Tunable magnetic properties in van der Waals crystals (Fe1−xCox)5GeTe2

We report the doping effects of cobalt on van der Waals (vdW) magnet Fe5GeTe2. A series of (Fe1−xCox)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 TC 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 TC = 363 K and then undergoes an antiferromagnetic (AFM) transition at TN = 335 K. Furthermore, magnetic-field-induced spin-flop transitions are observed for the AFM ground state. Our work reveals (Fe1−xCox)5GeTe2 as promising candidates for developing spin-related applications and proposes a method to engineer the magnetic properties of the vdW magnet.