2 GLYPH<s48>GLYPH<s46>GLYPH<s48> GLYPH<s48>GLYPH<s46>GLYPH<s53> GLYPH<s49>GLYPH<s46>GLYPH<s48> GLYPH<s49>GLYPH<s46>GLYPH<s53> GLYPH<s50>GLYPH<s46>GLYPH<s48> GLYPH<s32>GLYPH<s51>GLYPH<s50>GLYPH<s75> GLYPH<s32>GLYPH<s51>GLYPH<s48>GLYPH<s75> GLYPH<s32>GLYPH<s50>GLYPH<s53>GLYPH<s75> GLYPH<s32>GLYPH<s50>GLYPH<s48>GLYPH<s75> GLYPH<s32>GLYPH<s49>GLYPH<s53>GLYPH<s75> GLYPH<s69> GLYPH<s83> GLYPH<s82> GLYPH<s32>GLYPH<s115>GLYPH<s105>GLYPH<s103>GLYPH<s110>GLYPH<s97>GLYPH<s108>GLYPH<s32>GLYPH<s40>GLYPH<s97>GLYPH<s114>GLYPH<s98>GLYPH<s46>GLYPH<s32>GLYPH<s117>GLYPH<s110>GLYPH<s105>GLYPH<s116>GLYPH<s115>GLYPH<s41> GLYPH<s72>GLYPH<s40>GLYPH<s107>GLYPH<s79>GLYPH<s101>GLYPH<s41> GLYPH<s69>GLYPH<s117> GLYPH<s48>GLYPH<s46>GLYPH<s53> GLYPH<s75> GLYPH<s48>GLYPH<s46>GLYPH<s53> GLYPH<s70>GLYPH<s101> GLYPH<s50> GLYPH<s65>GLYPH<s115> GLYPH<s50> GLYPH<s61>GLYPH<s57>GLYPH<s46>GLYPH<s51>GLYPH<s54>GLYPH<s71>GLYPH<s72>GLYPH<s122> GLYPH<s48> GLYPH<s50> GLYPH<s52> GLYPH<s54> GLYPH<s32> GLYPH<s72>GLYPH<s40>GLYPH<s107>GLYPH<s79>GLYPH<s101>GLYPH<s41> GLYPH<s49>GLYPH<s53>GLYPH<s75> GLYPH<s51>GLYPH<s48>GLYPH<s75> GLYPH<s52>GLYPH<s48>GLYPH<s75> GLYPH<s53>GLYPH<s53>GLYPH<s75> GLYPH<s32> GLYPH<s69>GLYPH<s117> GLYPH<s48>GLYPH<s46>GLYPH<s53> GLYPH<s75> GLYPH<s111>GLYPH<s46>GLYPH<s53> GLYPH<s70>GLYPH<s101> GLYPH<s50> GLYPH<s65>GLYPH<s115> GLYPH<s50> GLYPH<s32> GLYPH<s61>GLYPH<s57>GLYPH<s46>GLYPH<s51>GLYPH<s54>GLYPH<s71>GLYPH<s72>GLYPH<s122> GLYPH<s32> GLYPH<s69>GLYPH<s83>GLYPH<s82>GLYPH<s32>GLYPH<s115>GLYPH<s105>GLYPH<s103>GLYPH<s110>GLYPH<s97>GLYPH<s108>GLYPH<s32>GLYPH<s40>GLYPH<s97>GLYPH<s114>GLYPH<s98>GLYPH<s46>GLYPH<s32>GLYPH<s117>GLYPH<s110>GLYPH<s105>GLYPH<s116>GLYPH<s115>GLYPH<s41> GLYPH<s32> GLYPH<s32> GLYPH<s32>GLYPH<s52>GLYPH<s75> FIG. 2: (Color online) Temperature evolution of the low-field absorption spectra of Eu 0$_{.}$5 K 0$_{.}$5 Fe 2 As 2 below T c . Lines are shifted for clarity. FIG. 1: (Color online) ESR spectra and corresponding fit curves of Eu 0$_{.}$5 K 0$_{.}$5 Fe 2 As 2 at different temperatures below and above T c . as a function of the static magnetic field H . The signal- to-noise ratio of the spectra is improved by recording the derivative dP/dH using lock-in technique with field modulation. The sample was measured in the form of fine powder. due to the penetration of magnetic flux in the Shubnikov phase above H$_{c1}$ of the type II superconductor. 27 Returning to the resonant absorption we determined the absolute value of the ESR spin susceptibility χ$_{ESR}$ above T$_{c}$ by comparison of the double-integrated sig- nal intensity with that of the reference compound Gd$_{2}$BaCuO$_{5}$ – the so called green phase – which exhibits an ESR signal with a similar linewidth. In Gd$_{2}$BaCuO$_{5}$ all Gd 3+ spins (with the same electron configuration 4 f 7 and spin S = 7 / 2 like Eu $^{2+}$) contribute to the ESR sig- nal and the corresponding susceptibility exhibits a Curie- Weiss law with Θ$_{CW}$ = − 23 K. 28 We find that about 50% of the Eu 2+ ions participate in the resonant absorption. Estimating the skin depth δ = ( ρ/µ$_{0}$ω ) 0 . 5 using the re- sistivity value ρ = 0.04 Ω cm at room temperature 24 and the microwave frequency 9 GHz we find a skin depth of δ ≈ 75 µ m. The typical grain size of powdered samples is of the same order of magnitude in agreement with the percentage of Eu 2+ spins contributing to the observed signal. The temperature dependence of the double-integrated signal intensity χ$_{ESR}$ is compared to the static suscepti- bility χ$_{dc}$ in Fig. 3(a). Note the different scales due to the fact that only 50% of the Eu spins contribute to the resonant absorption. Starting from high temperatures the Curie-Weiss behavior of χ$_{ESR}$ is interrupted just be- low T$_{c}$ , where it abruptly decreases and again increases on further lowering the temperature, while the static sus- ceptibility increases monotonously without any drop but only slightly deviating from the Curie-Weiss law at T$_{c}$ . The temperature dependence of the D/A ratio is shown in the inset of Fig. 3(a) together with the square root of the normalized conductivity ( σ/σ ( T = T$_{c}$ )) 0 . 5 in the normal state taken from Ref. 5. In the normal state the two quantities can be scaled to fall on top of each other, confirming that D/A is proportional to the penetration depth of the microwave. The D/A ratio drops by a fac- Figure 1 shows ESR spectra of a powdered polycrys- tal of Eu$_{0}$ . $_{5}$K$_{0}$ . $_{5}$Fe$_{2}$ As$_{2}$ for different temperatures. In all cases one observes a single exchange-narrowed resonance line which is well described by a Dyson shape, 26 i.e. a Lorentz line at resonance field H$_{res}$ with half width at half maximum Δ H and a contribution of the dispersion to absorption ( D/A ) ratio 0 ≤ D/A ≤ 1, resulting in an asymmetry typical for metals where the skin effect drives electric and magnetic components of the microwave field out of phase. The D/A ratio is determined as a fit pa- rameter and depends on sample size, geometry, and skin depth. If the skin depth is small compared to the sam- ple size, D/A approaches 1. Focussing on the low-field regime, below the superconducting transition tempera- ture T$_{c}$ the spectra as documented in Fig. 2 exhibit a dip- like signal at low fields typical for the magnetic shielding below the lower critical field H$_{c1}$ ≈ 100 Oe, 25 followed by a broad non-resonant microwave absorption feature III. EXPERIMENTAL RESULTS AND DISCUSSION