Coupled domain structures in superconductor/ferromagnet Nb-Fe/garnet bilayers

We investigate the magnetic structure in a superconducting Nb and ferromagnetic garnet hybrid film. Direct magneto-optical observations reveal a strong interaction between the superconducting vortices in Nb and the magnetic domains in the garnet, resulting in a unique coupled domain structure. The cooperative motion of vortices and domain walls results in an enhanced pinning and a modified electromagnetic response of the hybrid similar to that in type I superconductors. Application of ac field, routinely used for equilibration of domains, leads to their significant contraction. Our calculations explain this effect by a domain shrinkage instability induced by the vortex annihilation around oscillating domain walls.