Carrier-tunable RKKY magnetism in a crystalline magnet

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 $2\times 2\mathrm{R}{0}^{\circ }$ 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.

12 pages, 5 figures; Supplemental Material: 12 pages, 11 figures, 1 table