Observation of linear magnetoelectric effect in the van der Waals antiferromagnets N i 5 ( Te O 3 ) 4 X 2 ( X = Cl , Br )

Realization of linear magnetoelectric (ME) effect in single-phase van der Waals (vdW) compounds, combined with magnetic (spin), electric dipole, and dimensions degrees of freedom, could lead to new applications, such as high-density multistate data storage. Although the linear ME effect has been broadly studied for decades, vdW materials with an intrinsic ME effect have been rarely explored so far. Using a recently proposed design principle of two-dimensional vdW materials, transition metal tellurate halides, $\mathrm{N}{\mathrm{i}}₅{(\mathrm{Te}{\mathrm{O}}₃)}₄{X}₂ (X=\mathrm{Cl},\mathrm{Br})$ with linear ME coupling, have been synthesized and remarkable ME effect is demonstrated here. The onset of simultaneous anomalies in magnetic susceptibility and specific heat suggests the noncollinear antiferromagnetic order with the N\'eel temperature ${T}_{\mathrm{N}}\ensuremath{\sim}30$ K. In addition, a clear spin-flop transition appears below ${T}_{\mathrm{N}}$, driven by magnetic field along the $x$ axis, indicating a strong magnetic anisotropy. Associated with the antiferromagnetic transition, a shape anomaly in the displacement current implies the appearance of ME response below ${T}_{\mathrm{N}}$ with the ME coefficient ${\ensuremath{\alpha}}_xy$ up to 4.2 ps/m in $\mathrm{N}{\mathrm{i}}₅{(\mathrm{Te}{\mathrm{O}}₃)}₄\mathrm{C}{\mathrm{l}}₂$. This is evidenced by the evolution of electric polarization (${P}_x$) with magnetic field ($H$//$y$) along the $y$ axis at selected temperatures. The present study suggests that $\mathrm{N}{\mathrm{i}}₅{(\mathrm{Te}{\mathrm{O}}₃)}₄{X}₂$ is an unusual class of vdW materials hosting the linear ME behaviors, and thus an attractive platform for investigating the complicated interactions among spin order, electric polarization, and van der Waals force in layered vdW compounds.