High-Temperature ferromagnetism from site-selective filling in (Fe,Ni)$_{6-\delta}$GeTe$_2$

Tyler L. Werner; Jonathan T. Reichanadter; Xiang Chen; Pranab K. Nag; Luna Y. Liu; Yu-Tsun Shao; Hongrui Zhang; Mingyang Guo; Wenxin Li; Zhibo Kang; Han Wu; Makoto Hashimoto; Donghui Lu; Turgut Yilmaz; Elio Vescovo; Sung-Kwan Mo; Barat Achinuq; Alexei Fedorov; Jacob C. Ruff; Ming Yi; Qiong Ma; David A. Muller; Eduardo H. da Silva Neto; Robert J. Birgeneau; Jeffrey B. Neaton; Yu He

arXiv · 2609.11862

High-Temperature ferromagnetism from site-selective filling in (Fe,Ni)$_{6-\delta}$GeTe$_2$

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Abstract

The discovery of high-temperature ferromagnetism in the metallic van der Waals (vdW) system Fe$_N$GeTe$_2$ has brought two-dimensional (2D) magnets into technologically relevant temperature scales. Specifically at N = 5, dilution of magnetic moments by nickel substitution counterintuitively achieves a record high Curie temperature of 478~K. Unraveling the origin of this nickel-substitution-induced enhancement is complicated by the compound's structural complexity, coexistent itinerant and local magnetic contributions, and mesoscopic compositional domains. Through coordinated structural and electronic characterization, we identify that the high-T$_C$ magnetic phase arises from a strain-stabilized Fe$_6$GeTe$_2$ nano-precipitate. Combining first-principles calculations and spin- and angle-resolved photoemission spectroscopy (ARPES), we uncover a site-specific electronic landscape in which interior iron atoms primarily host localized moments while the outer iron atoms neighboring the tellurium layers produce spin-polarized itinerant carriers that cross the vdW gap. The large energy cost associated with homogeneous nickel substitution is found to favor the spontaneous precipitation of the crystallographically and electronically ``clean'' high-T$_C$ phase. Finally, we compare metal-rich vdW magnets with binary magnetic alloys, and discuss the unifying roles of nano-precipitates in stabilizing otherwise unattainable bulk phases. Our work provides mechanistic insights into the record-high T$_C$ ferromagnetism in (Fe,Ni)$_{5+\delta}$GeTe$_2$, establishing a rigorous foundation for the atomic engineering of vdW magnetic metals informed by direct electronic signatures.

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Tyler L. Werner, Jonathan T. Reichanadter, Xiang Chen, Pranab K. Nag, Luna Y. Liu, Yu-Tsun Shao, Hongrui Zhang, Mingyang Guo, Wenxin Li, Zhibo Kang, Han Wu, Makoto Hashimoto, Donghui Lu, Turgut Yilmaz, Elio Vescovo, Sung-Kwan Mo, Barat Achinuq, Alexei Fedorov, Jacob C. Ruff, Ming Yi, Qiong Ma, David A. Muller, Eduardo H. da Silva Neto, Robert J. Birgeneau, Jeffrey B. Neaton, Yu He. 2026-09-10. High-Temperature ferromagnetism from site-selective filling in (Fe,Ni)$_{6-\delta}$GeTe$_2$. https://arxiv.org/abs/2609.11862

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