Modulated Magnetic Nanowires for Controlling Domain Wall Motion

Modulated Magnetic Nanowires for Controlling Domain Wall Motion: Toward 3D Magnetic Memories Yurii P. Ivanov,*,† Andrey Chuvilin,‡,§ Sergei Lopatin,† and Jurgen Kosel*,† †

King Abdullah University of Science and Technology, Thuwal 23955, Saudi Arabia CIC nanoGUNE Consolider, Avenida de Tolosa 76, 20018 San Sebastian, Spain § IKERBASQUE, Basque Foundation for Science, Maria Diaz de Haro 3, 48013 Bilbao, Spain ‡

S Supporting Information *

ABSTRACT: Cylindrical magnetic nanowires are attractive materials for next generation data storage devices owing to the theoretically achievable high domain wall velocity and their efficient fabrication in highly dense arrays. In order to obtain control over domain wall motion, reliable and well-defined pinning sites are required. Here, we show that modulated nanowires consisting of alternating nickel and cobalt sections facilitate efficient domain wall pinning at the interfaces of those sections. By combining electron holography with micromagnetic simulations, the pinning effect can be explained by the interaction of the stray fields generated at the interface and the domain wall. Utilizing a modified differential phase contrast imaging, we visualized the pinned domain wall with a high resolution, revealing its three-dimensional vortex structure with the previously predicted Bloch point at its center. These findings suggest the potential of modulated nanowires for the development of high-density, three-dimensional data storage devices. KEYWORDS: domain wall motion, nanowires, assembled nanostructures, electron holography, 3D memory

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templates are porous membranes like self-ordered aluminum oxide templates (AOT). Data bits could be densely packed in 3D arrays of such NWs each carrying several bits along its length in the form of magnetic domains. Following the racetrack memory principle, the bits would be shifted by current-induced domain wall motion. A further benefit of using cylindrical NWs could arise for the operation speed, which should be an order of magnitude faster than for planar devices, due to the absence of the Walker breakdown phenomena.18,19 In order to implement such NW devices for data storage, it is crucial to introduce periodic energy potentials that are capable of pinning the DWs. A first approach to realize a periodic energy landscape along the NWs inside of an AOT was a diameter modulation. It can be realized via modulation of the AOT’s pore diameter20,21 or by means of chemical etching.22 However, until now, a reliable control over DW position has not been shown, which is also a consequence of the difficulties associated with DW observations.

he rapid development of data storage technology has ensured a continuous increase of the memory density over the past years. The current memory density of magnetic hard drives or solid state devices is already 1 Tb/ inch2.1 However, within the existing 2D device paradigm the theoretical density limit is almost reached. For example, the limit for commonly used magnetic hard drives is 10 Tb/inch2 using heat assisted magnetic recording.2 A further increase in memory capacity can only be accomplished via atypical approaches. A potential solution could be a utilization of the third dimension by increasing the number of layers in which the bits are stored.3 In case of magnetic devices, a promising concept is the so-called racetrack memory, where domain walls (DWs) are moved by spin-polarized current pulses.4 Although the racetrack memory conceptually includes 3D devices, in which the data bits are stored in tracks perpendicular to the substrate, it has so far only been realized in 2D in-plane thin film geometries.5 Magnetic cylindrical nanowires (NWs) growing inside of an isolating template perpendicular to the substrate are very promising candidates to realize 3D magnetic memory devices with a small footprint. They can be fabricated by template-assisted electrodeposition techniques, which provide a high level of control over morphology, geometry, crystal structure, and packing density of the NWs.6−17 The © XXXX American Chemical Society

Received: February 22, 2016 Accepted: May 3, 2016

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DOI: 10.1021/acsnano.6b01337 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. (a)−(b) SEM images of the cross section of Co and Ni NW arrays. (d)−(e) Corresponding XRD data. (c) SEM images of the cross section of multisegmented Co/Ni NW arrays with six segments. Though the Co−Ni−Co−Ni−Co staking in these arrays has been confirmed by energy filtered transmission electron microscopy (EFTEM) (see Figure 2), it is not distinguishable in the secondary electron SEM image due to the similar contrast from Co and Ni segments.

crystallinity with the ⟨100⟩ direction and the ⟨220⟩ direction for hcp Co and fcc Ni, respectively, which are close to the growth direction of the NWs. Note that this orientation implies that the magnetocrystalline anisotropy direction of the Co NWs is nearly perpendicular to the NWs.13 The morphology of the multisegmented Co/Ni NWs shown in Figure 1c is also characterized by a high uniformity. Figure 2a,b show a TEM image of the multisegmented NWs inside AOTs and a color map of Co and Ni distribution. The

