Flat bands and magnetism in $\mathrm{\mathbf{Fe_4 Ge Te_2}}$ and $\mathrm{\mathbf{Fe_5GeTe_2}}$ due to bipartite crystal lattices

TL;DR

This study combines model Hamiltonians and first-principles calculations to reveal that flat bands from bipartite lattices induce itinerant ferromagnetism in Fe4GeTe2 and Fe5GeTe2, with magnetic moments sensitive to external conditions.

Contribution

It introduces a bipartite lattice perspective to explain flat bands and magnetism in Fe4GeTe2 and Fe5GeTe2, providing new insights into their magnetic behavior.

Findings

Flat bands near the Fermi level originate from bipartite lattices.

Flat bands lead to itinerant ferromagnetism in the materials.

The magnetic moment of Fe5 is sensitive to Coulomb interaction and pressure.

Abstract

$\mathrm{Fe_{n=4,5}GeTe_2}$ exhibits quasi-two-dimensional properties as a promising candidate for a near-room-temperature ferromagnet, which has attracted great interest. In this work, we notice that the crystal lattice of $\mathrm{Fe_{n=4,5}GeTe_2}$ can be approximately regarded as being stacked by three bipartite crystal lattices. By combining the model Hamiltonians of bipartite crystal lattices and first-principles calculations, we investigate the electronic structure and the magnetism of $\mathrm{Fe_{n=4,5}GeTe_2}$. We conclude that flat bands near the Fermi level originate from the bipartite crystal lattices and that these flat bands are expected to lead to the itinerant ferromagnetism in $\mathrm{Fe_{n=4,5}GeTe_2}$. Interestingly, we also find that the magnetic moment of the Fe5 atom in $\mathrm{Fe_5 Ge Te_2}$ is distinct from the other Fe atoms and is sensitive to the Coulomb interaction $U$ and external pressure. These findings may be helpful to understand the exotic magnetic behavior of $\mathrm{Fe_{n=4,5} Ge Te_2}$.