Enhancement of the Magnetic Coupling in Exfoliated CrCl₃Crystals Observed by Low‐Temperature Magnetic Force Microscopy and X‐ray Magnetic Circular Dichroism

Abstract

Magnetic crystals formed by 2D layers interacting by weak van der Waals forces are currently a hot research topic. When these crystals are thinned to nanometric size, they can manifest strikingly different magnetic behavior compared to the bulk form. This can be the result of, for example, quantum electronic confinement effects, the presence of defects, or pinning of the crystallographic structure in metastable phases induced by the exfoliation process. In this work, an investigation of the magnetism of micromechanically cleaved CrCl3 flakes with thickness >10 nm is performed. These flakes are characterized by superconducting quantum interference device magnetometry, surface‐sensitive X‐ray magnetic circular dichroism, and spatially resolved magnetic force microscopy. The results highlight an enhancement of the CrCl3 antiferromagnetic interlayer interaction that appears to be independent of the flake size when the thickness is tens of nanometers. The estimated exchange field is 9 kOe, representing an increase of ≈900% compared to the one of the bulk crystals. This effect can be attributed to the pinning of the high‐temperature monoclinic structure, as recently suggested by polarized Raman spectroscopy investigations in thin (8–35 nm) CrCl3 flakes.

Emerging oxidized and defective phases in low-dimensional CrCl3

“…17 Low-resolution TEM images (Figure 1b and 1c) of an air-exposed CrCl 3 flake confirm the lamellar structure of CrCl 3 , with a few- layer (estimated to be composed of 5-8 layers) contour region of lateral dimension of 100-200 nm, while the inner region can be identified as "bulk-like" CrCl 3 flake with a thickness in a range of 5-10 nm. 16 As revealed by EDX maps of O K α , Cl K α , and Cr K α (Figure 1d,e, and f), apart from a ubiquitous presence of carbon (C K α map is not reported), the outer few-layer contour of the flake is characterized by the presence of Cr and O, while Cl is residual. This is confirmed by Figure 1g, where the EDX signals are measured along the yellow line scan in the TEM image crossing the bulk/contour (few-layer/substrate grid) region.…”

“…9 The research interest has primarily focused on few-layer CrI 3 , 10 but its rapid degradation upon air exposure, 11 which limits device fabrication, moved the attention also towards CrCl 3 . 8,[12][13][14][15][16] Interestingly, CrCl 3 can be experimentally studied even in reduced dimensions, as it easily exfoliates in large flakes down to the monolayer limit, starting from bulk material and remaining stable at ambient conditions without rapid degradation. 17 This enables real applications and multiple experimental investigations of CrCl 3 in reduced dimensions.…”

Determination of the refractive index and wavelength‐dependent optical properties of few‐layer CrCl within the Fresnel formalism

“…CrCl crystals were grown and characterized as in Ref. 26 Experimental details of the preparation of the exfoliated samples can be taken from a previously published work 25 . In particular CrCl showed to have longer degradation times of other tri‐halides, another reason of interest for its investigation 25 .…”

Massless Dirac magnons in the two dimensional van der Waals honeycomb magnet CrCl3

“…2(a), right axis]. We consider it to be the better estimate of T C than the maximum of the first derivative, which is located at a lower temperature by ∼ 2 K [36].…”