Magnetism and Optical Anisotropy in van der Waals Antiferromagnetic Insulator CrOCl

TLDR

Van der Waals magnetic insulators are important for research and technology, yet most 2D systems are unstable and highly symmetric. The study aims to develop stable, anisotropic 2D vdW magnetic insulators to enable property modulation and new applications. The authors synthesize the orthorhombic antiferromagnet CrOCl and systematically study its magnetism, phase transitions, and optical anisotropy. Raman spin‑phonon coupling indicates a magnetic superstructure, a structural transition near 27 K is seen from interplanar spacing changes, and pronounced in‑plane vibrational, reflective, and absorptive anisotropies reveal strong polarization sensitivity and linear dichroism, while second‑harmonic generation shows high polarization dependence with a nonlinear susceptibility of ~2.24×10⁻¹¹ m/V, positioning CrOCl as a promising material for polarization‑dependent nonlinear optics and spin‑dependent electronic/optoelectronic devices.

Abstract

van der Waals (vdW) magnetic insulators are of significance in both fundamental research and technological application, but most two-dimensional (2D) vdW magnetic systems are unstable and of high lattice symmetry. Stable 2D vdW magnetic insulators with anisotropic structure are needed to modulate the properties and unlock potential applications. Here we present a stable vdW antiferromagnetic material, CrOCl, with low-symmetry orthorhombic structure, and investigate systematically its magnetism, phase transition behavior, and optical anisotropy. Spin-phonon coupling effects are uncovered by the abnormal frequency shifts of Raman-active modes, suggesting the formation of a magnetic superstructure. The sizable abnormal change of interplanar spacing indicates the presence of a structural transition at around 27 K. Further in-plane vibrational, reflectional, and absorptional anisotropic properties are explored both experimentally and theoretically, revealing a highly polarization sensitive characteristic and linear dichroism in 2D CrOCl. Meanwhile, the particularly high polarization dependency of the second-harmonic generation and the nonlinear susceptibility of ∼2.24 × 10-11 m/V make it suitable in the field of polarization-dependent nonlinear optics. The findings on the intricate properties of 2D CrOCl lay foundations for future applications of low-symmetry vdW magnets in spin-dependent electronic and optoelectronic devices.