Ferromagnetism on surface of YBa2Cu3O7 particle

In recent experiments, a special ferromagnetism of YBa2Cu3O7 (YBCO) particles has been found at room-temperature. In this paper we mimic a YBCO particle using a small YBCO cluster and different YBCO surfaces. The magnetic properties of the small cluster and surfaces are calculated using ab initio density functional theory (DFT). According to our calculations, we infer that the ferromagnetism of the YBCO particle comes from the surfaces of the YBCO particle, which is contributed by both p electrons of oxygen atoms and d electrons of Cu atoms, and extends to several atomic layers beneath surface. The surface magnetism is similar to the itinerant magnetism and closely relative to low coordination number, spatial-inversion symmetry breaking and electric neutral condition. Our results indicate that the enhanced magneto-electric coupling induced by broken spatial inversion symmetry near surface layer plays an important role on the surface ferromagnetism of YBCO particle. The ferromagnetism on surface of YBCO particle can be significantly depressed in highly c-axis unique oriented films because the proportions of weak-links are small due to the reductions of pores, voids and micro-cracks.