Large pinning forces and matching effects in YBa2Cu3O7-δ thin films with Ba2Y(Nb/Ta)O6 nano-precipitates

The addition of mixed double perovskite Ba2Y(Nb/Ta)O6 (BYNTO) to YBa2Cu3O7−δ (YBCO) thin films leads to a large improvement of the in-field current carrying capability. For low deposition rates, BYNTO grows as well-oriented, densely distributed nanocolumns. We achieved a pinning force density of 25 GN/m3 at 77 K at a matching field of 2.3 T, which is among the highest values reported for YBCO. The anisotropy of the critical current density shows a complex behavior whereby additional maxima are developed at field dependent angles. This is caused by a matching effect of the magnetic fields c-axis component. The exponent N of the current-voltage characteristics (inversely proportional to the creep rate S) allows the depinning mechanism to be determined. It changes from a double-kink excitation below the matching field to pinning-potential-determined creep above it.

. J c could be strongly improved through incorporating this or related barium perovskites BaMO 3 (M transition metal) because they grow as globular or columnar structures which act as pinning centers. This has been reported for various techniques such as pulsed laser deposition (PLD) [11][12][13][14][15] or metal-organic chemical vapor deposition (MOCVD) 16 . In chemical deposition methods (CSD, MOD), usually randomly oriented, more or less isotropic nanoparticles are formed [17][18][19] . Secondary phases can furthermore introduce concurrent defects in the YBCO matrix. Strong positive correlations between nanostrain and J c for example suggest that nanostrain and strain-induced defects such as stacking faults 20 or dislocations 21 act as pinning centers. Nanoinclusions of the double perovskites Ba 2 YNbO 6 (BYNO) and Ba 2 YTaO 6 (BYTO) have been suggested 22 as promising pinning centers due to several advantages compared to BZO. Nb or Ta ions are less likely to substitute for Y in YBCO, which should lead to a smaller T c Scientific RepoRts | 6:21188 | DOI: 10.1038/srep21188 reduction 22 , (a minute Nb substitution on Cu was even reported to slightly increase T c 23 ). Furthermore, according to their melting points (ZrO 2 2715 °C, Nb 2 O 5 1512 °C and Ta 2 O 5 1872 °C), Ta-O and Nb-O species presumably have a larger mobility on the substrate surface and within the film during growth than Zr-O, leading to well aligned BYNO nanocolumns 24,25 even at high deposition rates 26 . BYTO 27 and Ba 2 Y(Nb/Ru)O 6 28 have been shown to form nanocolumns in PLD-grown films, whereas BYTO forms extended nanoparticles in CSD-grown films 29 , very similar to the single perosvkites. Mixing BYNO and BYTO effectively adjusts interfacial energies and strain as well as diffusion lengths.
As shown by Ercolano et al. 30 , double-doping of YBCO with BYNO and BYTO leads to a further improvement of the transport properties. Furthermore, these mixed-double perovskite Ba 2 Y(Nb/Ta)O 6 -doped YBCO films (BYNTO:YBCO) seem to show a more complex J c anisotropy than single-doped samples. Related structures are the so-called 2411-phases, Cu-containing mixed double perovskites, which also were shown to improve J c , however with less chemical stability and less tendency to form nanocolumns [31][32][33] . A deeper investigation of the material system BYNTO:YBCO is needed to provide a better understanding of its complex pinning landscape and the resulting effects on J c (H,T,θ). This detailed study shows the influence of the microstructure of such BYNTO:YBCO thin films grown under various deposition conditions on J c and N-value anisotropy as well as the field dependence of the pinning force density, F p (H), at liquid nitrogen temperatures. We explain the appearance of symmetric shoulders in J c (θ) and N(θ) by a matching effect which does not depend on the absolute magnitude of the applied magnetic field but rather on its c-component.

