Mechanism of magnetic phase transition in correlated magnetic metal: insight into itinerant ferromagnet Fe$_{3-\delta}$GeTe$_2$

TL;DR

This study elucidates the microscopic mechanisms behind magnetic phase transitions in Fe$_{3-\,delta}$GeTe$_2$, highlighting Hund's coupling, flat band formation, and site-specific Mott and Hund's physics, advancing understanding of correlated itinerant magnets.

Contribution

It provides new insights into the roles of Hund's coupling, flat bands, and site-dependent Mott and Hund's physics in the magnetic transition of Fe$_{3-\,delta}$GeTe$_2$, a correlated itinerant magnet.

Findings

Hund's coupling is essential for ferromagnetic order.

Formation of quasiparticle flat bands during transition.

Distinct Mott and Hund's physics at different Fe sites.

Abstract

Developing a comprehensive magnetic theory for correlated itinerant magnets poses challenges due to the difficulty in reconciling both local moments and itinerant electrons. In this work, we investigate the microscopic process of magnetic phase transition in ferromagnetic metal Fe$_{3-\delta}$GeTe$_2$. We find that Hund's coupling is crucial for establishing ferromagnetic order. During the ferromagnetic transition, we observe the formation of quasiparticle flat bands and an opposing tendency in spectral weight transfer, primarily between the lower and upper Hubbard bands, across the two spin channels. Moreover, our results indicate that one of the inequivalent Fe sites exhibits Mott physics, while the other Fe site exhibits Hund's physics, attributable to their distinct atomic environments. We suggest that ferromagnetic order reduces spin fluctuations and makes flat bands near the Fermi level more distinct. The hybridization between the distinctly flat bands and other itinerant bands offers a possible way to form heavy fermion behavior in ferromagnets. The complex interactions of competing orders drive correlated magnetic metals to a new frontier for discovering outstanding quantum states.