1. Introduction
High-temperature superconducting REBa₂Cu₃O_y (REBCO) coated conductors has attracted much attention for its superconducting application at liquid nitrogen temperature. For high performance of the magnets, it is expected to improve the critical current (I_c) of REBCO films in a magnetic field. Increasing the thickness of REBCO films is one of the ways to obtain the high I_c. However, I_c is not simply proportional to the thickness of the REBCO films in the cold-wall method, where the substrate is heated by radiation or conduction heat from a heater [1]. This problem is thought to be due to the lower growth temperature of the REBCO films and increase of a-axis oriented crystal that do not contribute to I_c. In our previous work, I_c in REBCO films increased proportionally to the film thickness up to 5 µm thickness by substrate self-heating technique method, in which the substrate itself is heated by a current through it and the substrate temperature is controlled by a non-contact radiation thermometer [2]. Since the critical current density (J_c) of REBCO films decreases rapidly in a magnetic field.

In this study, an artificial pinning centers (APCs) was introduced using substrate self-heating technique to improve the J_c under magnetic fields. BaMO₃ (M = Zr, Sn, Hf), one of the APCs, self-organize into nanorods in the REBCO films and improves flux pinning properties [3]. We investigated in-field superconducting properties of YBa₂Cu₃O_y (YBCO) films fabricated by changing the volume of BaHfO₃ (BHO) and the substrate temperature (T_s) by pulsed laser deposition with substrate self-heating technique.

2. Experimental method
Pure YBCO and BHO-doped YBCO films were deposited on IBAD-MgO substrate using pulsed laser deposition with a Nd: YAG (λ = 266 nm) laser at a repetition rate of 2 Hz. First, the YBCO films were deposited at T_s of 840 ºC using the BHO-doped YBCO polycrystalline targets with BHO contents of 0, 1.5 and 2.25vol.%, respectively. Second, the BHO-doped YBCO films were deposited using the 2.25vol.% BHO-doped YBCO target at various T_s in the range of 760-880 ºC, respectively. The laser energy density and the oxygen partial pressure during the deposition were 1.95 J/cm² and 200 Pa, respectively. The thicknesses of the films were 1.33 µm.

The crystalline orientation of the films was examined by x-ray diffraction analysis. The surface morphology and the microstructure of the films were observed by atomic force microscope and transmission electron microscope (TEM). The films were patterned into a bridges with a width of 50 µm and a length of 1 mm by laser etching. The J_cs of the films were measured with a physical properties measurement system (PPMS) by the standard four-probe method at temperatures from 65 K to 77 K, at magnetic fields B from 0 T to 9 T and at field angles θ from -45º to 135º. Here, θ is defined as 0º for B//c and 90º for B//ab. The electric field criterion was 1 µV/cm for J_c determination.

3. Results and discussion
Figure 1(a) shows a cross-sectional TEM image of the 2.25vol.% BHO-doped YBCO film fabricated at T_s of 840 ºC. In the image, YBCO is shown in the bright contrast and BHO is shown in the dark contrast as indicated by the white arrows. The YBCO grains grew with a diameter of 200 nm and were connected to each other by vertical grain boundaries along its c-axis. We observed the fireworks structure where the BHO nanorods grow from the center of the grain [4]. The nearer the BHO nanorods are to the grain boundaries, the larger the slope of the BHO nanorods as shown by the red lines.

Figure 1 (b) shows the field angular dependence of the J_c at 77 K and 1 T of the BHO-doped YBCO films fabricated at various T_ss. The minimum J_c (J_c ^min) in the BHO-doped YBCO films were higher than that for pure YBCO films. In the BHO-doped YBCO films, the J_c in the B //c direction was improved when T_ss were 860 and 880 ºC. This result indicates that vertical BHO nanorods are produced at high T_s [5]. On the other hand, the isotropy of J_c and J_c ^min were improved at low T_s, for exsample, the J_c ^min was 0.20 MA / cm² at T_s of 840 ºC. The result is probably due to flux pinning by the inclined BHO nanorods in the fireworks structure as shown in figure 1(a). In order to investigate the origin of the growth process of the characteristic fireworks structure of BHO nanorods obtained in this study, we will discuss the growth process from the flux pinning characteristics and microstructure observations of YBCO films fabricated by changing the amount of deposition per laser pulse.

・Acknowledgments
This work was partly supported by JSPS-KAKENHI (19K22154, 20H02682, 20K15217, 21H01872) and NEDO. Dr. Yasuhiro Iijima of Fujikura provided the IBAD substrate.

・References
[1] A. Ibi et al.: Physica C 445-448, 525-528 (2006).
[2] Y. Ichino et al.: J. Cryo. Super. Soc. Jpn. 55, 348-356 (2020).
[3] Macmanus J L et al.: Nat. Mater. 3, 439 (2004).
[4] A. Ichinose et al.: Physica C 468, 1627-1630 (2008).
[5] Y. Yoshida et al.: Supercond. Sci. Technol. 30, 104002 (2017).

Fig. (a) Cross-sectional TEM image of the 2.25vol.% BHO-doped YBCO coated conductor fabricated at substrate temperature (T_s ) of 840 ºC. (b) The field angular dependence of the J_c at 77 K and 1 T of the BHO-doped YBCO coated conductors fabricated at various T_ss.

Keywords: high-temperature superconductor, REBa2Cu3Oy film, substrate self-heating technique, pulsed laser deposition