1 Introduction

Critical current density (J c) of YBa2Cu3O7−δ (YBCO) superconducting film decays severely with applied magnetic field due to the motion of vortices [1]. How to immobilize the vortices to improve the in-field performance of YBCO film has captured the researchers’ attention. Although naturally grown defects [1], such as boundaries, in-plane or out-of-plane misorientation, dislocations and voids, can somewhat pin flux vortices, yet reports [2–6] have revealed that the density or effectiveness of these naturally grown defects was not high enough to retain high-field J c at a necessary level.

Substantial efforts on introducing artificial pinning centers (APCs) have been made to improve the flux pinning effects. For instance, nano-sized Y2O3 particles resulted from the excess of yttrium in YBCO films could form point, line and/or plane defects, thus enhanced the flux pinning effects [7–10]. Alternatively, perovskites such as BaZrO3 [10–15], BaSnO3 [16], BaHfO3 [17] and BaIrO3 [18] could also form randomly oriented particles or c-axis oriented columnar defects in YBCO matrix to improve the flux pinning effects. Additionally, inclusion of nanoparticles of Gd3TaO7 [19], Ba2GdTaO6 [20], Ba2YNbO6 [21], YBa2CuO5 [22] or Y2BaCuO5 [23] has also been demonstrated to be an effective way to raise the in-field performance of YBCO films. In addition to introduction of the above-mentioned extra particulates, the partial or complete substitution of lanthanides for yttrium to form REBCO (RE = rare earth) has also manifested the enhanced flux pinning effects. As an example, GdBa2Cu3O7−δ (GdBCO) was reported being of superior in-field performance to YBCO [24]. However, systematic research about the effects of Gd content on the properties of Gd x Y1−x Ba2Cu3O7−δ (GdYBCO) superconducting films has been barely reported [25].

Hence, the authors have applied metal organic chemical vapor deposition (MOCVD) to fabricate GdYBCO films with various Gd contents on the metallic substrates and systematically investigated the effects of Gd content on the crystalline orientation, in-plane and out-of-plane textures, surface morphology, self-field and in-field superconductivity at 77 K.

2 Experimental

The MOCVD system applying single mixed liquid precursor of metal organics was used to deposit GdYBCO films. And the home-made buffer stack of LaMnO3/homo-epi MgO/ion beam-assisted deposition (IBAD)-MgO/Y2O3/Al2O3/Hastelloy tape [26, 27], collectively termed as IBAD template, was used as substrate, which was preheated to 810 °C during the deposition of GdYBCO film. The precursor was prepared by dissolving solid-state metal organics of Y(tmhd)3, Gd(tmhd)3, Ba(tmhd)2 and Cu(tmhd)2 into tetrahydrofuran. To fabricate GdYBCO films with various Gd contents, the mole ratio of Gd to the constant total amount of Gd and Y in precursor was set as 0%, 10%, 30%, 50%, 70%, 90% and 100%, corresponding to x values of 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0 in GdYBCO film, respectively. More experimental details could be found elsewhere [28].

The crystalline orientation and texture of as-deposited GdYBCO films were examined by X-ray diffractometer (XRD, Bede D1). The microstructure was observed by scanning electron microscope (SEM, JEOL JSM-7001F). The critical transition temperature (T c) was measured by four-probe method using the current of 1 mA, and the GdYBCO samples were cooled by cold head of Sumitomo (RDK-101D). The J c at 77 K and 0 T (J csf) was obtained by induction method using Leipzig J c-scan system. The critical current in the magnetic field (B) from 0 to 1.1 T was measured on the sample bridge of 1 mm × 5 mm (width × length) with magnetic field direction parallel to the film normal, and the criteria to determine the critical current was 1 μV·cm−1. Additionally, the film thickness was tested by step profiler (Veeco, Dektak 150).

3 Results and discussion

500-nm-thick GdYBCO films with x values of 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0 were fabricated on IBAD-MgO templates. Figure 1 shows the detailed θ–2θ patterns, which exhibit clear and sharp peaks of (00l) GdYBCO and a few unexpected peaks of CuYO2 phases and (h00) GdYBCO. As x < 0.5, only (00l) GdYBCO peaks can be clearly observed, indicating that the corresponding films crystallize well and align their c-axis perpendicular to the substrate surface. However, the films mix with a few CuYO2 particles, demonstrated by the presence of CuYO2 peaks in Fig. 1. As x ≥ 0.5, the peaks of CuYO2 disappear, revealing that the increase in Gd content prevents the formation of CuYO2 precipitates and the composition ratio of the film body is closer to the stoichiometric value. However, the presence of (h00) GdYBCO peaks suggests that the film transfers from purely c-axis orientation to mixed orientation of c-axis and a-axis. It should also be noted that the intensity of (002) peak of the film becomes weaken with Gd content increasing. To give quantitative understanding, I (002) and I (003) are used to represent the intensities of (002) peak and (003) peak of the film, respectively, and the I (003)/I (002) values are summarized in Fig. 2. It is obvious that I (003)/I (002) value is enlarged as Gd content increases. As reported in Ref. [29], it is the intrinsic character of GdBCO that the (002) peak is much weaker than (003) peak. Hence, it is easy to understand that such increase in I (003)/I (002) ratio is ascribed to the change of film body from YBCO to GdBCO with Gd content increasing from 0 to 1. Conversely, the curve in Fig. 2 can also be used to semi-quantitatively determine the Gd content of GdYBCO film when the Gd content is uncertain.

