Magnetism and Optical Anisotropy in van der Waals Antiferromagnetic Insulator CrOCl

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.

van der Waals Magnets: Material Family, Detection and Modulation of Magnetism, and Perspective in Spintronics

“…Reproduced with permission. [ 93 ] Copyright 2019, American Chemical Society. c) The temperature‐dependent magnetic susceptibility measured on chemically exfoliated FeOCl nanoflakes using the field‐cooling mode (blue, magnetic field: 1T) and zero‐field‐cooling mode (orange), and the inset shows the field‐dependent magnetization measured at 2 and 300 K. Reproduced with permission.…”

“…[ 92 ] Strong spin‐phonon coupling in few‐layer CrOCl is revealed by the phonon anomaly at around 27 K from the temperature‐dependent Raman spectra (Figure 4b). [ 93 ] Besides, CrOCl presents optical linear dichroism and ambient stability at atomic scale. In addition, the T C of monolayer CrOCl is largely increased by isoelectronic atomic substitution, which is indicated by the theoretical calculations.…”

Strain engineering of two‐dimensional materials: Methods, properties, and applications

“…Recently, 2D materials have been proved to exhibit various interesting nonlinear optical properties, including second‐harmonic generation (SHG) and third‐harmonic generation (THG). SHG is a nonlinear optical process described by the second‐order susceptibility tensor, usually observed in materials with broken inversion symmetry, such as monolayer or odd‐layer transition metal dichalcogenides (TMDs), 96 and CrOCl 97 . SHG is sensitive to the lattice symmetry of materials, and the intensity can be greatly influenced by strain because strain will distort the lattice, thus changing the optical susceptibility of 2D materials.…”

Theoretical simulation and design of two-dimensional ferromagnetic materials

“…Cr 2 O 3 单层在双轴拉伸应变下具有较大的面 外MAE, 其基于伊辛模型的T C 理论值高达332 K [68] . 值 得注意的是, 最近对范德华层状块体CrOCl和CrSBr进 行的重复实验研究表明, 它们在空气中能够稳定存在, 且剥离成原子层厚度的可能性很大 [69,70] .…”