CoPt alloy thin films as ferromagnetic metal materials have attracted widespread attention for magneto-optical (MO) applications due to their high perpendicular magnetic anisotropy (PMA) and strong MO effect. However, quantitative determination of the off-diagonal dielectric permittivity tensor ${\varepsilon }_{ij}\left(i\ne j\right)$, which governs the MO effect, remains challenging in ultrathin multilayer films. In this study, we adopt an analytical approach combining experimental polar magneto-optical Kerr effect (P-MOKE) measurements with an inverse MO Fresnel transfer matrix method (TMM) to quantitatively extract ${\varepsilon }_{ij}\left(i\ne j\right)$ of the CoPt alloy thin films embedded in CoPt/AlN periodic multilayer structures. Beyond parameter extraction, the introduction of dielectric AlN spacer layers enables pronounced optical interference effects, leading to a substantial enhancement of the polar Kerr rotation through structural engineering. CoPt/AlN multilayers with varying stacking numbers ($N=1–10$) are fabricated via DC magnetron sputtering and ex situ postannealing to achieve high-quality interfaces between CoPt and AlN layers. Crystallographic and surface morphology characterizations confirm that annealing treatment leads to improved crystallinity and enhanced texturing. Magnetic characterizations done by vibrating sample magnetometer (VSM) further demonstrate that annealing treatment effectively increases both the coercivity and saturation magnetization. UV–Vis spectroscopy is employed to experimentally validate the optical parameters of the individual constituent layers. P-MOKE measurements reveal nonmonotonic Kerr rotation due to optical interference in the multilayer structures. Theoretical modeling based on MO Fresnel TMM show excellent agreement with experimental results. Furthermore, simulations comparing bulk-like thick films and multilayer stacked thin films elucidate the underlying mechanism of Kerr rotation reversal. Based on the experimental and simulation results, the extraction of ${\varepsilon }_{ij}\left(i\ne j\right)$ in ferromagnetic CoPt layers, which is critical for designing interference-enhanced MO devices, is realized.

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• Ferromagnetism
• Magneto-optical Kerr effect
• Magnetometry
• Magnetic multilayers
• Thin films
• Transition metal alloys