Results and Discussion
The growth of self-assembled nanocolumns is not only driven by interfacial energies and strain [34][35][36] but also by diffusion of the respective atomic species, which can be controlled by temperature [37][38][39][40] , time (i.e. laser repetition rate) 41,13 and diffusion lengths (i.e. distances between deposited material). Furthermore, secondary phases 39 as well as growth direction (vicinality of the substrate) 42 influence the growth kinetics of nanocolumns.
In this study, films with various laser repetition rates f Dep between 1/2 Hz and 50 Hz were grown at fixed (optimized) temperature in order to investigate the influence of different microstructures inside the BYNTO:YBCO matrix on the transport properties.

Microstructure
The YBCO matrix shows epitaxial growth on the STO substrates for all prepared films. As an example, Fig. 1 shows the Θ -2Θ scan for the film grown with a laser repetition rate of 1 Hz. For YBCO and BYNTO, just (00) and (00 2) peaks are visible, respectively. Y 2 O 3 could be found by the appearance of its (004) peak and Y 2 BaCuO 5 by its (220) peak. BYNTO nanoparticles are aligned cube-on-cube with the YBCO for all samples. This was confirmed by selected area electron diffraction (SAED) as well as pole figure measurements of YBCO (102) and BYNTO (220), Supp. Fig. S1. The intensity of the BYNTO (00) peaks and therefore the amount of biaxially incorporated BYNTO is increasing with decreasing f Dep (Supp. Fig. S2). TEM cross section images of the films grown at 1 Hz (Fig. 2a,b and Supp. Fig. S4) and 5 Hz (Supp. Fig. S3) show three types of defects: BYNTO nanocolumns (horizontal), Y 2 O 3 plates (vertical) and defects in the YBCO lattice like stacking faults (extra Y or CuO 2 planes) or anti-phase boundaries (APB). The density and morphology of the nanoparticles depend strongly on the laser repetition rate. With increasing f Dep the density of the nanorods increases (1 Hz: 1 column per 29.9 nm, 5 Hz: 1 column per 14.5 nm), their diameter decreases (1 Hz: (10 ± 2) nm, 5 Hz: (8 ± 4) nm), and the density of Y 2 O 3 plates seems to increase. Fig. 2c shows a large-area TEM cross section of 3200 nm width of the 1 Hz sample. Some of the nanorods seem to start or end within the film (red dots). However, if all columns are counted (black arrows plus red dots) an average column distance of d = 29.9 nm is calculated. This corresponds nicely to the value d = 30.5 nm obtained by plain-view TEM (Supp. Fig. S6). Therefore, we conclude that for f Dep up to 5 Hz, most of the nanorods are penetrating the whole film and some are slightly inclined and cut by the lamella preparation. The components, BYNTO nanorods and Y 2 O 3 particle, were further confirmed by EDX mapping of the contained elements (Supp. Fig. S5).

Critical current density and Pinning force
Films grown at f Dep of 1 Hz or 1/2 Hz show a transition temperature T c of ∼ 90 K, which is in the experimental range of undoped samples (90…92 K). In general, T c decreases for higher repetition rates (Fig. 3), while the transition width Δ T c (error bars in inset of Fig. 3) is increasing by a factor of 7 (Δ T c (1 Hz) = 0.5 K, Δ T c (50 Hz) = 3.3 K). The reduction in T c might be avoidable by adjusting other PLD parameters such as deposition temperature, energy density, oxygen partial pressure and target-substrate distance. Whereas the samples with 1 Hz and 1/2 Hz are similar in their current carrying capability, the samples grown at 5 Hz or higher show much smaller J c values at 77 K ( Fig. 4a and Table 1). This reduction is mainly due to the reduced T c value because the decrease in J c is smaller at lower temperatures such as 30 K, Fig. 4b. The critical current density in self field J c,sf at 77 K reaches values of 3.2 MA/cm 2 (1 Hz) and 4.0 MA/cm 2 (1/2 Hz) and decreases in an external field as it is expected for a high-temperature superconductors in the strong  pinning limit 43 . J c,sf values at 30 K as high as 28 MA/cm 2 can be achieved in BYNTO:YBCO films grown with laser repetition rates of 1 Hz or below. The critical current density is above 1 MA/cm 2 even for an external magnetic field of 9 T for all samples at this temperature (Fig. 4).The field dependence of J c at 77 K shows an unusual behavior of samples with f Dep = 1 Hz and 1/2 Hz ( Fig. 4a and Supp. Fig. S7). Above approx. 500 mT, J c stays almost constant for increasing fields.
