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arXiv · 2608.10574

Carrier-tunable RKKY magnetism in a crystalline magnet

Xiang S. W. Huang·Bruno K. Saika·Satoshi Hamao·Yuki Majima·Yuki Settai·Hideki Matsuoka·Yuki M. Itahashi·Masato Sakano·Taro Nakajima·Shinichiro Seki·Yoshihiro Iwasa·Kyoko Ishizaka·Masaki Nakano
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Abstract

In itinerant magnets governed by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, the exchange coupling depends on both the moment-moment distance $r$ and the Fermi wavevector $k_{\mathrm{F}}$, yet in bulk synthesis the two are tightly coupled: a change in composition typically alters both. Here, we use a thin-film approach to tune these two variables independently in a single crystalline host. Using molecular-beam epitaxy (MBE), we stabilize either $2\times2\mathrm{R}0^\circ$ Cr$_{1/4}$NbSe$_2$ or $\sqrt{3}\times\sqrt{3}\mathrm{R}30^\circ$ Cr$_{1/3}$NbSe$_2$ within the same NbSe$_2$ host through separate growth windows. Controlled post-growth annealing performed across a series of temperatures then modifies the carrier density while leaving the Cr superstructure intact below a structural-transition threshold. The two as-grown phases are distinct in electronic structure, magnetic ground state, and transport. Along this annealing series, the Hall response evolves systematically while the magnetic response changes in a structurally insensitive manner, with ferromagnetic order emerging within the same $\sqrt{3}\times\sqrt{3}\mathrm{R}30^\circ$ structural class only above a critical annealing temperature, experimentally disentangling carrier density and moment geometry. The Hall magnitude and sign evolution point to a low-carrier-density system in which $k_{\mathrm{F}}$ is susceptible to modest external tuning. Cr-NbSe$_2$ thus realizes carrier-sensitive RKKY magnetism in a single crystalline host, within an MBE-plus-annealing approach extensible across the intercalated transition-metal dichalcogenide family.

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Xiang S. W. Huang, Bruno K. Saika, Satoshi Hamao, Yuki Majima, Yuki Settai, Hideki Matsuoka, Yuki M. Itahashi, Masato Sakano, Taro Nakajima, Shinichiro Seki, Yoshihiro Iwasa, Kyoko Ishizaka, Masaki Nakano. 2026-08-11. Carrier-tunable RKKY magnetism in a crystalline magnet. https://arxiv.org/abs/2608.10574

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