Axial and radial ac susceptibility measurements on melt-processed single-grain Y-Ba-Cu-O discs

The field amplitude and frequency dependent complex ac susceptibility χ(Hm, f) of four Y-Ba-Cu-O discs made by a top-seeded melt growth technique has been measured at 77K with the ac field applied parallel to either the disc axis (the c axis of the grain) or its radius (the ab plane of the grain). The results are interpreted by considering the anisotropic nature of the critical-current density, Jc. The axial susceptibility is dominated by Jcab, c for current within the ab plane and field parallel to the c axis, whereas the radial susceptibility is dominated by J cc, ab for current along the c axis and field within the ab plane. It is concluded that the ratio of Jcab, c to Jcc, ab in a thin, round surface layer ranges between 2 and 5 for discs of different thicknesses, and both are much smaller than the value of Jcab, ab associated with intrinsic pinning. In addition, the measured susceptibilities of thicker grain discs suggest more granular behavior, indicating the existence of microcracks within the sample microstructure. © 2010 IOP Publishing Ltd.