Manipulating the Energy Balance of Perpendicular-Anisotropy Synthetic Antiferromagnets by He+-Ion Irradiation

${\mathrm{He}}^{+}$-ion irradiation enables controlled postdeposition modification of layered magnetic thin-film systems. The degree of modification and its depth profile can be tuned by the irradiation dose and energy. Here, we use magnetometry and magnetic force microscopy to explore the impact of gentle ${\mathrm{He}}^{+}$-ion irradiation on synthetic antiferromagnets, consisting of ferromagnetic $\mathrm{Co}/\mathrm{Pt}$ multilayers with perpendicular magnetic anisotropy, which are antiferromagnetically (AF) coupled via $\mathrm{Ru}$ interlayers. This system shows a rich variety of magnetic domain patterns due to the strong competition between different magnetic energies. We show that AF interlayer exchange and perpendicular interface anisotropy energy are gradually reduced by the ion irradiation while the demagnetization energy is mainly preserved, which thus results in multiple successive magnetic-phase transitions.