# In situ growth and magnetic characterization of Cr chloride monolayers

Giuseppe Buccoliero, Pamella Vasconcelos Borges Pinho, Marli dos Reis Cantarino, Francesco Rosa, Nicholas B Brookes, Roberto Sant

► To cite this version:

Giuseppe Buccoliero, Pamella Vasconcelos Borges Pinho, Marli dos Reis Cantarino, Francesco Rosa, Nicholas B Brookes, et al.. In situ growth and magnetic characterization of Cr chloride monolayers. Advanced Materials Interfaces, 2025, 12, pp.e00229. 10.1002/admi.202500229. hal-05443272

HAL Id: hal-05443272

https://hal.science/hal-05443272v1

Submitted on 6 Jan 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# In Situ Growth and Magnetic Characterization of Cr Chloride Monolayers

Giuseppe Buccoliero, Pamella Vasconcelos Borges Pinho, Marli dos Reis Cantarino, Francesco Rosa, Nicholas B. Brookes, and Roberto Sant*

Monolayer chromium dihalides and trihalides materials can be grown on a variety of substrates by molecular beam epitaxy regardless of the lattice mismatch thanks to the van der Waals epitaxy. In this work, the magnetic nature of Cr chloride monolayers are studied, grown on Au(111), Ni(111), and graphene-passivated Ni(111) from the evaporation in ultra-high vacuum of the same halide precursor. Structural, morphological, and magnetic characterizations are conducted in situ by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and X-ray magnetic circular dichroism (XMCD). Owing to opposite chemical behavior, Au(111) and Ni(111) promote the formation of two different valence compounds, i.e., $CrCl_3$ and $CrCl_2$, showing distinct magnetic properties at 4 K. When graphene is used to passivate the Ni(111) surface, the formation of $CrCl_3$ becomes allowed also on this substrate. The coexistence of $CrCl_3$ and $CrCl_2$, both showing few nm lateral size is demonstrated by XMCD spectra displaying two dichroic peaks at the characteristic $Cr^{3+}$ and $Cr^{2+}$ energies. *Site-selective magnetization measurements performed with the photon energy tuned on the two absorption edges show equivalent magnetic behavior of the two compounds, which is attributed to reduced cluster lateral size and superparamagnetic behavior.*

G. Buccoliero, P. Vasconcelos Borges Pinho, M. dos Reis Cantarino, N. B. Brookes, R. Sant

ESRF

The European Synchrotron

71 Avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9 Grenoble, France

E-mail: roberto.sant@polimi.it

P. Vasconcelos Borges Pinho

Université Paris-Saclay

CEA

Service de Recherche en Corrosion et Comportement des Matériaux

F-91191 Gif-sur-Yvette, France

F. Rosa, R. Sant

Dipartimento di Fisica

Politecnico di Milano

Piazza Leonardo da Vinci 32, I-20133 Milano, Italy

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202500229

© 2025 The Author(s). Advanced Materials Interfaces published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/admi.202500229

## 1. Introduction

Chromium dihalides ($CrX_2$, X = Cl, Br, I) and trihalides ($CrX_3$) van der Waals (vdW) materials exhibit intriguing layered magnetic order with different spin arrangements within the layer units. These configurations are governed by the competition between the antiferromagnetic (AFM) direct exchange among nearest-neighbours metal atoms and the ferromagnetic (FM) super-exchange between second neighbours mediated by halogen *p* states.[1,2] In $CrX_2$ compounds the metal cations are arranged in triangular nets and the materials develop AFM properties. When in-plane Jahn-Teller distortions are active, edge-sharing octahedra form ribbons that are characterized by helimagnetic spin order.[3,4] On the other hand, in $CrX_3$, the transition metal cations form honeycomb lattices where the in-plane interactions are of FM type.[5,6] Layer stacking gives rise in some cases to AFM interlayer coupling and overall AFM behavior, as for instance in $CrCl_3$.[7]

Single layers (SL) halides can be used as building blocks to incorporate magnetism into functional vdW 2D heterostructures, with promising applications in spintronics,[8] magnonics,[9] and spin-optoelectronics.[10,11] This can be achieved through their integration in vertical and lateral stackings with other 2D vdW materials,[12,13] on top of graphene,[10] or in twisted moiré superlattices.[14] The vdW nature of the interface minimizes effects such as hybridization, charge doping and other substrate-driven crystalline effects.

To build up devices with sharp and clean interfaces and scale up fabrication, bottom-up growth methods based on epitaxy are required. The push to synthesize single layer epitaxial halides via molecular beam epitaxy (MBE) has been driven by the simplicity of the process, which consists for most cases[15] in the evaporation under ultra-high-vacuum (UHV) of the very same stoichiometric powder compound. In this context, a single crystal substrate provides a suitable platform where low dimensional magnetism can be examined. Inert and reactive, as well as magnetic and non-magnetic substrate surfaces can be tested for this purpose, while suitable recipes have to be developed to control the growth in favor of the formation of a single targeted species.
Nevertheless, the synthesis of multiple coexisting phases can be desirable to build 2D patterned lateral heterostructures.[13] The freestanding properties of epitaxial 2D layers might be drastically altered when grown on tridimensional non-vdW substrates due to hybridization via dangling bonds and charge transfer. In other cases, magnetic proximity effects can be engineered on purpose to tune the pristine properties. The substrate choice must always be done considering the chemical affinity toward the precursors, since it might be detrimental to the stabilization of the desired phases. In similar cases, the substrate can be passivated by other 2D materials, such as graphene or boron nitride, to hinder the reactivity.[16,17]

Hereafter, we demonstrate this scenario for the specific case of two monolayer Chromium Chloride compounds, i.e., CrX$_{3}$ and CrX$_{2}$, which are the MBE growth products of CrCl$_{3}$ powder evaporated on top of distinct single crystal metal substrates.

