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Formation and protection of an Eu-Ir surface compound below hexagonal boron nitride

This study investigates the formation and structural evolution of Europium-Iridium surface compounds beneath hexagonal boron nitride on an Ir(111) substrate, demonstrating that while the hBN layer remains intact and partially protects the underlying EuIr $_2$ alloy from air exposure, different deposition temperatures and coverages yield distinct superstructures with varying electronic states and potential ferromagnetic properties.

Original authors: Alaa Mohammed Idris Bakhit, Khadiza Ali, Frederik Schiller

Published 2026-10-12

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CC BY 4.0

Original authors: Alaa Mohammed Idris Bakhit, Khadiza Ali, Frederik Schiller

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where the smallest building blocks of matter can be arranged like tiles on a floor to create materials with entirely new powers. Scientists are particularly interested in two-dimensional materials, which are sheets of atoms so thin they are essentially flat. These sheets can act as protective shields, hiding delicate substances underneath them from the harsh air we breathe. One such substance is europium, a rare metal that is incredibly useful for future electronics because it can be magnetic, but only if it stays in a specific chemical state. The problem is that europium is like a very eager sponge; the moment it touches the oxygen in our atmosphere, it reacts instantly, changing its nature and losing its magnetic potential. Researchers have long wondered if a thin, invisible sheet of hexagonal boron nitride could act as a perfect shield, keeping the europium safe and magnetic even after exposure to the air.

To test this idea, a team of scientists at the Centro de Física de Materiales in Spain and collaborators in Sweden and India set up an experiment involving a crystal of iridium, a hard metal often used as a foundation for these atomic experiments. They first grew a single, uniform layer of hexagonal boron nitride on top of the iridium. This layer forms a patterned surface, much like a honeycomb, which serves as a roof. Then, they introduced europium atoms underneath this roof, a process called intercalation, by heating the system to encourage the europium to slip between the boron nitride and the iridium. They carefully controlled how much europium they added and the temperature at which they added it, watching closely to see how the atoms arranged themselves and what kind of chemical state they ended up in.

The results revealed that the europium atoms did not just sit randomly; they organized themselves into distinct, ordered patterns depending on how many atoms were present and how hot the system was. When they added a small amount of europium, the atoms lined up in a single direction, creating a specific repeating pattern that was visible through their electron microscopes. As they added more europium, the pattern changed again, becoming more compressed and orderly. Finally, when they reached a specific amount of europium—about one-third of a single atomic layer covering the surface—the atoms settled into a highly organized structure where one europium atom sat for every two iridium atoms. This arrangement formed a new surface compound, a type of alloy, right beneath the protective boron nitride sheet.

Crucially, the chemical state of the europium changed based on where it was and how much of it was there. The europium atoms that successfully formed this ordered alloy underneath the roof remained in a "divalent" state, which is the specific condition required for them to be magnetic. However, any extra europium that did not fit into this neat pattern or that managed to sink deeper into the iridium metal turned into a "trivalent" state, which is not magnetic. This distinction was vital because it showed that the protective layer was doing its job for the atoms that mattered most, keeping them in the right state to potentially function as magnetic components.

To see if this protection held up against the real world, the researchers exposed their samples to air and oxygen, simulating the conditions a device would face outside a vacuum chamber. They found that while the unprotected europium would have instantly rusted and lost its properties, the europium hidden under the boron nitride was only partially affected. The protective layer did not stop every single atom from reacting, but it successfully preserved the core structure of the magnetic alloy. Even after exposure to air and subsequent heating to clean off some impurities, the underlying boron nitride sheet remained intact, and the electronic signature of the magnetic europium was still detectable. This suggests that using a thin sheet of boron nitride is a viable strategy for shielding reactive magnetic metals, offering a promising path toward creating stable, new types of electronic devices that rely on magnetism at the atomic scale.

Technical Summary: Formation and Protection of an Eu-Ir Surface Compound Below Hexagonal Boron Nitride

Problem Statement
Rare-earth metals, such as Europium (Eu), possess unique electronic and magnetic properties, particularly in their divalent state, which is promising for spintronic applications. However, Eu is highly reactive and oxidizes rapidly in ambient conditions, typically forming non-magnetic trivalent Eu $_2$ O $_3$ . While intercalating Eu beneath 2D materials like graphene or hexagonal boron nitride (hBN) has been explored to tune magnetic properties, the stability of Eu in such intercalation systems and the protective efficacy of the hBN layer against oxidation remain insufficiently characterized. Previous studies on Eu intercalation under graphene/Ir(111) and hBN/Pt(111) have shown valence-dependent magnetic behaviors, but the specific structural evolution and oxidation resistance of Eu beneath hBN on Ir(111) require further investigation.

