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 ${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2$ 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 Néel 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.

Figure 1

(a) Crystal structure of transition metal ($M$) dihalides $\mathrm{Co}{\mathrm{Cl}}_2$ and $\mathrm{Ni}{\mathrm{Cl}}_2$ from side and top views. $M$ atoms are shown as gray spheres, Cl atoms are shown as green spheres, and the unit cell is indicated by a dotted line. (b) Scanning electron microscopy (SEM)/energy-dispersive x-ray spectroscopy (EDS) images and mapping of ${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2$, where $x=0.25$. (c) Optical images of cobalt-nickel dihalides as the Ni composition ($x$) increases from 0 to 1. Further experimental details can be found in Appendices pp1, pp2, pp3.

Figure 2

Characterization of ${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2$. (a) X-ray diffraction results showing a linear shift in 2θ-$d$-spacing values as a function of $x$. (b) Raman spectrum. (c) Ultraviolet (UV)-visible (Vis) absorption spectra for a given compositional value ($x$) from $x=0$ (blue) to $x=1$ (yellow), normalized to the first peak. (d) The absorption spectra show the gradual shift of peak 1 (P1) as $x$ increases from 0 to 1. (e) Absorption spectra showing the reduction in absorption intensity of P3 as $x$ increases from 0 to 1. Further experimental details can be found in Appendices pp1 and pp3.

Figure 3

Magnetic characterization of ${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2$. (a) Susceptibility. (b) Inverse susceptibility with respect to temperature for different Ni concentrations ($x$). (c) Extracted Néel and Curie Weiss temperatures for a given composition value ($x$) in the $H_{\parallel }$ configuration. (d)–(f) In-plane and out-of-plane magnetization vs field measurements to show how the degree of magnetic anisotropy evolves with increasing Ni concentration. Note that both $\mathrm{Co}{\mathrm{Cl}}_2$ and $\mathrm{Ni}{\mathrm{Cl}}_2$ have in-plane moments.

Figure 4

Scanning electron microscopy (SEM)/energy-dispersive x-ray spectroscopy (EDS) images for $\mathrm{Co}{\mathrm{Cl}}_2$.

Figure 5

Scanning electron microscopy (SEM)/energy-dispersive x-ray spectroscopy (EDS) images for ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}{\mathrm{Cl}}_2$.

Figure 6

Scanning electron microscopy (SEM)/energy-dispersive x-ray spectroscopy (EDS) images for ${\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Cl}}_2$.

Figure 7

Scanning electron microscopy (SEM)/energy-dispersive x-ray spectroscopy (EDS) images for ${\mathrm{Co}}_{0.25}{\mathrm{Ni}}_{0.75}{\mathrm{Cl}}_2$.

Figure 8

Scanning electron microscopy (SEM)/energy-dispersive x-ray spectroscopy (EDS) images for $\mathrm{Ni}{\mathrm{Cl}}_2$.

Figure 9

Powder x-ray diffraction (XRD) spectra of $\mathrm{Ni}{\mathrm{Cl}}_2$.