It is shown that two-dimensional periodic magnetic systems with in-plane magnetization composed by a mixture of ferromagnetic materials have an effective diamagnetic behavior. Because of the formation of effective “surface magnetic charges” resulting from the magnetization contrast at the boundary between the two ferromagnetic materials, the demagnetizing field is parallel to the in-plane external magnetic field in some regions of the unit cell. Some examples of real systems exhibiting this behavior are given. The effective diamagnetic properties in a periodic mixture of ferromagnetic materials allow to describe periodic magnetic systems as metamaterials.