Magnetic anisotropy and domain patterning by means of ion irradiation

The control of magnetic properties and anisotropy in ferromagnetic thin films is of importance for many applications in spin electronics. Usually the uniaxial anisotropy in ferromagnetic alloys is set by applying a magnetic in-plane field during thin film deposition or adjusted by magnetic field annealing. In addition to temperature treatment, the alteration of magnetic properties in magnetic thin films by ion irradiation has gained increasing attention in recent years. However, most of the experiments focus on magnetic sandwiches or multilayers, where the magnetic properties depend strongly on the interface structure [1].
Here, we describe how to modify and tune the magnetic anisotropy axis and strength in single layer amorphous FeCoSiB soft magnetic films by ion irradiation with Co- and He-ions locally. Depending on the fluence and field angle during irradiation the uniaxial anisotropy can be realigned in the case of He-ion irradiation [2]. For Cobalt-implantation an increase of uniaxial anisotropy field with increasing fluence by more than a factor of two relative to the as-deposited anisotropy value (from Hk  15 Oe to Hk  40 Oe) is found. The ion treatment is used to “anneal” the films locally and thus to tailor the magnetic properties laterally. Periodic magnetic structures, consisting of regions of alternating anisotropy axis and strength, are generated using photo-lithographic processing. For that reason the films are partially masked with photo resist, patterned by optical lithography, and then irradiated with ions. Depending on the processing conditions and the size of the magnetic patterning, varying from mm in size down to sub-micrometer patterns, different kind of magnetic structures develop. The magnetization is modulated in accordance with the induced lateral anisotropy distribution. However, the domain walls do not align with the boundaries of the structures. Different types of periodic but rather complicated domain patterns are generated under different applied field conditions. The local change of magnetic properties due to the mixed anisotropy is limited by the micromagnetic feature size, not by the structural feature size. By structuring with a feature size smaller than the magnetic length scales, materials with novel magnetic properties are designed.

References
[1] J. Fassbender, D. Ravelosona, and Y. Samson, J. Phys. D: Appl. Phys. 37, R179 (2004).
[2] J. McCord, T. Gemming, L. Schultz, J. Fassbender, M.O. Liedke, M. Frommberger,
E. Quandt, Appl. Phys. Lett. 86, 162502 (2005).