Current-driven dynamics of skyrmions stabilized in MnSi nanowires revealed by topological Hall effect.

Skyrmions hold promise for next-generation magnetic storage as their nanoscale dimensions may enable high information storage density and their low threshold for current-driven motion may enable ultra-low energy consumption. Skyrmion-hosting nanowires not only serve as a natural platform for magnetic racetrack memory devices but also stabilize skyrmions. Here we use the topological Hall effect (THE) to study phase stability and current-driven dynamics of skyrmions in MnSi nanowires. THE is observed in an extended magnetic field-temperature window (15-30 K), suggesting stabilization of skyrmions in nanowires compared with the bulk. Furthermore, we show in nanowires that under the high current density of 10(8)-10(9) A m(-2), the THE decreases with increasing current densities, which demonstrates the current-driven motion of skyrmions generating the emergent electric field in the extended skyrmion phase region. These results open up the exploration of skyrmions in nanowires for fundamental physics and magnetic storage technologies.

Highly stable skyrmion state in helimagnetic MnSi nanowires.

Topologically stable magnetic skyrmions realized in B20 metal silicide or germanide compounds with helimagnetic order are very promising for magnetic memory and logic devices. However, these applications are hindered because the skyrmions only survive in a small temperature-field (T-H) pocket near the critical temperature Tc in bulk materials. Here we demonstrate that the skyrmion state in helimagnetic MnSi nanowires with varied sizes from 400 to 250 nm can exist in a substantially extended T-H region. Magnetoresistance measurements under a moderate external magnetic field along the long axis of the nanowires (H∥) show transitions corresponding to the skyrmion state from Tc ∼32 K down to at least 3 K, the lowest temperature in our measurement. When the field is applied perpendicular to the wire axis (H⊥), the skyrmion state was not resolvable using the magnetoresistance measurements. Our analysis suggests that the shape-induced uniaxial anisotropy might be responsible for the stabilization of skyrmion state observed in nanowires.

Three-dimensional mesoscale heterostructures of ZnO nanowire arrays epitaxially grown on CuGaO2 nanoplates as individual diodes.

We report a three-dimensional (3D) mesoscale heterostructure composed of one-dimensional (1D) nanowire (NW) arrays epitaxially grown on two-dimensional (2D) nanoplates. Specifically, three facile syntheses are developed to assemble vertical ZnO NWs on CuGaO2 (CGO) nanoplates in mild aqueous solution conditions. The key to the successful 3D mesoscale integration is the preferential nucleation and heteroepitaxial growth of ZnO NWs on the CGO nanoplates. Using transmission electron microscopy, heteroepitaxy was found between the basal planes of CGO nanoplates and ZnO NWs, which are their respective (001) crystallographic planes, by the observation of a hexagonal Moiré fringes pattern resulting from the slight mismatch between the c planes of ZnO and CGO. Careful analysis shows that this pattern can be described by a hexagonal supercell with a lattice parameter of almost exactly 11 and 12 times the a lattice constants for ZnO and CGO, respectively. The electrical properties of the individual CGO-ZnO mesoscale heterostructures were measured using a current-sensing atomic force microscopy setup to confirm the rectifying p-n diode behavior expected from the band alignment of p-type CGO and n-type ZnO wide band gap semiconductors. These 3D mesoscale heterostructures represent a new motif in nanoassembly for the integration of nanomaterials into functional devices with potential applications in electronics, photonics, and energy.

Observation of the magnetic skyrmion lattice in a MnSi nanowire by Lorentz TEM.

We report here the real-space observation of skyrmions and helical magnetic domains in a MnSi nanowire (NW) using Lorentz transmission electron microscopy (LTEM). The MnSi NW was thinned to a rectangular cross-section by focused-ion beam milling to reduce obstructive Fresnel fringes. Helimagnetic domains, imaged as alternating bright and dark contrast stripes with an 18 nm period, were observed to be the spontaneous magnetic ground state at 6 K, while the hexagonal skyrmion lattice (SkX) with a domain diameter of 18 nm was observed under a normal magnetic field of 210 mT. Temperature-dependent measurements reveal that the SkX is stable over a larger range in this NW system (6-35 K) compared to the narrow temperature regime of skyrmion phase in bulk MnSi (26-30 K) and thin films of MnSi (5-23 K).

A general method to measure the Hall effect in nanowires: examples of FeS2 and MnSi.