Bulk CrCl$_{3}$ is a vdW material consisting of FM layer units having in-plane long range magnetic order below 17 K. In the magnetic phase, bulk compounds have rhombohedral R-3 space group symmetry and the layers are stacked with AFM spin order, while the SL remains intrinsically FM. The individual layers of CrCl$_{3}$ are constituted by a Cr honeycomb net where Cr$^{3+}$ cations in S = 3/2 spin state are octahedrally coordinated by six Cl$^{-1}$ anions. Each Cl atom is shared between two Cr atoms and mediates the intra-layer FM super-exchange interaction.[7,15]

Also epitaxial monolayer CrCl$_{3}$ exhibits remarkably strong ferromagnetism below 13 K, as well as 2D-XY magnetic behavior, allowing the occurrence of a Berezinskii–Kosterlitz–Thouless (BKT) phase transition in a real 2D system.[15] Compared to other Chromium Trihalides, e.g., CrI$_{3}$, the relatively weaker spin-orbit coupling of CrCl$_{3}$ provide superior tunability for engineering its magnetic properties.[18]

On the other hand, early experimental works on bulk CrCl$_{2}$ compounds showed instead that this vdW compound possesses an orthorhombic unit cell and Pnnm space group symmetry.[19] Similarly to CrI$_{2}$, Cr atoms dimerize as a result of lattice deformation (Cr atoms are surrounded by a distorted Cl octahedron), forming in-plane chains along which spins have helical configuration.[3] The breaking of the threefold rotational symmetry is reported also for epitaxial monolayers and enables pseudo magnetic field in CrCl$_{2}$/graphene moiré with widely tunable twist angles.[20] However, an experimental study of the fundamental magnetic properties in epitaxial CrCl$_{2}$ monolayers is so far still lacking.

Although CrCl$_{3}$ monolayer was initially successfully synthesized on epitaxial graphene/SiC(0001),[15] direct synthesis of a monolayer CrCl$_{2}$ on the same substrate remains still challenging. In fact, while CrCl$_{3}$ can decompose into CrCl$_{2}$ under moderate thermal heating,[20–22] direct synthesis of a monolayer CrCl$_{2}$ by CrCl$_{3}$ annealing is hindered by the CrCl$_{3}$ desorption at slightly lower temperatures. This issue was first solved on graphene substrates by depositing a small amount of NbSe$_{2}$, which forms CrCl$_{2}$/NbSe$_{2}$ lateral heterojunctions, effectively suppressing CrCl$_{3}$ desorption during the subsequent decomposition reaction.[20]

In this work, we demonstrate that both Chloride species, CrCl$_{3}$ and CrCl$_{2}$, can be stabilized as isolated epitaxial monolayers exploiting different metal substrates, i.e., a non-magnetic and inert Au(111) and a magnetic but chemically reactive Ni(111).

The as grown monolayers are characterized by a combination of in situ techniques such as Low Energy Electron Diffraction (LEED), Scanning Tunneling Microscopy (STM), X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) in order to obtain complementary structural, morphological and spectroscopical information. In this context, a comprehensive study linking the spectroscopic information and the magnetic properties to structural and morphological characterizations is still missing in the literature, whereas it is of paramount importance 2D materials growths in view of device applications. We will show that Au(111) and Ni(111) promote the formation of Chloride species with different valence, i.e., CrCl$_{3}$ and CrCl$_{2}$, showing also distinct magnetic behaviors.

In order to prevent the reduction of the adsorbed molecules from Cr$^{3+}$ to Cr$^{2+}$, the Ni substrate was passivated by a single layer of graphene (SLG). SLG favors the partial formation of CrCl$_{3}$ domains in addition to CrCl$_{2}$ ones, both showing few nm lateral size and super-paramagnetic (SPM) features. The coexistence of the CrCl$_{2}$ and CrCl$_{3}$ monolayers is evidenced by XMCD spectra showing two dichroic peaks at the characteristic Cr$^{2+}$ and Cr$^{3+}$ absorption energies, and by site-selective magnetization measurements performed by tuning the probing photon energy to the two specific edges. This approach - allowed by the tunability of the synchrotron undulator source - revealed the equivalent behavior of magnetically heterogeneous species, which is attributed to reduced cluster lateral size and super-paramagnetism.

## 2. Results and Discussion

SL Cr Chlorides have been grown on Au(111), Ni(111), and graphene-passivated Ni(111), by evaporation from a Knudsen cell of an anhydrous CrCl$_{3}$ powder compound, as described in the *Experimental Section*. Morphology and structure of the as grown epitaxial 2D materials were first characterized in situ by LEED and STM in the same MBE setup and then transferred in UHV into the high-field-magnet (HFM) chamber connected with the synchrotron beam for the spectroscopic and magnetic characterization (XAS and XMCD). To assist in interpreting the acquired spectra, ligand field multiplet (LFM) calculations[23] were performed using the *Quanty* code.[24] Hereafter, we describe the results obtained for each type of sample.

