Phase transition in YBa2Cu3O7−δ and YBa2Cu3O7−δ/Y3Fe5O12 nanoparticles at low temperatures and high fields

• Nanoparticles of YBCO and YBCO/YIG have been studied.
• Ferromagnetism and superconductivity coexist in YBCO at 10 K.
• Existence of two pinning mechanisms in YBCO at 10 K.
• Vortex-surface barrier pinning at low fields.
• Spin–vortex pinning at high fields.

In order to interpret the low-H peak and the particular form of M(H) curves of pure YBa2Cu3O7−δ (YBCO) nanoparticles, many pellets of YBa2Cu3O7−δ/Y3Fe5O12 (YBCO/YIG) with different proportions of added YIG were prepared by pressing nanoparticles into disks of 5 mm diameter and 0.5 mm thickness and high temperature sintering. The crystal structure and phase composition of YBa2Cu3O7−δ and Y3Fe5O12 were characterized by X-ray diffraction (XRD) with CoKα radiation. Magnetization measurements vs. temperature T and applied field H were performed with a conventional magnetometer in 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 reveal that the application of a strong field leads to a phase transition: for H less than a threshold field H⁎, the superconducting nano-domains are prevalent in the sample. When the field increases beyond H⁎, superconducting nano-domains spins line up along the applied field direction to create a ferromagnetic phase. Phase diagrams are proposed. Field dependence of the critical current JC in YBCO pellet has revealed the existence of two pinning mechanisms: a pinning at low fields and a pinning at higher fields.