We report the growth and properties of epitaxial MgB2 thin films on (0001) Al2O3 substrates. The MgB2 thin films were prepared by depositing boron films via radio-frequency (rf) magnetron sputtering, followed by a postdeposition anneal at 850 °C in magnesium vapor. X-ray diffraction and cross-sectional transmission electron microscopy reveal that the epitaxial MgB2 films are oriented with their c-axis normal to the (0001) Al2O3 substrate with a 30° rotation in the (0001) plane with respect to the substrate. The critical temperature was found to be 35 K and the anisotropy ratio, Hc2∥/Hc2⊥, was about 3 at 25 K. The critical current densities at 4.2 and 20 K (at 1 T perpendicular magnetic field) are 5×106 and 1×106 A/cm2, respectively. The controlled growth of epitaxial MgB2 thin films opens a new avenue in both understanding superconductivity in MgB2 and technological applications.

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