Based on first-principles calculations, an antiferromagnetic Ising model on a triangular lattice has been proposed to interpret the order of Fe(1)-Ge pairs and the formation of 3×3 superstructures in the Fe5GeTe2 (F5GT), as well as to predict the existence of similar superstructures in Ni doped F5GT (Ni-F5GT). Our study suggests that F5GT systems may be considered as a structural realization of the well known antiferromagnetic Ising model on a triangular lattice. Based on the superstructures, a Heisenberg-Landau Hamiltonian, taking into account both Heisenberg interactions and 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 (x≤0.2), is responsible for the experimentally observed enhancement of the Tc to 478 K in Ni-F5GT. 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.
@article{arxiv.2209.09467,
title = {(Fe$_{1-x}$Ni$_{x}$)$_{5}$GeTe$_{2}$: an antiferromagnetic triangular Ising lattice with itinerant magnetism},
author = {Xunwu Hu and Dao-Xin Yao and Kun Cao},
journal= {arXiv preprint arXiv:2209.09467},
year = {2023}
}