Influence of ion implantation on the magnetic properties of thin $FeCo$ films

Modifications of $73-nm$-thick polycrystalline $FeCo$ films by magnetic-field-assisted implantation of $Ne$⁠, $Xe$⁠, and $Au$ ions have been investigated. For magnetic characterization, the longitudinal magneto-optic Kerr effect and magnetic force microscopy (MFM) in the remanent state have been used, while structural information has been gained from glancing-angle x-ray diffraction and Rutherford backscattering spectroscopy. The irradiated films show a soft-magnetic behavior with large magnetic anisotropy. The $Ne$ ions initially induce an increasing coercivity, possibly due to radiation defects accumulated during the ion implantation, while higher $Ne$ fluences anneal out the defects and reduce the coercivity. For the heavy ions the deposited energy density is high enough to reduce the coercivity at small fluences and then to increase it slightly for increasing fluence. Correlations between the magnetic anisotropy, coercivity and the ion species and fluence have been established. The MFM pictures feature the largest changes in the case of $Au$ ions. The magnetic anisotropy reflects the interplay between magnetocrystalline and magnetostrictive forces. For heavier ions and large fluences, the direction of the in-plane magnetic easy axis follows the orientation of the external magnetic field present during implantation.