Coexistence of superconductivity and ferromagnetism in nanosized YBCO powders

Pure YBa2Cu3O7-delta (YBCO) pellet and many pellets of YBa2Cu3O7-delta/Y3Fe5O12 (YBCO/YIG) of different concentrations of YIG were prepared by pressing nanoparticles into disks of 5 mm of diameter and 0.5 mm of thickness. X-ray diffraction (XRD) with CoK alpha radiation was used to check the phase formation and to determine the mean crystallite sizes from the half-width X-ray peaks using the Scherrer relation. Magnetization measurements vs. temperature T and applied field H were performed using a conventional magnetometer within the temperature range 4.2-110 K and in magnetic fields up to 10 T. The field was applied parallel to the pellet axis. Our results confirm the superconductivity and ferromagnetism coexistence in nanoparticles of YBCO at 4.2 K. For H less than a threshold field H*, the superconducting nano-domains are prevalent in the sample. When the field increases beyond H*, the spins of the superconducting nano-domains line up along the direction of the applied field to create a ferromagnetism phase that reinforces the ferromagnetic nano-domains. The present experimental findings corroborate Kopp et al.'s claims that superconductivity and ferromagnetism in singlet superconductors such as cuprates do not exist uniformly but are phase-separated in nano-domains. (c) 2013 Elsevier Ltd. All rights reserved.