Magnetic ordering and spin interactions are mediated by sulfur vacancies in few-layered correlated 2D vdW Ni 1− x Co x PS 3 nanosheets.

Magnetic skyrmions are topologically stable vortex-like spin structures that are promising for next generation information storage applications. Materials that host magnetic skyrmions, such as MnSi and FeGe with the noncentrosymmetric cubic B20 crystal structure, have been shown to stabilize skyrmions upon nanostructuring. Here, we report a chemical vapor deposition method to selectively grow nanowires (NWs) of cubic FeGe out of three possible FeGe polymorphs for the first time using finely ground particles of cubic FeGe as seeds. X-ray diffraction and transmission electron microscopy (TEM) confirm that these micron-length NWs with ∼100 nm to 1 μm diameters have the cubic B20 crystal structure. Although FeGe NWs are also formed, the two types of NWs can be readily differentiated by their faceting. Lorentz TEM imaging of the cubic FeGe NWs reveals a skyrmion lattice phase under small applied magnetic fields (∼0.1 T) at 233 K, a skyrmion chain state at lower temperatures (95 K) and under high magnetic fields (∼0.4 T), and a larger skyrmion stability window than bulk FeGe. This synthetic approach to cubic FeGe NWs that support stabilized skyrmions opens a route toward the exploration of new skyrmion physics and devices based on similar nanostructures.

In bulk chiral crystals, 3D structures of magnetic skyrmions form topologically protected skyrmion strings (SkS) that have shown potential as magnonic nano‐waveguides for information transfer. Although SkS stability is expected to be enhanced in nanostructures of skyrmion‐hosting materials, experimental observation and detection of SkS in nanostructures under an applied in‐plane magnetic field is difficult. Here, temperature‐dependent magnetic field‐driven creation and annihilation of SkS in B20 FeGe nanostructures (nanowires and nanoplates) under in‐plane magnetic field (H||) are shown and the mechanisms behind these transformations are explained. Unusual asymmetric and hysteretic magnetoresistance (MR) features are observed but previously unexplained during magnetic phase transitions within the SkS stability regime when H|| is along the nanostructure's long edge, which increase the sensitivity of MR detection. Lorentz transmission electron microscopy of the SkS and other magnetic textures under H|| in corroboration with the analysis of the anisotropic MR responses elucidates the field‐driven creation and annihilation processes of SkS responsible for such hysteretic MR features and reveals an unexplored stability regime in nanostructures.

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Magnetic skyrmions are a new form of magnetic ordering with whirlpool-like spin arrangements. These topologically protected particlelike spin textures were first discovered a decade ago in noncentrosymmetric magnetic materials. Confining magnetic skyrmions in nanostructures leads to interesting fundamental insights into skyrmion stability and could provide convenient platforms for potential practical applications of skyrmions in information storage technology. In this research update, we summarize the recent advances on studying magnetic skyrmions in nanostructures of skyrmion hosting noncentrosymmetric materials (especially the B20 materials) made via bottom-up synthesis or top-down fabrication methods. We discuss various real space imaging (such as Lorentz transmission electron microscopy or electron holography) or physical property measurement (such as magneto-transport) techniques that have been used to observe and detect these exotic magnetic domains in both nanostructure and bulk samples, which have proven to be critical to fully understanding them. We examine the importance of morphology and dimensionality of skyrmion hosting materials in stabilizing isolated magnetic skyrmions in confined geometry and their benefits for implementation in magnetic memory applications. We further highlight the need for experiments that allow the skyrmion research to move from the fundamental physics of skyrmion formation and dynamics to more applied device studies and eventual applications, such as the all-electrical writing and reading of skyrmions needed for skyrmion-based high density magnetic memory storage devices.

Magnetic skyrmions are topologically protected spin textures that are being heavily investigated for their potential use in next generation magnetic storage devices.However, transport studies of skyrmions in nanostructures is limited due to the difficulty of their detection. Here, magnetic skyrmions and other magnetic phases in Fe 1-x Co x Ge (x < 0.1) microplates (MPLs) newly synthesized via chemical vapor deposition were studied using both magnetic imaging and transport measurements. Lorentz transmission electron This article is protected by copyright. All rights reserved.2 microscopy revealed a stabilized magnetic skyrmion phase near room temperature (~280 K) and a quenched metastable skyrmion lattice via field cooling. Magnetoresistance (MR) measurements in three different configurations revealed a unique anomalous MR signal at temperatures below 200 K and two distinct field dependent magnetic transitions. The topological Hall effect (THE), known as the electronic signature of magnetic skyrmion phase, was detected for the first time in a Fe 1-x Co x Ge nanostructure, with a large and positive peak THE resistivity of ~32 nΩ•cm at 260 K. This large magnitude is attributed to both nanostructuring and decreased carrier concentrations due to Co alloying of the Fe 1-x Co x Ge MPL, which suggests alloying as a strategy to enhance the THE signals of skyrmion materials. A consistent magnetic phase diagram summarized from both the magentic imaging and transport measurements shows that the magnetic skyrmions are stabilized in Fe 1-x Co x Ge MPLs compared to bulk materials. This comprehensive electrical device and magnetic imaging study lays the solid foundation for future studies of skyrmion-based nanodevices to realize their full potential in information storage and processing technologies.

Iron monogermanide (FeGe) with the noncentrosymmetric cubic B20 structure is a well-known helimagnet and a magnetic skyrmion host with a relatively high ordering temperature (∼280 K). FeGe and related metal monogermanide compounds, such as CoGe and MnGe, have several structural polymorphs and typically require high pressure (∼4 GPa) and high temperature (∼1000 °C) to synthesize in the cubic B20 structure. Here, we report that the cubic B20 phase of both FeGe and alloys of Fe 1−x Co x Ge could in fact be formed without the application of high pressure by simply reacting elemental powders at modest temperatures (550 °C). Furthermore, the incorporation of Co into Fe 1−x Co x Ge (0.05 ≤ x ≤ 0.1) stabilizes the cubic B20 structure up to 650 °C, which we propose is caused by chemical pressure induced by the incorporation of Co into the lattice. Interestingly, chemical vapor transport reactions using the Fe 1−x Co x Ge alloys as precursors yield plentiful growth of large (0.1 to 1 mm) single crystals of pure FeGe. Magnetic susceptibility measurements of the Fe 0.95 Co 0.05 Ge alloy show evidence of a skyrmion phase not previously reported in the Fe 1−x Co x Ge system.

Magnetic skyrmions are topological spin textures that have shown promise for future nonvolatile memory devices. Herein, we report on the stability of magnetic skyrmions in alloyed cubic B20 Fe 1−x Co x Si nanowires (NWs) determined using off-axis electron holography and magnetotransport measurements. This study presents the real space observation of one-dimensional skyrmion lattice in a NW of Fe 1−x Co x Si which shows that the skyrmion phase in a Fe 0.75 Co 0.25 Si NW exists at lower applied magnetic fields (200 Oe) with a reduced domain size (28 ± 2 nm) in comparison to bulk and thin film samples. Magnetotransport measurements were used to observe the helimagnetic transition temperature dependence on the cobalt concentration in the Fe 1−x Co x Si NWs. Field-dependent magnetoresistance measurements of Fe 1−x Co x Si NWs under applied magnetic field parallel to the NW axis and their second derivative plots reveal the critical fields for the magnetic state transition at different temperatures. A representative magnetic phase diagram constructed with the results from transport measurements of a Fe 0.81 Co 0.19 Si NW clearly shows expanded stability region for magnetic skyrmions in the Fe 1−x Co x Si NWs.