The microstructural evolution of chemical disorder and ferromagnetism in He+ irradiated FePt3 films

This paper investigates the role of ion-induced disorder on the morphology and magnetic properties of chemically ordered FePt3 films. The effects are studied for 15 keV He+ ions as a function of the ion fluence for 0, 2 × 1016 and 2 × 1017 ions cm−2. Substitutional mixing of the L12-type Fe-Pt sites takes place within the region of the chemically ordered FePt3 film affected by the irradiation. This accompanies a paramagnetic-to-ferromagnetic transition, as determined by room-temperature magnetometry. Dark-field transmission electron microscopy (TEM) measurements confirm that the 15 keV He+ ions induce a 120 nm-thick chemically disordered layer into the sub-surface region of the nominally 280 nm-thick ordered FePt3 film. The average domain size and the fractional density of the chemically ordered domains within the irradiated FePt3 microstructure are found to mutually decrease with increasing ion fluence. Selected-area electron diffraction results demonstrate that the film's single crystallinity is preserved after irradiation, irrespective of the ion fluence. High-resolution TEM elucidates the coexistence of ordered domains and precipitate disordered domains in the near-surface, low-ion impacted regions of the FePt3 film. Collectively, this work provides detailed insights into the material-science relationship between ion-induced disorder and ferromagnetism in FePt3, as a step towards creating fully customisable, ion-beam-synthesised magnetic nano-elements.