Effect of artificial MgO pinning centers introduced by residual moisture in a deposition chamber on J _c –B–T characteristics and film structure of 10 μm thick MgB₂ films deposited on Cu substrates

Abstract

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.

New deposition method of MgB2 thin film with thermal evaporation of Mg and sputtering of B

“…After that, the MgB 2 crystals grew into a columnar shape along the c axis, and magnesium oxides also grew into a cylindrical shape due to the surrounding MgB 2 columnar structure. We previously reported a similar phenomenon with the MgB 2 film deposited in the chamber with residual moisture [18]. In the MgB 2 film deposited in the chamber with residual moisture, MgB 2 crystals also appear just above the substrate and surrounding magnesium oxides, and the MgB 2 crystals and magnesium oxides become larger as the sampling position becomes closer to the film surface.…”

“…The columnar MgB 2 crystals and magnesium oxides become larger closer to the film surface. The film has the columnar grain boundaries with high oxygen concentration, which are similar to the structure of films deposited by the co-evaporation system under relatively higher residual moisture vapor pressure [18].…”

“…As shown in figures 9 and 10, the MgB 2 film deposited by the new hybrid method has a relatively thick boron amorphous phase on the Ti barrier film. The MgB 2 film deposited by co-evaporation with EB did not have the boron amorphous phase [17,18]. The reevaporation rate of Mg particles from the substrate surface rapidly increases and the stay time of the Mg particles on the substrate rapidly decreases as the substrate temperature become higher because Mg has high vapor pressure.…”

“…MgB 2 films deposited by thin-film processes (e.g., hybrid physical-chemical vapor deposition (HPCVD) [13], molecular beam epitaxy (MBE) [14], and electron beam (EB) evaporation [15,16]) have high critical current density J c (>10 6 A mm −2 at 4.2 K and self-field). We also have developed 10 μm-thick MgB 2 films on copper substrates (plates and tapes) by using coevaporation of B and Mg, which uses EB evaporation for B and thermal evaporation from the effusion cell for Mg, and obtained high J c , which suggest that MgB 2 film is a promising candidate for next-generation superconducting tape conductors and coils [17,18].…”

Flexural properties of a MgB2 thin-film wire

“…Recently, MgB 2 thin-films have been fabricated by thinfilm processes such as hybrid-physical chemical-vapor deposition (HPCVD), 12) molecular-beam epitaxy (MBE), 13) and electron-beam evaporation (EBE). [14][15][16][17][18][19][20] Critical current density (J c ) under the magnetic field of a MgB 2 thin-film is generally several times higher than that of PIT-fabricated wires, and this excellent property has attracted much attention. In particular, MgB 2 thin films deposited by EBE on substrates heated at low temperatures (200 °C-300 °C) have high J c under a magnetic field, i.e., J c > 1 kA mm −2 at a temperature of 20 K and magnetic field strength of 5 T. [16][17][18][19][20] With that property in mind, we are developing MgB 2 thinfilm wires for use at 10-20 K and 5 T.…”