### 2.1. CrCl$_{3}$/Au(111)

A clean herringbone reconstructed Au(111) single crystal surface was exposed to a CrCl$_{3}$ flux at 150°C, as described in the *Experimental Section*. The LEED pattern in **Figure 1a** reveals that the overlayer grows oriented along two epitaxial directions, i.e., aligned and 30° off the substrate crystallographic axes, forming commensurate (2 × 2) and incommensurate (2 × 2)R30° reconstructions, the second characterized by a significantly larger mobile spread. The estimated overlayer surface lattice constant is ~5.8 Å, close to the in-plane lattice parameter of bulk CrCl$_{3}$ (5.96 Å).[7] The two rings crossing the overlayer spots indicate poor orientational alignment of the as-grown domains compared to the substrate and point to a weak interface bonding. This is a typical signature of the vdW epitaxy, which allows the growth of 2D films
<figure><img src="image_2.png" /><figcaption>**Figure 1.** CrCl₃/Au(111). a) LEED pattern acquired at 77 eV. Yellow spots and vectors correspond to substrate reflections and unit cell, while pink and violet ones indicate (2 × 2) and (2 × 2)R30° CrCl₃ reconstructions; the white dashed circumferences highlight the ring signals crossing the overlayer spots; inset: same LEED image without guidelines. b) STM image (225 × 225 nm²) showing SL CrCl₃ on Au(111) surface; second layer clusters are also visible on top of the CrCl₃ domains. c,d) XAS and XMCD spectra taken around Cr L₃,₂ edges at 4 K, under 9 T magnetic field applied along the beam direction, and in NI; the RCP and LCP XAS intensities are normalized to the average; the XMCD intensity is expressed in percentage units relative to the average XAS. e) Magnetization curves measured with 577.0 eV photon energy (Cr³⁺ edge, indicated by the dashed line in (c) and (d)), at 4 K and in NI. f) Simulations of the experimental XAS and XMCD spectra shown in (c) and (d).</figcaption></figure>

on top of the substrate also in the presence of a significant lattice mismatch.[25,26]

The STM image in Figure 1b shows the morphology of the overlayer. The domains consist of patches of several hundreds of nm lateral size and irregular contours. Brighter features, probably second layer clusters, are also visible. No periodic intensity modulation was observed on the surface, e.g., moiré superstructures or other kinds of patterning, but flat areas which are reminiscent of the denominated “pristine phase” recorded for CrI₃ grown on Au(111).[21] The apparent height of ~3 Å (see Figure S5b, Supporting Information) is compatible with the thickness of Cl-Cr-Cl trilayers bonded to tridimensional substrates with no vdW gap.

After the preliminary structural and morphological characterization, the sample was transferred into the HFM chamber for the spectroscopy measurements with synchrotron X-rays. Figure 1c shows the XAS spectra acquired with right-circular-polarized (RCP) and left-circular-polarized (LCP) X-rays around the Cr L₃,₂ edges at 4 K, under 9 T applied magnetic field and in normal incidence (NI) geometry. The spectra can be divided in the spin-orbit split L₃ (~577.0 eV) and L₂ (~585.5 eV) parts. An assessable XMCD signal is associated to element-specific magnetic polarization. On this sample, the XMCD intensity achieves the 98% of the average XAS intensity at the Cr L₃ edge under an applied

magnetic field of 9 T (Figure 1d). The spectra replicate the results obtained by Bedoya-Pinto et al.[15] and well reproduces the XAS lineshape of a commercial bulk CrCl₃ reference[27] measured in the same setup, as reported in the Supporting Information (Figure S1). This evidence confirms the hypothesis that the as grown monolayers are CrCl₃ species and that the main peak position (577.0 eV) corresponds to a Cr³⁺ valence state.

Magnetization cycles have been acquired by measuring the XMCD intensity as a function of the magnetic field varied between -6 and 6T, using the photon energy corresponding to the XMCD peak. The result, shown in Figure 1e, is a closed S-shaped loop saturating beyond ±2 T, with no remanence and zero coercive field. Moreover, no differences have been detected between normal and grazing incidence (Supporting Information, Figure S8). Low remanence and small coercive field are recurring characteristics in 2D vdW ferromagnets. In the specific, the coercive field diminishes with the layer number and tends to vanish when approaching the monolayer thickness.[28] It is worth noting that the coercive field measured by Bedoya-Pinto et al. at 7.5 K and 8 T is of the order of 10 mT.[15] The intrinsic remanence of the beamline superconducting magnet and the TEY signal noise while sweeping magnetic field across the zero severely limit the sensitivity of the measurements in this range and might have prevented from reproducing the same results. Despite that, a finite
<figure><img src="image_3.png" /><figcaption>**Figure 2.** CrCl₂/Ni(111). a) LEED pattern acquired at 70 eV. Brown spots and vectors correspond to substrate reflections and unit cell, while violet ones indicate the overlayer reconstruction; the dashed circumference highlights the ring signal crossing the overlayer spots. *inset*: same LEED image without guidelines. b) STM image (80 × 80 nm²) of CrCl₂ islands grown on the Ni(111) surface. (c,d) XAS and XMCD spectra taken around Cr L₃,₂ edges at 4 K, under 9 T magnetic field applied along the beam direction, and in NI; the RCP and LCP XAS intensities are normalized to the average; the XMCD intensity is expressed in percentage units relative to the average XAS. e) Magnetization curves measured with 575.5 eV photon energy (Cr²⁺ edge, indicated by the dashed line in (c) and (d)), at 4 K and in NI. f) Simulations of the experimental XAS and XMCD spectra shown in (c) and (d).</figcaption></figure>

magnetic dichroism of ~6% persists in the XMCD spectra measured in remanence after the application of a 9 T magnetic field, indicating a net residual magnetic moment (Supporting Information, Figure S7). In this respect, as grown SL CrCl₃/Au(111) can be described as a particularly *soft* ferromagnet at 4 K. Repeated magnetization measurements at 4, 15, and 25 K (Supporting Information, Figure S9) show that the low-temperature S-shaped profile evolves rapidly toward a linear field dependence as the temperature increases. This indicates that the onset of FM ordering occurs near, but below, 15 K, in agreement with the Curie temperature of 13 K reported in ref. [15].

