High intergrain critical current density in fine-grain (Ba0.6K0.4)Fe2As2 wires and bulks

Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable. This restricts choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K. Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form. So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices. The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven. Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-performance conductors of these remarkable compounds.

Grain boundary (GB) engineering of high critical temperature (T c) cuprate superconductors has been a critical issue in developing practical superconducting wires and tapes1, because their superconducting properties heavily depend on the misorientation angle (θ GB) at GBs; therefore, grains in cuprate superconductors must be highly textured to minimize deterioration of the critical current density (J c) across misoriented GBs. In a representative cuprate superconductor, YBa2Cu3O7–δ (YBCO), a fundamental study of intergrain J c in bicrystal GBs (J c BGB) has been performed using several types of bicrystal substrates2. Significantly misaligned adjacent grains cause J c BGB to decay exponentially as a function of θ GB from 3° to 40° (ref. 3). Therefore, to produce YBCO superconducting tapes with a high J c, it is necessary to insert well-aligned buffer layers with a small distribution of in-plane misalignment Δφ 50 T and weak thermal fluctuations with a Ginzburg number G i of 1.7×10−4 among iron pnictide materials19 20. In particular, cobalt-doped BaFe2As2 (BaFe2As2:Co) appears to have great potential for device applications15 16 17 21 22 23 24, because it is rather easy to grow films by pulsed laser deposition (PLD) and chemically stable in an ambient atmosphere15. It was previously reported that in-field transport properties across bicrystal GBs (BGBs) with 4 different θ GB=3°–24° formed in BaFe2As2:Co epitaxial films grown on [001]-tilt SrTiO3 bicrystal substrates suggested that even low-angle BGB with θ GB=6° obstructs supercurrent, and the weak-linked BGBs exhibit a similar behaviour to YBCO BGBs22. In this article, we report comprehensive results on transport properties of BaFe2As2:Co BGB junctions grown directly on insulating bicrystal substrates in the full range of θ GB from 3° to 45°. Among the results, it is particularly noteworthy that the critical angle θ c of ∼9° for BaFe2As2:Co is much larger than the previously reported value, and the decay slope is much smaller than that of cuprates. The large θ c allows a simpler and lower-cost fabrication process of superconducting tapes. This advantageous GB nature is demonstrated by the high J c>1 MA cm−2 of a BaFe2As2:Co superconducting tape fabricated on a polycrystalline flexible metal substrate25. Results BaFe2As2:Co bicrystal grain boundary (BGB) junctions BaFe2As2:Co epitaxial films with the optimal Co concentration of 8% were fabricated on [001]-tilt bicrystal substrates of MgO with θ GB=3°–45° and (La, Sr)(Al, Ta)O3 (LSAT) with θ GB=5°–45° by PLD with a Nd:YAG laser (λ=532 nm) at a substrate temperature of 850 °C (ref. 16). To date, different techniques that employ conductive buffer layers of SrTiO3, BaTiO3 (ref. 23) or Fe (ref. 26) have been proposed to enhance high-quality epitaxial growth. However, we have reported that it is possible to grow high-quality BaFe2As2:Co epitaxial films with high self-field J c>1 MA cm−2 directly on insulating MgO and LSAT single-crystalline substrates without any buffer layer, which has been achieved by optimizing the growth conditions16. The directly grown BaFe2As2:Co films exhibited an onset T c of 20.7 K for MgO bicrystal substrates and 21.6 K for LSAT bicrystal substrates with sharp transition widths (ΔT c) of 1.1 K (ref. 17). Figure 1a illustrates the device structure fabricated on [001]-tilt bicrystal substrates. To perform transport measurements of J c BGB, the BaFe2As2:Co epitaxial films were patterned into 300-μm-long, 8-μm-wide micro-bridge structures. To compare the intergrain J c (J c BGB) and intragrain J c (J c Grain), two types of micro-bridges were fabricated: one was a 'BGB junction' that contained a BGB each, and the other was a 'Grain bridge' that did not contain a BGB. The electrical contacts were formed with In metal pads and Au wires. The current–voltage (I–V) characteristics of BGB junctions and Grain bridges were measured by the four-probe method. High critical angle of strong-link—weak-link transition The correlation between the transport J c BGB and the θ GB is the most important index in characterizing the GB properties of superconductors. Figure 1b summarizes the self-field J c BGB(θ GB) measured at 4 and 12 K, and Figure 1c shows the J c BGB/J c Grain ratio at 4 K. For