The magnetization behavior of a single-crystalline [Fe-whisker/MgO (20 ML)/Fe-film (20 ML)] sandwich (ML denotes monolayer) was studied by depth-selective Kerr microscopy. Residual stray fields of the whisker domain walls were identified to be responsible for complex magnetization processes in the iron film. A 180° wall in the whisker magnetizes the film transversally to the wall direction, depending on the internal rotation alignment of the whisker wall and not on its surface rotation. We also found that 360° walls can be formed in the film for pure topological reasons if Néel walls of certain rotation alignments are facing each other.