The critical current behavior of REBa 2 Cu 3 O 7−δ (REBCO, with RE=Yb, Y, Ho, Dy, Gd, Eu, GdYb, DyY, GdY, EuY, SmY, GdEu, GdSm) superconducting tapes has been systematically studied for the temperature range of 10-77 K, magnetic field range of 0-5 T and field orientation (angle between the field and tape) of 0°-180°. We report that the RE size dependencies of infield critical current density (J c ) at low temperatures (below 40 K) are significantly different from those at 77 K. In a magnetic field of 1-3 T applied at 77 K, the J c of REBCO films increases with the RE ionic radius (r). EuBCO film, with the largest r in this study, shows the highest J c at 77 K in a magnetic field above 1 T, while YbBCO film, with the smallest r, has the lowest J c at 77 K. At temperatures below 40 K, however, the trend of in-field J c as a function of r reverses. There are crossovers among the functions of critical current density versus temperature for various RE ions and mixes. RE mixed films such as GdYBCO, EuYBCO, SmYBCO and GdEuBCO exhibit enhanced self-field critical current densities correlating with effective low field random pinning due to their suitable ion size variance. Strongly enhanced pinning by both random and correlated defects has been obtained in GdYbBCO films. Those pinning centers are especially effective at low temperatures. In a magnetic field of 5 T applied at 10 K, GdYbBCO gives the highest critical current density compared with other REBCO films, being 1.9 times greater than GdEuBCO film. This work provides a guideline regarding how to optimize RE ions in REBCO films for given application temperatures and magnetic fields.
