In Situ Growth and Magnetic Characterization of Cr Chloride Monolayers

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., CrCl₃ and CrCl₂, showing distinct magnetic properties at 4 K. When graphene is used to passivate the Ni(111) surface, the formation of CrCl₃ becomes allowed also on this substrate. The coexistence of CrCl₃ and CrCl₂, 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.