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Selective Chemical Vapor Deposition Growth of Cubic FeGe Nanowires That Support Stabilized Magnetic Skyrmions Matthew John Stolt, Zi-An Li, Brandon Phillips, Dongsheng Song, Nitish Mathur, Rafal E. Dunin-Borkowski, and Song Jin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04548 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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Selective Chemical Vapor Deposition Growth of Cubic FeGe Nanowires That Support Stabilized Magnetic Skyrmions Matthew J. Stolt1, Zi-An Li2,3, Brandon Phillips1, Dongsheng Song4, Nitish Mathur1, Rafal E. Dunin-Borkowski3, and Song Jin1 1

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Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin, 53706, United States

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Research Centre Jülich, 52425 Jülich, Germany

School of Materials Science and Engineering, Tsinghua University, 100084 Beijing, China *email: [email protected]

Abstract 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 non-centrosymmetric 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 Fe13Ge8 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 a higher applied magnetic field (~0.4 T), and a larger skyrmion stability window than bulk FeGe. This synthetic approach to cubic

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FeGe NWs that support stabilized skyrmions opens a route toward the exploration of new skyrmion physics and devices based on similar nanostructures. Keywords: cubic FeGe, nanowires, chemical vapor deposition, magnetic skyrmion, Lorentz TEM

Magnetic skyrmions are topological vortex-like spin textures1,

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that have attracted

considerable attention for their potential use in spintronic applications such as magnetic racetrack memory3 due to the relatively low current density that is required to move them.1, 2 Such magnetic whirlpools arise from a competition between a symmetric ferromagnetic exchange interaction and an anti-symmetric Dyshaloshinskii-Moriya (DM) exchange interaction.4 The DM interaction exists in systems that lack inversion symmetry, such as non-centrosymmetric crystal structures or ultra-thin films that have their inversion symmetry broken by local interfaces.1,

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Magnetic

skyrmions have been observed in both types of systems, but have most notably been studied in materials that have the cubic B20 structure, such as MnSi,4 Fe1-xCoxSi,6, 7 and FeGe.8-10 In bulk non-centrosymmetric crystals, skyrmions exist only in small windows of applied magnetic field and temperature, limiting their practical use in information storage systems.1 The existence of the magnetic skyrmion phase and the stability regime for a skyrmion lattice can be studied experimentally using neutron scattering,4,

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Hall effect measurements,10-13 magnetic

susceptibility measurements,14 or in real space using techniques such as Lorentz transmission electron microscopy (LTEM).7-9,

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The skyrmion stability window has been shown, both

theoretically18 and experimentally, to be expanded if skyrmions are confined to thin films9, 10, 19 or nanowires (NWs).20-23 Specifically, several experimental methods have shown that in MnSi NWs that the skyrmion lattice stability window is greatly enlarged even though the NW diameter of a

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few hundred nanometers is much larger than the skyrmion domain size in MnSi (~18 nm).20, 21, 23 The observed increase in stability is thought to result from the large surface/volume anisotropy inherent to nanostructures.18, 23 Bulk crystal systems such as FeGe9, 10 and Mn-Co-Zn alloys,15 whose skyrmion phases are stable only in a small window near room temperature, are also expected to benefit from nanostructuring. In addition to enhanced skyrmion stability, NWs also provide a natural platform for the implementation of racetrack memory devices due to their one dimensional (1D) nature, which acts as a guide for current-driven skyrmion motion.21, 24, 25 Thin films and bulk crystals of cubic FeGe were some of the first systems in which magnetic skyrmions were observed. It boasts the highest Néel transition temperature, TN, that has been reported for skyrmions in a material with the B20 crystal structure.1, 9 NWs of cubic FeGe promise to be interesting for the studies of skyrmion physics and potential devices because they are expected to provide an expanded region of skyrmion phase stability. However, the Fe-Ge binary system has a complicated phase diagram (Figure S1)26 with many possible phases including three stable structural polymorphs with the 1:1 Fe:Ge composition: the cubic (B20 type), the hexagonal, and the monoclinic structure. All three polymorphs have been synthesized in the form of large single crystals using sealed tube chemical vapor transport (CVT) methods.27 The growth zone temperature of the reaction dictates which polymorph is formed: the cubic at 450-575 °C, hexagonal at 600-720 °C, and monoclinic at ~740 °C. Phase conversion between any two polymorphs through annealing has proven to be nearly impossible due to the high kinetic barriers.27 In fact, it is already quite difficult to directly synthesize polycrystalline powders of the cubic FeGe phase as they requires high temperatures (800 °C) and high pressures (4 GPa) to form.9 These complex structural polymorphs of FeGe make the synthesis of cubic FeGe NWs especially challenging. Metal silicide28, 29 and germanide NWs and their axial heterostructures 3


