Using ion beams to locally modify material properties and subsequently drive magnetic phase transitions is rapidly gaining momentum as the technique of choice for the fabrication of magnetic nanoelements. This is because the method provides the capability to engineer in three dimensions on the nanometer length scale. This will be an important consideration for several emerging magnetic technologies (e.g., spintronic devices and racetrack and random-access memories) where device functionality will hinge on the spatial definition of the incorporated magnetic nanoelements. In this work, the fundamental sharpness of a magnetic interface formed by nanomachining FePt 3 films using He + irradiation is investigated. Through careful selection of the irradiating ion energy and fluence, room-temperature ferromagnetism is locally induced into a fractional volume of a paramagnetic (PM) FePt 3 film by modifying the chemical order parameter. A combination of transmission electron microscopy, magnetometry, and polarized neutron reflectometry measurements demonstrates that the interface over which the PM-to-ferromagnetic modulation occurs in this model system is confined to a few atomic monolayers only, while the structural boundary transition is less well-defined. Using complementary density functional theory, the mechanism for the ion-beam-induced magnetic transition is elucidated and shown to be caused by an intermixing of Fe and Pt atoms in antisite defects above a threshold density.