Interfacial Engineering and Tunable Interlayer Coupling in Perpendicularly Magnetized Co2FeSi/Pt Multilayer Films

Abstract

The influences of a Ta buffer layer and film thickness on the perpendicular magnetic anisotropy (PMA) of Pt/Co₂FeSi (CFS)/Pt multilayers was systematically investigated. Insertion of a Ta underlayer significantly improves interface quality and extends the CFS thickness window for strong PMA from 0.8–1.0 to 0.8–1.2 nm. The optimal PMA is achieved at a CFS thickness of 1.0 nm, yielding an effective anisotropy energy density (K_eff) of 1.89 × 10⁶ erg/cm³—a 56.1% enhancement over samples without the Ta layer. Further optimization of the Ta and Pt layer thicknesses to 8 nm and 16 nm, respectively, results in a pronounced increase in PMA, with K_eff reaching 3.86 × 10⁶ erg/cm³. In asymmetric CFS (0.8 nm)/Pt/CFS (1.2 nm) trilayers, increasing the Pt spacer thickness above 3 nm drives a transition from ferromagnetic coupling to a magnetically decoupled state. Moreover, with increasing repetition number (n), the out-of-plane hysteresis loops develop a pinched character, indicating a crossover in the magnetization reversal mechanism from domain-wall motion to nucleation-dominated behavior. The identified correlations between interfacial structure, layer thickness, and magnetic anisotropy provide a rational design framework for Co₂FeSi-based perpendicular multilayer systems.