Magneto-optical and high resolution electron microscopy studies of flux pinning at grain boundaries in Bi2Sr2CaCu2Ox crystals

Abstract

We have studied the flux pinning behavior of Bi2Sr2CaCu2Ox multicrystalline samples using a magneto-optical flux observation method. When a magnetic field is applied parallel to the ab plane, we find low flux density regions at the intersections of small angle grain boundaries. The flux density distribution can be explained by considering both flux pinning at the small angle grain boundaries and the intrinsic pinning effect at the BiO layers. High resolution electron microscopy observations show that amorphous patches with columnar structure exist at the small angle grain boundaries (<20°), while other defects such as misfit dislocations and distorted microstructures appear at general grain boundaries. The fact that flux pinning only occurs at small angle grain boundaries (<20°) indicates that the columnar amorphous structures could act as flux pinning centers.