transition and emergence of ferromagnetic ordering of Eu${}^{2+}$ moments in EuFe${}_{2-x}$Ni${}_x$As${}_2$
• Magnetotransport studies of EuFe${}_2$As${}_2$: the influence of the Eu${}^{2+}$ magnetic moments

Cited by in corpus (1)

• EPR study of superconductors