Effects of a magnetic substrate on the ac losses of superconducting films were investigated by measuring the losses for octagonal disks of an YBa2Cu3O7 layer on a magnetic Ni5at.% W substrate in perpendicular ac magnetic fields at 20Hz and 77K. At low fields, the losses depended on ac magnetic field amplitude B and film thickness t as B3t instead of B4t3 for a superconducting film on a nonmagnetic substrate. These results are described by considering the formation of a virtual infinite stack of superconducting films due to the magnetic mirror effect.

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