comparison, the generally accepted average J c BGB(θ GB) properties of YBCO BGB junctions measured at 4 and 77 K (ref. 2) are also plotted. The J c Grain for all of the BaFe2As2:Co Grain bridges are greater than 1 MA cm−2 at 4 K and 1.0–0.5 MA cm−2 at 12 K. For the BGB junctions with low θ GB≤∼9°, the J c BGB/J c Grain ratio remained almost at unity, indicating that the low-angle BGB junctions do not behave like a weak link. However, with the increase in θ GB from 9° to 45°, J c BGB decreases to ∼5%, which indicates that the transition from the strong link to the weak link occurs at θ c of ∼9°. In the YBCO system, the J c BGB values at 4 K for θ GB 5° showed a clear weak-link behaviour2 27. Although the reported data are somewhat scattered2, the typical values of θ c=3–5° are almost half the magnitude of those obtained for the BaFe2As2:Co BGB junctions. Gentle J c BGB decay in weak-link regime It is well known that the BGB junctions of the cuprates exhibit nearly exponential decay in their J c BGB(θ GB) curves in the weak-link regime with an empirical equation of J c0exp(−θ GB/θ 0), where θ 0 denotes the characteristic angle. The J c BGB(θ GB) curves are expressed by 3.0×107exp(−θ GB/4.3°) at 4 K and 7.0×106exp(−θ GB/4.2°) at 77 K. The present BaFe2As2:Co BGB junctions also show an exponential decay, approximated as 2.8×106exp(−θ GB/9.0°) at 4 K (the red line in Fig. 1b) and 1.5×106exp(–θ GB/8.5°) at 12 K (the blue line in Fig. 1b). It should be noted that the θ 0 values for the BaFe2As2:Co BGB junctions are twice as large as those for the YBCO BGB junctions, which indicates that the J c BGB(θ GB) of the BaFe2As2:Co BGB junctions shows a more gradual decrease than that of the YBCO BGB junctions. Consequently, the J c BGB of the BaFe2As2:Co BGB junction exceeds that of the YBCO BGB junctions at θ GB≥20° at 4 K. BGB junction characteristics Figure 2a shows the I–V characteristics measured at 12 K for 8-μm-wide BGB junctions with θ GB=4°, 16° and 45°. The BGB junction with θ GB=4° only exhibits a sharp resistivity jump at a large I c of 50 mA because of the normal state transition. Similar I–V characteristics were observed for all of the BGB junctions with θ GB ≤ 9°. On the other hand, the I–V curves of BGB junctions with θ GB=16° and 45° show nonlinear characteristics without hysteresis in the low-voltage region. In general, the shapes of the I–V curves of BGB junctions depend on the fractions of the Josephson current exhibiting resistively shunted junction (RSJ) behaviour (W RSJ) and a supercurrent showing flux flow (FF) behaviour (W FF). A phenomenological model to explain their fractions in I–V curves was previously proposed as follows28: I–V curves are expressed by the combination of the RSJ and the FF behaviours, where the RSJ current follows and the FF current follows I FF=I S−A exp(−V/V 0) (R N is the normal-state resistance of the barrier, and I S, A and V 0 are constants). The fitting results based on this model drawn by the blue lines reproduce the experimental I–V curves well. The fractions of the RSJ current W RSJ are approximately 0, 70 and 100% for the BGB junctions with θ GB=4°, 24° and 45°, respectively. The 100% RSJ current is further confirmed by good agreement with the commonly used Ambegaokar–Halperin (AH) model29 drawn by the red line for the BGB junctions with θ GB=45°. The RSJ behaviour in the I–V curves of the BGB junctions with θ GB=45° was observed in the whole temperature range below T c. The junction resistance R N A, where A is the cross-sectional area of the junction, provides information on the nature of the barrier in BGB junctions. The R N A products were estimated by fitting the above-mentioned model to the experimental I–V curves28. The R N A of the BaFe2As2:Co BGB junctions are 5×10−11 Ωcm2 for θ GB=16° and 5×10−10 Ωcm2 for θ GB=45°, which are one or two orders of magnitude smaller than those of the YBCO BGB junctions (6×10−9–8×10−8 Ωcm2 for θ GB=16°–45°, respectively). These results suggest that the BaFe2As2:Co BGB junctions work as superconductor–normal metal–superconductor junctions. On the other hand, the YBCO BGB junctions generally show hysteretic I–V curves at low temperatures well below T c, indicating relatively large junction capacitances and resistances due to the insulating nature of their junction barriers. In the case of the BaFe2As2:Co BGB junctions, the nonhysteretic curves, even at low temperatures, suggest the metallic nature of the junction barriers. Figure 2b,c show the Josephson junction properties of 3-μm-wide BGB junctions with a small θ GB=16° and a large θ GB=45°, respectively. The left figures show the magnetic field B dependencies of I c (I c−B) under B applied perpendicular to the film surfaces, and the right figures show the I–V