Periodic arrays of ferromagnetic structures with micrometer and submicrometer lateral sizes have been prepared at the surface of a paramagnetic austenitic stainless steel by means of ion beam nitriding through different types of shadow masks (such as transmission electron microscopy grids or self-assembled porous alumina membranes). This method takes advantage of the formation of the ferro-magnetic supersaturated nitrogen solid solution c N phase (i.e., expanded austenite) upon nitriding at moderate temperatures. The local character of the induced ferromagnetism is confirmed by magneto-optical Kerr effect measurements together with magnetic force micros-copy imaging. Furthermore, the influence of the nitriding temperature and time on the induced ferromagnetic and structural properties has been analyzed.