Antiferromagnetic order in Co-doped Fe5GeTe2 probed by resonant magnetic x-ray scattering

The quasi-two-dimensional van der Waals magnet ${\mathrm{Fe}}_{5\ensuremath{-}\ensuremath{\delta}}{\mathrm{GeTe}}_{2}$ has emerged as a promising platform for electronic and spintronic functionalities at room temperature, owing to its large ferromagnetic ordering temperature ${T}_{\text{C}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 315 K. Interestingly, by cobalt (Co) substitution of iron in F5GT, $\text{i.e.,}\phantom{\rule{4pt}{0ex}}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{5\ensuremath{-}\ensuremath{\delta}}{\text{GeTe}}_{2}$ (Co-F5GT), not only can its magnetic transition temperature be further enhanced, but the magnetic and structural ground states can also be tuned. Specifically, an antiferromagnetic (AFM) order is induced beyond the Co doping level $x\ensuremath{\ge}0.4$. Here, we investigate the magnetic properties of a Co-F5GT single crystal at $x=0.45(1)$, by utilizing the element-specific, resonant magnetic x-ray scattering technique. Our study reveals an A-type, Ising-like AFM ground state, with a transition temperature ${T}_{\text{N}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 340 K. In addition, our work unveils an important contribution from Co magnetic moments to the magnetic order. The application of the in-plane magnetic fields gradually polarizes the spin moments along the field direction, but without inducing incommensurate spin texture(s).