Figure 1f reports the polarization-dependent XAS and the XMCD spectra calculated using LFM theory with the *Quantity* code. Experimental conditions of beam incidence, temperature and external applied magnetic field, respectively set to NI, 4 K, and 9 T, have been recreated in the simulations. A D₃d symmetry has been adopted, which addresses the trigonal distortion of the pristine octahedral (Oₕ) coordination within the CrCl₆ cluster unit. The simulations are sensitive mostly to the crystal field interaction, the hybridization potentials and the exchange field. These and other details are reported in the Supporting Information. The simulations allow to estimate some noteworthy expectation values that cannot be retrieved experimentally such as crystal field and magnetic moments. The calculated lineshapes optimally agree with the experimental results. To this purpose, octa-

hedral and trigonal crystal field interactions have been optimized at 1.75 and 0.20 eV. The calculated spin angular momentum projection along the z-axis was found to be <Sz> = 2.98μ<sub>B</sub>, in agreement with the nominal value for a high spin Cr d³ configuration and the value extrapolated with corrected sum rules by Bedoya-Pinto et al.[15] The orbital moment is instead nearly quenched (<Lz> = -0.04μ<sub>B</sub>). These results confirm that the magnetic moment in CrCl₃ is purely spin-based.[7]

## 2.2. CrCl₂/Ni(111)

Ni(111) was chosen as a magnetic substrate for the growth of Cr Chloride monolayers to probe possible magnetic coupling effects. However, opposite to Au, Ni manifests a stronger chemical affinity toward ligand species such as chalcogens or halogens.[17] The growth process, similar to the CrCl₃/Au(111) case, is described in the *Experimental Section*. The LEED pattern of the as grown sample is shown in Figure 2a. It is characterized by six sets of spot quintuplets lying across a faint intensity ring internal to the hexagon marking the substrate reflections. The quintuplets are 30° rotated with respect to the substrate lattice vectors, and are interpreted as the overlap of six equivalent 2D crystal domains having a surface unit cell with lattice constants a = b = 3.60 Å and internal angle α = 50°. The unit cell length is
consistent with the one calculated in ref. [29], but it differs slightly from the one measured and then calculated in ref. [20] for an epitaxial layer of CrCl₂ grown on SLG/SiC(0001). Notably, the latter system represents a pure vdW interface, preserving the native distortion typical of CrX₂ materials,[³] whereas the strong interfacial bonds on Ni(111) tend to restore a threefold rotational symmetry. The LEED pattern has been simulated with the *LEEDpat4* software[³⁰] and the results are reported in Figure S4 (Supporting Information).

The STM measurements show that the overlayer grows in the form of interconnected ribbons with preferential directions at 120° that clusterize at the merging points (Figure 2b). A coverage of about 50% and an average thickness of ~3 Å were estimated. Stripe-shaped islands were observed also for CrI₂ on graphite and other dihalides epitaxially grown on Au(111).[²¹]

The XAS and XMCD spectra relative to this sample are shown in Figure 2c,d. The XAS whitelines are different from those measured on the Au(111) substrate (Figure 1c,d), and both L₃ and L₂ edges are shifted to lower energies by 1.5 eV, respectively at 575.5 and 584.0 eV. The new L₃ edge position can be associated to a lower Cr²⁺ oxidation state. This result suggests that the Ni substrate has reduced the CrCl₃ clusters nucleated on the surface, enabling the formation of CrCl₂ species. Compared to CrCl₃/Au(111), the XMCD signal has much lower intensity (~6% of the average XAS intensity) but similar lineshape. The magnetization curve (Figure 2e), measured by tuning the probe photon energy to the dichroic peak at 575.5 eV, displays a purely linear dependence. This trend is compatible with the behavior observed in AFM bulk CrI₂ in the same temperature window.[³]

We simulated the polarization-dependent XAS and XMCD spectra of CrCl₂/Ni(111) for the same geometrical and environmental conditions described for CrCl₃/Au(111). Fine agreement with the experiments was found in this case by adopting a pure octahedral symmetry ($O_h$) and the results are reported in Figure 2f. Noteworthy, the best fit of the CrCl₂ XAS has been obtained for a lower 10*Dq* (1.50 eV) than the CrCl₃ case, and for zero exchange field, this latter information supporting the non-ferromagnetic behavior observed in the experiments. Also in this case, all the relevant parameters are reported in the Supporting Information. Similarly to CrCl₃, the extrapolated orbital moment is quenched ($<Lz> = 0.004\mu_B$), whereas the spin angular momentum projection along the z-axis was found to be $<Sz> = -0.275\mu_B$, reflecting a low-spin case compatible with the weak magnetism of this sample. The low-spin Cr²⁺ oxidation state is unusual and is typically stabilized by ligands at the strong-field end of the spectrochemical series.[³¹] This aligns with our calculations, which show that the low-spin configuration is favored by large crystal field energy splitting, i.e., a larger separation separation between t₂g and e_g levels which force electrons to populate only the less energetic t₂g levels. However, given that Cl⁻ is generally regarded as a weak ligand, this effect is likely influenced by environmental factors, such as substrate doping. This discussion is better addressed in the Supporting Information.

## 2.3. CrCl₂(3)/SLG/Ni(111)

To hinder the chemical reactivity of Ni(111) and enable the formation of SL CrCl₃ monolayers - showing intense XMCD and

FM properties - also on top of a magnetic substrate, the Ni(111) surface was passivated with SLG. SLG was grown on Ni(111) by ethylene (C₂H₄) gas pyrolysis at 570°C, as described in the *Experimental Section*.

