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The manipulation of spin textures by spin currents is of fundamental and technological interest. A particularly interesting system is the 2D van der Waals ferromagnet Fe 3 GeTe 2, in which Néel-type skyrmions have recently been observed. The origin of these chiral spin textures is of considerable interest. Recently, it was proposed that these derive from defects in the structure that lower the symmetry and allow for a bulk vector Dzyaloshinsky-Moriya interaction. Here, we demonstrate current-induced domain wall motion in Fe 3 GeTe 2 flakes, in which the maximum domain wall velocity is an order of magnitude higher than those reported in previous studies. In heterostructures with Pt or W layers on top of the Fe 3 GeTe 2 flakes, domain walls can be moved via a combination of spin transfer and spin-orbit torques. The competition between these torques leads to a change in the direction of domain wall motion with increasing magnitude of the injected current., (© 2024. The Author(s).)

The family of two-dimensional (2D) van der Waals (vdW) materials provides a playground for tuning structural and magnetic interactions to create a wide variety of spin textures. Of particular interest is the ferromagnetic compound Fe 5 GeTe 2 that we show displays a range of complex spin textures as well as complex crystal structures. Here, using a high-brailliance laboratory X-ray source, we show that the majority (1 × 1) Fe 5 GeTe 2 (FGT5) phase exhibits a structure that was previously considered as being centrosymmetric but rather lacks inversion symmetry. In addition, FGT5 exhibits a minority phase that exhibits a long-range ordered (√3 × √3)-R30° superstructure. This superstructure is highly interesting in that it is innately 2D without any lattice periodicity perpendicular to the vdW layers, and furthermore, the superstructure is a result of ordered Te vacancies in one of the topmost layers of the FGT5 sheets rather than being a result of vertical Fe ordering as earlier suggested. We show, from direct real-space magnetic imaging, evidence for three distinct magnetic ground states in lamellae of FGT5 that are stabilized with increasing lamella thickness, namely, a multidomain state, a stripe phase, and an unusual fractal state. In the stripe phase we also observe unconventional type-I and type-II bubbles where the spin texture in the central region of the bubbles is nonuniform, unlike conventional bubbles. In addition, we find a bobber or a cocoon-like spin texture in thick (∼170 μm) FGT5 that emerges from the fractal state in the presence of a magnetic field. Among all the 2D vdW magnets we have thus demonstrated that FGT5 hosts perhaps the richest variety of magnetic phases that, thereby, make it a highly interesting platform for the subtle tuning of magnetic interactions.

The generation of spin currents and their application to the manipulation of magnetic states is fundamental to spintronics. Of particular interest are chiral antiferromagnets that exhibit properties typical of ferromagnetic materials even though they have negligible magnetization. Here, we report the generation of a robust spin current with both in-plane and out-of-plane spin polarization in epitaxial thin films of the chiral antiferromagnet Mn 3 Sn in proximity to permalloy thin layers. By employing temperature-dependent spin-torque ferromagnetic resonance, we find that the chiral antiferromagnetic structure of Mn 3 Sn is responsible for an in-plane polarized spin current that is generated from the interior of the Mn 3 Sn layer and whose temperature dependence follows that of this layer's antiferromagnetic order. On the other hand, the out-of-plane polarized spin current is unrelated to the chiral antiferromagnetic structure and is instead the result of scattering from the Mn 3 Sn/permalloy interface. We substantiate the later conclusion by performing studies with several other non-magnetic metals all of which are found to exhibit out-of-plane polarized spin currents arising from the spin swapping effect., (© 2023. The Author(s).)

Spin-triplet supercurrent spin valves are of practical importance for the realization of superconducting spintronic logic circuits. In ferromagnetic Josephson junctions, the magnetic-field-controlled non-collinearity between the spin-mixer and spin-rotator magnetizations switches the spin-polarized triplet supercurrents on and off. Here we report an antiferromagnetic equivalent of such spin-triplet supercurrent spin valves in chiral antiferromagnetic Josephson junctions as well as a direct-current superconducting quantum interference device. We employ the topological chiral antiferromagnet Mn 3 Ge, in which the Berry curvature of the band structure produces fictitious magnetic fields, and the non-collinear atomic-scale spin arrangement accommodates triplet Cooper pairing over long distances (>150 nm). We theoretically verify the observed supercurrent spin-valve behaviours under a small magnetic field of <2 mT for current-biased junctions and the direct-current superconducting quantum interference device functionality. Our calculations reproduce the observed hysteretic field interference of the Josephson critical current and link these to the magnetic-field-modulated antiferromagnetic texture that alters the Berry curvature. Our work employs band topology to control the pairing amplitude of spin-triplet Cooper pairs in a single chiral antiferromagnet., (© 2023. The Author(s).)

