Eu Intercalation Induces 2D Easy-Plane Magnetism and Spin Polarization in Graphene

Integrating magnetism with graphene represents a crucial advance for developing next-generation spintronic devices, and researchers are increasingly exploring the use of two-dimensional magnets in close proximity to graphene to achieve this. Ivan Sokolov, Dmitry Averyanov, and Oleg Parfenov, all from the National Research Center “Kurchatov Institute”, alongside colleagues, now demonstrate a successful method for coupling epitaxial graphene with a precisely ordered magnetic lattice created by incorporating europium atoms. This innovative approach overcomes previous challenges in inducing magnetism within graphene, revealing easy-plane magnetic behaviour and a transition temperature tunable with external magnetic fields. The resulting material exhibits both negative magnetoresistance and the anomalous Hall effect, indicating spin polarization of charge carriers within the graphene layer, and establishes a promising pathway towards realising graphene-based spintronic technologies.

Epitaxial Graphene with Europium Monolayer Magnetism

This research details the creation and characterization of a new material combining epitaxial graphene with a single layer of either europium or strontium on a silicon carbide substrate. The focus lies in understanding the magnetic properties of the graphene/europium structure and comparing it to a non-magnetic graphene/strontium control sample, demonstrating a novel way to introduce magnetism into graphene. Researchers successfully grew epitaxial graphene on silicon carbide and then integrated a single layer of europium or strontium. Advanced characterisation techniques confirmed the formation of the desired layered structure.

The graphene/europium structure exhibits clear ferromagnetic behavior, confirmed by measurements showing a hysteresis loop, indicating a persistent magnetic state. The magnetic moment responds to temperature and an applied magnetic field, demonstrating magnetic anisotropy, and applying a field shifts the temperature at which magnetism appears. The material also exhibits significant magnetoresistance, a change in electrical resistance in response to a magnetic field, dependent on field direction, and displays an anomalous Hall effect, a voltage generated by magnetization, further confirming magnetic order. Magnetic Force Microscopy visualized magnetic domains and measured the magnetic moment. Measurements of sheet resistance, magnetoresistance, and the anomalous Hall effect understood the material’s electrical properties, and Raman spectroscopy characterized the graphene layer. Density Functional Theory calculations helped understand the stability of different configurations of europium and strontium. This research demonstrates the successful integration of europium with graphene, creating a new two-dimensional material with interesting magnetic and electrical properties. This opens possibilities for developing new spintronic devices, sensors, and other applications leveraging the unique properties of graphene and magnetic materials, with the observed magnetoresistance and anomalous Hall effect particularly promising for practical devices.

Graphene Europium Heterostructures Grown by Epitaxy

The research team pursued an innovative approach to creating new magnetic materials by combining graphene with europium, aiming to induce magnetism within the material without compromising its exceptional electronic properties. A key challenge was achieving this magnetism in a controlled and stable manner, prompting the selection of a method involving precise layering of materials at the atomic level. The team employed molecular beam epitaxy to construct the heterostructure, a layered material combining graphene with europium and a silicon carbide substrate. This method allows for the deposition of individual atomic layers under ultra-high vacuum conditions, ensuring exceptional control over the material’s composition and structure.

They targeted a structure where europium atoms reside between the graphene layer and an underlying carbon layer on the silicon carbide, a configuration believed to optimize magnetic interactions and preserve graphene’s electronic characteristics. To verify successful formation of this layered structure, the researchers utilized advanced characterization techniques. Reflection high-energy electron diffraction, performed during growth, provided real-time monitoring of the surface structure, confirming orderly deposition of each atomic layer. Grazing incidence X-ray diffraction analyzed the atomic arrangement in detail, confirming the intended layering of graphene and europium.

These structural analyses validated the synthesis method and ensured creation of the desired heterostructure. Magnetic measurements assessed the magnetic properties of the resulting material. By carefully subtracting background signals from the substrate, the team isolated and characterized the magnetism induced within the graphene layer by the presence of europium. Precise electrical measurements investigated how europium affected the flow of electrons through the graphene, revealing evidence of spin polarization, a key characteristic for spintronic applications. This combination of structural, magnetic, and electrical characterization provided a comprehensive understanding of the material’s properties and its potential for future technological applications.

Europium Imprints Magnetism onto Graphene Layers

Researchers have achieved a significant step towards realizing graphene-based spintronic devices by successfully integrating it with a magnetic material, creating a new two-dimensional magnetic heterostructure. While graphene possesses exceptional electronic properties, its lack of inherent magnetism has limited its application in spintronics, a field that exploits electron spin for data storage and processing. This new approach overcomes this limitation by carefully introducing europium atoms between graphene and a silicon carbide substrate, effectively imprinting magnetism onto the graphene layer. The team focused on epitaxial graphene, grown directly on a substrate, offering advantages over conventionally extracted graphene flakes in terms of uniformity and compatibility with existing semiconductor manufacturing techniques.

By precisely controlling the intercalation process, the insertion of europium atoms, they created a stable structure where the magnetic properties of europium are coupled with the electronic characteristics of graphene. This resulted in a material exhibiting easy-plane two-dimensional magnetism, meaning the magnetic moments align within the plane of the material, and the strength of this magnetism can be tuned using relatively weak magnetic fields. Importantly, the introduction of europium did not degrade the desirable electronic properties of graphene; measurements confirmed the persistence of high-mobility charge carriers within the graphene layer. The resulting material demonstrates both negative magnetoresistance, a decrease in electrical resistance in the presence of a magnetic field, and the anomalous Hall effect, where a voltage develops perpendicular to both the current and the magnetic field, both indicative of spin polarization within the graphene.

In fact, the magnetoresistance exhibits a critical exponential behavior, suggesting a strong connection between the induced magnetic state and the material’s electrical response. This work expands the limited family of two-dimensional magnets and offers a promising pathway towards creating fully two-dimensional spintronic devices. By successfully combining the benefits of graphene, its high electron mobility and chemical stability, with the magnetic properties of europium, researchers have opened up new possibilities for designing advanced electronic components with enhanced performance and functionality. The ability to control the magnetic properties through external fields further enhances the potential for practical applications in areas like data storage, sensors, and quantum computing.