Tunable magnetic properties in van der Waals crystals (Fe_1−xCo_x)₅GeTe₂

We report the doping effects of cobalt on van der Waals (vdW) magnet Fe₅GeTe₂. A series of (Fe_1−xCo_x)₅GeTe₂ (0 $≤$ x $≤$ 0.44) single crystals have been grown, and their structural, magnetic, and transport properties are investigated. For x = 0.20, the Curie temperature T_C increases from 276 K to 337 K. Moreover, the magnetic easy axis is reoriented to the ab-plane from the c-axis in undoped Fe₅GeTe₂ with largely enhanced magnetic anisotropy. A complex magnetic phase diagram is identified on the higher doping side. The x = 0.44 crystal first orders ferromagnetically at T_C = 363 K and then undergoes an antiferromagnetic (AFM) transition at T_N = 335 K. Furthermore, magnetic-field-induced spin-flop transitions are observed for the AFM ground state. Our work reveals (Fe_1−xCo_x)₅GeTe₂ as promising candidates for developing spin-related applications and proposes a method to engineer the magnetic properties of the vdW magnet.