Vortex structures in the ferromagnet/type-II superconductor bilayer are investigated when the ferromagnet has domain structure and perpendicular magnetic anisotropy. It is found that two equilibrium vortex structures can be realized: straight vortices with alternating directions corresponding to the direction of the magnetization in the ferromagnetic domains and vortex semiloops connecting the ferromagnetic domains with opposite direction of the magnetization. These states are separated by an energy barrier. The values of the critical magnetization for the formation of these vortex structures are determined.
The surface impedance of type-II superconductors in the mixed state has been studied, taking into account the vortex elasticity and nonlocality (long-range interaction of vortices). An electromagnetic wave penetrates into a superconductor as a superposition of two exponentially decaying modes with different penetration lengths, in contrast with one mode in a normal conductor or in a superconductor in the Meissner state. The second mode is an elastic mode of the vortex array; it is crucial for incorporation of vortex elasticity and surface pinning into electrodynamics.
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