High field superconducting properties of Ba(Fe1−xCox)2As2 thin films

The intermetallic Fe-based superconductors (FBS) of ThCr2Si2-type crystal structure, such as electron or hole-doped AEFe2As2 (AE alkali earth metal), are characterized by a low Ginzburg number 1,2, high upper critical fields, 3, and very low anisotropies, 4,5, which tend to approach 1 at low temperatures. Although the in-plane coherence length, , is similarly small as in YBa2Cu3O7−δ (YBCO), grain boundaries (GBs) are slightly less detrimental to the current flow than in high cuprates6. The reason is mainly a difference in order parameter symmetry, being predominantly d-wave in YBCO but of s-wave type (presumably with a sign change, s±)7 in FBS. All these properties make FBS materials interesting for high-field applications at low temperatures since they combine advantageous properties of the low-temperature superconductors (narrow in-field transition, low Gi, low anisotropy) and high- cuprates (high upper critical fields ). Indeed, first prototypes of Co-doped BaFe2As2 (Ba-122) coated conductors8,9 and K-doped Ba-122 powder-in-tube wires10,11 have been reported. A well-investigated member of this family is Co-doped Ba-122 due to its relatively easy preparation as thin films by pulsed laser deposition and stability at ambient conditions12. Furthermore, Co-doped Ba-122 proved to be very susceptible to a high density of artificial pinning centers, and record values of the maximum pinning force density, , of around 70 GN/m3 for at 4.2 K have been reported for films on CaF2 substrate13. Lee et al. demonstrated that Co-doped Ba-122 films grown at relatively high O2 partial pressure14 or with oxygen-containing targets15 develop correlated c-axis-oriented columnar oxide defects. These defects lead to a huge contribution to pinning for fields applied parallel to the c-axis16. Katase et al. found similarly high Jc values for Co-doped Ba-122 on (La,Al)(Sr,Ta)O3 (LSAT)17. In most of these pinning-improved Co- or P-doped18,19 Ba-122 samples, a large density of c-axis correlated or extended random defects is introduced. This paper investigates the high-field transport properties of Co-doped Ba-122 thin films with several different natural growth defects, such as small-angle grain boundaries and stacking faults. It will be shown how these defect populations, in combination with the multi-band superconductivity, influence the vortex matter and the pinning properties in different regions of the H-T phase diagram.