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.

We have carried out a surface x-ray diffraction analysis of the geometric structure of a 13-monolayer-thick Fe film on W͑110͒. The Fe adlayer is characterized by a well-ordered two-dimensional array of misfit dislocations that have a periodicity of 35.84 and 50.76 Å along the ͓001͔ and ͓1 10͔ directions, respectively. In the ͑110͒ plane the average Fe structure is isotropically strained by ϩ1.2% with respect to ͑bulk͒ bcc Fe, corresponding to lattice constants of 2.901 Å along ͓001͔ and 4.103 Å along ͓1 10͔. In the surface normal direction ͓͑110͔͒ we find that the Fe film is also strained by ϩ0.22% (cϭ4.062 Å). This corresponds to a slightly laterally distorted bct phase with aϭbϭ2.887 Å, cϭ2.901 Å, and ␥ϭ89.4°. The intensity distribution along 14 satellite rods was analyzed quantitatively using a sine-wave modulation ansatz for the lateral and vertical displacements of the Fe atoms out of their average positions. We find maximum amplitudes in the order of about 0.7 Å for the normal and lateral modulations, respectively. The modulation amplitudes continuously decrease with distance from the Fe/W͑110͒ interface. The adlayer/substrate registry is characterized by a 37/34 coincidence between the Fe-the W atoms, where the vertical Fe corrugation is directly related to the Feadsorption site. The implications of the unexpected strain state of the film for its mechanical stress and magnetic anisotropy are discussed.

Transition-metal dichalcogenides intercalated with 3d-transition metals within the van der Waals (vdW) gaps have been the focus of intense investigations owing to their fascinating structural and magnetic properties. At certain concentrations the intercalated atoms form ordered superstructures that exhibit ferromagnetic or anti-ferromagnetic ordering. Here we show that the self-intercalated compound Cr1+δTe2 with δ ≈ 0.3 exhibits a new, so far unseen, three-dimensionally ordered (2×2×2) superstructure. Furthermore, high resolution X-ray diffraction reveals that there is an asymmetric occupation of the two inequivalent vdW gaps in the unit cell. The structure thus lacks inversion symmetry, which, thereby, allows for chiral non-collinear magnetic nanostructures. Indeed, Néel-type skyrmions are directly observed using Lorentz transmission electron microscopy. The skyrmions are stable within the accessible temperature range (100–200 K) as well as in zero magnetic field. The diameter of the Néel skyrmions increases with lamella thickness and varies with applied magnetic field, indicating the role of long-range dipole fields. Our studies show that self-intercalation in vdW materials is a novel route to the formation of synthetic non-collinear spin textures.

Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO 3-δ ( P- SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO 2.5 ( BM- SCO), is insulating and an antiferromagnet. Moreover, BM- SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally ( H -SCO) or vertically ( V -SCO) to the film’s surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H -SCO (antiferromagnetic insulator, AFI) can be converted to P -SCO (ferromagnetic metal, FM) and subsequently to V -SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H -SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices.

Dodecagonal oxide quasicrystals are well established as examples of long-range aperiodic order in two dimensions. However, despite investigations by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), photoemission spectroscopy as well as density functional theory (DFT), their structure is still controversial. Furthermore, the principles that guide the formation of quasicrystals (QCs) in oxides are elusive since the principles that are known to drive metallic QCs are expected to fail for oxides. Here we demonstrate the solution of the oxide QC structure by synchrotron-radiation based surface x-ray diffraction (SXRD) refinement of its largest-known approximant. The oxide QC formation is forced by large alkaline earth metal atoms and the reduction of their mutual electrostatic repulsion. It drives the n = 6 structure of the 2D Ti2O3 honeycomb arrangement via Stone–Wales transformations into an ordered structure with empty n = 4, singly occupied n = 7 and doubly occupied n = 10 rings, as supported by DFT.