Ferromagnetism in certain alloys consisting of magnetic and nonmagnetic species can be activated by the presence of chemical disorder. This phenomenon is linked to an increase in the number of nearest-neighbor magnetic atoms and local variations in the electronic band structure due to the existence of disorder sites. An approach to induce disorder is through exposure of the chemically ordered alloy to energetic ions; collision cascades formed by the ions knock atoms from their ordered sites and the concomitant vacancies are filled randomly via thermal diffusion of atoms at room temperature. The ordered structure thereby undergoes a transition into a metastable solid solution. Here we demonstrate the patterning of highly resolved magnetic structures by taking advantage of the large increase in the saturation magnetization of Fe 60Al40 alloy triggered by subtle atomic displacements. The sigmoidal characteristic and sensitive dependence of the induced magnetization on the atomic displacements manifests a sub-50 nm patterning resolution. Patterning of magnetic regions in the form of stripes separated by ∼40 nm wide spacers was performed, wherein the magnet/spacer/magnet structure exhibits reprogrammable parallel (↑/spacer/↑) and antiparallel (↑/spacer/↓) magnetization configurations in zero field. Materials in which the magnetic behavior can be tuned via ion-induced phase transitions may allow the fabrication of novel spin-transport and memory devices using existing lateral patterning tools. © 2013 American Chemical Society.