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could be synthesized by a controllable reaction of silicon or germanium NWs with metals,30-34 however, our attempts in converting germanium NWs have led to products with poorly controlled morphologies and complex phases. On the other hand, chemical vapor deposition (CVD) has proven to be a powerful approach for the synthesis of metal silicide NWs that have the cubic B20 structure such as FeSi,35, 36 CoSi,37, 38 MnSi,39-41 and their alloys such as Fe1-xCoxSi42, 43. In CVD, vapor phase metal halide or organometallic precursors are reacted at high temperatures to produce nanostructures or thin films over a solid substrate. It has also been used to synthesize NWs in the Fe-Ge binary system44, 45 as well as the Ni-Ge systems.46 However, there have been no reports of the synthesis of the cubic FeGe polymorph in NW morphology by CVD or any other methods. Interestingly, NWs of the monoclinic polymorph of FeGe have been synthesized recently using a solution method at relatively low temperatures (~300 °C).47 The synthesis of this polymorph was unexpected under these conditions as it is only thermodynamically stable at higher temperatures; this result suggests that the kinetics involved in the nucleation of the nanostructures could play an important role in determining which polymorph is formed. In an effort to control the kinetics in the formation of the cubic B20 FeGe phase, we developed a novel CVD approach that provides control over the initial stage of nanostructure growth by seeding a substrate with small crystals of the cubic B20 polymorph. This CVD method allows us to select a desired material phase out of a complex phase diagram, in this case the cubic FeGe phase, and then preferentially grow nanostructures of that desired phase. Herein we use this method to demonstrate the first direct chemical synthesis of single crystalline FeGe NWs with the cubic B20 structure and confirm the phase identity using a variety of structural characterization techniques. We further present evidence for a stabilized skyrmion phase in the cubic FeGe NWs using LTEM.

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Figure 1. a) (i, ii) Crystal structures of the two polymorphs of FeGe synthesized herein, cubic B20 and hexagonal, and (iii) the crystal structure of Fe13Ge8. b) SEM image of a Ge (100) substrate after a CVD reaction without using the seeding process. c-e) SEM images of a Ge (100) growth substrate with cubic FeGe seeds dropcast c) before and after CVD at d) 500 °C and e) 550 °C. In panel e) insets the scale bars are 2 µm, 2 µm, and 200 nm respectively. f) XRD patterns, shown alongside the standard patterns for cubic (red) and hexagonal FeGe (black) (JCPDS PDF#’s 65-6357 and 17-0228, respectively). The asterisks (*) above the top trace highlight peaks that are associated with the formation of the hexagonal FeGe background thin film.

In previous CVD reactions that grew metal silicide NWs with the B20 structure such as MnSi and Fe1-xCoxSi, a piece of a Si (100) wafer is placed in a tube furnace and reacted at high temperature with a vaporized metal halide precursor.39, 43 We initially carried out an analogous reaction using a piece of Ge (100) wafer and FeI2 precursor in a custom built CVD reactor. A tube furnace (Lindberg/BlueM) was used to heat a 1” quartz tube, which housed an alumina boat containing ~50 mg of the FeI2 precursor (Alfa Aesar, 97% anhydrous) and a 1 cm × 1 cm piece of Ge (100) wafer under a steady stream of 100 sccm of Ar and 50 sccm of H2 gases at a pressure of 500 torr. The alumina boat and the piece of Ge wafer were placed 1 cm upstream and 1 cm downstream of the furnace’s hot zone, respectively. Hour-long CVD reactions were carried out at temperatures of between 500 °C and 600 °C. However, these CVD reactions yielded mostly 5