Cr chloride grows on SLG/Ni(111) in the form of small tens of nm large flakes of irregular shapes distributed all over the SLG surface (Figure 3b). As consequence of the short range structural order and the morphological disorder, the LEED pattern shows a diffused background signal behind very broad and weak reflections oriented at 30° from the Ni(111) axes (Figure 3a). The morphology of the chloride flakes as observed by STM looks different from the monolayers grown on Au(111) and Ni(111). The lateral size of the 2D Chlorides could have been limited by the low diffusion rate of the adsorbed nuclei or by the morphology of the SLG layer underneath, e.g., the presence of point defects or the high density of domain boundaries.

Likewise the previous samples, Figure 3c,d report the XAS and XMCD spectra acquired at 4 K, 9 T and 0.1 V for the Cr Chlorides grown on SLG/Ni(111). At first glance, the XAS whiteline reproduces mostly the spectral shape acquired for CrCl₂/Ni(111) (Figure 2c) with the main L₃ absorption edge and the relative XMCD peak sitting exactly on the Cr²⁺ energy position, i.e., 575.5 eV. Noteworthy, the XAS profile along the L₃ post-edge region appears broader than in Figure 2c, while in the XMCD spectrum an additional peak emerges, which sits on the Cr³⁺ edge energy, i.e., at 577.0 eV. The double dichroism is likewise replicated on the L₂ peak, but was not present in the CrCl₂ spectrum measured on the bare Ni(111) substrate (Figure 2d). From this evidence, we deduce that both CrCl₂ and CrCl₃ phases coexist on the SLG/Ni(111) surface.

Focusing on the relative XMCD intensity of the two phases, it can be noted how CrCl₃, generates a magnetic response comparable to that of CrCl₂. The latter is however more abundant in the sample, as indicated by the overall Cr²⁺ character of the XAS lineshape. A quantitative estimation of the two phases amount was obtained by fitting the experimental average XAS with the linear combination of the CrCl₃ and CrCl₂ XAS lineshapes shown in Figures 1c and 2c, respectively measured on bare Au(111) and Ni(111), after polarization averaging. The weight factors associated to the single contributions reproduce the ratio between the CrCl₂ and CrCl₃ species. We estimated that the CrCl₃ species present on the sample is comprised between the 20% and 25% of the total overlayer amount. The calculated sum spectrum fitting best the experimental XAS together with the CrCl₃ and CrCl₂ components are shown in Figure 3e.

The results described so far for the two Ni-based substrates (bare and passivated by SLG) are reproducible: the Cr³⁺ peak is observed on Ni(111) only in presence of SLG, whereas it is not found when the growth is carried out on the bare surface. This fact supports the role of SLG in protecting the overlayer by physically decoupling it from the reactive Ni substrate.[¹⁷] CrCl₃ reduction to CrCl₂ does not occur instead on the Au(111) surface, owing to the chemical inertness of the substrate.

We then measured the magnetization loops on SLG/Ni(111) using two different photon probes tuned at the Cr²⁺ and Cr³⁺ absorption edges (more details are given in the *Experimental Section*). *Site-selective* measurements allowed us to address the single CrCl₂ and CrCl₃ phases separately and study the magnetic behavior of the different species found on this sample. The
<figure><img src="image_4.png" /><figcaption>**Figure 3.** CrCl₂(3)/SLG/Ni(111) (I). a) LEED pattern acquired at 42 eV. Violet spots and vectors correspond to overlayer reflections, while brown vectors indicate the substrate crystallographic axes directions (reflections are out of the image); the dashed circumference highlight the ring signal crossing the overlayer spots; inset: LEED pattern of SLG/Ni(111) substrate acquired at 80 eV. b) STM image (200 × 200 nm²) measured after CrCl₃ deposition on SLG/Ni(111). c,d) XAS and XMCD spectra taken around Cr L₃,₂ edges at 4 K, under 9 T magnetic field applied along the beam direction, and in NI; the RCP and LCP XAS intensities are normalized to the average; the XMCD intensity is expressed in percentage units relative to the average XAS. e) Experimental polarization average XAS of CrCl₃/Au(111) (gold) and CrCl₂/Ni(111) (gray) samples and their linear combination (blue). f) Magnetization curves taken at 4 K and in NI at the Cr²⁺ and Cr³⁺ edge energies indicated by the dashed lines in (c) and (d).</figcaption></figure>

magnetization curves corresponding to the two energies are shown in Figure 3f. They appear as two S-shaped closed loops with unsaturated magnetization even at high field (±6 T). These characteristics are commonly observed with SPM nanostructures and arise from the nanometric lateral dimensions of the chloride 2D particles, which behave as independent macroscopic “super-spin” in the presence of an external magnetic field.[32,33] In other words, CrCl₃ and CrCl₂, which exhibit distinct characteristic magnetic properties on Au(111) and Ni(111), display analogue behaviour on SLG.

Notably, when the interaction between SPM domains becomes more effective, collective behaviors associated to competing magnetic ordering (i.e., FM vs AFM) may manifest.[32,33] This effect has been observed for a second Chloride sample prepared on a different SLG/Ni(111) substrate, that has been reported in the Supporting Information.

