Increase in the infield critical current density of MgB₂ thin films by high-temperature post-annealing

Abstract

We propose a novel fabrication technique based on the formation of a Nb protective layer on a MgB2 thin film and high-temperature post-annealing to increase the critical current density (J c) of MgB2 films under an external magnetic field. Analyses of the crystal structure and the composition of the processed MgB2 films confirmed the suppression of the evaporation and oxidation of Mg by high-temperature annealing above 550 °C. The MgB2 film annealed at 650 °C exhibited a J c of 1.62 MA cm−2 under 5 T, which is the highest reported value for MgB2 films, wires, and bulk samples to date.

MgB2 thin films fabricated on Fe tape and effects of annealing on their properties

“…This suggests that the Nb protective layer was slightly oxidized during the annealing process. MgB 2 (002) peaks were not observed for any of the samples, in contrast to our previous study, 17) in which MgB 2 (002) peaks were observed in the XRD patterns of Nb/MgB 2 /B/Si samples. This could be because the Nb protective layer in this study was thicker than that in the previous study 17) (6 versus 3 μm).…”

“…MgB 2 (002) peaks were not observed for any of the samples, in contrast to our previous study, 17) in which MgB 2 (002) peaks were observed in the XRD patterns of Nb/MgB 2 /B/Si samples. This could be because the Nb protective layer in this study was thicker than that in the previous study 17) (6 versus 3 μm). Nb (110) peaks can be observed in all the samples whereas the small peaks of NbO (111) were seen only in the samples annealed for 3 h and 5 h. The presence of sufficient metal Nb and significantly small generation of NbO and Nb 2 O 5 phases after the annealing indicates that Nb layers blocked the oxygen diffusion from the atmosphere to the MgB 2 layer during the post-annealing.…”

“…This is because annealing increases the crystallinity of the MgB 2 layer. 17) Conversely, the T c values of the samples annealed for 3 and 5 h were lower than that of the non-annealed sample. The resistivities of the MgB 2 samples annealed for 3 and 5 h were similar to that of the Fe substrate and lower than those of the non-annealed and 1 h-annealed samples.…”

“…16,17) In our previous study, 17) an MgB 2 thin film was prepared on a single-crystal Si substrate using EBE at a substrate temperature of 280 °C, followed by the deposition of a Nb protective layer by RF sputtering method. The film, annealed at 650 °C for 1 h, exhibited a T c of 37.5 K and a J c of 1.4 MA cm −2 at 20 K under 6 T. 17) Although the T c of the thin film was comparable to that of PIT-processed MgB 2 wires, the infield J c was more than 100 times and several ten-fold higher than that of PIT-processed MgB 2 wires and the as-grown MgB 2 film, respectively. 17) Thus, annealing at 650 °C with a Nb protective layer can increase both the T c and infield J c of MgB 2 thin films.…”

“…Figure 3 demonstrates the magnetic-field dependence of the J c 's of the MgB 2 thin films at 20 K. The reported results of PIT-processed MgB 2 wires, 19) MgB 2 films prepared using HPCVD, 14) and non-annealed and annealed (650 °C for 1 h) MgB 2 films prepared on Si using EBE 17) are also shown for comparison. The magnetic field (0-6 T) was applied perpendicular to the film surface.…”