The field dependence of the pinning force density is equally unusual. For these BYNTO:YBCO thin films, it is not possible to describe ( ) F H P by one Dew-Hughes function 44 alone as it is possible for a YBCO film without any additions (bold green line in Fig. 4c). The field B max = μ 0 H(F P,max ) at which the maximum pinning force density is reached increases with laser repetition rate (1.  3 45 ) show similar F P,max values at 77 K. The previously mentioned plateaus in J c (H) at 77 K also end at these fields B max . Furthermore, B max is temperature independent as can be seen by the dashed lines in Fig. 4c,d for the pinning force density at 77 K and 30 K (see also Supp. Fig. S8). Therefore, this plateau can be explained by a matching effect. Up to this field, every flux line can be pinned by an individual BYNTO column. The mean column distance of d = 29.9 nm, as observed in TEM for the 1 Hz sample, corresponds to a matching field B m of approx. 2.3 T which corresponds quite well to the end position of the constant J c region, respective peak position in F P (H), dashed black line Fig. 4c,d (2.3 T). This value was also confirmed by a TEM plain view image    (1).
where 312 columns are visible in a 0.289 μm 2 wide area which leads to a matching field of (2.23 ± 0.14) T (Supp. (M … magnetization) 48 . Strictly, this relation only holds if S and N are measured at the same electrical field E. In general, N values determined from creep rates (equation above) are larger than from magnetization measurements 49 and those are larger than determined from transport E(J) curves 50 due to the negative curvature of E(J) below the glass-liquid transition. For the considerations below, this effect can be ignored, however.
For flux creep processes determined by the pinning potential  U k T 0 B and under the assumption of a logarithmic J-dependence of U 0 51 , = / N U k T 0 B and, hence, = / S k T U B 0 for N ≫ 1 (i.e. neglecting a log. time dependence term). This in turn means that J c , being determined itself by U 0 , scales with N. This is indeed found on a variety of samples in a wide range of N values with an empirical relation (N-1) ~ J c 1/p , p ~ 3, Fig. 5b. The origin of this power law is beyond the scope of this paper. As seen in Fig. 5, N is anticorrelated to J c in a wide magnetic field region below the matching field. The explanation for this peculiarity is a second mechanism for flux creep effects. In the presence of strongly correlated 1D or 2D pinning centers and for μ 0 H < B m , the flux creep rate is determined by the ability of flux half loops and double kink structures to form and move between the pinning centers, which consequently leads to an effective "hopping" of the flux lines from nanocolumn to nanocolumn. The net velocity of the flux lines and so the creep rate depends on the probability of finding a free nanocolumn in the neighborhood. Since this probability decreases with increasing magnetic field μ 0 H < B m , S decreases and N increases for μ 0 H →B m . This effect is the equivalent of a Mott insulator. As observed in magnetization measurements on irradiated YBCO single crystals with columnar defects 52 , matching effects are only visible for a very narrow orientation distribution (splay) of the correlated defects, in that case of less than 4°. This, in turn, leads to an overall decreased J c due to the increased creep rates. Larger splay values lead to lower creep rates and therefore higher J c values but no observable matching effects. Similar reductions in creep rate due to additional defects in YBCO films have been reported recently for increased film thickness and substrate decoration 53 , and nanoparticle addition (Y211, BZO, BYTO) in films grown by metall-organic deposition (MOD) 54,55 . In our samples, we observe high J c values and matching effects. This is a consequence of the intermediate Y 2 O 3 particles and stacking faults, which hinder the half loops and double kink structures from running parallel the nanocolumns and effectively jump to the next column 13 .

