Polar Magneto‐Optical Kerr Effect in CoPt/AlN Multilayers

ABSTRACT

Aiming to explore structurally stable and easily fabricable magneto‐optical (MO) platforms for potential refractive‐index‐sensing applications, this work investigates stacking‐number‐dependent low‐field‐enhanced polar magneto‐optical Kerr effect (P‐MOKE) in substrate/AlN buffer/[CoPt(3 nm)/AlN(22 nm)] _N /CoPt(3 nm) multilayer structures. With increasing stacking number N , the magnetic hysteresis loops gradually evolved into pronounced step‐like behaviors, indicating the coexistence of two magnetically distinct regions with different coercivity ( H _C ) values along the film thickness direction. Correspondingly, low‐field‐enhanced Kerr rotation ( θ _K ) hysteresis loops were observed, where the θ _K in the low‐field region exceeded that of the saturated state. To clarify the origin of this behavior, inverse MO Fresnel transfer matrix method analysis was performed by separating the multilayer structure into top and bottom CoPt regions with distinct magnetic reversal characteristics. The fitting results revealed that the two regions exhibit different complex Voigt vectors and contribute oppositely to the θ _K hysteresis behavior. The observed low‐field‐enhanced P‐MOKE was interpreted as a consequence of the competition between opposite MO polarities and different H _C values between these spatially separated magnetic regions along the film thickness direction. In addition, the extracted Voigt vector difference suggests that MO coupling may be influenced by the structural environment and residual‐stress‐related variations. Furthermore, the multilayer structures exhibited appreciable sensitivity to variations in the surrounding dielectric‐environment at 408 nm, indicating potential applicability in MO sensing.