Thickness-dependent magnetic properties in Pt/[Co/Ni]n multilayers with perpendicular magnetic anisotropy

We systematically investigated the Ni and Co thickness-dependent perpendicular magnetic anisotropy (PMA) coefficient, magnetic domain structures, and magnetization dynamics of Pt(5 nm)/[Co($t_{rm Co}$)/Ni($t_{rm Ni}$)]$_{5}$/Pt(1 nm) multilayers by combining the four standard magnetic characterization techniques. The magnetic-related hysteresis loops obtained from the field-dependent magnetization $M$ and anomalous Hall resistivity (AHR) $rho_{{xy}}$ showed that the two serial multilayers with $t_{rm Co} = 0.2$ nm and 0.3 nm have the optimum PMA coefficient $K_{rm U}$ as well as the highest coercivity $H_{rm C}$ at the Ni thickness $t_{rm Ni}= 0.6 $ nm. Additionally, the magnetic domain structures obtained by magneto-optic Kerr effect (MOKE) microscopy also significantly depend on the thickness and $K_{rm U}$ of the films. Furthermore, the thickness-dependent linewidth of ferromagnetic resonance is inversely proportional to $K_{rm U}$ and $H_{rm C}$, indicating that inhomogeneous magnetic properties dominate the linewidth. However, the intrinsic Gilbert damping constant determined by a linear fitting of the frequency-dependent linewidth does not depend on the Ni thickness and $K_{rm U}$. Our results could help promote the PMA [Co/Ni] multilayer applications in various spintronic and spin-orbitronic devices.