Observation of Magnetic Skyrmion Bubbles in a van der Waals ferromagnet Fe3GeTe2

Abstract: Two-dimensional (2D) van der Waals (vdW) magnetic materials have recently been introduced as a new horizon in materials science and enable the potential applications for next-generation spintronic devices. Here, in this communication, the observations of stable Bloch-type magnetic skyrmions in single crystals of 2D vdW Fe3GeTe2 (FGT) are reported by using in-situ Lorentz transmission electron microscopy (TEM). We find the ground-state magnetic stripe domains in FGT transform into skyrmion bubbles when an external magnetic field is applied perpendicularly to the (001) thin plate with temperatures below the Curie-temperature TC. Most interestingly, a hexagonal lattice of skyrmion bubbles is obtained via field cooling manipulation with magnetic field applied along the [001] direction. Owing to their topological stability, the skyrmion bubble lattices are stable to large field-cooling tilted angles and further reproduced by utilizing the micromagnetic simulations. These observations directly demonstrate that the 2D vdW FGT possesses a rich variety of topological spin textures, being of a great promise candidate for future applications in the field of spintronics.

• The paper demonstrates the transformation of magnetic stripe domains into Bloch-type skyrmion bubbles when a perpendicular magnetic field is applied.
• It employs in-situ Lorentz-TEM and micromagnetic simulations to reveal a high-density hexagonal lattice of skyrmion bubbles via a field-cooling process.
• The findings underscore Fe3GeTe2’s potential for spintronic devices by enabling controlled tuning of topological spin textures below its Curie temperature.

Observation of Magnetic Skyrmion Bubbles in a van der Waals Ferromagnet Fe$_3$GeTe$_2$

The paper presents the observation of Bloch-type magnetic skyrmion bubbles in two-dimensional (2D) van der Waals (vdW) ferromagnet Fe$_3$GeTe$_2$ (FGT), utilizing in-situ Lorentz transmission electron microscopy (TEM) to reveal topological spin textures. The results showcase the transformation of ground-state magnetic stripe domains into skyrmion bubbles upon applying an external magnetic field perpendicular to the (001) thin plate, below the Curie temperature ($T_C$).

In a remarkable display of topological stability, the paper reports the formation of a high-density hexagonal lattice of skyrmion bubbles achieved through a field-cooling process. These structures withstand large field-cooling tilted angles of the magnetic field and find further confirmation via micromagnetic simulations. This demonstration adds to evidence suggesting that 2D vdW FGT is a viable candidate for future spintronic applications, offering a rich variety of manipulatable topological spin textures.

Subsequent experimental details encompass the synthesis of single crystal FGT via self-flux techniques, confirmed by high-resolution structural characterization through X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). Magnetic characteristics such as the transition temperature ($T_C \approx 150$ K) and magnetic anisotropy manifest clearly in zero-field cooling (ZFC) and field-cooling (FC) magnetization curves, reinforcing the ferromagnetic metal characteristics extending from bulk to monolayer forms.

Moreover, using in-situ Lorentz-TEM, the researchers document the evolution of magnetic domain structures across varying temperatures and the effects of different external magnetic fields up to 920 Oe. An essential highlight is the gradual conversion of magnetic stripe domains into distinct skyrmion bubbles with increasing magnetic field, showcasing a dynamic morphological shift. Notably, as the magnetic field reaches 680 Oe, these bubbles emerge prominently, indicating successful isolation by employing an external magnetic field perpendicular to the FGT thin plate.

The study extends these observations by applying an oblique magnetic field to examine the stability and arrangement of skyrmion bubble lattices under varied tilt angles. Results highlight the formation and retention of hexagonal skyrmion lattices post-field exposure, corroborated by TIE analysis and theoretical simulations, illustrating Bloch-type skyrmions—a recurring motif in both centrosymmetric and non-centrosymmetric magnetic systems.

Implications of these findings are significant for the development of skyrmion-based devices in spintronics, leveraging the inherently stable properties of skyrmions in 2D vdW crystals. The capability to tune topological spin textures leveraging magnetic fields suggests potential avenues for enhancements in magnetic and magnetotransport properties pivotal to advancing technological applications.

The paper aligns with burgeoning research interest in 2D vdW materials, encouraging further exploration into the interplay of magnetic parameters such as thickness, spin-orbit coupling, and interfacial symmetry. Such inquiries may unlock a spectrum of tunable magnetic phenomena poised to enhance spintronic functionality, reinforcing FGT and its heterostructures as promising platforms for future innovations.

In conclusion, the detailed examination of magnetic skyrmion bubbles within FGT enriches understanding of 2D vdW systems, ultimately broadening the landscape of spintronic material science. The results prompt more nuanced inquiries into diverse topological spin configurations that these materials can sustain, setting the stage for scalable, novel applications in quantum materials.