Magnetic imaging of YBCO on RABiTS using a SQUID microscope

Using a scanning SQUID microscope we have magnetically imaged YBCO films grown upon Rolling-Assisted Biaxial Textured Substrates (RABiTS). The measurements clearly show a strong (about 3 Oe) and nonuniform spontaneous magnetization due to the Ni-based substrate. The YBCO film on a ferromagnetic substrate cannot be cooled in zero field. We can, however, compare images taken before and after application of a perpendicular magnetic field. Images obtained from the difference between signals in the presence and absence of an applied field are a measurement of the sample magnetization change. For all the values of applied field (from 2 mOe to 100 Oe), we measure screening currents at the sample edges. The hysteretic magnetization shows that some magnetic flux stays trapped by sample defects. AC measurements show both an in-phase screening component of the magnetization and an out-of-phase loss component. Both hysteretic and high frequency (greater than 5 kHz) eddy-current losses are observed. We address the source of the hysteresis and the AC losses by also measuring RABiTS tapes without YBCO and YBCO films on strontium titanate. (Author)