Critical current density anisotropy
The anisotropy of the critical current density J c (θ) at 77 K shows a maximum at 90° due to the electronic anisotropy of YBCO 56 and one at 180° due to c-axis correlated pinning 57 for all BYNTO:YBCO samples. Besides  Fig. 7a,b. For both samples, the N value maxima (and hence minima in S) appear once B c reaches the matching field B m just the same as for J c at H c. A similar effect has recently been observed by Trastoy et al. 58 on YBCO films with a periodic square pattern of artificial pin holes produced by ion irradiation. They have seen strong matching effects with μ 0 H θ ⋅ ( ) cos in field and angular dependence of resistivity and glass-liquid transition, concurrent with an increased mass anisotropy due to partial deoxygenation and a 2D glass-liquid transition. Our results show that this type of matching is independent of dimensionality (our samples are 3D) and periodicity (the BYNTO nanocolumns show a certain degree of density variance).
As calculated by Paulius et al. 59 for correlated irradiation defects and described by e.g. Ercolano et al. 30 and Jha et al. 60 for nanocolumns, the vortex is not pinned by one column alone but rather by several columns through a staircase-like path for angles θ smaller than a certain trapping or accommodation angle 56 . The flux line segments in between the vortices are pinned by additional defects, such as Y 2 O 3 , stacking faults and antiphase boundaries, as observed in TEM. The specific position ⁎ B m , where the θ ( ) J c curves have their maximum in J c (B c ) is slightly raised compared to the matching field and the maxima in the N value (2.0 T for f Dep = 1/2 Hz and 2.7 T for f Dep = 1 Hz, Fig. 7c,d). Apparently, the extrema in creep rate S and hence the N value are determined solely by the occupation of the c-axis aligned correlated defects. Therefore, they appear at the matching field B m . Additional intermediate defects contribute to J c for field directions closer to c and lead to a further increase in J c in a small region of m . This region (shaded area in Fig. 7) shows again anti-correlation between N and J c (i.e. creep rate S not determined by pinning potential U 0 ). However, this configuration is different from the case H c, since here the anti-correlation appears above B m , whereas it appears below B m for H c .
Besides the strongly pronounced shoulders, a second pair of shoulders can be seen for fields up to 8 T (Fig. 6, small arrows). They have a much smaller amplitude and their position is much closer to 90°. The position of those peaks does not scale with the c-axis component of the applied magnetic field, B c . A possible scenario for those peaks might be a combined effect of fractional occupation of the correlated defects and anisotropic pinning of the flux line segments at intermediate extended defects. This has to be investigated in more detail in future studies.

Conclusion
BYNTO-doped YBCO films were successfully grown using different laser repetition rates. Only for sufficiently long diffusion times ( f Dep = 1 Hz or lower), very high critical current densities were achieved. All films have a rich microstructure consisting of pinning centers with different size, shape and orientation distribution. The interplay between strongly c-axis correlated BYNTO nanocolumns, ab-stacking faults, c-axis oriented APBs, atomic disorder and biaxially oriented but randomly distributed Y 2 O 3 nanoparticles leads to additional new features in F P (H) and J c (θ). The critical current density shows almost constant values for magnetic fields up to several Tesla. The field where this plateau ends and where F P (H) has its highest value are equal to the matching field determined by TEM. A pinning force density of 25 GN/m 3 at 77 K at the matching field of 2.3 T was achieved for the film grown at 1 Hz, which is among the highest values reported for YBCO. At 30 K, deposition rates up to f Dep = 5 Hz lead to critical current densities of more than 1 MA/cm 2 even in external fields of up to 9 T for H c.
