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.

We are now developing a new in situ deposition process for MgB 2 film as a candidate method to massproduce MgB 2 thin film superconducting tape. In the new method, a MgB 2 film is deposited on a heated metal substrate by a hybrid deposition method, which consists of thermal evaporation of magnesium and sputtering of boron. By using the hybrid deposition method, the substrate temperature can raise from 250 to 350°C, while its fluctuation is kept less than 1°C, which will improve the quality and reproducibility of MgB 2 film in mass production. The J c of MgB 2 film deposited by the hybrid deposition method at 20 K and self-field was more than 30,000 A mm −2 , which was better than the results reported by the two-step in situ process using DC sputtering and 830°C high-temperature post annealing [1] or by the as-grown depostion using sputttering targets of Mg and B [2]. Although we obtained better J c than other deposition methods that use sputtering process, the J c is still lower than the value we obtained by using a co-evaporation method with electron beam (EB). We investigated the film structure and J c -B-T properties of the film made by the hybrid deposition method and compared them with those of the film made by co-evaporation. From the analysis results, we think the reasons for the lower J c are the larger amount of heterogeneous phases such as magnesium oxides in the film and the amorphous B phase under the MgB 2 . We expect to improve the the crystal qualities and superconductivities of the MgB 2 film deposited by the new method by removing impurities in Ar gas during sputtering and thinning the B amorphous phase by increasing the Mg deposition rate in the initial stage of deposition.

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.

MgB2 superconducting films with thicknesses of 10 μm were deposited under two conditions: the partial pressures of residual moisture in the deposition chamber were (a) 0.591 × 10−7 Pa and (b) 4.24 × 10−7 Pa, and superconducting critical current density (Jc) was measured in the temperature (T) range of 5–20 K and external magnetic field (B) range of 0–18 T. A magnetic field was applied perpendicularly to the film (B⊥film) or in parallel to the film (B//film). Both samples showed excellent Jc–B–T characteristics, but they were remarkably different in terms of anisotropy of Jc. At T = 20 K and B = 5 T, Jc values of the MgB2 film deposited under condition (a) were 7700 A mm−2 (B⊥film) and 5700 A mm−2 (B//film), which indicates small anisotropy. On the other hand, Jc values of the MgB2 film deposited under condition (b) were 14 000 A mm−2 (B⊥film) and 2200 A mm−2 (B//film), which indicates large anisotropy. However, Jc anisotropies of the film at low temperature (5–15 K) and high magnetic field (>5 T) deposited under condition (a) were larger than that of the film deposited under condition (b). The results of film structure analyses revealed that the film deposited under (b) has columnar magnesium oxides acting as artificial pinning centers, which are thought to cause the differences in Jc anisotropies.