arXiv:1001.1302v2 [cond-mat.supr-con] 1 Sep 2010 Magnetic fluctuations and superconductivity in Fe pnictides probed by Electron Spin Resonance N. Pascher, 1 J. Deisenhofer, 1 H.-A. Krug von Nidda, 1 M. Hemmida, 1 H. S. Jeevan, 2 P. Gegenwart, 2 and A. Loidl 1 $^{1}$EP V, Center for Electronic Correlations and Magnetism, Institute for Physics, Augsburg University, D-86135 Augsburg, Germany $^{2}$I. Physik. Institut, Georg-August-Universit¨ at G¨ ottingen, D-37077 G¨ ottingen, Germany (Dated: November 21, 2018) The electron spin resonance absorption spectrum of Eu 2+ ions serves as a probe of the normal and superconducting state in Eu 0$_{.}$5 K 0$_{.}$5 Fe 2 As 2 . The spin-lattice relaxation rate 1 /T ESR 1 obtained from the ESR linewidth exhibits a Korringa-like linear increase with increasing temperature above T c evidencing a normal Fermi-liquid behavior. Below 45 K deviations from the Korringa-law occur which are ascribed to enhanced magnetic fluctuations within the FeAs layers upon approaching the superconducting transition. Below T c the spin lattice relaxation rate 1 /T ESR 1 follows a T 1$_{.}$$^{5}$-behavior without any appearance of a coherence peak. PACS numbers: 76.30.-v,74.70.Xa has been deduced. 23 M¨ ossbauer spectroscopy measure- ments have established the coexistence of Eu 2+ short- range magnetic ordering with SC in Eu$_{0}$ . $_{5}$K$_{0}$ . $_{5}$Fe$_{2}$ As$_{2}$ be- low 4.5 K. 24 At the same temperature, a peak is found in the zero-field cooled magnetization, measured at low fields of 5 mT, 5 and a corresponding minimum occurs in the magnetic penetration depth λ ( T ), determined by radio-frequency technique. 25 Recently, the substitu- tion of Fe by Co in EuFe$_{2}$As$_{2}$ reportedly leads to an incomplete superconducting transition in the electrical resistivity. 20 SC has also been found in chemically pres- surized EuFe$_{2}$(As$_{0}$ . $_{7}$P$_{0}$ . $_{3}$)$_{2}$ at T$_{c}$ = 26 K, followed by fer- romagnetic Eu-ordering at 20 K. 19 The recent discovery of superconductivity in Fe-based pnictides and chalcogenides 1–4 has triggered enormous research efforts to understand the origin of superconduc- tivity and its relation to the inherent magnetism of iron. One class of these materials are the ternary A Fe$_{2}$As$_{2}$ sys- tems with A = Ba, Sr, Ca, Eu (122-systems) and T$_{c}$ values up to 38 K. 3,5–7 The parent compounds exhibit a spin-density wave (SDW) anomaly accompanied by a structural distortion. 8,9 Superconductivity (SC) appears, e.g., by substituting the A -site ions by K 3,5 or Fe by Co. 6,7 For underdoped Ba$_{1}$$_{−}$ x K x Fe$_{2}$As$_{2}$ and Co doped BaFe$_{2}$As$_{2}$ a coexistence of the SDW state and supercon- ductivity has been reported. 9–14 In this work we show that the ESR signal of Eu 2+ can be used as probe of the superconducting properties in the 122-family of Fe pnictides. In a metallic system the linewidth of the ESR absorption is a direct measure of the spin-lattice relaxation rate 1 /T ESR 1 , thus provid- ing information on the density of states at the Fermi en- ergy and the opening of the SC gap. In polycrystalline Eu$_{0}$ . $_{5}$K$_{0}$ . $_{5}$Fe$_{2}$ As$_{2}$ we observe a clear change of 1 /T ESR 1 from a normal Fermi-liquid like behavior with a Kor- ringa relaxation ∝ T above 45 K, the onset of magnetic fluctuations of the FeAs layers for T$_{c}$ < T < 45 K, and a ∝ T 1 . $^{5}$-law below T$_{c}$ = 32 K. A particularly interesting 122-system is EuFe$_{2}$As$_{2}$ with T$_{SDW}$ = 190 K, 15–17 the highest reported SDW tran- sition temperature in the pnictides. This system is of special importance among the 122 iron-pnictides, since the antiferromagnetic ordering of local Eu 2+ moments at T$_{N}$ = 19 K provides the opportunity to study the interplay between Eu and Fe magnetism and also the in- fluence of Eu magnetism on SC (under hydrostatic pres- sure 18 or doping $^{5,19,20}$). The appearance of a SDW gap in EuFe$_{2}$As$_{2}$ was evidenced by optical spectroscopy 17 and, recently, the opening of even two gaps with different char- acteristics was reported. 21 Moreover, electron spin reso- nance (ESR) in single crystalline EuFe$_{2}$As$_{2}$ revealed a drastic change in the magnetic properties of the Eu-spin system from a typical metallic-like behavior above T$_{SDW}$ to a behavior characteristic for a magnetic and insulating system in the SDW state. 22 Polycrystalline Eu$_{0}$ . $_{5}$K$_{0}$ . $_{5}$Fe$_{2}$ As$_{2}$ was prepared using a sintering method described in Ref. 5 and characterized by energy-dispersive X-ray (EDX) analysis, x-ray, elec- trical resistivity, magnetic susceptibility, and specific- heat experiments. 23 ESR measurements were performed in a Bruker ELEXSYS E500 CW-spectrometer at X- band frequencies ( ν ≈ 9 . 36 GHz) equipped with a con- tinuous He gas-flow cryostat in the temperature region 4 . 2 < T < 300 K. ESR detects the power P absorbed by the sample from the transverse magnetic microwave field Here, we focus on Eu$_{0}$ . $_{5}$K$_{0}$ . $_{5}$Fe$_{2}$ As$_{2}$ in which the iron SDW is completely suppressed by hole doping and SC is found below T$_{c}$ = 32 K. 5 The bulk nature of SC is confirmed by a clear specific-heat anomaly and diamag- netism found in DC-magnetization and AC-susceptibility measurements. After subtraction of the phonon contribu- tion, a specific-heat jump height of about 70 mJ/molK 2 II. EXPERIMENTAL DETAILS I. INTRODUCTION