RESULTS AND DISCUSSIONS In this work, we introduce an approach for modulation of energy potential inside cylindrical NWs, namely by alternating the composition and crystal structure of the wires, similar to alternating black and white stripes in the barcode. In this approach, the interfaces between segments of different magnetic materials with differently oriented magnetic anisotropy act as reliable pinning sites for DWs. We study in detail structure, composition, and magnetic behavior of Co/Ni “barcode” NWs. The pinning and the configuration of domain walls is investigated by a combination of magnetic force microscopy (MFM), differential phase contrast (DPC), and electron holography methods. The NWs are obtained by alternating electrodeposition of Co and Ni into AOTs. The AOTs with a long-range order of hexagonal symmetry have been grown by a controlled two-step anodization process.23 The pore diameter in these membranes was 80 nm and the pore center-to-center distance was 105 nm. Electrodeposition was carried out using two different aqueous electrolytes for Co and Ni at a constant bias. The deposition conditions were optimized to grow single crystalline hcp Co segments with magnetocrystalline anisotropy perpendicular to the NWs,13 whereas Ni segments were grown in fcc structure. The number of the segments was varied from 2 to 20 and the length of the segments was varied from ∼300 nm to ∼10 μm by controlling the deposition time. Figure 1a,b show Ni and Co NWs inside of AOTs. The bright contrasts in the scanning electron microscopy (SEM) images correspond to the Au electrode on the backside of the AOTs and to the NWs embedded inside of the AOTs. The gray and dark contrasts correspond to the alumina matrix and pores, which are not filled by NWs, respectively. The cylindrical morphology and the high uniformity of the length and diameter are clearly visible. The XRD data in Figure 1d,e reveal very high

Figure 2. (a)−(b) TEM and EFTEM images of the multisegmented Co/Ni nanowires. (c) EFTEM image of one interface between Co and Ni segments. (d) EELS profiles for Co, Ni, and O along the dashed arrow in (c). B

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ACS Nano interfaces between Co and Ni segments are sharp, yet inclined (Figure 2c), and are approximately at the same position for different NWs. Electron energy loss spectroscopy (EELS) confirms the sharp transition between Co and Ni segments, with an average interface width of about 30 nm, as well as the absence of oxygen at interfaces (Figure 2d). For a better understanding of the heteroepitaxial growth of Co and Ni segments on a top of each other, we performed a high resolution TEM (HRTEM) study of the interface (see Supporting Information). In the HRTEM image, the Co segment is oriented to the zone axis [011] with the ⟨100⟩ direction perpendicular to the interface and the Ni segment has orientation close to the [301] zone axis with the ⟨11−3⟩ vector perpendicular to the interface. The lattice mismatch between hcp Co and fcc Ni in this case is less than 2%. It is interesting to note that the interface between the segments is not always perpendicular to the NW axis. This may be due to the existence of preferential growth planes, which can in particular develop at the end of a growth cycle. Because the polycrystalline nature of the NWs, this growth plane is not necessarily perpendicular to the growth direction, which defines the inclined interface, when the next component is deposited. The study of the magnetic properties of individual cylindrical NWs is not a trivial task and requires a combination of different techniques. In this case, we studied the stray fields of individual NWs by MFM, and we imaged the in-plane component of the magnetic induction by electron holography. Additionally, a recent modification of the DPC method in scanning TEM (STEM) mode was employed for magnetic imaging that has the advantage of multiscale capabilities. In opposition to conventional DPC with specially designed segmented detector(s),24−26 we introduced the concept of the unitary virtual bright field detector (VBF).27 In this scheme, the VBF detector is the area of the standard high angle annual dark field detector limited by the selected area (SA) aperture. Using the field free low magnification (LM)-STEM mode, 0.6 mrad convergence angle, 40 μm SA aperture, which results in 0.2 mrad collection angle, the VBF-DPC method allows to visualize the magnetic fields with better than 4 nm resolution.27 Figure 3a,b show a TEM image and a composition map of a multisegmented Co/Ni NW, clearly revealing the barcode structure. The corresponding magnetic structure at remanence, visualized with the VBF-DPC method for two orthogonal inplane components of the magnetic field is shown in Figure 3d,e. The axial component Bx (Figure 3d) shows that the NW is magnetized in the x direction with a strong periodic contrast (red and white) that perfectly correlates with the barcode structure (Figure 3c). This periodic contrast is caused by the difference in the magnetization values, which is significantly higher in the Co than in the Ni segments. This difference also produces the stray fields outside of the Co segments in the opposite direction to the magnetization, which can be seen as periodic blue contrasts. It emanates from the interfaces (red and blue contrasts), as clearly shown by the By component of the field in Figure 3e. Figure 3d also shows that outside of the Ni segment the stray field is zero (green). To complete the picture, we also performed electron holography measurements, which enable a quantitative study of magnetic fields inside of NWs with a spatial resolution in the nanometer range.28,29 Figure 4a−c show images reconstructed from the electron holography of a part of the NW with several sections (marked by the yellow dashed rectangle in Figure 3c). Strong stray fields are clearly visible at the interfaces and stray

