Microscopic Origin of Polarization-Controlled Magnetization Switching in FePt/BaTiO$_3$

Electric-field driven magnetization switching in FePt/BaTiO$_3$ (001) is demonstrated through first-principles calculations. The magnetic easy axis of FePt layer undergoes a transition from in-plane to perpendicular direction upon ferroelectric polarization reversal, a process sensitively controlled by epitaxial strain with threshold strain strain($\eta$) $\eta\approx\%$. At this phenomena, a large interfacial magnetoelectric coupling ($\alpha_I = 3.6 \times 10^{-10}$ G$\cdot$cm$^2$/V) is responsible, stemming from the orbital reconstruction. In particular, the redistribution of Pt-$d$ orbital occupancy alters spin-orbit coupling, thereby tuning the competition between magnetic anisotropy ($K_i$) and magnetoelastic energy ($b_1$). Our work clarifies the fundamental physics of strain-engineered magnetoelectricity and suggests a concrete pathway for designing ultra-low-power voltage-controlled magnetic memory.