Previously, we have explicitly formulated the hypothesis that CrCl₂ formation on SLG/Ni(111) occurs due to the electron charge transferred from the graphene layer, similarly to other cases reported in the literature.[34,35] Despite the fact that the graphene passivation leads to a significant amount of CrCl₃ unachievable on bare Ni(111), CrCl₂ still represents the dominant phase found on the sample. Besides this hypothesis, other possible mechanisms for CrCl₂ formation were considered. First, the Cr reduction by Ni atoms could be in this circumstance only a partial explanation, since it requires the direct contact between the two materials, whereas we expect a relatively small portion of un-

covered areas on the Ni(111) surface after SLG growth. Second, past works suggest that the surface thermodynamics of halide species deposited on SLG/metal implies a variety of possible microscopic events, including trihalide dissociation into dihalide compounds and halogen atoms.[36,37] The latter might finally intercalate SLG. All these processes are thermally activated and they can be controlled by means of the annealing temperature, which is typically reported between 150 and 200°C.[36,37] However, during the chloride growths, the substrate was maintained at temperature lower than 150°C to prevent CrCl₃ dissociation. Actually, the mechanism enabling CrCl₃ dissociation and SLG intercalation by Cl and Cr atoms cannot be fully established with our techniques. However, our STM images do not show profile modulation and roughness such as due to localized Cl nanodomains intercalated between the SLG and the metallic substrate.[38] Moreover, reconstructions of intercalated adatoms do not appear in the LEED pattern upon growth, though they could be invisible due to structural disorder. Charge transfer from SLG toward Chlorides remains therefore a plausible argument to explain the coexistence of CrCl₂ and CrCl₃ on top of SLG. In this respect, it is reasonable to assume that CrCl₂ formation occurs randomly within the CrCl₃ layers, i.e., there is no net CrCl₂ and CrCl₃ phase segregation, but a “puzzle” of patches separated by a dense network of grain boundaries. The simultaneous formation of MX₂ and MX₃ species on the same substrate at ~100°C is possible by MBE growths due to the narrow stability temperature windows of the two species.[21,39]
Finally, we address the role of magnetic Ni(111) substrate in the as built vdW stacking. One may wonder in fact whether the magnetism in the overlayer could be controlled through the heterostructure stacking, i.e., by the modulation via SLG of the interlayer exchange between the Ni(111) substrate and the 2D Chlorides. For instance, in 0D magnetic molecular systems deposited on SLG/Ni(111), the spin alignment of the metal center is determined by the coupling distance. Epitaxial SLG is used to reduce the out-of-plane exchange interaction by increasing the separation between substrate and molecules, and transform the FM exchange into AFM.[40] However, in the hysteresis cycles measured on such systems, the saturation point is the same for the overlayer and the substrate and their XMCD sign is parallel or antiparallel as a function of their mutual distance. In our case instead, no distance-modulated dichroism sign has been experimentally observed by XMCD between Ni and 2D Chloride, and no correlation between saturation fields in Cr and Ni magnetization has been detected (Ni has much smaller coercive field than those measured for chlorides in this work). We conclude that the Ni(111) substrate does not represent a coupling term for the grown SPM Chlorides, and SLG does not work as a suitable filter through which the magnetic properties of Cr Chloride can be controlled.

### 3. Conclusion

In this work we described the structural, morphological and magnetic nature of Cr Chloride (sub)monolayers grown by evaporation in UHV of the same $CrCl_3$ powder precursor on Au(111), Ni(111), and SLG/Ni(111) substrates. The as grown 2D materials were characterized by LEED, STM and XMCD using synchrotron soft X-rays. We showed that the two metallic substrates, i.e., Au(111) and Ni(111), promote the formation of different Chloride species, i.e., $CrCl_3$ and $CrCl_2$, showing respectively weak FM and non-FM behaviors. In order to hinder the chemical reactivity of the Ni substrate toward the $CrCl_3$ clusters upon their adsorption on the surface, the Ni(111) was passivated with SLG. We showed that SLG favors the partial formation of nanometric $CrCl_3$ islands in addition to $CrCl_2$ ones. The latter are probably generated by the electron transfer channel from SLG. The $CrCl_3$ amount is correlated to the SLG synthesis process and final morphology, and the highest estimated $CrCl_3$ concentration was found ~25%. The coexistence of $CrCl_2$ and $CrCl_3$ phases is evidenced by two dichroic peaks in the XMCD spectra matching the characteristic $Cr^{3+}$ and $Cr^{2+}$ energy positions, and by site-selective magnetization measurements performed by tuning the probing photons to the two absorption edges. The latter reveals the SPM nature of both $CrCl_2$ and $CrCl_3$ nano-domains, which is intimately related to the surface morphology of the SLG underneath. No evidence of SLG-mediated coupling between the Ni(111) substrate and any Chloride species was instead observed. These results demonstrate the variety of possible chemical and magnetic phases that can be found on a single crystal substrate surface upon chloride MBE growths, which adds to the already known complexity of the morphology at the microscopy level (moiré, domain shapes, presence of single and double layers, etc.). The results invoke deeper and more specific microscopy studies to understand the mechanisms behind the phase formations, in view of more uniform structural and magnetic order in Chlorides and, more generally, in 2D epitaxial transition metal halides.

### 4. Experimental Section

**Sample Growth:** Sample growth and preliminary in situ characterizations were carried out in the MBE setup of the XMCD endstation at the ESRF-ID32 beamline.[41,42] The metallic Au(111) and Ni(111) substrates were cleaned using standard sputtering (1 kV, $1 \times 10^{-5}$ mbar of Ar pressure) and annealing (550–600°C) cycles.

SLG was grown on Ni(111) by ethylene ($C_2H_4$) pyrolysis.[43] During the process, the substrate temperature was kept at around 570°C and the ethylene gas exposure was estimated between 500 and 650 L (Langmuir). The sample was then slowly cooled under ethylene atmosphere, until an homogeneous self limited and fully covering SLG was obtained.[42]

The growth of $CrCl_3$ monolayers was conducted in UHV environment (background pressure below $10^{-9}$ mbar) by MBE using a Knudsen cell evaporator filled with anhydrous $CrCl_3$ powder (provided by Sigma Aldrich, 99.99% pure) heated up to 530°C. The deposition rate was calibrated a posteriori by STM to 0.10 ML/min, and the time adjusted for the desired target coverage. Both on Au(111) and Ni(111), a minimum sub-monolayer coverage of 50% was achieved. The substrate temperature was kept below 150°C to limit the substrate reactivity toward the $CrCl_3$ precursor species and to avoid halogen desorption.

