(Fe$_{1-x}$Ni$_{x}$)$_{5}$GeTe$_{2}$: an antiferromagnetic triangular Ising lattice with itinerant magnetism

TL;DR

This study uses first-principles calculations to model the magnetic properties of Fe$_{1-x}$Ni$_{x}$GeTe$_{2}$, revealing how Ni doping influences antiferromagnetic interactions and enhances the Curie temperature, suggesting potential for high-temperature 2D ferromagnetism.

Contribution

It introduces a combined Ising and Heisenberg-Landau model to explain magnetic superstructures and doping effects in Fe$_{5}$GeTe$_{2}$ and Ni-doped variants, providing new insights into their magnetic behavior.

Findings

Ni doping increases the Curie temperature to 478 K.

Low doping levels preserve the magnetic phase diagram of monolayer and bulk.

Frustrated magnetic interactions drive the enhanced magnetism with doping.

Abstract

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 $\sqrt{3}$ $\times $ $\sqrt{3}$ superstructures in the Fe$_5$GeTe$_2$ (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 \leq 0.2 $), is responsible for the experimentally observed enhancement of the $T_c$ 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.