Interaction between ring permanent magnets and bulk Dy–Ba–Cu–O superconductors

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A combination of bulk Dy–Ba–Cu–O superconductors and permanent magnets can be used for various rotational applications such as flywheel energy storage and magnetic bearings. For practical applications, there are two important parameters: the levitation force and the stiffness. Since the superconductor and magnets are installed in a closed space, the attractive force is another important parameter that we should take care. In this study, we measured the levitation force and the stiffness by changing the thickness of a ring permanent magnet. We used ring Fe–Nd–B magnets 120mm in outer diameter and 70mm in inner diameter with the thicknesses of 5–40mm. For superconductors, we used single-domain bulk Dy–Ba–Cu–O 47mm in diameter and 10mm in thickness. Six pellets of Dy–Ba–Cu–O were placed concentrically such that the inner diameter becomes 70mm. The levitation forces increased with increasing the thickness of the permanent magnet but tended to saturate.