Strain release and twin structure in GdBa2Cu3O7-Φ films on (001) SrTiO3 and NdGaO3

Ultrathin films of GdBa 2 Cu 3 O 7 - Φ grown by pulsed laser deposition on NdGaO 3 (001) and SrTiO 3 (001) were studied by grazing incidence x-ray diffraction. The critical thicknesses t c was found to be 50 nm on SrTiO 3 (001). On NdGaO 3 (001). the shear distortion introduced by the substrate is released in two stages at 15 nm and 40 nm thickness by the formation of disclinations with orthogonal pseudo-Burgers vectors. The twin structure forms in the films at the first stage of strain relief, and as a consequence the twin boundaries are aligned unidirectionally. Calculated values for the critical thickness are too small for GdBCO/STO suggesting the existence of other mechanisms of strain relief, which is corroborated by our experimental findings.