Magnetic Anisotropy Modulation via van der Waals Gap Engineering in 2D Ferromagnet Fe ₄ GeTe ₂

Abstract Magnetic anisotropy modulation is central to spintronics. 2D ferromagnetic materials (2D FMs), with their atomic‐level thickness, tunable electronic structures, and high sensitivity to external stimuli, provide unprecedented opportunities for precise magnetic control. Particularly, the van der Waals (vdW) gap holds promise for effective magnetic anisotropy modulation. However, the microscopic mechanisms remain elusive. Here, it is demonstrated that epitaxial growth of α‐Al ₂ O ₃ /Fe ₄ GeTe ₂ induces a pronounced expansion of the vdW gap (up to 0.51 Å) at the interface, leading to a robust enhancement of in‐plane magnetic anisotropy (IMA) and suppression of the spin reorientation temperature ( T _SR ) from 288 K to undetectable levels. This counterintuitive behavior contrasts with conventional thickness‐dependent perpendicular magnetic anisotropy (PMA). Combined experimental and theoretical analyses reveal that vdW gap expansion reduces Te p _x /p _y orbital overlap, diminishing their contribution to magnetocrystalline anisotropy energy and suppressing PMA. These findings establish interface gap engineering as a novel route for tailoring magnetic anisotropy in 2D FMs, advancing the design of next‐generation spintronic devices.