Antiferromagnets with non-collinear spin structures display various properties that make them attractive for spintronic devices. Some of the most interesting examples are an anomalous Hall effect despite negligible magnetization and a spin Hall effect with unusual spin polarization directions. However, these effects can only be observed when the sample is set predominantly into a single antiferromagnetic domain state. This can only be achieved when the compensated spin structure is perturbed and displays weak moments due to spin canting that allows for external domain control. In thin films of cubic non-collinear antiferromagnets, this imbalance is previously assumed to require tetragonal distortions induced by substrate strain. Here, it is shown that in Mn 3 SnN and Mn 3 GaN, spin canting is due to structural symmetry lowering induced by large displacements of the magnetic manganese atoms away from high-symmetry positions. These displacements remain hidden in X-ray diffraction when only probing the lattice metric and require measurement of a large set of scattering vectors to resolve the local atomic positions. In Mn 3 SnN, the induced net moments enable the observation of the anomalous Hall effect with an unusual temperature dependence, which is conjectured to result from a bulk-like temperature-dependent coherent spin rotation within the kagome plane., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

The fabrication of three-dimensional nanostructures is key to the development of next-generation nanoelectronic devices with a low device footprint. Magnetic racetrack memory encodes data in a series of magnetic domain walls that are moved by current pulses along magnetic nanowires. To date, most studies have focused on two-dimensional racetracks. Here we introduce a lift-off and transfer method to fabricate three-dimensional racetracks from freestanding magnetic heterostructures grown on a water-soluble sacrificial release layer. First, we create two-dimensional racetracks from freestanding films transferred onto sapphire substrates and show that they have nearly identical characteristics compared with the films before transfer. Second, we design three-dimensional racetracks by covering protrusions patterned on a sapphire wafer with freestanding magnetic heterostructures. We demonstrate current-induced domain-wall motion for synthetic antiferromagnetic three-dimensional racetracks with protrusions of up to 900 nm in height. Freestanding magnetic layers, as demonstrated here, may enable future spintronic devices with high packing density and low energy consumption., (© 2022. The Author(s).)

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 Cr 1+δ Te 2 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., (© 2022. The Author(s).)

The current-induced spin-orbit torque switching of ferromagnets has had huge impact in spintronics. However, short spin-diffusion lengths limit the thickness of switchable ferromagnetic layers, thereby limiting their thermal stability. Here, we report a previously unobserved seeded spin-orbit torque (SSOT) by which current can set the magnetic states of even thick layers of the chiral kagome antiferromagnet Mn 3 Sn. The mechanism involves setting the orientation of the antiferromagnetic domains in a thin region at the interface with spin currents arising from an adjacent heavy metal while also heating the layer above its magnetic ordering temperature. This interface region seeds the resulting spin texture of the entire layer as it cools down and, thereby, overcomes the thickness limitation of conventional spin-orbit torques. SSOT switching in Mn 3 Sn can be extended beyond chiral antiferromagnets to diverse magnetic systems and provides a path toward the development of highly efficient, high-speed, and thermally stable spintronic devices.

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 Fe 3 GeTe 2 (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., (© 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

We investigate the low temperature magnetic properties of a S=5/2 Heisenberg kagome antiferromagnet, the layered monodiphosphate Li&#95;{9}Fe&#95;{3}(P&#95;{2}O&#95;{7})&#95;{3}(PO&#95;{4})&#95;{2}, using magnetization measurements and ^{31}P nuclear magnetic resonance. An antiferromagnetic-type order sets in at T&#95;{N}=1.3  K and a characteristic magnetization plateau is observed at 1/3 of the saturation magnetization below T^{*}∼5  K. A moderate ^{31}P NMR line broadening reveals the development of anisotropic short-range correlations concomitantly with a gapless spin-lattice relaxation time T&#95;{1}∼k&#95;{B}T/ℏS, which may point to the presence of a semiclassical nematic spin-liquid state predicted for the Heisenberg kagome antiferromagnetic model or to the persistence of the zero-energy modes of the kagome lattice under large magnetic fields.

