Ferromagnetic resonance study of ion irradiated Co/Ni multilayers with perpendicular magnetic anisotropy

Ferromagnetic resonance (FMR) spectroscopy was used to investigate the effect of helium ion-irradiation on the magnetic properties and the magnetization dynamics of Co/Ni multilayer films. The anisotropy in these materials is associated with interfaces, which can be systematically disordered with light ion-irradiation without inducing major structural changes to the films. [Pd/Co]$\times 2|$[8\AA Ni/1.4\AA Co]$\times 3 |$Pd$|$Co$|$Pd$|$ have been exposed to He$^+$ irradiation with fluence up to $10^{15}$ ions/cm$^{2}$ [1]. FMR was conducted with a broad band coplanar waveguide up to 30 GHz. The resonance field and the FMR linewidth were determined as a function of frequency for dc magnetic fields in-plane, out-of-plane and for selected field angles. The perpendicular anisotropy decreases linearly with fluence, and at a particular fluence the direction of easy magnetization switches from perpendicular to in-plane. The Gilbert damping constant of the films irradiated at the higher and lower fluence is about the same: 0.03$\leq \alpha \leq$0.04. However, the linewidth exhibits a non-monotonic dependence on fluence, with a maximum at intermediate fluence. We will discuss this effect as well as possible explanations in terms of the changes in interface structure as a function of fluence. [1] Stanescu $et$ $al.$, J. Appl. Phys. (2008).