Synergetic pinning centres in BaZrO₃-doped YBa₂Cu₃O_7-x films induced by SrTiO₃ nano-layers

Large-scale power applications of YBa₂Cu₃O_7−x (YBCO) films processed as long-length coated superconductors depend on the development of low-cost methods for the preparation of thick films with a high critical current density (J_c) to obtain a high critical current per centimetre of width (I_c-w) [1]. For applications in moderate and high magnetic fields, high J_c and I_c-w values require a high density of defects (pinning centres, both natural and artificial) preventing motion of the magnetic flux induced by Lorentz force and thermal energy. The initial methods, such as substrate decoration [2–4] or deposition from composite targets, e.g. of YBCO with BaZrO₃ (BZO) nanoinclusions [5] or with Gd₂Ba₄CuWO_y [6], targeted one specific type of pinning centre. The improvement of J_c could be further increased by the proper choice of the architecture of the films, e.g. using a multilayer structure in which YBCO layers are separated by a thin insulating layer such as CeO₂ [7]. Recent approaches have targeted several types of pinning centres by using double-doping, for example, with BZO and Y₂O₃ [8, 9], or Gd₃TaO₇ and YBa₂NbO₆ [10]. More recently, a large pinning force and matching effects were reported by the addition of mixed double perovskite Ba₂Y(Nb/Ta)O₆ which resulted in an unusual pinning landscape consisting of Ba₂(Y/Gd)(Nb/Ta)O₆ segmented nano-rods parallel to the c-axis, and (Y/Gd)₂O₃ plate-like nanoparticles parallel to the ab-plane [11] with a record irreversibility field of 11 T in the case of additions of Ba₂YTaO₆ and mixed Ba₂YTaO₆+Ba₂YNbO₆ nano-columnar inclusions [12].

We reported previously that the combination of the three methods—substrate decoration, BZO nano-inclusions and quasi-multilayer architecture—brings together the advantages of each approach, creating a very complex landscape of synergetic pinning centres. By combining substrate decoration with Ag nano-dots (which increased the critical current density, allowed the growth of thicker films without the usual drastic decrease in J_c and has been proven to promote the columnar growth of YBCO [13]) with BZO-doping, BZO nano-rods and nanoparticles act as effective pinning centres for the magnetic field in both directions: along the c-axis and in the ab-plane [14].

Over a decade ago, a radically different concept for high-T_c superconducting devices was proposed: coated conductor cylinders. This approach involves sequential deposition of YBCO and insulating layers onto a heated cylinder (a Ni or NiW alloy with bi-axially textured buffer layers; a cylinder with ion beam assisted deposition or inclined substrate deposition buffers). To produce a multilayer superconducting coil in situ, insulating layers are deposited by pulse laser deposition (PLD) followed by YBCO layers to electrically isolate one superconducting layer from another. These individual superconducting layers can also be 'cut' into trenches in situ using a laser beam, resulting in a multi-turn approach, and can be connected together in series or in parallel after the whole deposition and 'patterned' for a whole variety of power applications [15]. As for the insulating layers, CeO₂, SrTiO₃ (STO) and yttrium stabilized zirconia were tested [16], the best results being obtained for STO. Multi-layers of (120 nm STO/450 nm YBCO)_n with n up to 6 (a total thickness of YBCO up to 2.7 μm) were successfully grown on buffered curved NiW RABiTS without any significant degradation in the crystalline structure and in the grain boundary orientation, thus proving the feasibility of this approach [17]. Those studies have been performed using pure YBCO targets. Moreover, field-dependent critical current studies on such structures have not been performed to the best of our knowledge.

The aim of this work was to grow and study the superconducting properties of such multilayers, this time with artificial pinning centres, and to investigate the influence of the STO on the pinning properties of YBCO layers, knowing that very thin (nm scale) layers have been shown to provide additional strong pinning along the ab-plane [18].

Ag nano-dots were deposited on the STO substrate by PLD with the optimum conditions for deposition as reported in [13, 14]. Two BZO-doped YBCO layers and the separating STO nano-layer were grown at a temperature of 800 °C and a laser frequency of 4 Hz using a 4 wt.% BZO-doped YBCO target prepared by a citrate–gel method described in detail elsewhere [19], and a STO target, respectively. The thickness of the BZO-doped YBCO layers was 1.5 μm while the thickness of the insulating STO layer was 15 nm, 30 nm and 45 nm, respectively, with a total film thickness slightly above 3 μm. For comparison, a single-layer film of BZO-doped YBCO was fabricated under the same conditions as the bilayer films. The 3 μm-thick single layer film was of poor quality, so the comparison was made with a good quality 2 μm-thick single layer film.

