Spectral evidence for local-moment ferromagnetism in the van der Waals metals Fe3GaTe2 and Fe3GeTe2

Magnetism in two-dimensional (2D) materials has attracted considerable attention recently for both fundamental understanding of magnetism and its tunability towards device applications. The isostructural ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ and ${\mathrm{Fe}}_{3}{\mathrm{GaTe}}_{2}$ are two members of the Fe-based van der Waals (vdW) ferromagnet family, but exhibit very different Curie temperatures (${T}_{C}$) of 210 and 360 K, respectively. Here, by using angle-resolved photoemission spectroscopy and density functional theory, we systematically compare the electronic structures of the two compounds. Qualitative similarities in the Fermi surface can be found between the two compounds, with expanded hole pockets in ${\mathrm{Fe}}_{3}{\mathrm{GaTe}}_{2}$ suggesting additional hole carriers compared to ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$. Interestingly, we observe almost no band shift in ${\mathrm{Fe}}_{3}{\mathrm{GaTe}}_{2}$ across its ${T}_{C}$ of 360 K, compared to a small shift in ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ across its ${T}_{C}$ of 210 K. The weak temperature-dependent evolution strongly deviates from the expectations of an itinerant Stoner mechanism. Our results suggest that itinerant electrons have minimal contributions to the enhancement of ${T}_{C}$ in ${\mathrm{Fe}}_{3}{\mathrm{GaTe}}_{2}$ compared to ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$, and that the nature of ferromagnetism in these Fe-based vdW ferromagnets must be understood with considerations of the electron correlations.