Lithium‐Induced Thickness‐Dependent Magnetic Structure Transition in 2D Fe ₃ GaTe ₂

The nonvolatile control of magnetic structures in 2D ferromagnets is essential for advancing spintronics. Here, gate‐tunable lithium intercalation is demonstrated as an effective strategy for modulating the magnetic properties of Fe 3 GaTe 2 in a pronounced thickness‐dependent manner. In flakes thicker than 27 nm, partial Li intercalation induces a functional ferromagnetic–antiferromagnetic vertical heterostructure, evidenced by an enhanced coercive field and a giant exchange bias of ∼0.31 T. Conversely, in flakes thinner than 20 nm, full Li penetration leads to a mixed‐phase transition, resulting in reduced coercivity and no exchange bias is observed. Real‐space magnetic force microscopy (MFM) imaging, combined with in situ atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM), directly reveals the thickness‐dependent evolution of magnetic domains and lattice distortions, providing a structural basis for the observed modulation. Density functional theory (DFT) calculations support these findings, confirming a lithium‐induced ferromagnetic‐to‐antiferromagnetic phase transition accompanied by lattice expansion. These results highlight the exceptional magnetic tunability of Fe 3 GaTe 2 via ionic control and establish gate‐controlled intercalation as a reconfigurable platform for engineering topological spin textures and energy‐efficient magnetic memory devices.