Dimensional control of magnetic phases and gate-tunable magnetism in van der Waals ferromagnet Fe4GeTe2

Abstract

van der Waals (vdW) ferromagnet Fe4GeTe2, notable for its high Curie temperature, is investigated for its potential in electrically controlled spintronic devices. We report a systematic study of thickness-dependent magnetic properties and ionic liquid gating effects in mechanically exfoliated Fe4GeTe2 flakes using anomalous Hall effect (AHE) measurements. As thickness decreases from bulk down to 25 nm, significant changes in magnetic behavior are observed, including a 215% enhancement in AHE amplitude, a transition from magnetic phase coexistence in thicker flakes (e.g., 877 nm) to a homogenized phase below a critical thickness (e.g., 67 nm), and a strong thickness dependence of coercivity (Hc). A spin reorientation transition around 100 K is also identified in both bulk and thin flakes. Furthermore, we demonstrate effective electrical control over magnetism in 25 nm Fe4GeTe2 devices using ionic liquid gating. Applying gate voltages of ±3 V at 100 K resulted in a substantial modulation of magnetic properties, increasing the saturation Hall resistance (Rs) by up to 74% and Hc by up to 64%, attributed to gate-induced modification of hole carrier concentration. These findings highlight the tunability of magnetism in Fe4GeTe2 via dimensionality and electric fields, presenting opportunities for developing vdW material-based electrically controlled spintronic applications.