Exchange bias in two-dimensional van der Waals heterostructures

Informations projet

ExBiaVdW

N° de convention de subvention: 101068014

DOI

10.3030/101068014

Projet clôturé

Date de signature de la CE 20 Juin 2022

Date de début 1 Juillet 2023

Date de fin 30 Juin 2025

Financé au titre de

• Marie Skłodowska-Curie Actions (MSCA)

Coût total

Aucune donnée

Contribution de l’UE

€ 189 687,36

Coordonné par

JOHANNES GUTENBERG-UNIVERSITAT MAINZ
Germany

Periodic Reporting for period 1 - ExBiaVdW (Exchange bias in two-dimensional van der Waals heterostructures)

Période du rapport: 2023-07-01 au 2025-06-30

Résumé du contexte et des objectifs généraux du projet

Magnetic materials play a pivotal role in our modern world, enabling technologies that range from hard disk drives to magnetic sensors in smartphones and vehicles. A key breakthrough in this domain was the discovery of exchange bias, a magnetic phenomenon that allows engineers to control the orientation of magnetic domains in thin films with great precision. This control is essential for reliable data storage and sensing, and it revolutionized the magnetic recording industry in the late 1990s—laying the foundation for today’s high-density storage devices.

Exchange bias typically arises at the interface between two types of magnetic materials: a ferromagnet (FM)—where magnetic moments align in the same direction—and an antiferromagnet (AFM)—where adjacent magnetic moments cancel each other out. When these materials are layered together, interactions at their interface can “pin” the magnetic direction of the ferromagnet, stabilizing it against external magnetic fields. This asymmetric behavior, known as exchange bias, has been harnessed for decades in magnetic read heads and spintronic devices.

However, traditional exchange bias systems rely on bulk thin films grown using high-temperature, high-vacuum techniques. These interfaces often suffer from roughness, strain, and unwanted and difficult to control chemical reactions, which complicate device performance and limit miniaturization.

This changed dramatically with the advent of van der Waals (vdW) materials—a new class of atomically thin crystals that can be stacked like LEGO blocks without concern for lattice mismatch. The groundbreaking discovery of graphene in 2004 (which won the Nobel Prize in Physics in 2010) demonstrated that 2D materials can exhibit extraordinary properties when isolated from their 3D bulk forms. Since then, researchers have identified an entire family of vdW materials—including semiconductors, insulators, and more recently, magnetic materials.

In 2017, the first magnetic van der Waals materials were discovered that order down to the monolayer limit, ushering in a new era for magnetism at the atomic scale. Unlike conventional thin films, vdW magnets can be exfoliated down to a single layer, maintain clean interfaces without strain, and can be freely combined to form heterostructures—stacks of 2D materials with tailored properties. These heterostructures open new possibilities for designing magnetic systems with unprecedented control over interlayer coupling and spin interactions.

Despite these exciting advances, the nature of exchange bias in vdW heterostructures remains poorly understood. Unlike in conventional systems, vdW materials offer new degrees of freedom: spins can lie in-plane or out-of-plane, interfaces can be tuned atom-by-atom, and magnetic properties can be controlled using electric fields, strain, or twist angles between layers. In this context, ExBiaVdW commits to illuminating this less explored yet exciting field of spintronics research by the following key objectives:

1. Fundamental Investigation of Exchange Bias in van der Waals Heterostructures: We aim to uncover the microscopic origins of exchange bias in 2D systems, distinguishing intrinsic interfacial effects from extrinsic factors such as oxidation or defects.
2. Exploration of Spin Configuration Effects: By systematically studying systems where the ferromagnet and antiferromagnet have either parallel (collinear) or perpendicular (orthogonal) spin orientations, we will determine how spin geometry affects exchange bias strength and stability.
3. Modulation of Exchange Bias via Interfacial and External Controls: Investigate how exchange bias can be tuned through interfacial engineering, layer thickness variation, and external stimuli such as electric fields or gating. This could lead to reconfigurable magnetic devices for future spintronic and quantum technologies.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

ExBiaVdW undertook a comprehensive investigation into the origin and tunability of exchange bias (EB) in van der Waals (vdW) heterostructures composed of ferromagnetic (FM) and antiferromagnetic (AFM) layered materials. The primary objective of this work was to understand how factors such as interface quality, spin orientation, and material-specific characteristics influence exchange bias—an effect of critical importance in magnetic memory and spintronic sensor technologies. The research was structured around three major objectives, each focused on a distinct material system and interfacial configuration.

The first objective involved the fabrication and characterization of Fe₃GeTe2/CrPS₄ heterostructures. CrPS₄ was chosen rather than the conventionally used CrI₃ due to its superior air stability and the presence of accessible spin-flop and spin-flip transitions, which make it a more practical and robust AFM candidate for experimental studies under ambient conditions. Initial measurements showed that the pristine Fe₃GeTe2/CrPS₄ interface, despite having a fully uncompensated and collinear interfacial spin orientation [see Schematic Figure (a)], exhibited a weak exchange bias and a low blocking temperature, indicating minimal interfacial coupling. However, the presence of a naturally formed or deliberately introduced oxidized layer on Fe₃GeTe2 (referred to as O-FGT) resulted in a dramatic enhancement of the EB effect [ACS Nano 18, 11, 8383–8391 (2024)]. Specifically, the exchange bias field increased by a factor of ten at 5 K, and the blocking temperature rose more than sevenfold, reaching 150 K. Detailed structural analysis revealed the presence of multiple magnetic phases within the O-FGT layer, helping to explain the observed temperature dependence of EB. These findings underscore the potential of disordered or defective magnetic interfacial layers to significantly influence interfacial magnetic coupling in vdW systems.

