Speaker
Description
Critical current density $J_c$ properties of 7 core Cu/Ag/(Ba, K)Fe$_2$As$_2$ (Cu/Ag/Ba122) tapes are investigated in low temperature and high magnetic field. The tape was fabricated by a powder-in-tube method with a flat-roll process [1]. The hysteresis behavior in magnetic field and its angular dependences of $J_c$ were observed at 4.2 K and low field. The $J_c$ values in increasing filed are smaller than those in decreasing field and upturn behaviors in field dependence of $J_c$ appear only in increasing field. These Jc hysteresis behaviors appear in all magnetic field angle at 4.2 K below 8 T although the hysteresis width decreases when the magnetic field angle approaches to $B$//c. The upturn behavior of field dependent $J_c$ in increasing field can be understood by Gurevich model based on the flux pinning of Abrikosov-Josephson vortices at grain boundaries [2]. The anomalous angular dependence of $J_c$, which shows a broad peak around B//c and a dip at $B$//ab, is observed at 4.2 K and low fields and it almost disappears at 18 T. These $J_c$ properties can be represented by the flux pining model by nano-scaled spherical pinning centers [3, 4]. Since no any additional inclusions are introduced as pinning centers in the Ba122 tapes, the strain induced nano-scaled spherical pinning centers are considered from TEM study. It is a key to improve $J_c$ in Ba122 tapes. The detail current transport mechanism and flux pinning will be discussed at the presentation.
[1] S. Liu $et$ $al.$, Supercond. Sci. Technol., 30 (2017) 115007.
[2] A. Gurevich and L.D. Cooley, Phys. Rev. B., 50 (1994) 13563.
[3] T. Okada $et$ $al.$, to be submitted.
[4] J. Luo $et$ $al.$, IEEE TAS under review.
