SuperconductingYBa2Cu3O7−x(YBCO) thin films with a surface resistance of 0.86 m&OHgr; at 10 GHz and 76 K have been grown on polycrystalline ferrite yttrium iron garnet (YIG) substrates. The chemical and structural mismatches between YBCO and YIG are solved by using a double buffer layer of biaxially oriented yttria-stabilized zirconia (YSZ) andCeO2,where YSZ is deposited by an ion-beam-assisted-deposition technique. The YBCO films arecaxis oriented with an in-plane mosaic spread [full width at half maximum of an x-ray &fgr;-scan on (103) reflection] of less than 8°. The films have a superconductive transition temperature above 88 K with a transition width less than 0.3 K, giving a critical current density above106 A/cm2in self field at 75 K. At 75 K in an external magnetic field of 1 T perpendicular to the film surface, the films maintain a critical current density over2×105 A/cm2.©1998 American Institute of Physics.