High infield performance and critical temperatures in post-annealed MgB₂ films

Abstract

Critical temperatures (Tc) comparable to those of bulk materials were achieved by post-annealing MgB2 thin films grown at a low temperature of 280 °C. The Tc was improved to 36.7 K under annealing conditions of 550 °C for 50 h or more. Under these annealing conditions, a critical current density several tens of times higher at than that of MgB2 wires processed by a powder-in-tube method was achieved at 20 K under 5 T. This is the highest value reported in MgB2 bulk wires and films. Film-based MgB2 is a promising candidate for next-generation MgB2 wires.

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

“…nonannealed film to 0.18 MA cm −2 , which was approximately 90% lower than that in the self-magnetic field under a magnetic field of 5 T. The decrease in the J c of the annealed sample under a magnetic field of 5 T was approximately 40%, which was significantly lower than that of the nonannealed sample. The MgB 2 film that was annealed at 650 °C exhibited a J c of 1.62 MA cm −2 at 20 K under 5 T. This was not only more than 2.5 times higher than the previous highest J c of 0.64 MA cm −2 for the EBE-prepared MgB 2 films that were annealed at 550 °C in vacuum 12) but also more than 100 times higher than the J c for the PIT-processed wires. 13) The results of our study demonstrated that high-temperature annealing and the Nb protective layer significantly increased the J c of the EBE-prepared MgB 2 thin films under high magnetic fields.…”

“…The presence of Nb (110) and MgB 2 (002) peaks in the XRD pattern of the nonannealed films revealed the formation of c-axis oriented MgB 2 thin films. 12) The intensity of the XRD peaks of MgB 2 was low owing to the deposition of the Nb protective layer on MgB 2 film; therefore, the MgB 2 (001) peak was not observed in this study. The XRD pattern of the film that was annealed at 450 °C was not significantly different from that of the nonannealed film.…”

“…In one of our previous studies, we demonstrated the effects of in situ annealing under ultrahigh vacuum (<4 × 10 −7 Pa) on MgB 2 thin films that were grown by EBE at a low substrate temperature of 280 °C. 12) The film that was annealed at 550 °C for 100 h exhibited a T c of 36.7 K and an infield J c of 0.64 MA cm −2 at 20 K under 5 T. The T c of the film was comparable to that of PIT-processed MgB 2 wires. However, the infield J c of the film was several times higher than that of as-grown MgB 2 films and several tens of times higher than that of PIT-processed MgB 2 wires.…”

“…Figure 4 shows the dependence of the J c of the MgB 2 films at 20 K on the magnetic fields under various annealing conditions. The results for the EBE-prepared MgB 2 film that was annealed in vacuum, 12) the PIT-processed MgB 2 wire, 13) and the HPCVD-prepared MgB 2 film 14) are also plotted in the figure to facilitate comparison. In all data, the magnetic fields were applied perpendicular to the film surfaces.…”

“…The J c of the nonannealed MgB 2 film in a selfmagnetic field (1.65 MA cm −2 ) was almost identical to that of the MgB 2 films that were annealed at 450 °C and 550 °C in the present experiment. There was a decrease in the J c of the The results for the EBE-prepared MgB 2 film that was annealed in vacuum (light blue dotted line), 12) the PIT-processed MgB 2 wire (gray dotted line), 13) and the HPCVD-prepared MgB 2 film (orange dotted line) 14) are also plotted for comparison. In all the magnetic applied perpendicular to the film surfaces.…”