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microparticles and few nanostructures independent of the reaction conditions. Scanning electron microscopy (SEM) images of the samples after reaction were obtained using a LEO 1530 electron microscope and show little to no nanostructuring (Figure 1b), while powder X-ray diffraction (PXRD, Figure 1f, orange) confirmed that only the hexagonal FeGe polymorph (space group P6/mmm, a = 5.0027 nm, c = 0.40548 nm, crystal structure shown in Figure 1aii) was formed. In order to grow FeGe NWs of the cubic instead of the hexagonal polymorph, we used a non-traditional method of seeding the Ge substrate with small particles of cubic FeGe. We first grew large single crystals of cubic FeGe via a chemical vapor transport (CVT) method described by Richardson.27 The resulting FeGe single crystals had a cubic B20 structure (space group P213, a = 0.4700 nm, shown in Figure 1ai) confirmed using PXRD (Figure S2b). The large increase in relative signal intensity of the (111) peak revealed a preferential orientation of the FeGe single crystals, which is consistent with the triangular faceting of the crystals (Figure S2a). The large single crystals were ground in an agate mortar and pestle for ~15 minutes to yield a finely ground powder of cubic FeGe. Prior to CVD reactions, we seeded a Ge (100) substrate by successively drop-casting two 30 µL aliquots of a suspension of the finely ground cubic B20 FeGe particles dispersed in isopropyl alcohol (IPA) at a concentration of ~1 mg/mL. SEM image (Figure 1c) of this seeded substrate showed many particles with sizes ranging from 10s of nanometers to ~10 µm. After we carried out CVD reactions over these seeded substrates at 500 °C and 550 °C using essentially the same CVD reaction conditions described above, there was significant formation of NWs and other nanostructures on the entire surface of the substrate and the seeds themselves (Figure 1d and 1e, respectively), in stark contrast to the initial reactions without the seeding step that produced few nanostructures (Figure 1b). The reaction temperature also had a significant effect on both the density and dimensions of the nanostructures formed. A reaction 6


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temperature of 500 °C resulted in a lower density of NWs that were ~100 to 500 nm in diameter and ~1 to 10s of micrometers in length. The reaction temperature of 550 °C yielded wires in much higher density that were generally ~200 nm to 5 µm in diameter and ~1 to 10s of micrometers in length. Upon closer examination, it is apparent that two different types of NWs are formed, those with some well-defined faceting and some with a more rounded morphology (Figure 1e insets). Along with the NWs, a number of other nanostructure morphologies, such as tetrahedra and octahedra were found in both reactions, as shown for the reaction at 550 °C (Figure 1e inset and more images shown in Figure S3). Powder XRD (Figure 1f) clearly demonstrated the impact of the cubic FeGe seeding on the phases of the CVD reaction products. Powder XRD recorded from the Ge (100) wafer piece covered with cubic FeGe seeds (Figure 1f, green) shows very weak peaks that are indexed to cubic FeGe only. The intensities of the Bragg peaks of the FeGe seeds prior to reaction are expected to be low because of the low density of seeds on the substrate surface and the poor crystallinity of the seeds as a result of the grinding process. After CVD reaction over the seeded Ge (100) wafer piece at 550 °C, the cubic FeGe peaks that were previously barely visible before the reaction are now prominent (Figure 1f, purple), while peaks that were hidden in noise are now visible. This result is in clear contrast to the PXRD of the product after the same CVD reaction without using cubic FeGe seeds (Figure 1f, orange). The reaction at 500 °C also produced a noticeable change in cubic FeGe peak intensity (Figure 1f, blue), although not to the same degree as for the 550 °C reaction (Figure 1f, purple). A few minor impurity peaks (marked by the asterisks in Figure 1f) in the 550 °C reaction pattern cannot be indexed to cubic FeGe. However, they can be indexed to the hexagonal FeGe, and they likely originate from areas of the substrate where the FeI2 vapor has reacted with the Ge substrate directly (rather than the seeds) thereby forming a background film

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of the hexagonal FeGe polymorph. All of the diffraction patterns were recorded from samples of similar size and using identical collection settings (i.e. exposure time, step size, etc.) in order to ensure that the intensities of the patterns were as comparable as possible. Although small variations in absolute intensity between different samples is expected in PXRD, the observed intensity difference of several orders of magnitude between the before and after reaction samples cannot be accounted for by sample variation alone. This suggests that a significant quantity of cubic FeGe nanostructures was produced by these CVD reactions only when seeds of cubic FeGe were present. In traditional vapor-liquid-solid (VLS) NW growth, metal nanoparticle seeds are reacted with a precursor vapor to create a eutectic mixture that promotes anisotropic one dimensional (1D) growth.48, 49 Unlike in VLS reactions, we suspect that the seeds that are used in this study play a different role of promoting the growth of one crystallographic phase by acting as pseudo-epitaxial nucleation sites for the resulting nanostructures.

