(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">Fe</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mrow><mml:msub><mml:mi mathvariant="normal">Ni</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mn>5</mml:mn></mml:msub></mml:mrow><mml:msub><mml:mi mathvariant="normal">GeTe</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>: An antiferromagnetic triangular Ising lattice with itinerant magnetism

Abstract

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.

Topics & Concepts