The proximity-coupling of a chiral non-collinear antiferromagnet (AFM) 1-5 with a singlet superconductor allows spin-unpolarized singlet Cooper pairs to be converted into spin-polarized triplet pairs 6-8 , thereby enabling non-dissipative, long-range spin correlations 9-14 . The mechanism of this conversion derives from fictitious magnetic fields that are created by a non-zero Berry phase 15 in AFMs with non-collinear atomic-scale spin arrangements 1-5 . Here we report long-ranged lateral Josephson supercurrents through an epitaxial thin film of the triangular chiral AFM Mn 3 Ge (refs. 3-5 ). The Josephson supercurrents in this chiral AFM decay by approximately one to two orders of magnitude slower than would be expected for singlet pair correlations 9-14 and their response to an external magnetic field reflects a clear spatial quantum interference. Given the long-range supercurrents present in both single- and mixed-phase Mn 3 Ge, but absent in a collinear AFM IrMn 16 , our results pave a way for the topological generation of spin-polarized triplet pairs 6-8 via Berry phase engineering 15 of the chiral AFMs., (© 2021. The Author(s).)

Different amounts of graphene-wrapped magnesium oxide (G@MgO) powders are uniformly dispersed in poly(vinyl alcohol) (PVA) solution in different experiments to obtain solutions which are coagulated to obtain solid materials, which are then hot pressed at 413 K and 3 t of pressure to finally obtain 1 mm thick freestanding G@MgO/PVA composite sheets in which the constituents, namely, graphene and MgO (in the form of G@MgO), are the nanofillers in PVA matrix. During synthesis of G@MgO powder, MgO nanoparticles are in situ wrapped by the graphene nanosheets as revealed by electron microscopy. Uniformity of G@MgO dispersion in PVA was confirmed by secondary electron micrographs and the consistency in X-ray diffraction and Raman scattering data collected from different locations of the samples. Temperature (303-393 K) dependent complex permittivity of G@MgO/PVA composite sheets (including those prepared by casting) in low frequency (20 Hz to 2 MHz) and high frequency (i.e., X-band, 8.2-12.4 GHz) ranges are measured. In both frequency ranges, G@MgO/PVA composite sheets prepared by coagulation exhibited dielectric properties superior to those of PVA and G@MgO/PVA composite sheets prepared by casting. A strong interfacial polarization is observed in coagulated and as-cast G@MgO/PVA composite sheets. It is noticed from the calculated activation energies that conduction is the dominating mechanism for energy transfer in both composite sheets' cases, while it is predominating in coagulated composite sheets due to the better network formation of the fillers in the coagulated samples than in the cast composite samples. The electromagnetic interference shielding effectiveness (EMI SE) values in the X-band frequency range (i.e., 8.2-12.4 GHz) of the G@MgO/PVA composite sheets prepared by coagulation are more than those prepared by casting for a particular weight fraction of G@MgO. At 393 K, for a particular G@MgO/PVA composite sheet prepared by coagulation, an excellent EMI SE of ∼27.5 dB is measured. It is also experimentally elucidated that the absorption is the dominating mechanism for EMI SE in the prepared composite sheets.

We report the magnetic and dielectric behavior of Pb 6 Ni 9 (TeO 6 ) 5 , a new compound comprising the honeycomb-like layers of S = 1 spins, through detailed structural, magnetic and dielectric investigation. An antiferromagnetic-type transition at 25 K (T N ) with weak-ferromagnetic behavior is revealed. Interestingly, a large value of coercive field of 1.32 T at 2 K is observed. The isothermal magnetization after zero-field-cooled condition, it exhibits the presence of large spontaneous exchange bias (SEB) with a magnitude of 0.19 T at 2 K; which is rare in single bulk materials, especially without external doping. The value of |H EB | further enhances to 0.24 T under 16 T field-cooled condition, confirming the presence of large exchange bias in the material.