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 MgB
₂
filmssuch as Si and Ni. This property inspires MgB
₂
fabrication on a large-scale Al substrate as a new route to MgB
₂
tapes. Here we report microstructural factors influencing critical current density,
J
_c
, in MgB
₂
/Al films. MgB
₂
/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 × 10
⁶
A cm
^-2
for sample A and 2.7 ×10
⁶
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 MgB
₂
lattices. The [001] texture of columnar MgB
₂
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 MgB
₂
polycrystals, resulting in the higher
J
_c
enhancement for sample A.