Above-Room-Temperature Antiferromagnetism in Ultrathin Fe₃Co_2.5GeTe₂ Nanosheets Enabled by a Hybrid Magnetic State

The development of van der Waals (vdW) magnetic materials with robust magnetic ordering above room temperature is critical for advancing spintronic technologies. Here, we demonstrate a doping-engineered strategy to create Fe₃Co_2.5GeTe₂, a vdW crystal with an exceptionally high Néel temperature (T_N) of 361 K. Through targeted Co-substitution, we transform the parent ferromagnet Fe₅GeTe₂ into a complex hybrid magnetic state, characterized by intralayer ferrimagnetism coexisting with interlayer antiferromagnetism. This unique ground state is unambiguously confirmed by its hallmark experimental signatures: a characteristic two-step spin-flip transition and a small remanent magnetization, observed in both bulk magnetometry and nanoscale anomalous Hall effect measurements. Crucially, we show that this hybrid magnetic order is robust in the 2D limit, with the spin-flip signature persisting in devices as thin as 9 nm and the magnetic ordering remaining stable up to 330 K. Our findings position Fe₃Co_2.5GeTe₂ as a premier platform for high-temperature antiferromagnetic spintronics and validate chemical substitution as a powerful method for designing complex magnetic states in vdW materials.