( Fe1−xNix)5GeTe2 : An antiferromagnetic triangular…

Based on first-principles calculations, an antiferromagnetic Ising model on a triangular lattice has been proposed to interpret the order of $\mathrm{Fe}(1)$-Ge pairs and the formation of $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ superstructures in the ${\mathrm{Fe}}_{5}{\mathrm{GeTe}}_{2}$ (F5GT) as well as to predict the existence of similar superstructures in Ni doped F5GT (Ni-F5GT). Our paper suggests that F5GT systems may be considered as a rare structural realization of the well-known antiferromagnetic Ising model on a triangular lattice. Based on the superstructures, a Heisenberg-Landau Hamiltonian, taking into account longitudinal spin fluctuations, is implemented to describe magnetism in both F5GT and Ni-F5GT. We unveil that frustrated magnetic interactions associated with Fe(1), tuned by a tiny Ni doping, is responsible for the experimentally observed enhancement of the ${T}_{c}$ to 478 K in Ni-F5GT. Itinerant magnetism, reflected by longitudinal spin fluctuations, are found to only affect the ${T}_{c}$'s mildly with a modification of $\ensuremath{\sim}5%$ with respect to that obtained with standard Heisenberg interactions. Our calculations show that at low doping levels, monolayer Ni-F5GT has almost the same magnetic phase diagram as that of the bulk, which indicates a pervasive beyond room-temperature ferromagnetism in this Ni-doped two-dimensional system.