Fig. 1
figure 1

XRD θ–2θ patterns of GdYBCO films fabricated at various Gd contents (x value). Symbols asterisk and number sign indicating diffraction peaks of MgO and Hastelloy tape, respectively

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Fig. 2
figure 2

Relationship between ratio value of I (003)/I (002) and Gd content. I (003) and I (002) representing peak intensities of (003)GdYBCO and (002)GdYBCO, respectively

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Figure 3 shows χ-scans [30] on (102) planes of all GdYBCO films. As x < 0.5, there is only one peak around 57° corresponding to (102) plane of c-axis oriented GdYBCO grains, whereas there arises one more peak around 33° corresponding to (102) planes of a-axis oriented GdYBCO grains when x further increases to 1.0. The above-obtained results are in agreement with those yielded from 2θ scans, further demonstrating that the orientation of GdYBCO films transfer from c-axis orientation to a-axis orientation. Such conversion of orientation suggests that the MOCVD conditions optimized for YBCO film deposition are not appropriate any more, since the film body has transferred from YBCO to GdBCO as x > 0.5. For example, with experiments it has been found that the deposition temperature of GdBCO film was about 15 °C higher than that of YBCO film.

Fig. 3
figure 3

XRD χ-scans of (102)GdYBCO of as-fabricated films

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To determine the out-of-plane and in-plane textures of the films, the ω-scans and Φ-scans were performed, respectively, on (005) GdYBCO and (103) GdYBCO, and the corresponding full width at half maximum (FWHM) values (Δω, ΔΦ) are summarized in Fig. 4. As Gd content rises up to 0.5, Δω hardly changes, while ΔΦ decreases from 5.0° to 4.3°, indicating that Gd substitution for Y deeply improves the in-plane texture rather than out-of-plane texture. Such improvement on in-plane texture would be owed to the disappearance of CuYO2 impurities. However, ΔΦ is enlarged as Gd content further increases, suggesting that the in-plane texture deteriorates. And this deterioration is attributed to the appearance of a-axis oriented growth, which results from the above-mentioned improper deposition conditions of GdYBCO films.

Fig. 4
figure 4

Relationship of FWHMs of ω-scans on (005) planes and Φ-scans on (103) planes of GdYBCO films with various Gd contents

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SEM images of GdYBCO films are shown in Fig. 5, exhibiting dense and crack-free microstructure as well as some surface particles. As x = 0, there is a large number of particles identified as Y–Cu–O phases by energy dispersive spectroscopy (EDS) analysis (not shown here), which is consistent with the XRD examinations in Fig. 1. As x is enlarged, the number of Y–Cu–O particle decreases, which is in agreement with the weakening of diffraction peak of CuYO2 phase in Fig. 1. However, the a-axis oriented GdYBCO grains (rectangular shape particles in Fig. 5) also arise with Gd content increasing, especially for x > 0.5. When x > 0.5, the size and density of a-axis grains increase with Gd content increasing, consistent with the higher (h00) GdYBCO peaks at larger x in Fig. 1. Such microstructure with large number of a-axis grains for x > 0.5 further demonstrates that the deposition conditions are not appropriate anymore and the substrate temperature is somewhat low, especially for those of higher Gd content.

Fig. 5
figure 5

SEM images of GdYBCO films with various Gd contents: a x = 0, b x = 0.1, c x = 0.3, d x = 0.5, e x = 0.7, f x = 0.9 and g x = 1.0

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The superconductivity of GdYBCO film is characterized by the measurements of critical transition temperature (T c) and J csf, of which the results are depicted in Fig. 6. In Fig. 6a, the T c almost linearly increases from 90.75 to 92.25 K, whereas the transition width (ΔT) firstly increases and then decreases with the increase in Gd content. And ΔT is 0.65 and 0.93 K for YBCO and GdBCO, respectively, and reaches its maximum of ~2 K at x = 0.5. Owing to the higher T c of GdBCO than YBCO, such variation of ΔT results from the co-existence of YBCO phase and GdBCO phase at 0 < x < 1.0. In Fig. 6b, J csf increases from 1.8 to 2.8 MA·cm−2 as 0 ≤ x ≤ 0.5, which benefits from the rising of T c and the above-mentioned improvement of in-plane texture yielded from the decrease in CuYO2 impurities. When Gd content is above 0.5, the film body transfers to GdBCO and the improper deposition conditions leads to the degradation of in-plane and out-of-plane textures as well as the presence of misoriented grains including (103) GdYBCO and (h00) GdYBCO. The texture degradation and grain misorientation are harmful to the transmission of superconducting current, consequently resulting in the drop of J csf from 2.8 to 0.8 MA·cm−2.

