Microstructures of MgB2 films on yttria-stabilized-zirconia (YSZ)/Hastelloy substrates were studied by analytical transmission electron microscopy. The MgB2 films with Mg-rich compositions were fabricated by pulsed laser deposition followed by annealing at 600°C or 680°C under Ar atmosphere. Fine grains of MgB2 and MgO crystals 5-30 nm in size that shows a good mutual connectivity are formed in the MgB2 films. The MgB2 films also form voids 5-100 nm in size that may contain amorphous or oxide phases. Two-dimensional elemental analyses revealed that Mg and O atoms are enriched near the MgB2/YSZ interface. From a comparison in microstructures between the 600°C- and 680°C-annealed MgB2 films, it is suggested that the films composed of finer grains of MgB2 exhibit a higher critical current density under high magnetic fields.
