Strongly enhanced irreversibility field and flux pinning force density in SmBa₂Cu₃O_y-coated conductors with well-aligned BaHfO₃ nanorods

Conductors coated with the rare-earth-based cuprate superconductor REBa₂Cu₃O_y (REBCO, RE = rare earth element or Y) are promising for various electric power applications.^1–4) Therefore, many researchers have developed REBCO-coated conductors. However, the critical current density (J_c) and irreversibility field (B_irr) in fields applied parallel to the c-axis of the film (B ∥ c) require improvement because of the large anisotropy. The introduction of c-axis correlated pinning centers into the REBCO matrix has been effective for improving the J_c and B_irr via enhanced flux pinning. Among the c-axis correlated pinning centers, self-organized nanoscale columnar defects (nanorods) have received particular attention because of their strong pinning force. It is known that perovskite oxide BaMO₃ (BMO, M = Zr, Sn, or Hf) and double-perovskite oxide BaREM'O₆ (BREM'O, M' = Nb or Ta) form nanorods in the REBCO matrix by adding a small amount of M or M' via the vapor phase epitaxial deposition method.^5–10) Recently, a high flux pinning force density F_p (= J_c × B: magnetic field) of 25 GN/m³ for B ∥ c was achieved at a measurement temperature (T) of 77 K for YBCO films with Ba₂Y(Nb/Ta)O₆ nanorods fabricated with an optimum deposition rate.¹¹⁾

According to Ref. 12, the irreversibility temperature (T_irr) depends on the critical temperature (T_c) and matching field (B_ϕ) of the nanorods, for B > B_ϕ (the range of B_ϕ is 1.5–5 T). T_irr/T_c at 7 T increases with B_ϕ, which means that a high-T_c REBCO film with high-B_ϕ nanorods should achieve a high T_irr and B_irr. The B_ϕ is calculated by multiplying the nanorod density by the flux quanta. Awaji et al. reported that well-aligned nanorods play an important role in achieving the high performance of the B_irr.¹³⁾ Horii et al. reported that B_irr at T/T_c = 0.9 increased with B_ϕ up to ∼5 T and became saturated (or decreased) for B_ϕ > 5 T.¹⁴⁾ Therefore, we expect that the introduction of well-aligned nanorods with B_ϕ ≈ 5 T into the REBCO film with little or no degradation of T_c is an effective methodology for improving B_irr at 77.3 K. In this study, to achieve a high B_irr and J_c, especially at a liquid-nitrogen temperature of 77.3 K, we fabricated a SmBCO film with a high T_c (91.1 K), including well-aligned BHO nanorods with the appropriate B_ϕ ≈ 5.8 T.

The BHO-doped SmBCO film was deposited on a metallic substrate using the pulsed laser deposition method with a KrF excimer laser (λ = 248 nm), pulsed at a repetition rate of 10 Hz. The substrate temperature during deposition, the laser energy density, and the distance between the substrate and targets were 840 °C, 2.0 J/cm², and 52.5 mm, respectively. The thickness of the BHO-doped SmBCO superconducting layer was 240 nm. A Ag stabilizing layer with a thickness of 400 nm was deposited on the film by using a direct-current magnetron sputtering apparatus.

The metallic substrate used was 10-mm-wide metallic tape covered with multiple oxide buffer layers, including a biaxially textured MgO layer fabricated via ion beam-assisted deposition. The textured CeO₂ top layers had a full width at half maximum (FWHM) of 1.6° for the CeO₂(220) reflection and ϕ-scan profile (Δϕ = 1.6°).

The microstructures of the film were investigated using high-resolution transmission electron microscopy (TEM). The volume fraction of the BHO within the film was 3.8 vol %, which was measured via inductively coupled plasma-atomic emission spectrometry. We performed transport measurements at various temperatures under magnetic fields of up to 17.5 T using the four-probe method at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Japan. The J_c values were determined by using the 1 µV/cm criterion, and a transport current density of 100 A/cm² was used for the resistivity measurement. Before the transport measurement, the film was processed into a micro-bridge 27 µm in width and ∼1 mm in length via YAG laser etching with a metal mask.

We first discuss the microstructures of the 3.8 vol % BHO-doped SmBCO film using examples of typical cross-sectional and plan-view TEM images. The cross-sectional image [Fig. 1(a)] reveals the straight-shaped BHO nanorods unidirectionally from the bottom to the surface. The red arrows indicate some of the BHO nanorods. The low-magnification plan-view TEM image in Fig. 1(b) shows numerous BHO nanorods. We determined the diameter of the BHO nanorods using high-magnification plan-view TEM images, as shown in the inset of Fig. 1(b). Furthermore, we defined the interface between the BHO nanorod and the SmBCO matrix as a boundary with a relatively large difference in contrast (for example, a strained region of the SmBCO matrix). The average diameter, number density, matching field B_ϕ, and FWHM of the inclination-angle distributions of the nanorods were 5.4 ± 0.9 nm, 2.8 ± 0.1 × 10³ µm⁻², 5.8 ± 0.3 T, and 1.1°, respectively. The inclination angle of the BHO nanorods were measured from the cross-sectional TEM images. We previously reported the FWHM of the BHO nanorods within SmBCO films,^15–17) and the range of the FWHM was 3.4–48.8°. Therefore, the 3.8 vol % BHO-doped SmBCO film in this study had well-aligned BHO nanorods.

