Effect of oxygenation process on flux pinning in pristine and BaHfO3 nanocomposite GdBa2Cu3O7 superconducting thin films

Pristine and BaHfO3 (BHO) nanocomposite GdBa2Cu3O7 (GdBCO) superconducting thin films were deposited at 800 °C, 0.4 mbar O2 partial pressure using a Nd:YAG laser (λ=355 nm) with 10 Hz frequency on MgO substrate. The influence of the oxygenation process on flux pinning and electrical transport properties was investigated by changing the annealing temperature (T ann) between 450 °C and 780 °C and the holding time (t hold) between 10 and 30 min. The irreversibility field shows the largest shift toward higher fields for T ann = 450 °C and thus the largest in-field J c. Angular dependence of J c for most of the pristine GdBCO films exhibits a large peak for B‖c, however the sample with T ann = 550 °C has a large peak in the 90°-120° region, i.e. near B ‖ ab. For BHO nanocomposites, a decrease in T ann increases in-field J c by a factor of 2.


Introduction
RE Ba 2 Cu 3 O 7-δ (RE -rare earth, RE BCO) based superconductors were thoroughly investigated during last decades due to possible applications in superconducting motors, fault current limiters, generators and superconducting electronics. RE BCO superconducting thin films have relatively high critical temperatures (T c ), which allows operation at liquid nitrogen temperatures and in addition high critical current density (J c ). One of the advantages of RE BCO thin films is the presence of growth-related defects and artificial pinning centers (APCs), which results in high infield J c values due to the interaction of flux lines with structural defects. In the seminal work of J. L. MacManus-Driscoll et al. [1] on the example of YBa 2 Cu 3 O 7 (YBCO) thin films containing BaZrO 3 (BZO) nanoparticles 1.5-5 fold improvement of in-field J c by introduction of secondary phases has been demonstrated. Additionally, as it was shown in the studies of A. Goyal et al. [2] and Y. Yamada et al. [3] on YBCO films with BZO or yttria-stabilized zirconia nanocomposites that secondary phases can form columnar-like defects in the YBCO matrix. Regardless of many studies on the influence of deposition parameters, such as substrate temperature (T sub ), laser repetition rate (f ), oxygen partial pressure (p O 2 ) and laser energy density [4][5][6][7][8][9], post-annealing treatment of such thin films with APCs has not been studied widely. During deposition, the insulator RE Ba 2 Cu 3 O 6 tetragonal phase is formed and an oxygenation procedure is necessary to induce additional oxygen to form CuO chains of orthorombic phase, which are essential for the superconductivity [10,11]. Moreover, T c and self-field as well as in-field J c strongly dependent on the oxygen content, and thus can affect both self-field and in-field J c values. Recent studies of J. W. Lee et al. [12] and W. J. Oh et al. [13] on oxygenation of GdBa 2 Cu 3 O 7 (GdBCO) coated conductors (CCs) showed that different oxygen pressures and holding times (t hold ) can lead to a reduction of stacking faults density and increase in in-field J c . Regardless of huge amounts of research and publication in the field of superconducting RE BCO thin films influence of the oxygenation process in (PLD-grown) RE BCO films with APCs is still not clarified. Therefore, we studied the influence of the annealing temperature (T ann ) on transport and pinning properties in superconducting GdBCO and GdBCO+2.5wt% BaHfO 3 (BHO) thin films.

Experimental methods
Thin films were deposited using 3 rd harmonic (λ = 355 nm) Nd:YAG laser on MgO (100) single crystals from pristine GdBCO and mixed GdBCO+2.5wt% BHO targets. The laser beam was focused by a lens into the spot of 2 mm diameter, resulting in a laser energy density of 2.5 J/cm 2 . For each sample 6000 pulses were used to achieve films thickness of 200 -250 nm. For both systems, T sub = 800°C, f = 10 Hz, p O 2 = 40 Pa is found to be an optimum [9]. After depositon, all samples were cooled down to T ann between 450°C and 780°C, with further input of O 2 up to 400 mbar, kept at T ann for 10-30 min (holding time, t hold ) and cooled to room temperature. t hold was increased from 10 to 30 min for lower T ann due to lower diffusion rates of oxygen into GdBCO matrix [14,15]. To check the phase purity and microstructure films were investigated by x-ray diffraction (XRD) on a Bruker D8 diffractometer with Cu K α radiation. Transition temperatures were determined with a criterion 10% of the normal state resistance from resistivity measurements. Afterwards, microbridges of 1 mm length and 20-30 µm width were prepared by photolithography and wet-chemical etching for transport measurement in four-point geometry. Quantum design PPMS with magnetic field up to 14 T used for transport measurements at different temperatures, magnetic fields and field orientation in maximum Lorentz force configuration. 1 µV/cm electrical field was used as criterion for J c and R(T )/R(100K) ≈10 −7 criterion for the irreversibility temperatures. Bright-Field (BF) scanning transmission electron microscope (STEM) was conducted using a Cs-corrected JEOL JEM 2200FS device, operated at 200 kV. Samples for BF-STEM were prepared by cutting a cross-sectional lamella via the Focused Ion Beam (FIB) technique in a FEI Nova 600 Nanolab Dual Beam FIB-SEM. The lamella were extracted using the in-situ lift out procedure with an Omniprobe extraction needle [16].

