Growth of strongly biaxially aligned MgB2 thin films on sapphire by postannealing of amorphous precursors

MgB2 thin films were cold grown on sapphire substrates by pulsed laser deposition followed by postannealing in mixed, reducing gas, Mg rich, Zr gettered, environments (pO2∼10−24 atm) at 750 and 950 °C. The films had Tc in the range 29–34 K, Jc (20 K, H=0) in the range 3×104–3×105 A cm−2, and irreversibility fields H* at 20 K of 4–6.2 T. An inverse correlation was found between Tc and H*. The films had grain sizes of ∼0.1–1 μm and a strong biaxial alignment was observed in the 950 °C annealed film. (111) oriented MgO was also observed. Mg coating of films during crystallization appeared to improve film Tc.