curves of the BGB junctions irradiated with microwaves at a frequency of 1.39 GHz for θ GB=16° and 2.0 GHz for θ GB=45° measured at 16 K. The I c−B pattern of the BGB junction with θ GB=45° is distinct from the ideal Fraunhofer pattern, probably due to the inhomogeneous current distribution along the BGB; however, the junctions exhibit an I c modulation of almost 100%, which corresponds to the fact that the BGB junction with θ GB=45° exhibits the 100% RSJ current. The BGB junction with θ GB=16° exhibits an I c modulation of only 35% due to the excess current attributable to the FF current. Furthermore, both devices clearly show Shapiro steps with periodic current steps. The measured step voltage heights of 2.9 μV for θ GB=16° and 4.1 μV for θ GB=45° are consistent with the Josephson relation V RF=nhfRF/2e, where f RF is the frequency of the applied microwaves. Atomic structures of BGBs Here we examined the microstructure and the local chemical composition deviation of the BGBs to check the effect of an impurity phase on the weak-link junction behaviours. Figure 3a–c show plan-view high-resolution transmission electron microscope (HR-TEM) images of the BaFe2As2:Co BGB junctions with θ GB=4°, 24° and 45°, respectively. The [100]-axes of BaFe2As2:Co are indicated by the arrows. Symmetrically tilted junctions were formed in almost the entire region of the BGBs for all of the junctions. In the BGB junctions with θ GB=4° and 24°, periodic misfit dislocations at intervals of ∼5.0 nm for θ GB=4° and 1.2 nm for θ GB=24° are clearly observed along the BGBs. On the other hand, the BGB with θ GB=45° has blurred lattice fringes across the entire region. Using a geometric tilted boundary model, the grain boundary dislocation spacing D is given by D=(|b|/2)/sin(θ GB/2), where |b| is the norm of the corresponding Burgers vector3. With the lattice constant a=0.396 nm of BaFe2As2 (ref. 30), D is estimated to be 5.7 and 1.0 nm for θ GB=4° and 24°, respectively. The estimated D values are very similar to the D values observed above. For θ GB=45°, the D value is estimated to be 0.5 nm. This value is almost the same as the lattice parameter; therefore, we cannot observe periodic misfit dislocations in the BGB with θ GB=45° in Figure 3c. Energy dispersive spectroscopy line spectra across the BGBs and parallel to the BGBs confirmed that the chemical compositions of the BGBs and the film region are homogeneous, and no secondary phase was observed in the BGB regions. Next, we discuss the relationship between θ c and the BGB dislocation spacing D observed by TEM. For the BaFe2As2:Co BGB junctions, the observed critical angles θ c are ∼9°, which correspond to a D value of approximately 2.8 nm. This is comparable with or slightly larger than the coherence length ξ ab(T) of 2.6 nm at 4 K estimated from the reported ξ ab(0 K)=2.4 nm for BaFe2As2:Co (ref. 19). The above relationship supports the notion that strong current channels still remain between the dislocations when θ GB is below θ c, whereas coherent superconducting current cannot pass through the BGBs at θ GB>θ c and behaves like a weak link. Note that there would be other factors that affect the GB transport properties. For instance, the dislocation cores formed along BGBs can produce residual strains, which have been considered to have one of the major roles in current blocking at BGBs of cuprates31 32. This possibility would help us to obtain a more informative insight into the weak-link behaviour in iron pnictide superconductors; however, further microstructure and strain analyses are necessary to evaluate the strain field and discuss their effects. In-field characteristics of BGB junctions To investigate the weak-link behaviour in a B, J c BGB(B) values for the BGB junctions withθ GB=3°–45° were measured at B up to 9 T applied parallel to the c-axis. Figure 4a shows the J c BGB(B) curves measured at 4 K, and the inset figure shows a magnified view in the low B region up to 0.2 T. The J c Grain measured for the Grain bridge on the 3° MgO bicrystal substrate is also plotted by the black squares. For the BGB junctions with θ GB=3° and 4°, the J c BGB(B) values are almost indistinguishable from those of the J c Grain(B) curves, and a reduction in J c BGB is not observed. The other BGB junctions with larger θ GB show more rapid decreases than those of the J c Grain(B), even in a low magnetic field. The J c BGB(B) of the BGB junctions with large θ GB=24° and 45° decrease sharply to 2 and 0.8% of the J c Grain(B) on the application of 0.1 T. For the BGB junctions with θ GB=8° and 11°, the J c BGB(B) curves show an intermediate behaviour between the strongly linked and weakly linked states, where the rapid decrease in J c BGB(B) at 4 because the critical currents become