Deposition and substrate contamination were regularly checked by Auger electron spectroscopy (AES) after each growth or sputtering/annealing cycle. XAS spectra in the Supporting Information demonstrate the absence of Oxygen in the sample upon growth. All the samples were characterized in situ by LEED and STM prior to the X-ray measurements, and results are consistent between growths conducted before and during the beamtime.

**X-Ray Measurements:** XAS/XMCD experiments have been carried out at the XMCD station of the ID32 beamline at the ESRF.[41,42] The measurements were performed inside the high field magnet (HFM) chamber, in a background pressure lower than $3 \times 10^{-10}$ mbar, by scanning the energy around the $Cr~L_{3,2}$ edges using right- and left-circularly polarized (RCP, LCP) X-rays. The synchrotron beam presents a degree of polarization of almost 100% and the monochromator has a resolving power better than 5000. The samples were studied in the 4–300 K temperature with magnetic field varying between 0 and 9 T, applied along the beam direction. Measurements were performed both in normal (90°) and grazing (30°) incidence geometries. The absorption signal was measured in total-electron yield (TEY) mode and normalized by the intensity collected on a gold mesh placed in front of the sample stage. Polarization-dependent XAS are plotted after normalization to the average XAS intensity. XMCD intensity is plotted in percentage units relative to the average XAS intensity. Site selective magnetization curves were obtained by measuring the XMCD spectra at field variance between 6 and –6 T, and between –6 and 6 T, and then by integrating the signal under the selected $Cr^{2+}$ or $Cr^{3+}$ peaks (integration width: 0.25 eV). The exact magnetization is proportional to the plotted XMCD signal.

### Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

### Acknowledgements

The authors acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities under proposal number HC-5442 and the authors would like to thank all the past and present ID32 beamline staff for the technical and scientific support. This work was partially performed within the MUSA-Multilayered Urban Sustainability Action project, funded by the European Union-NextGenerationEU, under the National Recovery and Resilience Plan (NRRP) Mission 4 Component 2 Investment Line 1.5: Strengthening of research structures and creation of R&D, "innovation ecosystems" set up of "territorial leaders" in R&D. The authors thank in particular
Prof. A. Brambilla from Politecnico di Milano and Dr. K. Kummer from ESRF-ID32 for the fruitful discussions. Funded by European Union NextGenerationEU - PNRR M4 C2 11.5 - Project MUSA CUP D43C22001410007.

Open access publishing facilitated by Politecnico di Milano, as part of the Wiley - CRUI-CARE agreement.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

2D materials, CrCl₂, CrCl₃, MBE, van der Waals, XMCD

Received: March 14, 2025

Revised: August 7, 2025

Published online: September 12, 2025

[1] V. V. Kulish, W. Huang, *J. Mater. Chem. C* **2017**, *5*, 8734.

[2] M. A. McGuire, *Crystals* **2017**, *7*, 121.

[3] J. A. Schneeloch, S. Liu, P. V. Balachandran, Q. Zhang, D. Louca, *Phys. Rev. B* **2024**, *109*, 144403.

[4] C. Liu, W. Ren, S. Picozzi, *Phys. Rev. Lett.* **2024**, *132*, 086802.

[5] M. A. McGuire, H. Dixit, V. R. Cooper, B. C. Sales, *Chem. Mater.* **2015**, *27*, 612.

[6] S. Tian, J.-F. Zhang, C. Li, T. Ying, S. Li, X. Zhang, K. Liu, H. Lei, *J. Am. Chem. Soc.* **2019**, *141*, 5326.

[7] M. A. McGuire, G. Clark, S. K C, W. M. Chance, G. E. Jellison Jr, V. R. Cooper, X. Xiaodong, B. C. Sales, *Phys. Rev. Mater.* **2017**, *1*, 014001.

[8] T. Song, M. W.-Y. Tu, C. Carnahan, X. Cai, T. Taniguchi, K. Watanabe, M. A. McGuire, D. H. Cobden, D. Xiao, W. Yao, Xiaodong Xu, *Nano Lett.* **2019**, *19*, 915.

[9] A. T. Costa, M. I. Vasilevskiy, J. Fernández-Rossier, N. M. Peres, *Nano Lett.* **2023**, *23*, 4510.

[10] G. Tenasini, D. Soler-Delgado, Z. Wang, F. Yao, D. Dumcenco, E. Giannini, K. Watanabe, T. Taniguchi, C. Moulsdale, A. Garcia-Ruiz, et al., *Nano Lett.* **2022**, *22*, 6760.

[11] J. Dang, T. Wu, S. Yan, K. Watanabe, T. Taniguchi, H. Lei, X.-X. Zhang, *Nat. Commun.* **2024**, *15*, 6799.

[12] M. M. Hammo, S. Froeschke, G. Haider, D. Wolf, A. Popov, B. Büchner, M. Mertig, S. Hampel, *Nanoscale Adv.* **2025**, *7*, 2351.

[13] P. Li, N. Liu, J. Zhang, S. Chen, X. Zhou, D. Guo, C. Wang, W. Ji, D. Zhong, *ACS Appl. Mater. Interfaces* **2023**, *15*, 19574.

[14] H. Xie, X. Luo, G. Ye, Z. Ye, H. Ge, S. H. Sung, E. Rennich, S. Yan, Y. Fu, S. Tian, et al., *Nature Physics* **2022**, *18*, 30.

