Strong Magnetic Field Dependence of Micro-YBCO Superconductor Levitated Above Halbach Guideway

For a regular-geometry superconductor, due to a nonuniform external magnetic field, the effect of the field dependence (i.e., J_c anisotropy and J_c ( H ) property) on the levitation force is complex and indistinct. For a relative microsample, its applied field is uniform, and the phenomenon caused by the field dependence should be more obvious. A novel instrument with a parallel-structure is therefore developed to accurately measure the micro force, and the device achieves a 4 mN precision. Based on this specialized apparatus, the experimental results present several rare phenomena caused by the field dependence. (a) Force attenuation (near the guideway during the superconductor descent) occurs in the small-width superconductor at the c -axis⊥ H condition, which is attributed to J_c ( H ). (b) The maximum levitation force (MLF) position offset of bulks do not appear in the stack. (c) For the stack, the MLF curve varies in a sinusoidal way with the lateral position and fits well with the applied field | B_z |, which depicts that the anisotropic feature of the stack is much stronger than that of the bulk. An analytical critical-state model based on the assumptions of a uniform external field and saturated magnetization is proposed for describing these results that accurately reflect the experimental phenomenon. In addition, the finite element simulation of the micro superconductor in COMSOL is discussed, and a force formulation from this analytical model improves the calculated result.