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

Abstract

For a regular-geometry superconductor, due to a nonuniform external magnetic field, the effect of the field dependence (i.e., <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> anisotropy and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> ) 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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis⊥ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> condition, which is attributed to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> ). (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 | <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B_z</i> |, 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.