The high-temperature superconductor REBa₂Cu₃O_y (REBCO, RE = rare earth) shows an excellent superconducting property even under a high magnetic field [1]. In addition, the REBCO has a high critical current density J_c at a low temperature of 4.2 K, which is expected to be applied to NMR that generates a magnetic field ~ 45.5 T [2]. Therefore, further enhancement of J_c is required at a low temperature and in a high magnetic field where oxygen vacancy is considered to be effective in improving J_c. In fact, we reported that the addition of BaHfO₃ (BHO) improved J_c in REBCO films at a low temperature, which could be due to the oxygen vacancy in the REBCO films [3]. In this study, the annealing temperature T_a was controlled to improve J_c of non-doped and BHO-doped SmBa₂Cu₃O_y (SmBCO) films at a low operating temperature.

Non-doped and BHO-doped SmBCO films were prepared on CeO₂-buffered IBAD-MgO substrates by the pulsed laser deposition method. The films were processed at annealing temperatures T_a of 150-400 ºC in an oxygen atmosphere. Field dependences of the J_c were measured at 4.2 K with B//c-axis to calculate the flux pinning force density F_p.

Fig. 1(a) shows field dependence of the F_p at 4.2 K of the BHO-doped SmBCO films annealed at T_a. The F_p was the highest at T_a = 250 ºC. Fig. 1(b) shows T_a dependence of the F_p of the non-doped and the BHO-doped SmBCO films at 4.2 K and 9 T. The maximum F_p of the BHO-doped films was 592 GN/m³ at T_a = 250ºC, which is 2.3 times higher than one of the non-doped films. In addition, a narrow range of T_a with high F_p of the BHO-doped films was confirmed, compared with the non-doped films. We will discuss the above results by analyzing whether the enhanced J_c of the BHO-doped films are due to flux pinning by the BHO phase or the oxygen vacancy.

This work was partly supported by JSPS (19K22154, 20H02682, 20K15217), JST-A-STEP, and NEDO. The IBAD-MgO substrates were provided by Dr. Y. Iijima of Fujikura Ltd.

Fig. 1. (a) Field dependences of the flux pinning force density F_p of the BaHfO₃-doped SmBa₂Cu₃O_y films at 4.2 K processed at various annealing temperatures T_a. (b) T_a dependences of F_p of the non-doped and the BaHfO₃-doped SmBa₂Cu₃O_y films at 4.2 K and 9 T.

Reference:
[1] Y. Shiohara et. al., J. J. App. Phys. 51, 010007 (2012).
[2] S. Hahn et. al., Nature 570, 496-499 (2019).
[3] S. Miura et. al., Supercond. Sci. Technol. 30, 084009 (2017).

Keywords: REBCO, artificial pinning center, coated conductor, critical current