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# Scaling behavior of the critical current in clean epitaxial Ba(Fe$_{1}$$_{−}$ x Co x )$_{2}$As$_{2}$ thin films 

K. Iida, ∗ J. H¨ anisch, T. Thersleff, F. Kurth, M. Kidszun, S. Haindl, R. H¨ uhne, L. Schultz, and B. Holzapfel 

IFW Dresden, P. O. Box 270116, D–01171 Dresden, Germany (Dated: June 4, 2018) 

The angular-dependent critical current density, J c (Θ), and the upper critical field, H c2 (Θ), of epitaxial Ba(Fe $^{1}$− x Co x ) 2 As 2 thin films have been investigated. No J c (Θ) peaks for H ‖ c were ob- served regardless of temperatures and magnetic fields. In contrast, J c (Θ) showed a broad maximum at Θ = 90 $^{◦}$, which arises from intrinsic pinning. All data except at Θ = 90 ◦ can be scaled by the Blatter plot. H c2 (Θ) near T c follows the anisotropic Ginzburg-Landau expression. The mass anisotropy increased from 1.5 to 2 with increasing temperature, which is an evidence for multi-band superconductivity. 

Tremendous efforts on the fabrication of supercon- ducting iron pnictide thin films involving fluorine-doped (Rare-earth elements)FeAsO, 1–4 Co-containing (alkaline elements)Fe$_{2}$As$_{2}$, 5–11 FeSe 12 and Te-containing FeSe 13 have been made for both investigating their intrinsic properties and exploring possible device applications. For such investigations, Co-containing BaFe$_{2}$As$_{2}$ (here- after, Ba-122) films are, perhaps, the most feasible with regard to their easy growth by pulsed laser deposition (PLD) and, most importantly, their chemical stability against ambient atmosphere. 8 

Recent investigations on this material revealed numer- ous similarities to the so-called 123-cuprates in terms of physical parameters albeit a large difference in the superconducting transition temperature, T$_{c}$ , of ≈ 70 K. YBa$_{2}$Cu$_{3}$O$_{7}$ (Y-123) and Co-containing Ba-122, for ex- ample, have a similar in-plane coherence length, ξ$_{ab}$ , of around 2 nm at low temperatures, which originates from both their small Fermi velocity and low carrier concentration. 14 Therefore, grain boundaries (GBs), even with small misorientation angles of 3-5 ◦ , are predicted to become a critical issue for multiband Co-containing Ba- 122 system, and indeed the current-limiting effect of GBs has been reported. 9 

In this Letter, we report on the transport proper- ties of Ba(Fe$_{1}$$_{−}$$_{x}$Co$_{x}$)$_{2}$As$_{2}$ thin films on (La,Sr)(Ta,Al)O$_{3}$ (LSAT) substrates and evaluate their pinning perfor- mance. In a previous publication, 11 the films on SrTiO$_{3}$ (STO) had a better epitaxial quality with a higher T$_{c}$ compared to those obtained on LSAT. However, the STO substrate becomes electrically conducting under UHV conditions, 15 presenting an obstacle to transport mea- surements particularly near the normal-superconducting transition. Therefore, we have selected the LSAT sub- strate in the present investigation. 

High-quality epitaxial Co-containing Ba-122 thin films of 30 nm thickness have been deposited on (100)- oriented cubic LSAT substrates by PLD, where the BaFe$_{1}$$_{.}$$_{8}$Co$_{0}$$_{.}$$_{2}$As$_{2}$ sintered target was ablated by 248 nm KrF radiation at a frequency of 5 Hz, an energy den- sity of 3 − 5 Jcm $^{$_{−}$2}$, and a base pressure of 10$^{$_{−}$9}$mbar. Co-containing Ba-122 films seem to be weakly volatile against ambient atmosphere albeit they have been re- 

PACS numbers: 74.70.Xa, 74.78.-w, 74.25.F-, 74.25.Sv, 74.25.Wx 

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ported to be stable. 8 Therefore, after the deposition, a Cr cap layer of 5 nm was deposited on the resulting films at room temperature in order to prevent any degradation of superconducting properties during measurements. A detailed description of the film preparation can be found in ref. $^{11}$. 

For transport measurements, the films were cut into slabs measuring 1-2 mm in width and 5-8 mm in length with a razor blade. Silver paint was employed for elec- trical contacts. Superconducting properties were mea- sured in a commercial Physical Property Measurement System (PPMS, Quantum Design) by a standard four- probe method with a criterion of 1 µ Vcm $_{−}$1 for evaluat- ing J$_{c}$ . In the angular-dependent J$_{c}$ measurements, the 