The critical current density in the magnetic fields along the c-axis was determined from DC magnetisation loops of square 5 mm × 5 mm samples measured on a quantum design magnetic properties measurement system (MPMS-XL) in a DC field up to 4.5 T, using a critical state model, as in [13] and [14]. The critical temperature (T_c) was measured on an MPMS-XL by recording the temperature dependence of the AC susceptibility in a small excitation field of 0.05 Oe. In magnetic fields of arbitrary orientation, J_c was determined from the angle-dependent transport measurement using a voltage criterion of 1 μV cm⁻¹ on 550 μm long and 10 μm wide micro-bridges produced by conventional photolithography and chemical etching. The angle-dependent measurements were conducted on a quantum design physical properties measurement system (PPMS). Due to the limited access to PPMS, transport measurements were performed for one bi-layer sample only, and the results are compared with other types of YBCO films, as stated above. Using the same equipment with a different set-up, pinning potentials were estimated from frequency-dependent AC susceptibility measurements using a technique and data analysis as described in detail elsewhere [20]. Such AC susceptibility measurements are currently on-going on other bi- and multi-layers, and the results will be reported elsewhere. The micro- and nanostructure of the samples was analysed by scanning electron microscopy (JEOL 7000) and transmission electron microscopy (TEM, Technai F20 operated at 200 kV). The cross-sectional TEM samples were prepared by focused-ion-beam milling on an FEI Quanta 3D FEG.

The superconducting properties of the bilayers were first examined by AC susceptibility measurements during slow temperature scans of 0.2 K min⁻¹. All the films show quite a sharp transition at a high T_c of 88.5–89.5 K, as can be seen in figure 1, where S15, S30 and S45 indicate the STO thickness of 15, 30 and 45 nm. S0 denotes the single-layer of BZO-doped YBCO film (2 μm-thick) deposited on a Ag-decorated STO substrate. The critical current density (J_c) for the bilayer films with different thicknesses of an insulating STO layer is shown in figure 2 in comparison with the J_c of S0. J_c was measured at a temperature of 65 K (temperature of interest for superconducting devices in pumped nitrogen) and applied fields up to 4.5 T perpendicular to the film surface, and at 77.3 K in fields up to 3.5 T. At 77.3 K, the J_cs of all the 3 μm-thick bi-layers are slightly smaller than that of the 2 μm-thick single layer film. At 65 K, the bilayer film with a thickness of the STO layer of 30 nm has the highest J_c in a self-field of about 3.86 MA cm⁻², while the J_cs of the bilayer films with an STO layer of 15 and 45 nm are of 2.86 and 3.53 MA cm⁻², respectively. The self-field J_c of all bilayer films at this temperature is higher than that of the single-layer BZO-doped YBCO film deposited on the Ag-decorated STO substrate, which is about 2.67 MA cm⁻². The J_c of the bilayer films continues to be higher than that of the single layer film in fields up to 1 T for S30, up to 0.5 T for S45 and up to 0.1 T for S15. It can be seen that the highest J_c is for the film with a 30 nm-thick STO layer, but the reason for this is unclear. In high applied fields, for example at 4 T, the J_c of the single-layer film is the same as in the bilayer films with a thickness of the STO layer of 30 and 45 nm (≈0.3 MA cm⁻²). Some of the values of the critical current density for the samples discussed above are shown in table 1. The differences in J_c are rather small, perhaps even in the limit of variability between subsequent depositions, so we can conclude that in an applied field perpendicular to the film surface, no significant improvement or significant degradation in J_c for bilayer films is observed in comparison with the J_c of single-layer film.

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In applied fields parallel to the film surface, the J_c of bilayer films obtained by transport measurement (1 μV cm⁻¹ criterion) shows significant improvement in comparison with single-layer films, even when the single-layer films are thinner than the bilayer films, as shown in figure 3. This figure compares the J_c of the 3 μm bilayer film with the 45 nm STO layer with the 2 μm single-layer film on a bare STO substrate, and a 2 μm single-layer film on the Ag-decorated substrate in applied fields up to 6 T and a temperature of 77.3 K. It is well known that for the same composition and growth conditions, the J_c of a 3 μm film is smaller than that of a 2 μm film, the critical current density decreasing with increasing thickness above an optimum one of several hundred nm [1], so we can safely conclude that the STO layer promotes the improvement of J_c for fields parallel to the ab-plane.

