Magnetization reversal in irradiation-fabricated nanostructures

We study the magnetization reversal mechanisms of buried submicron magnetic nanostructures fabricated by ion irradiation through a patterned 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. Magnetization reversal in these nanostructures exhibits specific features: Low field nucleation centers and preferred domain wall propagation paths are located at the borders between irradiated and nonirradiated areas. The magnetization reversal dynamics is limited everywhere by domain wall motion, ensuring a relatively weak spread of coercive forces. In contrast with usual magnetic nanostructures, the coercivity of irradiation-fabricated lines decreases when reducing their size. Modeling of the irradiation process when performed through a mask indicates that in such a geometry, the irradiation effects are not homogeneous. It arises from collateral damage around the mask. An irradiation overshoot occurs in the vicinity of the mask edges: Depending on mask width, it can extend to a few hundred nanometers and reach up to +34% of the nominal fluence. Both magnetically hard and soft area coercivities are affected by this overshoot. A semiquantitative analysis of these dependencies is presented. It allows us to understand the magnetic properties of irradiation-fabricated nanostructures in a wide size interval. (C) 2000 American Institute of Physics. [S0021-8979(00)05112-4].