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

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 $\Gamma$-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 N\'eel 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.