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In a self-field, the J_c of the bilayer film (pink stars) shows a 100% increase in comparison with the J_c of the single-layer film (blue squares). In an applied field of 6 T, the J_c of the bilayer film (70 kA cm⁻²) is 43% higher than that of single-layer film on a bare substrate with a J_c of 49 kA cm⁻², while the J_c of the film on a Ag-decorated substrate (white-brown hexagons) is in-between with the value of 59 kA cm⁻². It is obvious that the STO layer separating the two BZO-doped YBCO layers helps to increase J_c at 77 K both in a self-field, and in fields up to 6 T parallel to the ab-plane.

It has been previously reported [7] that, in applied fields below 2.5 T, J_c in BZO-doped YBCO films is larger for magnetic fields applied along the c-axis of YBCO, with a maximum of ${{J}_{{\rm{c}}}}^{{\rm{c}}}/{{J}_{{\rm{c}}}}^{{\rm{ab}}}=2,$ where ${{J}_{{\rm{c}}}}^{{\rm{c}}}$ is the critical current density for the magnetic field along the c-axis and ${{J}_{{\rm{c}}}}^{{\rm{ab}}}$ is the critical current density for the in-plane magnetic field. The same property can be seen in the bilayer film with a 45 nm STO layer as shown by the angular dependence of J_c at a temperature of 77.3 K presented in figure 4. In an applied field of 0.6 T, the ${{J}_{{\rm{c}}}}^{{\rm{c}}}$ of the bilayer film at 0, 180 and 360° is twice the ${{J}_{{\rm{c}}}}^{{\rm{ab}}}$ at 90 and 270°. The ${{J}_{{\rm{c}}}}^{{\rm{c}}}/{{J}_{{\rm{c}}}}^{{\rm{ab}}}$ ratio reduces to nearly 1 in an applied field of 2.5 T, and decreases further with an increase of the applied field, returning, for high fields, to the 'normal' situation in which ${{J}_{{\rm{c}}}}^{{\rm{c}}}/{{J}_{{\rm{c}}}}^{{\rm{ab}}}=1/\gamma ,$ where $\gamma$ is the YBCO anisotropy factor.

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In an applied field of 2.5 T, the J_c of all three samples—bilayer film, a single layer on bare STO substrate and a single layer on a Ag-decorated STO substrate—show a low anisotropy of J_c with ${{J}_{{\rm{c}}}}^{{\rm{c}}}/{{J}_{{\rm{c}}}}^{{\rm{ab}}}\approx 1$, as shown in figure 5. Again, the J_c for a field along the ab-plane of the 3 μm bilayer film (blue squares) is higher than that of a 2 μm single-layer film on a Ag-decorated STO substrate (red circles) and of the single-layer film on bare substrate (black triangles). The J_c for a field along the c-axis of the single-layer film on a Ag-decorated substrate, however, is higher than that of the bilayer film due to the thickness difference, as discussed above. As shown in figure 5, the J_c for fields along the c-axis of the bilayer is almost the same as in the single-layer film on the decorated substrate in an applied field of 2.5 T.

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In bilayers, for the field directions close to the ab-plane, the peaks of J_c at 90° and 270° are split into two peaks, as can be seen in figure 5. The cross-sectional TEM image of the bilayer film shown in figure 6 reveals the presence of fragments of planar defects parallel to the STO layer. If the average length of these fragments, schematically shown in figure 7 by thick black lines, is L, and the average distance between them along the c-axis is l, then the optimal angle φ at which pinning is at its strongest, and segmented vortices fit best to the planar defects, is given by relation:

$$\begin{eqnarray}&&\tan (\varphi )=l/L.\end{eqnarray} \tag{ 1 }$$

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The TEM image in figure 6 allows for very rough estimates of L as 150 nm and l as 50 nm, which results in φ of approximately 18°. Of course, just from one TEM image, statistical analysis of the observed defects along the ab-plane is very difficult to perform, so the values selected for L and l are just for qualitative analysis. The resulting value for φ is close to that observed in figure 4 with positions of split maxima situated at 90° ± φ and 270° ± φ.

