  We present a ferromagnetic resonance (FMR) study of the effect of Helium ion
irradiation on the magnetic anisotropy, the linewidth and the Gilbert damping
of a Co/Ni multilayer coupled to Co/Pd bilayers. The perpendicular magnetic
anisotropy decreases linearly with He ion fluence, leading to a transition to
in-plane magnetization at a critical fluence of 5x10^{14} ions/cm^2. We find
that the damping is nearly independent of fluence but the FMR linewidth at
fixed frequency has a maximum near the critical fluence, indicating that the
inhomogeneous broadening of the FMR line is a non-monotonic function of the He
ion fluence. Based on an analysis of the angular dependence of the FMR
linewidth, the inhomogeneous broadening is associated with spatial variations
in the magnitude of the perpendicular magnetic anisotropy. These results
demonstrate that ion irradiation may be used to systematically modify the
magnetic anisotropy and distribution of magnetic anisotropy parameters of
Co/Pd/Co/Ni multilayers for applications and basic physics studies.
