Structural modifications induced by FIB implantation in magnetic thin films

Recently, there is a rise of interest in fabrication and investigation of nanometre sized magnetic objects. Magnetic properties can easily be manipulated by ion beam implantation. Focused-ion beam (FIB) techniques are one way to combine both nanopatterning and implantation. The main difference between standard ion implantation and FIB implantation is the beam current density, which could lead to differences in the structural and magnetic properties. The aim of this work is to compare both implantation techniques in terms of structural changes and to relate them to magnetic property changes.
For our investigation we have used 50 nm thick non-ordered nano-crystalline permalloy (Ni81Fe19) films modified by a 30 keV Ga+ ion beam. The magnetic properties were characterised via magneto-optic Kerr effect measurements at room temperature. Both types of implantation demonstrate a degradation of saturation magnetisation with increasing ion fluence. For structural investigations we have applied several techniques. We have used the advantage of non-destructive X-ray techniques to study the structural changes. Besides X-ray diffraction, providing the long-range order information, EXAFS measurements to probe the local structure were performed. Both methods are statistical ones, whereas TEM imaging provides information on a local scale.
Implantation leads to a crystallite growth from ~12 nm up to ~25 nm and further texturing of the material towards (111) direction at almost constant lattice parameter. In the case of FIB implanted samples the TEM images show crystallites growing through the entire film at high implantation fluences. The EXAFS analysis shows an almost perfect near-order coordination, corresponding to an fcc cell; only in the vicinity of the Ga atoms a small local deviation could be observed. In general FIB and standard implantation demonstrate similar behaviour with a shift in the fluence value. These results let us conclude that the simple presence of the Ga atoms is the dominating effect leading the degradation of saturation magnetisation.