Influence of oxygenation on flux pinning in pristine GdBCO thin films
As it can be seen from Table 1, all GdBCO films have similar T c values around 92.7 K. The c-axis lattice parameter slightly increases with decrease of T ann and reaches it maximum at 450°C and t hold = 30 min, which means the oxygen content goes from overdoped region around c = 1.1726 nm to an optimally doped with c = 1.1732 nm. In addition c-axis lattice parameters correspond well to other studies on RE BCO thin films [17,18]. Regardless of T c , self-field J c for pristine samples range from 1.2 to 4.7 MA/cm 2 and is in good agreement with other works on pristine GdBCO growth on single crystals [19,20]. 4.7 MA/cm 2 is achieved for the sample annealed at 650°C and t hold = 30 mim. However, the highest pinning force density of 6.2 GN/m 3 at 77 K is achieved at T ann = 450°C. All pristine GdBCO thin films with T ann = 450°C-780°C and t hold = 10-30 min show strong GdBCO (00l ) peaks in θ-2θ XRD scans, Fig. 1, which indicates highly c-axis oriented growth. Only the sample at T ann = 700°C and t hold = 20 min shows (h00) peaks from ab-grains. Cross-sectional BF-STEM images for T ann = 780°C (         T ann = 780°C, the sample with 550°C has triangular and trapezoidally shaped Gd 2 O 3 particles and ≈ 20-50 nm in size. Irreversibility lines (IL), representing transition between vortex-glass and vortex-liquid state, Fig. 3, is lowest for the sample annealed at 780°C and t hold = 10 min and the biggest shift toward high field region is achieved at T ann = 450°C, t hold = 30 min. The GdBCO films with T ann = 650°C, 550°C and 450°C have the highest in-field J c values at 77 and 65 K, Fig. 4. The sample with T ann = 780°C has the strongest dependence of J c on magnetic field. However, the situation changes at low temperatures region: the film at T ann = 780°C outperforms the films at T ann = 550°C and 700°C but the samples at 650°C and 450°C still have the highest J c in whole range of magnetic fields. The difference in J c (B ) behaviour can be the result of twin boundaries and planar defects in GdBCO. At high T ann = 780°C, the diffusion rate of oxygen is higher while oxygen in GdBCO is limited due to the phase stability diagram and thus does not contribute to the formation of CuO chains but increases the stacking fault density [21]. This is in a good agreement with the calculated c-axis lattice parameter for GdBCO films, which is in particular directly related to the oxygen content in RE BCO thin films, and BF-STEM images of film microstructure. In case T ann = 700°C self-field and in-field J c values drop at low measurement temperatures due to presence of ab-grains, which are known to be large in size and responsible for blocking the current. GdBCO films are characterized by strong anisotropy regarding the field orientation J c (θ)(θangle between B and c-axis), Fig. 5. The sample annealed at high temperature shows behaviour similar to YBCO films with 2 ab-peaks and small peak at 180°. A small peak in the c-direction appear due to threading dislocations and the small density of Gd 2 O 3 nanoparticles, as it seen on Fig. 2 (A-B). At T ann = 700°C, J c values for ab-peaks decrease and peak in c-direction appear, but the overall J c values are the lowest due to presence of ab-grains. For low T ann = 450 -650°C we observed 2 different behaviours. 1 st at T ann = 450°C and 650°C a large peak in c-direction appears at 1 T for 77 and 65 K. This type of behaviour was seen for GdBCO films on MgO substrate by K. Matsumoto et al. [6] and by W. J. Oh et al. [13] and was explianed as an effect of post-annealing of GdBCO CCs. This type of large peak in c-direction appears due to pinning on anti-phase boundaries and a-axis domains. At T ann = 450°C, the ratio between peak values of J c at 180°and 90°is larger compared to T ann = 650°C, thus confirming the fact that the density of the stacking faults can be reduced by decrease of the oxygenation temperature. 2 nd at T ann = 550°C for 1 T and 77 K there is flat region between 140°and 180°and additionally large peak between 90°and 120°region which further creates "volcano" in ab-direction. However, peak in ab-direction appears at higher magnetic fields. This type of behaviour at 1 T and 77 K is due to combination of planar and spherical defects in GdBCO, as it can be seen from BF-STEM images. Recently, in the studies of A. K. Jha et al. [22] on YBCO films with Y 2 BaCuO 5 nanoparticles it was shown that the pinning potential dependence on the field orientation on planar defect has a gap at 90°, while for spherical defects it has a peak behaviour with maximum values at 90°, thus combination of these 2 types of defects gives a peak behaviour between 90°and 120°. Similar behaviour with "volcano" in ab-direction can be seen in the work by T. Sueyoshi et al. [23] for YBCO films with crossed columnar defects which can be also applied to the case of 550°C sample due to triangular and and trapezoidal boundaries of Gd 2 O 3 , which have approximately 60°-120°between sides.

