Controlling the Magnetic Properties of the van der Waals Multiferroic Crystals Co$_{1-x}$Ni$_{x}$I$_2$

TL;DR

This study explores how chemical substitution in Co$_{1-x}$Ni$_{x}$I$_2$ crystals allows precise tuning of their structural and magnetic properties, revealing a continuous evolution and phase transition in a multiferroic van der Waals system.

Contribution

It demonstrates the successful synthesis and characterization of the entire Co$_{1-x}$Ni$_{x}$I$_2$ solid solution, revealing structural transitions and magnetic phase diagram evolution.

Findings

Crystal symmetry changes from $P\bar{3}m1$ to $R\bar{3}m$ beyond $x=0.2$

Magnetic properties evolve smoothly with composition

Metamagnetic transition persists across the solid solution

Abstract

The structurally related compounds NiI$_2$ and CoI$_2$ are multiferroic van der Waals materials, in which helimagnetic orders exist simultaneously with electric polarization. Here, we report on the evolution of the crystal structure and of the magnetic properties across the solid solution Co$_{1-x}$Ni$_{x}$I$_2$. We have successfully grown crystals of the whole range of the solid solution, i.e. $x = 0-1$, by employing the self-selecting vapor growth (SSVG) technique and by carefully tuning the synthesis conditions according to the chemical composition. Our structural investigations show that the crystal symmetry changes from $P\bar{3}m1$ to $R\bar{3}m$ when Ni substitutes for Co beyond $x = 0.2$. Both the lattice parameters and magnetic properties evolve continuously and smoothly from one end member to the other, showing that they can be finely tuned by the chemical composition. We also observe that the Ni substitution degree in the solid solution affects the metamagnetic transition typical for CoI$_2$ at high magnetic fields. In particular, we find the existence of the metamagnetic transition similar to that for CoI$_2$ in the NiI$_2$ structure. Based on magnetic measurements we construct the phase diagram of the Co$_{1-x}$Ni$_{x}$I$_2$ system. Controlling the magnetic properties by the chemical composition may open new pathways for the fabrication of electronic devices made of two-dimensional (2D) flakes of multiferroic van der Waals materials.