X-ray spectroscopy for the magnetic study of the van der Waals ferromagnet CrSiTe₃ in the few- and monolayer limit

Abstract

The study of magnetic order in few- and monolayer van der Waals materials poses a challenge to the most commonly employed magnetic characterization techniques as they normally lack magnetic sensitivity and/or lateral resolution enabling their thickness-dependent probing. Here we demonstrate the usefulness of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr L 2,3 and Te M 5 edges, for the study of the ferromagnetic semiconductor CrSiTe3 (CST) in the form of single- and few-layer flakes. By scanning the sample under the incident X-ray beam, a map of the exfoliated system was obtained, which reproduced the optical micrographs showing the detailed distribution and thicknesses of the flakes. In this way, XAS/XMCD was performed at selected sample areas, revealing the thickness-resolved spectroscopic and magnetic properties of the flakes, such as the spin and orbital magnetic moments. The spin moment, in line with the saturation field, is decreasing with film thickness, revealing a single-domain and out-of-plane magnetization for the thinnest films. For CST, the electronic properties are governed by the strong covalent bond between the Cr 3d(eg) and Te 5p states, giving rise to a superexchange scenario. We observed a gradually increasing ratio of orbital to spin moment for thinner flakes, which could be due to a further increase of the covalent mixing. Hysteresis loops were recorded at the Cr L3 edge, showing an open loop for 10 down to ∼3 layers, while the bulk shows a wasp-waist shaped loop. With the transition temperature from the soft to the hard ferromagnetic state decreasing with thickness, the monolayer shows a narrowed, closed loop at 10 K, suggesting its transition temperature >10 K. Our study demonstrates the unique capabilities of XAS/XMCD for the study of few-layer van der Waals magnets, correlation and ferromagnetism in CST.

Surface‐Sensitive Detection of Magnetic Phase Transition in Van Der Waals Magnet CrSBr

“…Figure 4b exhibits the Cr L 2,3 XMCD spectra with left-and right-circular-polarized X-rays at 78 K, clearly showing that the intrinsic ferromagnetism resulted from Cr atoms instead of any possible magnetic impurities. The XMCD spectra were similar to those of CrGeTe 3 , [40] CrSiTe 3 , [41,42,43] and Cr 2 Ge 2 Te 6 , [45] which showed a pronounced multiplet structure with sharp peaks. We also noticed the presence of a nonzero XMCD signal pointed by the red arrow in Figure 4b, which also appeared in the XMCD results of CrGeTe 3 [40] and CrSiTe 3 , [41,42,43] and was attributed to the Te 5p spin polarization due to mixing with the Cr e g bonding states.…”

“…The XMCD spectra were similar to those of CrGeTe 3 , [40] CrSiTe 3 , [41,42,43] and Cr 2 Ge 2 Te 6 , [45] which showed a pronounced multiplet structure with sharp peaks. We also noticed the presence of a nonzero XMCD signal pointed by the red arrow in Figure 4b, which also appeared in the XMCD results of CrGeTe 3 [40] and CrSiTe 3 , [41,42,43] and was attributed to the Te 5p spin polarization due to mixing with the Cr e g bonding states. We speculate that this feature might indicate a hybridization of S 3p or Br 4p electrons with Cr 3d electrons, [50,51] which leads to the halogen-mediated (Cr─Br─Cr) and chalcogen-mediated (Cr─S─Cr) strong intraplanar superexchange interactions.…”

“…For example, the electron escape length is estimated to be only 1.7-2.5 nm for metallic FM Fe, Co, and Ni, [35] 0.4-1.4 nm for compound semiconductors, [36] and relatively larger values for insulators, such as 4.5 nm for Fe 3 O 4 , [37] and 5.5 nm for NiO. [38] In recent XAS studies of 2D vdW magnetic materials, [39][40][41][42][43][44][45] the probing length was considered to be in the range of 3-5 nm. The techniques of X-ray magnetic linear dichroism (XMLD) and X-ray magnetic circular dichroism (XMCD) are developed based on XAS measurements, thus possessing a surface-sensitive capability of detecting AFM and FM magnetic orders.…”

Two-dimensional chalcogenide-based ferromagnetic semiconductors

“…The atomically thin CrSiTe 3 flakes was reported to occur a transition from the soft to the hard FM state as the thickness of samples decreases down to several nanometers, accompanied by a drop of the T c from 33 K for bulk to ∼17 K for the 4 nm sample [159]. This phenomenon is also evidenced by x-ray absorption spectroscopy and x-ray magnetic circular dichroism [92].…”

“…The 2D magnetism can be controlled by gate voltage [55]. Compared to CrGeTe 3 , CrSiTe 3 has strong electronic correlations at both high-and low-temperature regimes, significantly weakens the interlayer FM coupling [56,92]. The CrGeTe 3 and CrSiTe 3 are FM semiconductors that have been experimentally obtained, and more TMAX 3 semiconductors with atomiclayer thickness are predicted to be FM, leaving for experimental verification.…”