Hybrid magnetic materials created by local ion irradiation

Lateral patterning of thin ferromagnetic films allows the modification of the magnetic parameters below certain intrinsic magnetic length scales like the domain wall width. In contrast to the creation of isolated micro- and nanostructures, magnetic patterning by means of local ion irradiation results in direct exchanged coupled regions in the thin film, which have different magnetic properties. At the lateral interfaces of these regions magnetic domain walls can be trapped and thus easily investigated. The modification of the material properties, e.g. saturation magnetization and magnetic anisotropy, also directly affects the intrinsic length scales. As a model system periodic patterns of Ni80Fe20 stripes with alternating saturation magnetization value located either inside a Ni80Fe20 matrix or isolated are used. During the magnetic reversal with the external field parallel to the stripes, magnetic domain walls are created between adjacent stripes due to a noneparallel alignment of the stripe’s magnetization
(Fig. 1). Fig. 1: Magnetization distribution in periodic pattern of 10 μm wide stripes with alternating saturation magnetization value. The magnetization directions are indicated by arrows. (Non-) irradiated regions with (unchanged) decreased saturation magnetization are indicated with (gray) white shaded areas at the bottom. The location of a 180° domain wall is indicated by the green rectangle. As the wall profile for stripe widths in the sub-μm regime is not easily accessible in experiment, changes of the possible orientations of the magnetization inside the stripes with respect to the stripe orientation are used to obtain informations about the domain walls. For special pattern configurations these domain walls are able to mediate an exchange spring behavior through the storage of energy [1]. By scaling the stripes width down, a crossing between the patterning size and the intrinsic length scales associated with the domain walls is expected. This is combined with a transformation to an effective magnetic medium, showing neither the properties of the fully irradiated nor the unirradiated material (so-called hybrid material). Local ion irradiation is accomplished by irradiation in combination with a lithographically defined mask or the by the use of a focused ion beam. For comparision the ion energies for both methods are choosen to be 30 keV. For a Ni80Fe20 film thickness of 20 nm the resulting penetration depths of the ions cover the complete film as well as the interface to the seed layer. In order to understand the ion irradiation induced changes in the magnetic parameters of the film, ions of different species (Ar+, Ga+, Cr+, Ni+, Co+, Si+, O+) as well as different seed layers are used. Changes of the magnetic and structural properties are studied as function of ion species, fluence and seed layer material. The uniaxial magnetic anisotropy of the Ni80Fe20 film is not affected by the implanted ion species, but by the species of the recoils originating from the seed layer. Using SiO2 as seed layer material, the anisotropy can be successively annihilated with increasing fluence. Irradiation using a Ga+ focused ion beam with its high current density induces a grain growth to the material [2], limiting therefore also the minimum patterning size (Fig. 2). Fig. 2: Bright field plane view TEM image of 200 nm wide stripes. The focused ion beam irradiated areas are observable by the increased grain size.
References: [1] McCord J. et al. Advanced Materials 20 (2008) 2090. [2] Ozkaya D.L. et al. Journal of Applied Physics 91 (2002) 9937.