Effect of ion irradiation on the magnetic properties of CoPt films

The possibility of using He$^+$ ion implantation with an energy of 20 keV for modifying the domain structure and magnetic properties of CoPt films formed by electron beam evaporation with different compositions – Co$_{0.45}$Pt$_{0.55}$ and Co$_{0.35}$Pt$_{0.65}$ – has been investigated. For the irradiated CoPt samples of both compositions, a decrease in the coercivity (narrowing of the hysteresis loop on the magnetic field dependences of the Faraday angle and magnetization) with an increase in the He$^+$ ion fluence from 2 $\times$ 10$^{14}$ to 4 $\times$ 10$^{14}$ cm$^{-2}$ was found. In this case, the remanent magnetization of the Co$_{0.35}$Pt$_{0.65}$ films coincides with the value of saturation magnetization, while for Co$_{0.45}$Pt$_{0.55}$ , a decrease in the remanent magnetization is observed. Magnetic force microscopy has shown that for the Co$_{0.45}$Pt$_{0.55}$ alloy, with an increase in the ion fluence up to 3 $\times$ 10$^{14}$ cm$^{-2}$, the largest number of isolated circular domains (skyrmions) is formed, while for He+ irradiation with a fluence of 4 $\times$ 10$^{14}$ cm$^{-2}$ for Co$_{0.35}$Pt$_{0.65}$, in addition to isolated circular domains, 360-degree domain walls (1D skyrmions) are observed. At the same time, the study of CoPt films by the Mandelstam-Brillouin spectroscopy method revealed an increase in the shift between the Stokes and anti-Stokes components of the spectrum and thus a significant increase of the Dzyaloshinsky–Moriya interaction for the irradiated samples. Simulation using the SRIM software showed that the applied ion irradiation causes the asymmetric mixing of Co and Pt atoms and thus, this may underlie the mechanism of the of the ion irradiation on magnetic properties and domain structure in CoPt films.