Interfacial Magnetic Order Controls Valley Polarization in WS ₂ /Fe ₃ GaTe ₂ Heterostructures

heterostructures. By comparing bottom- and top-stacked architectures, we disentangle intrinsic magnetic proximity effects from oxidation-driven interfacial reconstruction. The protected bottom-stacked interface preserves ferromagnetic order, yielding pronounced valley hysteresis and a giant valley exciton Zeeman splitting of 13.7 meV, equivalent to an effective magnetic field of ∼59 T. In contrast, the exposed top-stacked interface exhibits only 1.26 meV splitting. Polarization-resolved photoluminescence, high-field magneto-optical measurements, and first-principles calculations reveal that oxidation induces an interfacial transition from ferromagnetic to antiferromagnetic order, suppressing the proximity exchange field through weakened Fe-W orbital hybridization. These results establish interfacial magnetic reconstruction as the key factor governing magneto-valley coupling.