TWEED STRUCTURE OF FE-DOPED YBA2CU3O7-DELTA

Fe-doped YBa2Cu3O7-delta single crystals have been prepared by a flux method from an Y-Ba-Cu-O precursor with copper partially substituted by iron. Electron diffraction and microscopy were applied to study the domain structure of the material. Superposed on a broad twin band texture, as also observed in undoped material, all samples reveal a finer "tweed" structure of overlapping lenticular domains oriented along [120] and [11BAR0]. This suggests a two-step formation process of the domain structure. The displacement field, corresponding with the tweed structure, resembles that of microtwinning in undoped, quenched YBa2Cu3O7-delta samples. The randomly dispersed Fe-ions act as pinning centers or fragment the structure by the induction of twin interfaces. Monte Carlo simulations support this idea.