We have measured the levitation forces acting on a small permanent magnet by epoxy-YBCO superconducting samples with different grain size under both field cooling and zero field cooling conditions. It is found that the differences in the magnetic levitation forces between zero field cooling and field cooling samples are larger for larger grain size samples. It is also found that there exists a critical magnetic field Bo such that the field cooling magnetic levitation forces become very small for all applied magnetic fields smaller than Bo. This critical field Bo is also larger for a larger grain size sample. A model, consistent with the critical state model, is proposed to explain this field cooling phenomenon and good agreements have been obtained.
Chinese Journal of Physics=中國物理學刊 38(6)，頁1079-1083