Hard magnetic properties in nanoflake van der Waals Fe3GeTe2

Abstract: Two dimensional (2D) van der Waals (vdW) materials have demonstrated fascinating optical, electrical and thickness-dependent characteristics. These have been explored by numerous authors but reports on magnetic properties and spintronic applications of 2D vdW materials are scarce by comparison. By performing anomalous Hall effect transport measurements, we have characterised the thickness dependent magnetic properties of single crystalline vdW Fe3GeTe2. The nanoflakes of this vdW metallic material exhibit a single hard magnetic phase with a near square-shaped magnetic loop, large coercivity (up to 550 mT at 2 K), a Curie temperature near 200 K and strong perpendicular magnetic anisotropy. Using criticality analysis, we confirmed the existence of magnetic coupling between vdW atomic layers and obtained an estimated coupling length of ~ 5 vdW layers in Fe3GeTe2. Furthermore, the hard magnetic behaviour of Fe3GeTe2 can be well described by a proposed model. The magnetic properties of Fe3GeTe2 highlight its potential for integration into vdW magnetic heterostructures, paving the way for spintronic research and applications based on these devices.

• The paper demonstrates a single hard magnetic phase in Fe3GeTe2 nanoflakes, with coercivities up to 550 mT at 2 K and near square-shaped hysteresis loops.
• The paper employs anomalous Hall effect measurements and a refined Stoner-Wohlfarth model to elucidate strong perpendicular anisotropy and interlayer magnetic coupling.
• The paper highlights the potential of FGT nanoflakes for spintronic devices, particularly in vdW heterostructures featuring enhanced spin-orbit torque and magnetoresistance.

Insights into Hard Magnetic Properties of Fe$_3$GeTe$_2$ Nanoflakes

The study presented in this paper investigates the thickness-dependent magnetic properties of single crystalline van der Waals (vdW) Fe$_3$GeTe$_2$ (FGT) nanoflakes. Utilizing anomalous Hall effect transport measurements, the authors offer a comprehensive examination of the hard magnetic properties inherent to these vdW metallic nanostructures, highlighting their potential in future spintronic applications.

Overview of Findings

The key finding of this study is the manifestation of a single hard magnetic phase in FGT nanoflakes of specific thicknesses (<200 nm). Significantly, these nanoflakes exhibit a near square-shaped magnetic loop and coercivity values up to 550 mT at 2 K, as well as strong perpendicular magnetic anisotropy. The coercivities and the ratio of remanence to saturation magnetization (M$_R$/M$_S$) approach unity for nanoflakes of 82 nm, 49 nm, and 10.4 nm thicknesses, marking an improvement compared to bulk FGT, which suffers from a smaller coercivity and M$_R$/M$_S$ ratio.

Through criticality analysis, the study deduces a magnetic coupling length of approximately five vdW layers, indicating a notable interlayer magnetic coupling within the atomic layers of FGT. This coupling is crucial for maintaining 3D magnetic characteristics even as the thickness approaches the quasi-2D limit.

Implications for Spintronics

The study underscores that the unique hard magnetic phase of FGT thin flakes enhances their applicability within vdW heterostructure-based spintronics. The robust coercivity and strong perpendicular anisotropy, combined with 2D metallic ferromagnetism, render FGT an ideal candidate for integration into vdW heterostructures that exploit properties like spin-orbit torque, giant magnetoresistance, and tunneling magnetoresistance. The perpendicular anisotropy that persists due to the crystalline field in FGT adds to its utility in these contexts.

Theoretical Contributions

A principal theoretical contribution of the paper is the refinement of the Stoner-Wohlfarth model to accurately describe the angular dependence of coercivity in FGT nanoflakes. The modified model considers thermal agitation energy, multi-domain flipping dynamics near coercivity, and the impact of demagnetization effects. This model aids in corroborating the single domain behavior of the nanoflakes in the magnetic field regime, away from coercivity, due to strong anisotropic energy.

Future Directions

The study suggests further exploration into the evolution of ferromagnetic behavior as thickness decreases down to monolayers, bridging the gap between 3D and 2D ferromagnetism. Scaling analyses near the Curie temperature will be integral to fully capturing this transition. Additionally, although the presence of an amorphous oxide layer on FGT flakes doesn't impede the core findings, ultra-clean fabrication methods refined during this study establish a path for future investigations into pristine vdW ferromagnetic materials.

Conclusion

This investigation of FGT nanoflakes provides significant insights into the behavior of vdW ferromagnetic metals and their potential role in next-generation spintronic devices. By elucidating the thickness-dependent magnetic characteristics and developing a nuanced theoretical model, the research contributes to the foundational understanding necessary for realizing vdW heterostructure-based applications. The paper's conclusions point towards a bright prospect for FGT nanoflakes in revolutionary spintronic technologies, offering a rich avenue for both practical implementations and further academic inquiry into 2D material magnetism.