  A multilayered structure with a single superconductor layer and a single
insulator layer formed on a bulk superconductor is studied. General formulae
for the vortex-penetration field of the superconductor layer and the magnetic
field on the bulk superconductor, which is shielded by the superconductor and
insulator layers, are derived with a rigorous calculation of the magnetic field
attenuation in the multilayered structure. The achievable peak surface field
depends on the thickness and its material of the superconductor layer, the
thickness of the insulator layer and material of the bulk superconductor. The
calculation shows a good agreement with an experimental result. A combination
of the thicknesses of superconductor and insulator layers to enhance the field
limit can be given by the formulae for any given materials.