Fig. 6
figure 6

Dependence of superconductivity of as-deposited GdYBCO films on Gd content: a critical transition temperature (T c) and transition width (ΔT) and b J c at 77 K and 0 T

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The dependence of critical current (I c) on B was measured on the samples with Gd content of 0, 0.3, 0.5, 0.7 and 1.0 at 77 K and plotted in Fig. 7a, which shows that I c of all samples declines with B. To intuitively exhibit the effects of Gd content, the curves in Fig. 7a are normalized by I c(B = 0) and replotted in Fig. 7b. It is clearly illustrated that the increase in Gd content slows down the declining rate of normalized I c/I c(B = 0) with B increasing, which benefits from the flux pinning effects induced by substituting Gd for Y [24, 25]. At B = 1 T and 77 K, the normalized I c/I c(B = 0) corresponding to Gd content of 1.0 retains about 27%, twice more than that of Gd content of 0, demonstrating that GdBCO film has in-field performance quite superior to that of YBCO film at 77 K. As reported, I c(B) is proportional to B −α in the field range of 0.1–1.0 T (α value is calculated from the I c–B curve in Fig. 7) and the smaller α value means the better in-field performance [1]. By fitting the data points (shown in inset of Fig. 7b), the α values corresponding to Gd content of 0, 0.3, 0.5, 0.7 and 1.0 are 0.400, 0.397, 0.394, 0.375 and 0.332, respectively, among which the smallest value of 0.332 is comparable to those reported α values [12, 13, 15, 31]. Figure 8 shows the magnetic field dependence of flux pinning force (F p), where F p = J c(B) × B [31]. And J c(B) is deduced from the data in Fig. 7a through the following equation:

$$J_{\text{c}} (B) = \frac{{I_{\text{c}} (B)}}{w \times t}$$
(1)

where w and t represent the width and thickness of the micro-bridge of GdYBCO films used in I c measurements, being 1 and 500 nm, respectively.

Fig. 7
figure 7

Dependence of a critical current (I c) and b normalized I c/I c (B = 0) on magnetic field (B) with B perpendicular to normal of sample surface

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Fig. 8
figure 8

Magnetic field (B) dependence of flux pinning force (F p), where F p = J c × B

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In view of the various J csf shown in Fig. 6b, the comparative analysis of Gd content of 0 and 1.0 as well as 0.3 and 0.7 is more illustrative. With regard to Gd content of 0 and 1.0, F p of the former is higher than that of the latter as B < 0.2 T, which is attributed to the much higher J csf of 1.8 MA·cm−2 of the former than 0.8 MA·cm−2 of the latter. As 0.2 T < B < 1.0 T, F p of Gd content of 1.0 is almost same with that of Gd content of 0, indicating that the flux pinning effect of GdBCO is stronger than that of YBCO at this moment. As for Gd content of 0.3 and 0.7, they have the same J csf and their F p is same too as B < 10 mT, whereas F p of the latter is increasingly higher than that of the former as B increases to 1.0 T, suggesting that the latter is of superior flux pinning effect. All the above discussion suggests that substitution of Gd for Y is beneficial to enhance the flux pinning effect of the film.

4 Conclusion

Gd content deeply affects the in-plane texture, morphology and superconducting performance of GdYBCO films. The increase in Gd content from 0 to 0.5 could suppress the formation of CuYO2, thus improves the in-plane texture and microstructure of the films and finally enlarges J csf from 1.8 to 2.8 MA·cm−2. As Gd content further increases from 0.5 to 1.0, the growth of a-axis GdYBCO grains arises abundantly and becomes much severer at Gd content of 1.0, which consequently deteriorates the texture and thus drops J csf from 2.8 to 0.8 MA·cm−2. However, the increasing Gd content brings the linear increase of T c from 90.75 to 92.25 K and the slower decay of normalized I c/I c(B = 0) as well as the stronger flux pinning effect. Even though the flux pinning force of GdBCO is smallest due to the lowest J c, the authors believe that further modification on MOCVD process would raise J c and then enhance the flux pinning force.