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.
