Effects of heat treatments on microstructure formation in MgB2/YSZ/Hastelloy film

Annealing processes in MgB₂ films deposited on yttria-stabilized zirconia (YSZ)-buffered Hastelloy substrates were studied by transmission electron microscopy. The MgB₂ films with Mg-rich compositions were fabricated at room temperature using a pulsed laser deposition (PLD) method. The as-deposited films do not exhibit clear superconducting transitions, and have inhomogeneous microstructures composed of amorphous regions and a small amount of crystalline MgB₂ and MgO with several tens of nm in size. Various sizes of voids are also incorporated within the amorphous regions. After the annealing at 600 or 680°C, the films exhibit clear superconducting transitions. The amorphous regions crystallize into fine grains of MgB₂ or MgO 5-50 nm in size. The voids remain in the annealed films and their sizes depend on the annealing condition. Magnesium-and oxygen-rich layers are formed near the MgB₂/YSZ interface and the film surface. It is suggested that the finer grains of MgB₂ and MgO obtained by the lower-temperature annealing give a larger critical current density under a high magnetic field.