Figure 2. a) Low magnification TEM image of a cubic FeGe NW with rounded faceting and b-d) the corresponding SAED patterns along the zone axis (ZA) of b) (001), c) (101), and d) (101) that confirm the B20 cubic structure. e) Low magnification TEM image and h) HRTEM image of a cubic FeGe NW with a [111] growth direction determined from the indexed FFT (inset). f) Low magnification TEM image, and i) HRTEM image, and g) the corresponding FFT of a cubic FeGe NW with the [010] growth direction. j) Representative EDS spectrum recorded from the cubic FeGe NWs.

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Since PXRD is a technique that averages over many nanostructures on a large surface area, we further employed transmission electron microscopy (TEM) and selected area electron diffraction (SAED) to study individual nanostructures. The nanostructures were transferred onto a Cu TEM grid by lightly dragging it across a reacted Ge substrate. Extensive SAED analysis was performed on the rounded NWs (Figure 2a – 2d) using a FEI Tecnai TF-12 TEM operated at 120 keV. A total of at least 20 rounded wires from both reaction temperatures (500 °C and 550 °C) were studied and indexed. Typically, a NW was tilted from one zone axis (ZA) to another ZA to allow the determination of multiple lattice spacings. However, some NW diameters were too large to allow the collection of more than one SAED pattern. For both reactions, at least one rounded NW was indexed along all three ZAs of (001), (101), and (102), as shown for one representative NW in Figure 2a – 2d. The complete results of indexing and lattice spacings are summarized in Table 1. In all cases, we were able to identify the rounded NWs as having a cubic B20 FeGe crystal structure. Table 1. Summary of the lattice spacings measured using SAED from rounded NWs. Lattice plane

Number of NWs studied

Expected dspacing (nm)

% Deviation

20

Avg. d-spacing and standard deviation (nm) 0.47 ± 0.01

(001)

0.4700

0

(011)

8

0.34 ± 0.01

0.3323

2.3

(201)

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0.216 ± 0.004

0.2102

2.76

Both the crystallographic orientation and the structure of the NWs can influence how the skyrmions in the material behave. Therefore, we recorded high resolution TEM (HRTEM) images (Figure 2b and e) on a Tecnai TF-30 microscope operating at 300 keV to further confirm the cubic FeGe structure and to study the growth direction of each NW. The measured lattice spacings of 0.47 and 0.34 nm for the NW shown in Figure 2b are consistent with the expected values of 0.4700 9


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nm and 0.3323 nm for the (100) and (110) lattice spacings of cubic FeGe, respectively. The measured lattice spacing of 0.48 nm for the NW shown in Figure 2e matches the expected (100) lattice spacing of cubic FeGe. Fast Fourier transforms of the HRTEM images (Figure 3b inset and Figure 3d) were used to determine the growth direction of the individual wires, which are [111̅] and [010], respectively. Notably, the B20 monosilicide NWs previously reported, such as MnSi and FeSi NWs, showed NW growth only in the [110] direction.35, 39 It is perhaps not surprising that the NW growth direction differs between the FeGe NWs here and previously reported silicide NWs, whose synthesis does not involve a seeding step. The observation of more than one NW growth directions is also not surprising for this reaction as the exposed surfaces of the FeGe seed particles were not controlled, although some preferential growth directions are likely. We also used HRTEM to confirm that the triangular faceted tetrahedral and octahedral nanocrystals seen in Figure 1e also have the cubic B20 FeGe crystal structure (see details in Figure S4). Then the average elemental composition of the NWs was determined in the TEM via electron dispersive spectroscopy (EDS). Fitting of the EDS spectra of the cubic FeGe NWs (Figure 1j) using NORAN System software provided an average Fe:Ge ratio of 1:1.1. The presence of C and Cu in the EDS spectra can be explained by the lacey carbon coated Cu TEM grids used. Alongside the cubic FeGe NWs that exhibited rounded faceting, we also could observe NWs with clear hexagonal faceting (Figure 1e inset and Figure 3a) among the products of seeded CVD reactions. From SAED patterns (Figure 3b and 3c), we were able to show that these NWs with clear facets have the hexagonal Fe13Ge8 crystal structure (space group P6/mmm, a = 0.7976 nm, c = 0.4993 nm, crystal structure shown in Figure 1a iii). The Fe13Ge8 structure has the same space group, P6/mmm, as the hexagonal FeGe polymorph (Figure 1a, ii), but the large difference in lattice parameters between the two phases allows them to be differentiated using SAED.