Influence of oxygenation in BHO nanocomposite GdBCO thin films
In our previous work on GdBCO-BHO nancomoposite films [9], we have shown that 800°C, 10 Hz, 0.4 mbar can be considered as optimum deposition condition in our experimental set-up. Therefore, we used these conditions also for this study. Maximum self-field J c of 2 MA/cm 2 and the maximum pinning force density of F p,max = 11.9 GN/m 3 at 77 K is achieved at T ann = 700°C and t hold = 20 min, Table 1. It can be seen that the presence of columnar defects in GdBCO matrix decreases T c and J c values due to strain, which BHO nanocolumns induce and concurrent oxygen-deficient regions surrounding BHO nanocolumns [24]. This effect of secondary phases were shown in the work of K. Ko on GdBCO films with BaSnO 3 on CeO 2 -buffered IBAD-MgO templates [25], and similar results were achieved in our previous work on influence of T sub and v dep on transport properties in GdBCO+2.5wt% BHO thin films grown on LaAlO 3 single crystals. All samples show GdBCO (00l ) peaks indicating strong c-axis orientation, Fig.  6. Regardless of BHO introduction into GdBCO matrix, additional peaks from BHO can not be seen due to strong overlap of BHO(002) peak with strong MgO (002) substrate peak. A cross-sectional BF-STEM image of the sample annealed at 700°C shows BHO columns with diameter of ≈ 1-3 nm along the c-direction and with a splay up to 20°. The elongated growth of BHO nanocolumns appears due to low lattice mismatch between GdBCO and BHO [26]. All nanocomposite samples show the typical S-shape of the IL, Fig. 8, with matching field B Φ ≈ 6 T, which is another confirmation of the nancolumn formation in the GdBCO matrix. Similar shape was shown in the references [27,28]. In contrast to pristine films in GdBCO-BHO nanocomposite the highest in-field J c is achieved at higher T ann = 700°C and t hold = 20 min, Fig. 9. This effect of higher oxygenation temperatures may appear due to oxygen-deficient areas around columnar defects, and higher diffusion rates may be necessary for not only the formation of the CuO chains in GdBCO, but also for regeneration of those areas. Furthermore, T ann = 700°C and t hold = 20 min can be considered as an optimum oxygenation process for GdBCO+2.5wt% BHO because longer t hold can possibly induce atomic diffusion at the boundaries between GdBCO and BHO, thus forming sharper and well defined interfaces and therefore lower splay, which may correspond to a clearer S-shape of IL. The maximum pinning force density F p,max = 11.9 GN/m 3 at 77 K and 65.2 GN/m 3 at 65 K for the sample T ann = 700°C and t hold = 20 min are similar to reported by P. Cayado et al. [29] for CSD-grown (chemical solution deposition) GdBCO+12     Figure 9: Field dependence of critical current density, J c , for BHO containing GdBCO films annealed at 550°C-780°C and t hold =10-30 min.   [30] on PLD grown GdBCO+1.5 vol% BHO on tapes. Additionally, sample with T ann = 700°C and 550°C have similar in-field J c at 77 and 65 K, but situation changes at low measurement temperatures. At 30 and 10 K in-field J c for T ann = 700°C at 4-14 T magnetic field region is higher by a factor of 2 compare to T ann = 550°C. To show influence of the oxygenation process on transport properties in GdBCO films with APCs, Fig. 10 compares J c (θ) of 2.5wt% BHO nanocomposite GdBCO films annealed at 550°C and 780°C. Both samples show clear peaks in ab-direction. Furthermore, T ann = 550°C gives a large peak at 180°which is present for 1 and 5 T at 77 K, as well as in all magnetic field at 65 K which indicates strong correlated pinning on BHO nanocolumns. J c for B c of the 550°C sample are 2-3 times larger of the 780°C sample. The 780°C sample has 2 additional peaks in the 90°-120°area at 1 T, 77 and 65 K which can appear due to pinning on nanoparticles as it was shown in work of A. K. Jha et al. [22] or splayed and inclined short nanocolumns. Comparison of J c anisotropy at 2 different T ann shows that change in post-annealing procedure for GdBCO films with columnar defects can not only increase J c by a factor of 2 but also lead to a difference in defect morphology. Different defects may originate from increased atomic diffusion near the interfaces or in extreme cases even particle diffusion (for high T ann and long t hold ).

Conclusion
Pristine GdBCO and GdBCO+2.5wt% BHO nanocomposite thin films were deposited at 800°C, 10 Hz frequency, 0.4 mbar oxygen partial pressure and annealed at T ann = 450°C-780°C with t hold = 10-30 min in order to investigate influence of post-annealing process on tranport and pinning properties. In pristine GdBCO decrease of T ann from 780°C to 450°C decreases the oxygen content from overdoped region to optimally-doped and decreases the stacking faults density and induces formation of Gd 2 O 3 nanoparticles. In BHO-containing GdBCO films decrease of T ann from 780 C to 550°C increases J c by a factor of 2.

Acknowledgments
We thank A. Marz for technical assistance as well as P. Cayado, M. Langer and S. Meyer for numerous fruitful discussions.