We present a general methodology for measuring the Hall effect on nanostructures with one-dimensional (1D) nanowire morphology. Relying only on typical e-beam lithography, the methodology developed herein utilizes an angled electrode evaporation technique so that the nanowire itself is a shadow mask and an intimate sidewall contact can be formed for the Hall electrodes. A six-contact electrode scheme with offset transverse contacts is utilized that allows monitoring of both the longitudinal resistivity and the Hall resistivity which is extracted from the raw voltage from the transverse electrodes using an antisymmetrization procedure. Our method does not require the use of a highly engineered lithographic process to produce directly opposing Hall electrodes with a very small gap. Hall effect measurements on semiconducting iron pyrite (FeS2) nanowire devices are validated by comparing to Hall effect measurements in the conventional Hall geometry using FeS2 plate devices. This Hall effect measurement is further extended to MnSi nanowires, and the distinct anomalous Hall effect signature is identified for the first time in chiral magnetic MnSi nanowires, a significant step toward identifying the topological Hall effect due to skyrmions in chiral magnetic nanowires.

Synthesis, characterization, and variable range hopping transport of pyrite (FeS2) nanorods, nanobelts, and nanoplates.

We report the growth, structural, and electrical characterization of single-crystalline iron pyrite (FeS2) nanorods, nanobelts, and nanoplates synthesized via sulfidation reaction with iron dichloride (FeCl2) and iron dibromide (FeBr2). The as-synthesized products were confirmed to be single-crystal phase pure cubic iron pyrite using powder X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. An intermediate reaction temperature of 425 °C or a high sulfur vapor pressure under high temperatures was found to be critical for the formation of phase pure pyrite. Field effect transport measurements showed that these pyrite nanostructures appear to behave as a moderately p-doped semiconductor with an average resistivity of 2.19 ± 1.21 Ω·cm, an improved hole mobility of 0.2 cm² V⁻¹ s⁻¹, and a lower carrier concentration on the order of 10¹8-10¹9 cm⁻³ compared with previous reported pyrite nanowires. Temperature-dependent electrical transport measurements reveal Mott variable range hopping transport in the temperature range 40-220 K and transport via thermal activation of carriers with an activation energy of 100 meV above room temperature (300-400 K). Most importantly, the transport properties of the pyrite nanodevices do not change if highly pure (99.999%) precursors are utilized, suggesting that the electrical transport is dominated by intrinsic defects in pyrite. These single-crystal pyrite nanostructures are nice platforms to further study the carrier conduction mechanisms, semiconductor defect physics, and surface properties in depth, toward improving the physical properties of pyrite for efficient solar energy conversion.

Spin polarization measurement of homogeneously doped Fe(1-x)Co(x)Si nanowires by Andreev reflection spectroscopy.

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x̅ = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.

Mechanistic investigation of the growth of Fe1-xCoxSi (0 ≤ x ≤ 1) and Fe5(Si1-yGey)3 (0 ≤ y ≤ 0.33) ternary alloy nanowires.

We present the chemical vapor deposition (CVD) reactions of the single source precursor Fe(SiCl(3))(2)(CO)(4) over Si, Ge, CoSi(2)/Si, and CoSi/Si substrates to explore the growth and doping processes of silicide nanowires (NWs). Careful investigation of the composition and morphology of the NW products and the intruded silicide films from which they nucleate revealed that the group IV elements (Si, Ge) in the NW products originate from both the precursor and the substrate, while the metal elements incorporated into the NWs (Fe, Co) originate from vapor phase precursor delivery. The use of a Ge growth substrate enabled the successful synthesis of Fe(5)Si(2)Ge NWs, the first report of a metal silicide-germanide alloy NW. Further, investigation of the pyrolysis of the CoSiCl(3)(CO)(4) precursor revealed independent delivery of Co and Si species during CVD reactions. This understanding enabled a new, more robust two-precursor synthetic route to Fe(1-x)Co(x)Si alloy NWs using Fe(SiCl(3))(2)(CO)(4) and CoCl(2).

Signature of helimagnetic ordering in single-crystal MnSi nanowires.

We report the synthesis, structural characterization, and magnetotransport of single-crystalline nanowires of manganese monosilicide, MnSi. Bulk MnSi has unusual magnetic orderings, helimagnetism, and skyrmions at ambient pressure, and high pressure studies have revealed partial magnetic ordering and non-Fermi liquid behavior. MnSi nanowires were synthesized using chemical vapor deposition of MnCl(2) onto silicon substrates. The morphology, structure, and composition of these nanowires were analyzed using electron microscopy and X-ray spectroscopy. The low-temperature magnetoresistance characteristics of MnSi nanowires reveal the first signature of helimagnetism in one-dimensional nanomaterials.