Tunability of the magnetic properties in Ni intercalated transition metal dichalcogenide NbSe$_2$

TL;DR

Ni intercalation in NbSe2 creates a tunable magnetic landscape on a triangular Ni lattice, transitioning between stripe AFM and FM states as the Coulomb interaction and spin-orbit coupling vary. The study maps NN exchange couplings $J_{\parallel}$ and $J_{\perp}$ via DFT+U, revealing a strong out-of-plane coupling and a smaller in-plane interaction, with a $U_C$-driven FM↔AFM2 ground-state switch that SOC lowers to about $1.1$ eV. SOC further imposes a $z$-axis easy axis and modifies Fermiology, shifting the Van Hove singularity away from the Fermi level and generating Γ-point electron pockets, thereby suppressing potential instabilities while preserving metallicity. In the broader T$_{0.25}$MX$_2$ class, altermagnetism coexists with weak ferromagnetism or ferrimagnetism depending on Néel-vector orientation, accompanied by sizable orbital moments and an AHE across orientations. Overall, Ni$_{0.25}$NbSe$_2$ provides a versatile platform to study and tune competing magnetic phases near degeneracy, with clear implications for spintronic control and correlated electronic behavior in intercalated TMDs.

Abstract

We study the magnetic and electronic properties of Ni-intercalated NbSe$_2$.We calculate the magnetic exchanges of Ni$_x$NbSe$_2$ ($x = 1/3, 1/4,$ and $1$) and find that the out-of-plane magnetic coupling depends on the Ni connectivity: it is ferromagnetic when Ni atoms stack on top of each other, and antiferromagnetic otherwise. Focusing on Ni$_{0.25}$NbSe$_2$, we identify a ground-state transition from a stripe antiferromagnetic phase with Kramers degeneracy to a ferromagnetic phase above a critical Coulomb interaction U$_C$. Spin--orbit coupling lowers U$_C$, aligns the easy axis along $z$, and stabilizes collinear AFM and FM states over the competing 120$^\circ$ phase. Ni intercalation also strongly modifies the electronic structure, replacing the $Γ$-point hole pocket of pristine NbSe$_2$ with an electron pocket and shifting the Van Hove singularity away from the Fermi level, thereby suppressing potential instabilities. Finally, we investigate the altermagnetic phase in the broader class T$_{0.25}$MX$_2$, finding that spin--orbit effects induce orbital antiferromagnetism with weak ferromagnetism or ferrimagnetism depending on the Néel vector orientation. Our results demonstrate that Ni-intercalated NbSe$_2$ provides a versatile platform to explore and tune multiple competing magnetic phases that lie close in energy.

Figures (16)

• Figure 1: Four different possible magnetic structures of Ni$_{0.25}$NbSe$_2$. The magnetic configuration that breaks time-reversal symmetries are the ferromagnetic (FM) in panel (a) and the altermagnetic (AM) in panel (b). Once we double the supercell, we obtain two antiferromagnetic phases that we define as AFM1 in panel (c) and AFM2 in panel (d). The red (blue) arrows represent the atoms with the majority of spin-up (spin-down). The Ni, Nb and Se atoms are represented by grey, green and yellow balls, respectively. The NbSe$_6$ octahedra are in transparent green. The circular arrows in (a) represent the nearest-neighbor exchange interactions $J_{\parallel}$ and $J_{\perp}$.
• Figure 2: Energy difference of AM, AFM1, AFM2 configurations relative to FM configuration per Ni as a function of U ranging from 0 to 3 eV. These data refer to the calculations performed without SOC. The energy difference of the NM configuration is out of the scale, since it reaches the values of 98 and 188 meV for U=2 eV and 3 eV, respectively.
• Figure 3: The out-of-plane magnetic coupling J$_{||}$ has different connectivity in (a) Ni$_{1/3}$NbS$_2$ with respect to (b) Ni$_{0.25}$NbSe$_2$. Due to this different connectivity, the coupling of Ni$_{1/3}$NbS$_2$ is antiferromagnetic, while the coupling of Ni$_{0.25}$NbSe$_2$ is ferromagnetic. (c) Side and (d) top view of NiNbSe$_2$. Red and blue arrows represent the spin-up and spin-down sites. We highlight the C$_2$ rototranslational symmetry that connects the spin-up and spin-down states, generating alternagnetism due to the absence of a translational symmetry. The C$_2$ spin rotation is represented by the orange arrow. Ni, Nb, and chalcogen atoms are represented by gray, green and yellow atoms, respectively.
• Figure 4: Brillouin zone for the space group no.194 and no.4 of Ni$_{0.25}$NbSe$_2$. Left panel: Brillouin zone of the unit cell with space group no. 194. Right panel: Brillouin zone of the supercell with space group no. 4. The high-symmetry points E and D are not equivalent because of the doubling of the unit cell along one direction.
• Figure 5: Electronic band structure of Ni$_{0.25}$NbSe$_2$ for different magnetic states: (a) NM (U=0), (b) AM (U=2 eV), (c) FM (U=3 eV), (d) AFM1 (U=2 eV), and (e) AFM2 (U=2 eV). Red/blue denote spin-up/down (merging into purple under Kramers' degeneracy). Left: zoom near $\Gamma$; right: full $k$-path. AFM2 and FM are shown at their ground-state $U$ values.