Pinning effects in ceramic ${\text{SmO}}_{1-x}{\text{F}}_x\text{FeAs}$ as revealed by microwave absorption

Modulated microwave absorption (MMA) measurements have revealed strong pinning of vortices in ceramic superconducting ${\text{SmO}}_{1-x}{\text{F}}_x\text{FeAs}$ compounds with $x=0.06$, 0.08, and 0.1. Different behavior of MMA in small and strong fields enables to discriminate between intergranular and intragranular effects. Irreversibility lines due to the intragranular pinning exhibit a steep ascent comparable with that of the ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7$ ceramics which is known to possess the highest pinning strength among cuprate high-temperature superconductors. A weak dependence of the critical current density on the magnetic field in the underdoped samples $\left(x=0.06,0.08\right)$ indicates the presence of additional pinning centers. The analysis of the data together with the theoretical modeling yields a conclusion that strong pinning in ${\text{SmO}}_{1-x}{\text{F}}_x\text{FeAs}$ is due to nonsuperconducting regions intermixed on a nanoscale with the superconducting phase.

Figure 1

(a) SEM image of a polycrystalline ${\text{SmO}}_{0.9}{\text{F}}_{0.1}\text{FeAs}$ sample. White stripes are the superconducting grains and black spots are voids. (b) Histogram distribution of lateral grain dimensions for the ${\text{SmO}}_{0.9}{\text{F}}_{0.1}\text{FeAs}$ sample. An average lateral dimension is $2.5\mu \text{m}$ and the length of grains varies between $5–15\mu \text{m}$.

Figure 2

(Color online) Superconducting transition temperatures of the ${\text{SmO}}_{1-x}{\text{F}}_x\text{FeAs}$ samples with different fluorine doping as determined from the temperature dependence of (a) resistivity, (b) ac susceptibility, and (c) static magnetization. Onset critical temperatures are 39, 49, and 55 K for the samples with the fluorine content $x=0.06$, 0.08, and 0.1, respectively. Error bars reflect the uncertainty of the estimation of the demagnetization factor and of the sample volume.

Figure 3

Hysteresis loops recorded at different temperatures below $T_{\text{c}}$ for a polycrystalline ${\text{SmO}}_{0.9}{\text{F}}_{0.1}\text{FeAs}$ sample. Horizontal arrows show the sweep direction. Vertical arrows indicate the location of the irreversibility field $H_{\text{irr}}$. Dashed middle line indicates the constant, field-independent level of absorption.

Figure 4

(Color online) MMA hysteresis loop for the ${\text{SmO}}_{0.9}{\text{F}}_{0.1}\text{FeAs}$ sample recorded in the magnetic field around 0.25 T at $T=50\text{K}$: solid line, experimental loop; dashed line, simulated loop. Long-dashed middle line indicates the constant, field-independent level of absorption.

Figure 5

MMA hysteresis loops recorded for the ${\text{SmO}}_{0.94}{\text{F}}_{0.06}\text{FeAs}$ sample at different temperatures: (a) $T=24.8\text{K}$, (b) $T=27.3\text{K}$, and (c) $T=31.5\text{K}$. Dashed middle line indicates the constant, field-independent level of absorption.

Figure 6

(Color online) (a) Irreversibility lines for ${\text{SmO}}_{1-x}{\text{F}}_x\text{FeAs}$ samples reduced to the onset critical temperature: triangles, $x=0.06$; circles, $x=0.08$; and squares, $x=0.1$. (b) Irreversibility lines for polycrystalline HTSC compounds: squares, ${\text{SmO}}_{0.94}{\text{F}}_{0.06}\text{FeAs}$; triangles, YBCO (Ref. 21); diamonds, LSCO (Ref. 22); and circles, BSCCO (Ref. 23).

Figure 7

(Color online) Experimental (solid line) and theoretical (dashed line) MMA hysteresis loops for the ${\text{SmO}}_{0.92}{\text{F}}_{0.08}\text{FeAs}$ sample at $T=39\text{K}$. Long-dashed middle line indicates the constant, field-independent level of absorption.

Figure 8

(Color online) Temperature dependence of the MMA hysteresis amplitude at the applied magnetic field $H=0.2\text{T}$ for the ${\text{SmO}}_{1-x}{\text{F}}_x\text{FeAs}$ samples with different fluorine doping: (a) $x=0.1$, (b) $x=0.08$, and (c) $x=0.06$. Solid line is the result of numerical calculations, symbols are the experimental data.

Figure 9

(Color online) Magnetic field dependence of the critical current density $j_{\text{c}}$ at $T=25\text{K}$ for ${\text{SmO}}_{1-x}{\text{F}}_x\text{FeAs}$ samples with different fluorine doping: circles, $x=0.06$; diamonds, $x=0.08$; and squares, $x=0.1$.