Influences of Microstructure on Critical Current Properties in $\hbox {MgB}_{2}/\hbox {Al}$ Film

The metal Al is lighter in weight than other substrate materials for MgB₂ filmssuch as Si and Ni. This property inspires MgB₂ fabrication on a large-scale Al substrate as a new route to MgB₂ tapes. Here we report microstructural factors influencing critical current density, J_c, in MgB₂/Al films. MgB₂/Al films were prepared by the following steps: deposit a boron layer of 3 nm in thickness on an Al substrate heated at 280 ^°C ; deposit Mg and boron on the boron layer (sample A). For comparison, Mg and boron were deposited directly on an Al substrate heated at 265 ^°C (sample B). The microstructure was observed by transmission electron microscopy (TEM) and scanning TEM. J_c values at 20 K in the self-field were 4.9 × 10⁶ A cm^-2 for sample A and 2.7 ×10⁶ A cm^-2 for sample B. Both the samples form an oxygen-rich layer of 10 nm in thickness at the substrate surface. This oxygen-rich layer may suppress Al diffusion into MgB₂ lattices. The [001] texture of columnar MgB₂ crystals grown on the substrate is stronger in sample A than in sample B. This indicates that the boron layer deposition on the Al substrate is effective for fabricating well-textured MgB₂ polycrystals, resulting in the higher J_c enhancement for sample A.