Methodology
The study investigates the intercalation of Eu atoms beneath a monolayer of hBN grown on an Ir(111) substrate. The experimental approach involved:

Sample Preparation: A clean Ir(111) substrate was prepared via ion sputtering and annealing. A uniform hBN monolayer was synthesized using Chemical Vapor Deposition (CVD) with borazine precursor at temperatures between 1120 K and 1220 K.
Intercalation: Eu was deposited onto the hBN/Ir(111) system at three distinct substrate temperatures (800 K, 920 K, and 940 K) to vary coverage and induce different structural phases. Coverage was estimated by comparing the intensity of the divalent Eu $^{2+}$ 3d core-level peak in XPS spectra against a reference sample with a known 1/3 monolayer (ML) coverage.
Characterization: Structural and electronic properties were analyzed using Low Energy Electron Diffraction (LEED), Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS), and Angle-Resolved Photoemission Spectroscopy (ARPES).
Oxidation Testing: To evaluate the protective capability of the hBN layer, samples were exposed to 1000 L of oxygen or ambient air conditions, followed by annealing, and subsequently re-analyzed.

Key Contributions and Results

Structural Evolution and Superstructures:
Depending on the deposition temperature and Eu coverage, three distinct superstructures were identified relative to the Ir substrate:
- (5 × M) Superstructure: At low coverage (~0.10 ML) and 800 K, a (5 × M) superstructure (where M > 2) formed. This phase preserved the underlying hBN/Ir Moiré pattern and exhibited unidirectional ordering of Eu atoms along the <11 $\bar{2}$ > directions, while alignment along < $\bar{1}$ 10> remained disordered.
- (5 × 2) Superstructure: At higher coverage (~0.26 ML) and 920 K, a (5 × 2) superstructure emerged. This indicated a compression of the intercalated layer where Eu atoms occupied positions every two Ir rows. STM analysis revealed this structure consists of three rotational domains. Excess Eu atoms in this regime were found to diffuse into the Ir bulk in a trivalent state.
- ( $\sqrt{3}$ × $\sqrt{3}$ )R30° Superstructure: At saturation coverage (1/3 ML) and 940 K, a ( $\sqrt{3}$ × $\sqrt{3}$ )R30° superstructure formed. This corresponds to the formation of a EuIr $_2$ surface alloy beneath the hBN layer with a stoichiometry of one Eu atom per two Ir atoms.
Electronic and Valence States:
- Valence Configuration: XPS analysis revealed that Eu atoms at the interface (in the ordered superstructures) exist primarily in a divalent (Eu $^{2+}$ ) state, which is associated with the formation of the surface alloy. Conversely, Eu atoms that diffuse into the Ir bulk or exist in excess are found in a trivalent (Eu $^{3+}$ ) state.
- Band Structure: ARPES measurements showed that the hBN $\pi$ band shifts by approximately 2 eV to higher binding energies upon Eu intercalation, indicating a strong interaction between the hBN layer and the underlying EuIr $_2$ interface. The divalent Eu 4f emissions were observed at approximately 1.6 eV.
- Oxidation Dynamics: In the absence of protection, Eu deposited directly on Ir oxidizes rapidly, with a mean lifetime of approximately 1.9 hours in UHV before significant trivalent oxidation occurs.
Protective Role of hBN:
The study assessed the stability of the intercalated EuIr $_2$ alloy after exposure to oxygen and ambient air.
- Partial Protection: While the EuIr $_2$ surface alloy was partially protected, air exposure led to a significant increase in trivalent Eu species (Eu $_2$ O $_3$ and Eu-OH) and a reduction in the divalent signal.
- Integrity of hBN: Crucially, the hBN layer itself remained intact. ARPES analysis confirmed that the hBN $\pi$ band persisted after air exposure, albeit with some blurring and a partial recovery of its binding energy position. This contrasts with unprotected Eu-Ir alloys, which degrade rapidly even under UHV conditions.

Significance
The paper demonstrates that intercalating Eu beneath hBN on Ir(111) allows for the controlled formation of distinct superstructures, including a EuIr $_2$ surface alloy with divalent Eu atoms that suggest potential ferromagnetic properties. While the hBN layer does not provide complete immunity against oxidation for the underlying Eu-Ir alloy, it effectively preserves the structural integrity of the 2D material itself. This work advances the understanding of rare-earth intercalation mechanisms and highlights the potential of using hBN as a partial protective barrier for reactive metallic intercalants in the development of 2D ferromagnetic materials with tailored electronic properties.

Technical Summary: Formation and Protection of an Eu-Ir Surface Compound Below Hexagonal Boron Nitride

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