For applications of high-temperature cuprate superconductors in magnetic fields at 77 K, a RBa2Cu3Ox (R123; R = Nd, Sm, Gd, etc.) film is one of the promising materials due to its high critical temperature Tc and high irreversibility field Birr. Especially, a substitution between R and Ba sites that results in a T c degradation is not likely to occur in GdBa2Cu 3Ox (Gd123) films, compared to the cases of Nd and Sm ions. This is an advantage for an application of Gd123 film. We studied the c-axis oriented Gd123 film for obtaining a high critical current density J c in a magnetic field at 77 K. The substrate temperature to deposit the epitaxial films on MgO(100) was varied between 700-800°C under an oxygen partial pressure of 200 mTorr. Tc values of the films were within 89.8-92.7 K. Interestingly, the Jc at 77 K for 5 T (B//c) reached 0.2 MA/cm2. The values are three times higher than those of Y123 films, which indicate that Gd123 films have the superior potentiality in magnetic fields at 77 K.
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