E L E C T R O N I C S T R U C T U R E , M A G N E T I S M A N D D Y N A M I C S

S C I E N T I F I C H I G H L I G H T S

1 1 6 H I G H L I G H T S 2 0 2 1 I

transport properties of graphene, which makes this material particularly attractive. Particular attention has been paid to the synthesis of the sample. 2D magnetic materials are usually produced by exfoliation from the bulk or a chemical reaction between the elements. Here, in making monolayer EuC6, a graphene sheet was modified with Eu atoms. This delicate procedure was implemented by reactive molecular beam epitaxy, a technique ensuring control over chemical processes on the monolayer level.

Element-selective X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) was employed at the L3-edge of europium to shed light onto the complex nature of magnetism in monolayer EuC6. The extremely high sensitivity and excellent performance of beamline ID12 made these challenging experiments feasible. Firstly, XAS/XMCD spectra (Figure 96b) unambiguously demonstrated that the valence state of Eu in EuC6 monolayer is 2+ without any admixture of non-magnetic 3+ ions. This observation ruled out the hypothesis of a mixed valence state (or partial oxidation of the Eu/graphene material) to be the reason of reduced sample magnetisation. The valence state of Eu suggests some chemical bonding between Eu and the graphene sheet. Europium donates electrons to the π-electron system of graphene but is unlikely to form any directed bonds with C atoms; the bonding between Eu and graphene is mostly ionic.

The magnetic properties of europium in EuC6 monolayer were then probed with XMCD, which is an inherently element-selective technique. Unfortunately, the L-edges of the XMCD spectra alone do not allow the magnetic moment carried by Eu to be determined. To rationalise the XMCD spectra, a study of their dependence on external magnetic fields (up to 17 Tesla) and temperatures (down to 3 K) was performed. The results are presented in Figure 96c together with the fit, assuming the total XMCD signal to be a sum of ferromagnetic (≈ 2.5 µB) and paramagnetic

Fig. 96: a) Schematic of the XMCD experiment on monolayer EuC6. b) Normalised XAS spectra in monolayer EuC6 at the Eu L3 edge: right (red) and

left (green) circular polarisations; their difference (blue) makes a normalised XMCD signal. The spectra are recorded at 3 K in a magnetic field of 17 Tesla. c) Magnetic

field dependence of the Eu magnetic moment (scaled from the Eu L3 XMCD signal) in monolayer EuC6 at different temperatures and their fits shown as continuous lines.

contributions (7 µB). The existence of paramagnetic Eu2+ ions seems to be a possible reason for reduced magnetic moments in EuC6 observed in low-field macroscopic magnetisation measurements.

In conclusion, combined XAS/XMCD studies demonstrate that EuC6 monolayer is a prominent member of the family of Eu-based 2D magnets, combining the celebrated properties of graphene with the strong magnetism of europium. This example is yet another illustration of a rather general phenomenon antiferromagnetic compounds thinned down to a single monolayer exhibit ferromagnetism.

PRINCIPAL PUBLICATION AND AUTHORS

Emerging 2D magnetic states in a graphene-based monolayer of EuC6, I.S. Sokolov (a), D.V. Averyanov (a), F. Wilhelm (b), A. Rogalev (b), O.E. Parfenov (a), A.N. Taldenkov (a), I.A. Karateev (a), A.M. Tokmachev (a), V.G. Storchak (a), NanoResearch 15, 408-413 (2022); https:/doi.org/10.1007/s12274-021-3494-9 (a) National Research Center Kurchatov Institute, Moscow (Russia) (b) ESRF

REFERENCES

[1] D.L. Cortie et al., Adv. Funct. Mater. 30, 1901414 (2020). [2] D.V. Averyanov et al., ACS Appl. Mater. Interfaces 10, 20767-20774 (2018). [3] I.S. Sokolov et al., Mater. Horiz. 7,1372-1378 (2020).

This work is supported by the Ministry of Science and Higher Education of Russia (Agreement No. 075-15-2021-1351).