Tunable magnetic and optical properties of transition metal dihalides by cation alloying

Alloying has been a tradition in materials science that has enabled groundbreaking discoveries in semiconductor technologies, optics, and photovoltaics, among others. While alloying in traditional systems is relatively well established, the effects of alloying in the emerging van der Waals (vdW) two-dimensional (2D) magnets are still in their infancy. Using ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{Cl}}_{2}$ as a testbed system, our results show that chemical vapor transport of stoichiometric mixtures of $\mathrm{Te}{\mathrm{Cl}}_{4}$, Co, and Ni enables the synthesis of highly crystalline vdW magnetic alloys with excellent control over the Ni concentration ($x$) without any tellurium impurities or phase separation. The method is advantageous compared to binary $\mathrm{Co}{\mathrm{Cl}}_{2}$ and $\mathrm{Ni}{\mathrm{Cl}}_{2}$ precursor mixtures which only produce small-sized crystals with a large compositional variation. Magnetic measurements show that the degree of magnetic anisotropy, Weiss temperature, and N\'eel temperature (${T}_{\mathrm{N}}$) strongly correlate to the Ni concentration, offering a tune-knob to engineer the magnetic behavior of transition metal dihalides. First-principles calculations offer further insights into how the increasing Ni content influences the interlayer and intralayer magnetic couplings and the resulting magnetic response. Overall, our findings provide an important avenue toward metal cation alloying in dihalide 2D vdW magnets and offer means to tune their magnetic behavior on demand.