so large that the Josephson penetration depth λ J becomes smaller than the junction width w. The dependencies of the J c BGB(T) curves of the BaFe2As2:Co BGB junctions are distinctly different from those reported for the YBCO BGB junctions because the latter closely follow the linear relation α(1−T/T c) over a wide temperature range below T c (ref. 34). Discussion The doubly larger θ c and the much gentler slope decay than those of YBCO BGB junctions make it easier to produce high J c BaFe2As2:Co superconducting tapes because the formation of a buffer layer with Δφ≤9° is much easier than those used for the cuprates, which require much smaller Δφ of <5°, but such buffer layers have been achieved only by a few groups using an extra MgO or CeO2 buffer layers35 36. Therefore, the large θ c allows us to use a simpler and lower cost production process of superconducting tapes, and the iron pnictide superconducting tapes would find practical applications under a higher magnetic field, if further improvement in J c will be achieved. The powder-in-tube technique has been rather progressing as an alternative technology for superconducting wires also in iron pnictides37; however, their J c values still remains at ∼104 A/cm2 (ref. 38) due probably to existence of large angle GBs with θ GB much greater than θ c=9°. The grain boundary issue in iron pnicides will be largely relaxed by the present finding. Actually, we recently succeeded in obtaining high transport J c=3.5 MA cm−2 with a BaFe2As2:Co-biaxially textured thin film on a polycrystalline Hastelloy tape with an ion-beam-assisted deposition-MgO-textured buffer layer25. In conclusion, we fabricated high-quality BaFe2As2:Co films with large J c Grain on bicrystal substrates with the entire range of θ GB=3–45° and comprehensively examined the grain boundary nature of the iron pnictide. The primary point clarified by the present study is that the BaFe2As2:Co BGB junctions exhibit a large θ c of ∼9°. The low-angle BGBs with θ GB≤9° consist of long-period dislocation cores and, therefore, J c BGB is similar to J c Grain; whereas the high-angle BGBs show a weak-link behaviour with a gradual decay of J c BGB(θ GB) expressed by the exponential equation of 2.8×106exp(−θ GB/9.0°). Such grain boundary natures together with the high B c2(0) make the iron pnictides to be more promising materials for application to high J c superconducting tapes. Methods BaFe2As2:Co epitaxial films on bicrystal substrates BaFe2As2:Co epitaxial films were fabricated by PLD on [001]-tilt bicrystal substrates of MgO with θ GB=3°–45° and also of LSAT with θ GB=5°–45°. A Nd:YAG laser (wavelength: 532 nm, INDI-40, Spectra-Physics) typically used for epitaxial growth of iron pnictide films13 39 with a repetition rate of 10 Hz on the PLD target of a high-purity BaFe1.84Co0.16As2 polycrystalline disk was used as the excitation source16. Films with 250–350 nm in thickness were grown at a temperature of 850 °C and the thickness of each film was measured precisely with a surface profiler. The base pressure in our PLD chamber was ≤1×10−6 Pa, and film deposition was carried out in a vacuum at approximately 10−5 Pa. The BaFe2As2:Co epitaxial films grown under these conditions showed high J c values of 1–4 MA cm−2 at 4 K, which were confirmed by I–V characteristic measurements with a 1-μV cm−1 criterion17. Transport properties through BGB The BaFe2As2:Co films were patterned using photolithography and Ar-ion milling into 300-μm-long and 8-μm-wide micro-bridge structures (Fig. 1a) to perform 4-terminal I–V measurements of the J c BGB across the BGB and of the J c Grain not across the BGB and under magnetic fields perpendicular to the film surface. The critical current (I c) and the asymptotic junction resistance (R N A) were estimated from the I–V characteristics28. Microstructure and chemical composition analysis of BGB The microstructures around the BGBs were examined by plan-view HR-TEM. The TEM samples were prepared by a focused-ion-beam micro-sampling technique in which the area near the BGBs was mechanically cutout, and that area was only thinned by focused-ion-beam technique. All of the operations were performed in a high-vacuum chamber. The chemical composition of the bulk film and the BGBs was analysed by energy dispersive X-ray spectroscopy with a spatial resolution of approximately 1 nm. Author contributions T. Katase, Y.I., A.T., H.H. and T. Kamiya performed the experiments and analyzed the data. K.T. and H.H. designed the study. All the authors contributed to discussion on the results for the manuscript. Additional information How to cite this article: Katase, T. et al. Advantageous grain boundaries in iron pnictide superconductors. Nat. Commun. 2:409 doi: 10.1038/nocmms1419 (2011).