Tailoring Magnetism by Light-Ion Irradiation

Due to the reduced dimension, the magnetic properties of ultrathin magnetic films and multilayers, e.g. magnetic anisotropies and exchange coupling, often depend strongly on the surface and interface structure. In addition chemical composition, crystallinity, grain sizes and their distribution govern the magnetic behaviour. All these structural properties can be modified by light-ion irradiation in an energy range from 5 - 150 keV due to the energy loss of the ions in the solid along their trajectory. Consequently the magnetic properties can be tailored by ion irradiation. Similar effects can also be observed using Ga+ ion irradiation, which is the common ion source in focused ion beam lithography.

Examples for ion induced modifications of magnetic anisotropies and exchange coupling are presented. This review is limited to radiation induced structural changes giving rise to a modification of magnetic parameters. Ion implantation is discussed only in special cases.

Due to the local nature of the interaction, magnetic patterning without affecting the surface topography becomes feasible, which may have an applied interest. The main patterning technique is homogeneous ion irradiation through masks. Focused ion beam and ion projection lithography are commonly only relevant for larger ion masses. The creation of magnetic feature sizes below 50 nm is shown. In contrast to topographic nanostructures the surrounding area of these nanostructures can be left ferromagnetic leading to new phenomena at their mutual interface.

Most of the material systems discussed here are important for technological applications. The main areas are magnetic data storage applications, such as, hard magnetic media with a large perpendicular magnetic anisotropy or patterned media with an improved signal to noise ratio, and magnetic sensor elements. It will be shown that light-ion irradiation has many advantages for design of new material properties and the fabrication technology of actual devices.