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Figure 3. a) Low magnification TEM image of a well-faceted Fe13Ge8 NW. b, c) Corresponding SAED patterns indexed to the Fe13Ge8 structure along the b) (100) ZA and c) (210) ZA. d) HRTEM image of the Fe13Ge8 NW and the corresponding FFT indexed to the Fe13Ge8 structure (inset).

However, the low concentration of these Fe13Ge8 NWs made them undetectable using PXRD. From SAED patterns (Figure 3b and 3c), we calculated lattice spacings of 0.69, 0.51, 0.40, and 0.25 nm corresponding to the (010), (001), (1̅20), and (002) lattice plane spacings respectively, which are all in good agreement with the expected values of 0.691 (010), 0.499 (001), 0.398 (1̅20) and 0.250 nm (002) for the Fe13Ge8 structure.50 HRTEM images (Figure 3d) further showed the two lattice spacings of the NW are 0.29 and 0.21 nm, which are in good agreement with the expected values of 0.28 and 0.21 nm for the (2̅21) and (02̅2) lattice spacings, respectively. We note that Fe13Ge8 NWs have been synthesized previously in a very different CVD/CVT reaction,44 but they were labeled as Fe1.3Ge instead of Fe1.625Ge, even though the structure described in that paper matches the Fe13Ge8 phase.44 In our reactions presented above, both the cubic FeGe and Fe13Ge8 phase NWs were formed in roughly equal quantities (based on faceting and EDS analyses) 11


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and in high density over the entire Ge (100) substrate. However, slight adjustments to the reactions conditions and seeding process, such as heating the Ge substrate (~50 °C) on a hotplate during the dropcasting process to better disperse the seeds, have shown promise in reducing the amount of the impurity Fe13Ge8 phase formed alongside the cubic FeGe NWs. Although we have not found a way to completely eliminate these Fe13Ge8 NWs as an impurity phase from the cubic FeGe NWs during synthesis, the two types of NWs can be readily differentiated by their faceting, therefore this does not pose a problem for the Lorentz TEM measurements or any future device studies of individual cubic FeGe NWs. After confirming that we had successfully synthesized FeGe NWs with the cubic B20 structure, we performed LTEM experiments to study the existence and stability of the skyrmion phase in these FeGe NWs (Figure 4). LTEM is a special form of microscopy that allows direct visualization of magnetic structure with nanometer spatial resolution. We carried out the LTEM experiments using an FEI Titan 80-300 TEM operated at 300 keV in the Lorentz mode. At a specimen temperature of 233 K and in zero applied magnetic field, the over-focus and under-focus images of a cubic FeGe NW with a diameter of ~200 nm and a length of 2 μm showed a striped pattern (Figure 4c and 4e), which is associated with a helimagnetic ground state that has been seen previously in other skyrmion hosting B20 materials including FeGe.9 An incomplete skyrmion lattice phase that only has a width of two skyrmions appears under the application of a small magnetic field of ~0.07 T. The skyrmions packed in the “zig-zag” pattern (Figure 4d and 4f) have a diameter of ~72 nm, which is in good agreement with previous reports for cubic FeGe.8-10 At this temperature, the skyrmion lattice is stable up to an applied magnetic field of ~0.2 T, as shown by Supplementary Video 1. Beyond such magnetic field, the skyrmion phase then transitions into a ferromagnetic or conical state. We also performed another set of LTEM experiments at a specimen

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Figure 4. a,b) In-focus LTEM images showing no magnetic contrast in zero field and in an applied magnetic field of 0.1 T at 233 K. c,e) Over-focus and under-focus LTEM images of the NW with no applied magnetic field showing the striped magnetic contrast of the helimagnetic phase. d,f) Over-focus and under-focus images of the NW in an applied magnetic field of 0.1 T showing a clear zig-zag skyrmion lattice.