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We now discuss the flux pinning properties. Figure 2(a) shows the magnetic-field dependence of F_p for B ∥ c at 65 and 77.3 K. The maximum F_p values of 120 and 32.5 GN/m³ were obtained at 65 and 77.3 K, respectively. Figure 2(b) shows J_c as a function of the magnetic field at 65 and 77.3 K for B ∥ c. The J_c decreased as the magnetic field increased; however, field-insensitive behavior can be observed at fields ranging from 1 to 5 T. This behavior is typical of c-axis correlated pinning. The endpoints of the behavior (5 T) roughly correspond to B_ϕ ≈ 5.8 T. Under fields within the regions of the field-insensitive behavior, the single-vortex pinning caused by the BHO nanorods dominates. As a result, the BHO nanorods strongly enhance J_c for B ∥ c; i.e., high F_p values were attained via the flux pinning of the BHO nanorods. Although J_c decreases with an increasing magnetic field above B_ϕ, J_c remains in high magnetic fields up to 17 T at 77.3 K. Figure 3 shows the J_c–B curve under strong magnetic fields and the field dependence of the resistivity ρ under magnetic fields of 16–17.5 T at 77.3 K for B ∥ c. We considered the ρ of the Ag layer (∼2.9 × 10⁻⁹ Ω m) when we calculated the resistivity of the superconducting layer. We approximated the ρ values using a linear approximated line (green solid line) for simplicity, which was calculated as follows: ρ = −1.87 × 10² + 11.7 × B. If we define B_irr as 10 nΩ cm, B_irr = 16.8 T at 77.3 K. This criterion corresponds to J_c = 100 A/cm², which is determined by the 1-µV/cm criterion.

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To investigate the reasons why the BHO-doped SmBCO film showed such a high B_irr, we compare the morphology of nanorods and B_irr–T line of the film with those of high B_irr samples earlier reported. In this section, we refer to the 3.8 vol % BHO-doped SmBCO film on a metallic substrate as Sm3.8M. We reported B_irr values of 15.8, 15.1, and 15.0 T for a 1.5 vol % BHO-doped GdBCO film on a metallic substrate (Gd1.5M),¹³⁾ a 5.6 vol % BHO-doped SmBCO film on an LaAlO₃ single crystalline substrate (Sm5.6L),¹⁸⁾ and a 3.7 vol % BHO-doped SmBCO film on an LaAlO₃ substrate (Sm3.7L),¹⁵⁾ respectively. Here, the Sm5.6L was fabricated via a low-temperature growth technique to introduce a high number density of nanorods into the SmBCO film with little degradation of T_c.¹⁹⁾ Table I shows the B_irr, the matching field B_ϕ, T_c, and the FWHM of the inclination-angle distributions of the nanorods, for all four samples. Figure 4 shows the temperature dependence of B_irr for B ∥ c. The inset shows the T/T_c dependence of B_irr. The B_irr lines show a kink shape. This is a common characteristic in REBCO films with c-axis correlated pinning centers. At 77–77.3 K, the magnitude relation of B_irr is Sm3.8M > Gd1.5M > Sm5.6L ≈ Sm3.7L. The shape of the B_irr line for the Sm3.8M film is similar to that for the Gd1.5M film (see the inset), because the films have almost the same B_ϕ (5.8 ± 0.3 T and 4.1 T). Therefore, the higher B_irr of 16.8 T for the Sm3.8M film results from the higher T_c. The B_irr of the Sm5.6L film is lower than that of the Sm3.8M film even though the Sm5.6L film has the same T_c (∼91 K) and a higher B_ϕ of 9.9 T. This depends on the FWHM; i.e., the reason for the lower B_irr was the larger FWHM (9.3°) of the Sm5.6L film. The BHO nanorods inclined with a strong deviation from the c-axis of the film cannot act as c-axis correlated pinning centers. The Sm3.7L film has a high T_c of 92.3 K and a small FWHM of 3.4°, but the B_ϕ is low (1.5 T). As a result, the B_irr is lower than that of the Sm3.8M film. Consequently, the high B_irr of 16.8 T for the Sm3.8M film results from the high B_ϕ (5.8 ± 0.3 T) of the well-aligned BHO nanorods and the high T_c (91.1 K).

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Figure 5 shows the temperature dependence of the maximum F_p and the peak field B_p, of the F_p–B curves for B ∥ c at various temperatures (4.2–40 K) for the Sm3.8M film, as shown in the inset. The maximum F_p values increase as the temperature decreases, reaching 1.3 TN/m³ at 4.2 K. This F_p value is one of the highest values reported thus far. The maximum F_p values of 1,170, 870, and 400 GN/m³ were achieved at temperatures of 10, 20, and 40 K, respectively. The c-axis correlated pinning due to the BHO nanorods is still dominant at the temperature of 10 K, because the peak fields are almost independent of the temperature above 10 K and roughly correspond to the matching field B_ϕ ≈ 5.8 T.

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In summary, the high-performance irreversibility field B_irr and flux pinning force density F_p were realized for the 3.8 vol % BHO-doped SmBCO film on a metallic substrate with an IBAD-MgO buffer layer. At a liquid-nitrogen temperature of 77.3 K, maximum F_p values of 32.5 GN/m³ and B_irr of 16.8 T were achieved for the BHO-doped SmBCO film for B ∥ c. At a subcooled liquid-nitrogen temperature of 65 K, the film also exhibited a high F_p of 120 GN/m³. These values are the highest values reported thus far for REBCO films, to our knowledge. The well-aligned BHO nanorods with a high number density (equivalent vortex matching field of approximately 5.8 T) and high T_c play important roles in attaining the high B_irr. In addition, the film exhibited high flux pinning performance not only at high temperatures of 65–77.3 K but also at low temperatures of 4.2–50 K.

Acknowledgments