The effect of different field cooling processes on the levitation force and attractive force of single-domain YBa₂Cu₃O_7-x bulk

The effects of different field cooling processes on the levitation force and attractive force of single-domain YBa₂Cu₃O_7-x (YBCO) bulk have been investigated. The forces were measured at 77 K by changing the field cooling distance Z_fc between the YBCO bulk and a permanent magnet. We found that the stiffness between the YBCO and magnet increases with the decrease of Z_fc; the maximum levitation force (F_mlf) of the sample decreases from 74 N to 48 N with a decrease of the Z_fc from 7 cm to 0.7 cm; but the maximum attractive force (F_maf) increases from 2.96 N to 30.6 N with a decrease of Z_fc from 7 cm to 0.1 mm. Both of them obey the first decreasing exponential law. It is also found that the distance Z_maf, corresponding to F_maf, decreases in an exponential law with the decrease of Z_fc. The F_mlf, F_maf and Z_maf are all saturated in the range of Z_fc from 3 to 4 cm. All the relations between F_mlf, F_maf and Z_fc are closely related with the axial magnetic field distribution of the magnet, which allow us to easily predict the values between the F_mlf, F_maf and Z_fc, which is very helpful for practical designing and for applications.