temperature of 95 K, and observed a “zig-zag” skyrmion lattice in an applied magnetic field window of ~0.15 to 0.4 T, as shown in Supplementary Video 2. Based on the results obtained, it is clear that the skyrmion lattice phase in FeGe NWs is more stabilized compared with bulk FeGe, in which skyrmions only exist up to an applied magnetic field of up to ~0.1 T at temperatures of between ~225 K and ~275 K.9, 51 The skyrmions’ stability window in these NWs appears to be similar to what was observed in the 15 nm thick epitaxial thin films of (111) FeGe, which can host a skyrmion lattice up to ~0.3 T of applied magnetic field at temperatures of between ~25 K and ~275 K.9 Either the confinement of the skyrmions or the bulk/surface anisotropy of the NWs may be responsible for this increased stability. An attempt was made to perform transport of intensity

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equation (TIE) analysis on the LTEM images. However, the dominant Fresnel fringe contrast at the specimen edges and thickness variations of the NWs overshadow the weak magnetic contrasts of the NWs, making the TIE analysis extremely difficult. Furthermore, under higher applied magnetic fields at 95 K, the “zig-zag” skyrmion lattice transitioned into a skyrmion chain state, and eventually transformed into a ferromagnetic state, as shown in Figure 5. This transformation is similar to that observed in top-down fabricated nanoribbons and nanodisks of cubic B20 FeGe prepared by a sophisticated focused ion beam (FIB) process.8, 52 After transforming into a single chain, the skyrmions are repelled to the center of the NW by the magnetic edge states of the NW.53 Notably, as the applied magnetic field is further

Figure 5. A series of LTEM images of an FeGe NW recorded at 95 K showing the transition from a skyrmion chain state to a ferromagnetic state under increasing applied magnetic fields. a-e) LTEM images taken at increasing applied fields, showing the progression of a skyrmion chain state. f) LTEM image of a final uniform ferromagnetic state. Scale bar: 200 nm.

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increased, the skyrmions are further reduced in size. At an applied magnetic field of ~0.5 T, only a few skyrmions remain (Figure 5d and 5e). Finally, a ferromagnetic state is formed (Figure 5f). Such a skyrmion chain state is promising as it closely resembles a desired magnetic state in a future racetrack memory device that could be utilized for information storage. In conclusion, we have developed a CVD growth method that uses cubic B20 FeGe seed particles to selectively synthesize novel nanowires and other nanostructures of the cubic B20 FeGe phase out of the complex binary system with many possible phases and three FeGe structural polymorphs. Structural characterization using PXRD, TEM, SAED, and EDS analysis conclusively confirmed the formation of single crystalline FeGe NWs with the B20 cubic crystal structure. Although some impurity NWs of the Fe13Ge8 phase were also observed, the two types of NW can be readily differentiated from one another based on their faceting using SEM or TEM imaging, thereby allowing single NWs of cubic FeGe to be selected for skyrmion studies. Furthermore, we observed both a helimagnetic and skyrmion magnetic phases in the B20 FeGe NWs using Lorentz TEM and a skyrmion chain state was also observed under higher applied magnetic fields. The observed skyrmion state showed greater temperature and magnetic field stability than that of the bulk FeGe. The successful synthesis of cubic FeGe NWs and the observation of stabilized skyrmions within them lays the foundation for future fundamental studies and device applications of skyrmions in these novel NW materials. The seeding approach described here could also provide a new approach for the selective synthesis of new nanoscale materials from compounds that can adopt several possible crystallographic phases.

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ASSOCIATED CONTENT Supporting Information. Included in the supplementary information is: an Fe-Ge binary phase diagram, details on the CVT synthesis and characterization of the cubic FeGe single crystals, additional SEM images of the reacted samples including other nanostructure morphologies produced by the seeded CVD reactions, representative SAED of the cubic FeGe NWs, and representative HRTEM analysis of the cubic FeGe tetrahedrons. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research is supported by NSF grant ECCS-1609585. M.J.S. also acknowledges support from the NSF Graduate Research Fellowship Program grant number DGE-1256259. The Lorentz TEM imaging has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ ERC grant agreement number 320832.

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