Long-range supercurrents through a chiral non-collinear antiferromagnet in lateral Josephson junctions.

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 pairs6-8, thereby enabling non-dissipative, long-range spin correlations9-14. The mechanism of this conversion derives from fictitious magnetic fields that are created by a non-zero Berry phase15 in AFMs with non-collinear atomic-scale spin arrangements1-5. Here we report long-ranged lateral Josephson supercurrents through an epitaxial thin film of the triangular chiral AFM Mn3Ge (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 correlations9-14 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 Mn3Ge, but absent in a collinear AFM IrMn16, our results pave a way for the topological generation of spin-polarized triplet pairs6-8 via Berry phase engineering15 of the chiral AFMs.

Current-induced domain wall motion in a van der Waals ferromagnet Fe3GeTe2.

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 Fe3GeTe2, 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 Fe3GeTe2 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 Fe3GeTe2 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.

Chiral antiferromagnetic Josephson junctions as spin-triplet supercurrent spin valves and d.c. SQUIDs.

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 Mn3Ge, 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.

Three-dimensional racetrack memory devices designed from freestanding magnetic heterostructures.

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.

Observation of Néel-type skyrmions in acentric self-intercalated Cr1+δTe2.

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.

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

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 Fe3 GeTe2 (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.

Author Correction: Large spontaneous exchange bias in a weak ferromagnet Pb6Ni9(TeO6)5.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Graphene-Wrapped MgO/Poly(vinyl alcohol) Composite Sheets: Dielectric and Electromagnetic Interference Shielding Properties at Elevated Temperatures.

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.

Large spontaneous exchange bias in a weak ferromagnet Pb6Ni9(TeO6)5.

We report the magnetic and dielectric behavior of Pb6Ni9(TeO6)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.