The in-plane defects are only observed in the areas near the STO substrate and close to the STO insulating layer, while the c-axis columns of BZO appear in the middle of BZO-doped YBCO layers, as shown in figure 8. The combination of the ab-plane and the c-axis defects act as active pinning sites maintaining high J_c, not only in the direction of applied magnetic fields parallel to the c-axis, but also for fields in the ab-plane, and this very peculiar pinning landscape is responsible for the split of the ab-plane peak in the J_c(θ). This split due to pinning centres with different orientations resembles, somewhat, the split in the field-dependence of the pinning force due to the presence of different types of pinning centres [21]. Another origin of pinning in the BZO-doped YBCO films is in the presence of Y₂O₃ nano-particles frequently observed in the bilayer sample and shown in figure 9. Although in [8], Y₂O₃ nano-particles were added to the BZO-doped YBCO target to create more effective pinning, in our bilayer films, the presence of Y₂O₃ nano-islands occurs naturally during film deposition.

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The critical current density can also be calculated from the AC susceptibility while a DC field is applied to the sample. Brandt [22] showed that, for superconducting thin films, the position h^* of the maximum in the AC-field dependence of the out-of-phase susceptibility χ''(h_ac) is related to the critical current density by the equation

$$\begin{eqnarray}&&{J}_{c}=\displaystyle \frac{{h}^{* }}{\alpha d},\end{eqnarray} \tag{ 2 }$$

where d is the film thickness and α is a constant between 0.8 and 0.9 that depends slightly on the geometry (disc, square, rectangle). An example of such frequency-dependent χ''(h_ac) measured curves is shown in figure 10 for a bi-layer with a 30 nm-thick STO nanolayer at 77.3 K and in a 7 T DC field. From the frequency (time) dependence of the critical current density presented in figure 11, one can estimate the pinning potential, as shown in detail in [20]. In brief, our data are very well described by a straight line in a double logarithmic plot in figure 11:

$$\begin{eqnarray}&&\mathrm{ln}\,{J}_{c}=a-b\,\mathrm{ln}\left(\displaystyle \frac{{f}_{0}}{f}\right)\end{eqnarray} \tag{ 3 }$$

where f₀ is a macroscopic attempt frequency of about 10⁶ Hz. The effective pinning potential, U₀, can be related [20] to the slope b in equation (3) of our experimental data:

$$\begin{eqnarray}&&{U}_{0}={k}_{{\rm{B}}}T\left(1+\displaystyle \frac{1}{b}\right).\end{eqnarray} \tag{ 4 }$$

Using equation (4) and the data in figure 11, we estimated the effective pinning potential of the synergetic pinning centres in this film at 77.3 K. The values (in K, k_B = 1) are 389 K (in 5 T), 271 K (in 6 T) and 176 K (in 7 T). It is worth mentioning that, due to limitations in the AC-field amplitude (hence, in the probing current), the experimental window in which such a technique can be used is limited to high fields in thicker films, as in our case.

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In conclusion, the introduction of 15–40 nm thick STO films between YBCO layers does not degrade the critical current density. The combination of the three methods (substrate decoration, multilayers, BZO doping) is effective in increasing the J_c of the film for magnetic fields oriented in both the c-axis and the ab-plane. It appears that a 30 nm STO layer gave the best results, for reasons yet unclear, but the differences in J_c for fields along the c-axis is rather small. A significant increase of J_c for fields along the ab-plane of the bilayer film has been achieved. The presence of the fragments of ab-plane defects explains the split of the J_c peak in its dependence on magnetic field orientation close to the ab-plane of YBCO. In a perpendicular configuration, frequency-dependent AC susceptibility measurements allowed estimations of quite high effective pinning potentials in high magnetic fields.

This work has been supported by the COST Action MP 1201 'Nanoscale SuperConductivity: Novel Functionalities through Optimized Confinement of Condensate and Fields' and by the Romanian Ministry of National Education and Research through the Core Programme PN16-480102 'Synthesis and characterisation of nanostructured materials, thin films and heterostructures'; through PCCA 138/2012; and through the POC Project P_37_697 (28/01.09.2016) 'Boron- and rare-earths-based advanced functional materials'. VSD acknowledges financial support from the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.99-2015.81.