Magnetic Skyrmions in a Thickness Tunable 2D Ferromagnet from a Defect Driven Dzyaloshinskii–Moriya Interaction

Abstract

There is considerable interest in van der Waals (vdW) materials as potential hosts for chiral skyrmionic spin textures. Of particular interest is the ferromagnetic, metallic compound Fe3GeTe2 (FGT), which has a comparatively high Curie temperature (150–220 K). Several recent studies have reported the observation of chiral Néel skyrmions in this compound, which is inconsistent with its presumed centrosymmetric structure. Here the observation of Néel type skyrmions in single crystals of FGT via Lorentz transmission electron microscopy (LTEM) is reported. It is shown from detailed X‐ray diffraction structure analysis that FGT lacks an inversion symmetry as a result of an asymmetric distribution of Fe vacancies. This vacancy‐induced breaking of the inversion symmetry of this compound is a surprising and novel observation and is a prerequisite for a Dzyaloshinskii–Moriya vector exchange interaction which accounts for the chiral Néel skyrmion phase. This phenomenon is likely to be common to many 2D vdW materials and suggests a path to the preparation of many such acentric compounds. Furthermore, it is found that the skyrmion size in FGT is strongly dependent on its thickness: the skyrmion size increases from ≈100 to ≈750 nm as the thickness of the lamella is increased from ≈90 nm to ≈2 µm. This extreme size tunability is a feature common to many low symmetry ferro‐ and ferri‐magnetic compounds.

History-dependent domain and skyrmion formation in 2D van der Waals magnet Fe3GeTe2

“…With this in mind, we tentatively suggest that in such thick samples, any present interfacial DMI from the oxide layer may no longer be sufficient to stabilise Néel-type domain walls, and this would provide strong indication that the DMI observed in FGT flakes must be due to some interfacial effect. However, recent studies have suggested that the DMI could instead be due to some intrinsic property of the bulk material, either due to vacancy driven DMI 38 or additional higher order effects 39 . Further work may be required to elucidate the true origin of the monochrial Néel-type domain walls.…”

“…Recently, the formation of magnetic skyrmions—topological nanoscale whirls in the spin structure of magnetic materials 30 , 31 —has been reported in FGT flakes 32 – 38 . This discovery opens up further prospective applications such as 2D skyrmion racetrack or neuromorphic computing devices 40 , 41 .…”

“…To this end, the possibility of exploiting the atomically flat and well-defined interfaces offered by vdW materials presents significant advantages over typical sputtered multilayer thin-film skyrmion systems 42 , 43 . However, there have been a variety of conflicting mechanisms proposed for skyrmion formation in FGT flakes, including the dipolar interaction 32 , an intrinsic interfacial Dzyaloshinskii-Moriya interaction (DMI) between the Te and Fe layer interface 37 , an interfacial DMI between pristine FGT and its naturally oxidised top and bottom layers 33 , 35 , DMI induced by intrinsic defects within the bulk of the material 38 , or more complex higher-order contributions 39 . The matter has been further complicated by the observation of both Bloch 32 , 35 and Néel-type 33 – 37 skyrmions.…”

Hidden interplay of current-induced spin and orbital torques in bulk Fe$_3$GeTe$_2$

“…In some instances, if multiple pairs of layers have displaced in different ways, the inversion symmetry could be broken in this fashion. Moreover, a recent experimental work reports a discrepancy in the occupancy of the central Fe atom in the A layer versus the B layer [49]. This implies that the unit cell is no longer inversion symmetric, providing the avenue required for bulk torque generation.…”

“…by surfaces and interfaces [14,15], inversion sym-metry present in ideal bulk FGT ultimately suppresses the overall torque on the magnetization. However, the reality is that there are a variety of "intrinsic" mechanisms which can drive the inversion symmetry breaking in this bulk system [15,49]. One of these effects is that of magnetostriction [50].…”