Effect of preloading on the relaxation of the levitation force in bulk Y-Ba-Cu-O superconductors

We have studied the effect of preloading on the relaxation of the levitation force of bulk Y-Ba-Cu-O superconductors. The levitation force was measured by applying the external field with a superconducting solenoid. We set the standard magnetic induction (B_st) to be 1.2 T supposing that the bulk Y-Ba-Cu-O disk experiences this field in operation. Preloading was applied such that the external field was first ramped to 1.1B_st and reduced to B_st. We confirmed that the preloading was effective in reducing the relaxation of the levitation force, although the effective levitation force was largely depressed compared to that without preloading. The employment of an extra step to lower the field below B_st before reaching B _st was effective in improving the effective levitation force without sacrificing the ability to reduce the relaxation rate. This is qualitatively understood in terms of the field distribution inside the disk based on the critical state model.