The second objective was to study MnPS₃/Fe₃GeTe2 heterostructures. These were chosen to investigate exchange bias at interfaces involving a compensated antiferromagnet as indicated in Schematic Figure (b). Conventional theory predicts negligible exchange bias in such systems due to the absence of net interfacial spin. However, the experimental results revealed a large exchange bias (~170 mT at 5 K), among the highest recorded in vdW heterostructures [Adv. Mater. 36, 2403685 (2024)]. This unexpected behavior was attributed to the emergence of a weak ferromagnetic order in MnPS₃ below 40 K, arising from a spin-reorientation transition that effectively pins the adjacent FM layer. This emergent magnetism was confirmed through single-spin nitrogen vacancy (NV) magnetometry. Furthermore, high-resolution transmission electron microscopy (HRTEM) showed the formation of amorphous interfacial layers resulting from inter-diffusion across the vdW gap, especially under thermal cycling. These thermally modulated interfaces led to exchange bias variations as large as 500%, demonstrating that interfacial dynamics, often seen as a drawback, can be harnessed as a powerful tuning mechanism.

The third objective was to explore Fe₃GeTe2/CrSBr heterostructures, where the FM and AFM components have orthogonal spin alignments —Fe₃GeTe2 with strong out-of-plane anisotropy and CrSBr with an in-plane Néel vector as shown in Schematic Figure (c). This system served as a platform to probe non-collinear exchange coupling mechanisms. Despite the orthogonal spin configuration, a sizeable exchange bias was observed. The origin of this effect was linked to asymmetric magnetization switching in Fe₃GeTe2, induced by proximity to CrSBr [arXiv e-prints pp.arXiv:2508.00082. arXiv:2508.00082]. High-resolution cross-sectional electron holography revealed domain formation in Fe₃GeTe2 during magnetization along the biasing field, followed by abrupt mono-domain switching in the opposite direction. This switching asymmetry provided a direct microscopic explanation for the observed exchange bias.

Collectively, these three investigations significantly advance the state of the art in vdW spintronics. The first two demonstrate that interfacial disorder and defectiveness, traditionally regarded as undesirable, can be exploited to enhance or tune interfacial magnetic phenomena such as exchange bias. The third offers a rare insight into the microscopic origin of exchange bias in orthogonally coupled systems, highlighting the role of domain formation leading to asymmetric switching that results in exchange bias.

These findings not only expand the fundamental understanding of exchange bias in 2D systems but also introduce new design strategies for developing reconfigurable and energy-efficient spintronic memory and sensing devices. Moving forward, the project aims to explore energy-efficient modulation strategies, including electrostatic gating to control interfacial magnetism and twist engineering to manipulate interlayer exchange interactions by tailoring spin registry. These approaches are expected to deliver robust, scalable, and low-power solutions for future vdW-based spintronic technologies.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

ExBiaVdW has made significant strides in advancing the understanding and control of exchange bias (EB) phenomena in van der Waals (vdW) magnetic heterostructures, surpassing current knowledge in several key areas:

1. Redefining the Role of Interfacial Defects
Contrary to the conventional view that defects degrade interfacial magnetic performance, our work demonstrates that defective and amorphous interfacial layers—either introduced intentionally (e.g. via oxidation or thermal cycling) or formed naturally—can enhance exchange bias strength by over an order of magnitude and increase blocking temperature significantly. This insight redefines the design paradigm for magnetic interfaces in 2D materials.
2. Uncovering Emergent Magnetism in Nominally Compensated AFMs
We discovered that compensated antiferromagnets such as MnPS₃ can exhibit unexpected exchange bias due to the emergence of weak ferromagnetic phases at low temperatures. This challenges long-standing theoretical assumptions and opens new possibilities for using a broader class of AFMs in spintronic devices.
3. Revealing Asymmetric Switching via Orthogonal Spin Coupling
Through high-resolution electron holography, we provided the first microscopic evidence of asymmetric domain switching in orthogonally coupled vdW systems (Fe₃GeTe2/CrSBr), identifying a novel mechanism of exchange bias rooted in spin geometry rather than interfacial spin compensation.
4. Dynamic and Reconfigurable Interfaces
We demonstrated that thermal cycling can be used to modulate exchange bias up to 500%, exploiting the dynamic nature of vdW interfaces—a feature largely absent in conventional thin-film architectures.

These breakthroughs position this project at the forefront of 2D magnetism and spintronics, offering new design rules and material combinations for reconfigurable, scalable, and energy-efficient magnetic devices.

FM/AF spin states: a) collinear–uncompensated, b) collinear–compensated, c) orthogonal–uncompensated