[15] A. Bedoya-Pinto, J.-R. Ji, A. K. Pandeya, P. Gargiani, M. Valvidares, P. Sessi, J. M. Taylor, F. Radu, K. Chang, S. S. Parkin, *Science* **2021**, *374*, 616.

[16] A. Lodesani, A. Picone, A. Brambilla, D. Giannotti, M. S. Jagadeesh, A. Calloni, G. Bussetti, G. Berti, M. Zani, M. Finazzi, et al., *ACS Nano* **2019**, *13*, 4361.

[17] R. Sant, M. Cattelan, S. Agnoli, G. Granozzi, *Journal of Vacuum Science & Technology A* **2021**, *39*, 5.

[18] T. A. Tartaglia, J. N. Tang, J. L. Lado, F. Bahrami, M. Abramchuk, G. T. McCandless, M. C. Doyle, K. S. Burch, Y. Ran, J. Y. Chan, et al., *Sci. Adv.* **2020**, *6*, eabb9379.

[19] A. Hermann, B. Vest, P. Schwerdtfeger, *Phys. Rev. B: Condens. Matter Mater. Phys.* **2006**, *74*, 224402.

[20] Z. Cheng, N. Liu, J. Deng, H. Zhang, Z. Pan, C. Zhu, S. Lu, Y. Bai, X. Lin, W. Ji, et al., *Mater. Horiz.* **2025**, *12*, 473.

[21] P. Li, C. Wang, J. Zhang, S. Chen, D. Guo, W. Ji, D. Zhong, *Sci. Bull.* **2020**, *65*, 1064.

[22] D. Mastrippolito, L. Ottaviano, J. Wang, J. Yang, F. Gao, M. Ali, G. D'Olimpio, A. Politano, S. Palleschi, S. Kazim, et al., *Nanoscale Adv.* **2021**, *3*, 4756.

[23] F. De Groot, *Coord. Chem. Rev.* **2005**, *249*, 31.

[24] M. W. Haverkort, *J. Phys.: Conf. Ser.* **2016**, *712*, 012001.

[25] A. Koma, *Thin Solid Films* **1992**, *216*, 72.

[26] L. A. Walsh, C. L. Hinkle, *Appl. Mater. Today* **2017**, *9*, 504.

[27] *HQgraphene*, https://www.hqgraphene.com (accessed: March. 2025).

[28] B. Li, Z. Wan, C. Wang, P. Chen, B. Huang, X. Cheng, Q. Qian, J. Li, Z. Zhang, G. Sun, et al., *Nat. Mater.* **2021**, *20*, 818.

[29] X. Bo, L. Fu, X. Wan, S. Li, Y. Pu, *Phys. Rev. B* **2024**, *109*, 014405.

[30] *Leedpat4*, https://www.fhi.mpg.de/958975/LEEDpat4 (accessed: March. 2025).

[31] C. Scarborough, S. Sproules, C. Doonan, K. Hagen, T. Weyhermuller, K. Wieghardt, *Inorg. Chem.* **2012**, *51*, 6969.

[32] D. De, S. Goswami, M. Chakraborty, *J. Magn. Magn. Mater.* **2023**, *565*, 170175.

[33] D. Peddis, K. Trohidou, M. Vasilakaki, G. Margaris, M. Bellusci, F. Varsano, M. Hudl, N. Yaacoub, D. Fiorani, P. Nordblad, et al., *Sci. Rep.* **2021**, *11*, 7743.

[34] M. Suzuki, I. S. Suzuki, *Phys. Rev. B* **1998**, *57*, 10674.

[35] Y. Li, A. Li, J. Li, H. Tian, Z. Zhang, S. Zhu, R. Zhang, S. Liu, K. Cao, L. Kang, et al., *ACS Nano* **2023**, *17*, 20112.

[36] A. V. Generalov, K. A. Simonov, N. A. Vinogradov, E. M. Zagrebina, N. Martensson, A. B. Preobrajenski, A. S. Vinogradov, *J. Phys. Chem. C* **2015**, *119*, 12434.

[37] N. A. Vinogradov, K. A. Simonov, A. V. Generalov, A. S. Vinogradov, D. V. Vyalikh, C. Laubschat, N. Mårtensson, A. B. Preobrajenski, *J. Phys.: Condens. Matter* **2012**, *24*, 314202.

[38] R. Sant, S. Lisi, V. D. Nguyen, E. Mazaleyrat, A. C. Gomez Herrero, O. Geaymond, V. Guisset, P. David, A. Marty, M. Jamet, et al., *J. Phys. Chem. C* **2020**, *124*, 12397.

[39] P. Li, N. Liu, J. Zhang, S. Chen, X. Zhou, D. Guo, C. Wang, W. Ji, D. Zhong, *ACS Appl. Mater. Interfaces* **2023**, *15*, 19574.

[40] V. Corradini, A. Candini, D. Klar, R. Biagi, V. De Renzi, A. L. Rizzini, N. Cavani, U. del Pennino, S. Klyatskaya, M. Ruben, et al., *Nanoscale* **2018**, *10*, 277.

[41] N. B. Brookes, F. Yakhou-Harris, K. Kummer, A. Fondacaro, J. Cezar, D. Betto, E. Velez-Fort, A. Amorese, G. Ghiringhelli, L. Braicovich, et al., *Nucl. Instrum. Methods Phys. Res., Sect. A* **2018**, *903*, 175.

[42] K. Kummer, A. Fondacaro, E. Jimenez, E. Velez-Fort, A. Amorese, M. Aspbury, F. Yakhou-Harris, P. Van Der Linden, N. B. Brookes, *J. Synchrotron Radiat.* **2016**, *23*, 464.

[43] M. Batzill, *Surf. Sci. Rep.* **2012**, *67*, 83.