Lithium-Induced Thickness-Dependent Magnetic Structure Transition in 2D Fe₃GaTe₂

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 Fe₃GaTe₂ 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 Fe₃GaTe₂ via ionic control and establish gate-controlled intercalation as a reconfigurable platform for engineering topological spin textures and energy-efficient magnetic memory devices.