Particle size dependence of the superconductivity and ferromagnetism in YBCO nanoparticles

Magnetic properties of superconducting yttrium barium copper oxide (YBa₂Cu₃O_7-δ) nanoparticles (31–43 nm) prepared by a chemical route have been studied. These nanoparticles have been found to clearly exhibit ferromagnetism at room temperature while superconducting transition is observed at lower temperatures. The low temperature hysteresis loops show evidence suggesting the presence of a large paramagnetic contribution in addition to the superconducting contributions from the particles. Bulk YBCO obtained by pelletizing and heating the same nanoparticles at a high temperature, displays the usual superconducting characteristics and gives no trace of ferromagnetism down to 10 K. The superconducting transition temperature of the nanoparticles is lower than for the bulk YBCO and there is a trend of decreasing T _c with smaller size of the particles. In contrast the ferromagnetic moment increases with decreasing particle size. The development of ferromagnetism is attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. The simultaneous decrease of superconducting T _c and the increase of ferromagnetism with decreasing size considered as being reflective of the increased role of finite size and surface defects that weaken the superconductivity and enhance the ferromagnetism. Possible coexistence of surface ferromagnetism and bulk superconductivity at lower temperatures is discussed.