A comparative study of levitation force and magnetic stiffness of bulk YBCO and YBCO thin films

摘要

Comparative experiments of levitation force and vertical magnetic stiffness on melt processed, granular and film samples of YBa_2CU_3O_(7-δ) are presented. The melt processed and the film samples show a strong hysteretic behavior as expected from their enhanced pinning properties. The vertical stiffness is determined from the slope of minor loops of the force vs. distance curve. When plotted as a function of magnetic field it shows a crossover from B~(1.5) to a B~2-behavior for all samples except for the granular one. This can be explained by flux line lattice effects. In addition a quantitative estimation for the pinning strength of the samples can be obtained from the stiffness measurements. Furthermore a 3-d positioning system is used to map the spatial distribution of levitation force and trapped flux providing a good characterization of sample homogeneity.