FISHTAILS, SCALES AND MAGNETIC-FIELDS

The granularity of high T(c) single crystals and the cause of the secondary peak in m - H loops (known as the 'fishtail' effect) are still areas of active debate and are crucial to our understanding of the nature of the superconductivity and pinning in high T(c) materials. This paper demonstrates the use of the length scale technique to rule out large scale granularity as a cause of the fishtail effect. Magnetic moment and flux creep Measurements are discussed as a function of field on single crystals of YBCO; these results exclude several of the other models previously proposed to explain the origin of the fishtail feature, but support an explanation in terms of microstructural regions that are turned normal in an increasing field, and so act as additional pinning centres.