Stoichiometry-Controlled Structural Order and Tunable Antiferromagnetism in $\mathrm{Fe}_{x}\mathrm{NbSe_2}$ ($0.05 \le x \le 0.38$)

TL;DR

The study addresses how Fe intercalation in NbSe$_2$ tunes structural order and magnetic ground states. The authors synthesize Fe$_x$NbSe$_2$ across $0.05 \le x \le 0.38$, verify composition by EDS, and characterize structure with XRD/LEED while probing magnetism and transport. They observe a non-monotonic magnetic phase sequence—paramagnetism, spin glass, antiferromagnetism, and back to spin glass—with a maximum Néel temperature of $T_{\mathrm{N}} = 175$ K at $x=0.25$, where a well-ordered $2a_0 × 2a_0$ Fe superlattice forms. The results tie the magnetic ground states to commensurate Fe superstructures, mediated by RKKY interactions and modulated by electron doping and chalcogen vacancies, providing a route to altermagnetic or switchable AFM behavior in van der Waals materials.

Abstract

Transition metal dichalcogenides (TMDs) enable magnetic property engineering via intercalation, but stoichiometry-structure-magnetism correlations remain poorly defined for Fe-intercalated $\mathrm{NbSe_2}$. Here, we report a systematic study of $\mathrm{Fe}_{x}\mathrm{NbSe_2}$ across an extended composition range $0.05 \le x \le 0.38$, synthesized via chemical vapor transport and verified by rigorous energy-dispersive X-ray spectroscopy (EDS) microanalysis. X-ray diffraction, magnetic, and transport measurements reveal an intrinsic correlation between Fe content, structural ordering, and magnetic ground states. With increasing $x$, the system undergoes a successive transition from paramagnetism to a spin-glass state, then to long-range antiferromagnetism (AFM), and ultimately to a reentrant spin-glass phase, with the transition temperatures exhibiting a non-monotonic dependence on Fe content. The maximum Néel temperature ($T_{\mathrm{N}}$ = $\mathrm{175K}$) and strongest AFM coupling occur at $x=0.25$, where Fe atoms form a well-ordered $2a_0 \times 2a_0 $ superlattice within van der Waals gaps. Beyond $x = 0.25$, the superlattice transforms or disorders, weakening Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions and reducing $T_{\mathrm{N}}$ significantly. Electrical transport exhibits distinct anomalies at magnetic transition temperatures, corroborating the magnetic state evolution. Our work extends the compositional boundary of Fe-intercalated $\mathrm{NbSe_2}$, establishes precise stoichiometry-structure-magnetism correlations, and identifies structural ordering as a key tuning parameter for AFM. These findings provide a quantitative framework for engineering altermagnetic or switchable antiferromagnetic states in van der Waals materials.

Figures (6)

• Figure 1: Structural and surface signatures of commensurate superstructures in Fe_xNbSe2.(a, d) Side views of the crystal structure for $x = 1/4$ and $x = 1/3$, respectively. (b, e) Top-down projections along the $c$ axis showing Fe atoms (orange) at ordered positions in Fe_1/4NbSe2 and Fe_1/3NbSe2, respectively. (c, f) Corresponding LEED patterns recorded at 105 eV and 115 eV, respectively. The red overlays denote the enlarged reciprocal-unit cells, directly confirming the real-space superstructures induced by Fe ordering in the van der Waals gaps.
• Figure 2: (a) Powder X-ray diffraction patterns of Fe_xNbSe2 ($0.05 \leq x \leq 0.38$) at room temperature. All samples crystallize in the hexagonal 2H-NbSe2 structure. (b) Magnified view of the (204) peak showing a systematic shift to lower $2\theta$ with increasing Fe content $x$. (c) Low-angle region displaying superlattice peaks arising from the $2a_0 \times 2a_0$ (stars) and $\sqrt{3}a_0 \times \sqrt{3}a_0$ (inverted triangles) superstructure.
• Figure 3: Evolution of the lattice parameters of Fe_xNbSe2 with Fe content $x$ ($0 \leq x \leq 0.38$), obtained from Rietveld refinement of powder XRD data.
• Figure 4: Temperature-dependent magnetic susceptibility $\chi(T)$ of Fe_xNbSe2 under a magnetic field of 1 T. (a) paramagnetic behavior at low Fe content ($x \leq 0.10$); (b) spin-glass freezing for $x = 0.15$–0.18; (c) antiferromagnetic ordering at intermediate concentrations ($x = 0.20$–0.33); and (d) reentrant spin-glass state at high doping ($x = 0.38$). ZFC and FC data are shown for selected Fe concentrations $x$.
• Figure 5: Magnetic phase diagram of Fe_xNbSe2 as a function of Fe concentration $x$. Squares denote the magnetic transition temperature $T_{\mathrm{tr}}$ from susceptibility data; circles show the Weiss temperature $\theta_{\mathrm{CW}}$ from Curie–Weiss fits. The system evolves from paramagnetic (PM) to spin-glass (SG), antiferromagnetic (AFM), and back to SG with increasing $x$.