Irradiation-induced magnetic patterning in magnetic multilayers

Irradiation with light ion (He+) can modify in a precisely controlled way the magnetic properties of thin films, with negligible change of surface roughness and optical indices. In Co/Pt multilayers with perpendicular easy axis of magnetization, we have shown that the magnetic anisotropy decreases with the irradiation fluence. In FePt alloys, partial chemical ordering and high perpendicular anisotropy have been induced by irradiation at moderate processing temperatures. Also, we have shown that perpendicular anisotropy in FePd and CoPt3 alloys can be decreased depending on the temperature of irradiation. These features allow the realization of a new type of magnetic nanostructures. As the structural modifications are localized in the close vicinity of the ion path in matter, planar magnetic patterning at the sub-50-nm scale can be achieved when the irradiation is performed through a mask. By means of 30 keV He+ ion irradiation of Co/Pt multilayers, we have produced a nearly optical contrast-free, planar array of magnetically hard lines embedded in a softer matrix. Such irradiation-fabricated nanostructures exhibit specific magnetization reversal processes: low field nucleation centers and preferred domain wall propagation paths are located at the borders between irradiated and non-irradiated areas. The magnetization reversal dynamic is limited everywhere by domain wall motion, ensuring a relatively weak spread of coercive forces. Striking images of nucleation and domain wall motion in such arrays are displayed and interpreted within a novel theoretical framework of irradiation through a mask.