The field dependence of the N value (the exponent of the E(J) curves) and its scaling behavior with the corresponding J c value clearly show different regions of flux depinning mechanisms, being dominated by the creation of half loop and double kink structures below the matching field and by usual flux creep processes due to weaker, uncorrelated, pinning centers above the matching field.
The anisotropy of the critical current density J c (θ) as well as N(θ) are composed of two additional shoulder formations besides the maxima at 0° and 90° which appear at different angles if the applied magnetic field strength is changed. The existence of these shoulders can be explained by a staircase-like path of the vortices and additional pinning at intermediate defects. At a certain angle, the magnetic fields c-axis component μ 0 H ·cos θ reaches the matching field B m which is accompanied by a maximum of the N value. Because of further contribution of intermediate defects to J c but not to N, the maximum at the critical current density anisotropy ⁎ B m is slightly higher than the matching field B m .
Due to their ability to grow as very uniformly sized, distributed and oriented nanocolumns, Ba 2 Y(Nb/Ta)O 6 nano-precipitates in YBa 2 Cu 3 O 7−δ thin films are not only effective artificial pinning centers for increased J c and F P values but also a model system to study matching effects and flux creep.

Methods
BYNTO:YBCO films were prepared by pulsed laser deposition (PLD) using a YBCO target with 2.5 mol% BYNO and 2.5 mol% BYTO. The target was prepared by mixing the precursor oxides (barium oxide, yttrium oxide, tantalum oxide and niobium oxide) in the appropriate amount with YBCO powder and grinding the mixture in an agate mortar. The powder was pressed in pellets and sintered at 950 °C in flowing O 2 for 24 h. A Lambda Physics LPX305i KrF excimer laser (λ = 248 nm, t pulse = 25 ns) was used with an energy density of 1.6 J/cm 2 at the target surface to grow BYNTO:YBCO films of approx. 250 nm thickness on single crystalline (100)-oriented SrTiO 3 (STO). An oxygen partial pressure of 0.4 mbar was maintained during the deposition process. The substrate temperature was set to 840 °C and checked with a HEITRONICS ceramic pyrometer. The laser repetition rate f Dep was varied between 0.5 Hz and 50 Hz. Oxygen loading of the YBCO films took place in 400 mbar O 2 for 1 hour at 770 °C. A silver cap layer of several nm thickness was deposited afterwards to improve the contact resistance.
Surface morphology and film thickness were analyzed by scanning electron microscopy using a JEOL JSM-6510 and an FEI Helios Nanolab 600i to cut cross sections by a focused gallium ion beam. Transmission electron microscopy (TEM) analysis was carried out on an FEI Osiris microscope, equipped with a 'Super-X' wide solid angle EDX detector, operated at 200 kV for bright field transmission electron microscopy (BFTEM), high angle annual dark field scanning transmission electron microscopy (HAADF STEM) and energy dispersive X-ray spectroscopy (EDX). FEI Titan 'cubed' electron microscope operated at 300 kV, equipped with an aberration corrector for the probe-forming lens as well as a high-brightness gun was used for high-resolution transmission electron microscopy (HRTEM). X-ray diffraction was carried out on a Bruker D8 Advance with a Co anode in a modified parallel-beam geometry for Θ -2Θ scans. Pole figures were measured at a Philips X'Pert PW3040 with a Cu anode in Bragg-Brentano geometry.
Bridges of 800 nm length and approx. 20 μ m width for transport current measurements were prepared by laser cutting. Field and angular dependencies of critical current density J c and exponent N of the V(I) curves were measured in a four-point assembly in magnetic fields up to 9 T with a Quantum Design physical properties measurement system (PPMS). J c was defined by an electrical field criterion of 1 μ V/cm on V(I) curves fitted as Ẽ J N in the first decade of E above E c . The angular dependence of the critical current density was measured under maximum Lorentz force configuration. To correct for small heating effects, the absolute value of some of those measurements were corrected to the value measured at H c where thermal contact was better.