Grain growth induced by focused ion beam irradiation in thin magnetic films

Focused ion beam irradiation can be used as a tool for creation of magnetic nanos-tructures. Previous studies have shown that FIB irradiation of thin metallic films could induce significant grain growth and therefore modify magnetic properties [1-2]. In this work we compare X-ray diffraction studies with magnetic properties that were characterized by magneto-optic Kerr effect sensing both the in-plane and the out-of-plane components of the magnetization.
A 50nm thick permalloy layer (Ni80Fe20) irradiated with different fluences of Ga+ ions was chosen for the investigations of grain size and microstrain. Due to the small irradiated areas produced by FIB irradiation XRD measurements were carried out on an optimized X-ray laboratory setup with a focused X-ray beam of 200µm as well as on the ESRF ID01 facility using a 1µm focused beam. Figure 1a shows rocking curves of the (111) permalloy reflection and demonstrates the material texturing with increase of ion fluence as there is a narrowing in FWHM and a rise of intensity. Low irradiation fluences up to 6.24*10E15 ions/cm² modify the material and induce crystal-lite growth, whereas larger fluences completely destroy the crystalline structure. A further material crystallization should improve the magnetic properties. Due to the incorporation of Ga+ ions into the lattice additional softening of the permalloy film occurs and leads to the degradation of magnetic properties. In figure 1b a corre-sponding saturation Kerr rotation measurement is shown. It demonstrates a decrease of the magnetic moment with increasing ion fluence.

[1] C.M. Park and J. A. Bain., J. of Appl. Phys. 91, 6830 (2002).
[2] W.M. Kaminsky et al., Appl. Phys. Lett. 78, 1589 (2001).