Irradiation-induced magnetic patterning in magnetic multilayers

Irradiation with light ion (He+) can modify in a precisely controlled way t he magnetic properties of thin films, with negligible change of surface rou ghness and optical indices. In Co/Pt multilayers with perpendicular easy ax is of magnetization, we have shown that the magnetic anisotropy decreases w ith 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 i rradiation. 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-5 0-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 prod uced a nearly optical contrast-free, planar array of magnetically hard line s embedded in a softer matrix. Such irradiation-fabricated nanostructures e xhibit specific magnetization reversal processes: low field nucleation cent ers and preferred domain wall propagation paths are located at the borders between irradiated and non-irradiated areas. The magnetization reversal dyn amic is limited everywhere by domain wall motion, ensuring a relatively wea k spread of coercive forces. Striking images of nucleation and domain wall motion in such arrays are displayed and interpreted within a novel theoreti cal framework of irradiation through a mask. (C) 2001 Elsevier Science B.V. All rights reserved.