Figure 3. (a)−(b) TEM and EFTEM images of a multisegmented Co/Ni nanowire. (c) EELS-STEM mapping. (d)−(e) x component (Bx) and y component (By) components of the in-plane magnetic field obtained by VBF-DPC. Blue and red contrasts show the opposite directions of the magnetic field component parallel (d) and perpendicular (e) to the NW length. (f) Schematic of the NW magnetization and stray fields emanating from the interfaces. The yellow rectangles in (c), (d), and (e) mark the areas that were evaluated by electron holography in Figure 4.

fields can also be found outside of the Co sections. These fields are emanating from the Co sections, due to their larger magnetization. To get a deeper insight into the magnetic structure at the interface, we performed micromagnetic finite elements simulations (details in the Methods section). As shown in Figure 4d, the spins are pointing along the NW in the Ni side, where the shape anisotropy dominates, and forming a complex vortex structure in the Co side, which has a pronounced crystalline anisotropy perpendicular to the NW axis. As can be seen from the cross section view, the 3D vortex in the Co segment has a large component of magnetization in the center parallel to the NW,30,31 which is in contrast to the in-plane vortices found in flat nanodots.32,33 Figure 4e,f shows the line profiles of the reconstructed projected magnetic field in T*m taken perpendicular to the NW (through its center) in the Ni and Co segments. The figures also include simulated profiles that were calculated assuming the magnetization in the segments being homogeneous and parallel to the NW. Although the Ni profile is very close to the one of a homogeneously magnetized NW, the Co profile has a pronounced center. This is caused by the vortex structure near the surface and confirms the micromagnetic simulation results (Figure 4d). The vortex structure leads to magnetization components that are parallel to the beam direction, to which the electron hologram is insensitive. The magnetizations of the Ni and Co segments can be calculated by integration of the reconstructed magnetic field amplitude value across the NW diameter using the cross sections in Figure 4e,f.28 The value found for the Ni segment is close to the bulk one, whereas the value of Co segment is much smaller than the bulk one (Figure 4g), which can again be associated with the 3D vortex.13,17,30,31 The results obtained by VBF-DPC and electron holography are in good agreement. The difference in the error bar for the DPC C

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Figure 4. (a)−(c) In-plane component of the magnetic field of the area marked with the dashed yellow rectangle in Figure 3 reconstructed from the electron hologram: (a) magnetic phase shift of the electrons, (b) magnetic induction map, and (c) color map of the magnetic field direction. (d) Micromagnetic simulation results at the Co/Ni interface marked by the red dashed rectangle in (b). The cones show the magnetization direction, and their color corresponds to the component of magnetization parallel to the NW. The circles show the component of the magnetization parallel to the NW in the cross sections at different position near the interface. (e−f) Line profile of the reconstructed projected in-plane component of the magnetic field in T*m along the Co and Ni segments. (g) Comparison between the bulk values of the Co and Ni and the measured ones using the single domain approximation.

method mainly arises from averaging over different segments along the NW. The possibility to pin the DW at the interface between Co and Ni segments has been investigated first by means of micromagnetic simulations. They indicate that a DW, nucleated in one of the segments, propagates along the NW and is pinned at the interface between two segments. To study DW pinning experimentally we utilized MFM with variable magnetic field. At remanence, the MFM images of barcode NWs show strong periodic stray fields (as found in Figures 3 and 4) arising from the interfaces between Co and Ni segments (see Supporting Information). When applying an external magnetic field (around half of the switching field) in the direction opposite to the magnetization direction of the NW, the stray field of the MFM tip placed at a segment near the NW’s end nucleates a DW, which propagates toward the interface and switches the magnetization of the segment. This process occurs with a high reliability and could be reproduced many times in any Co/Ni NWs with various numbers of segments as well as different lengths of the segments. DW pinning in a NW with one Co and one Ni segment of 50 nm diameter and 4 μm length each is detailed in Supporting Information. If the NW consists of more than two segments (each 750 nm long), several DWs can be pinned repeatable at the same time at different interfaces (Supporting Information), thus creating a

multidomain NW. In terms of data storage applications, this means that several data bits could be stored along such a NW. The reliable DW pinning at the interfaces between Co and Ni segments enabled visualization of the magnetic structure of the 3D vortex DW, for which we utilized the VBF-DPC method. Figure 5a shows a DPC image of a DW pinned at the Co/Ni interface. The field vectors are simply constructed from two measured orthogonal components. These vectors visualize the direction of the magnetic moments and stray field. Interestingly, the DW is located in a distance of about 100−120 nm (approximately equal to its size) away from the interface. It shows a strong stray field that is larger than the ones at interfaces without DWs or at NW tips. At the center of the DW there is a small area, with a size close to the imaging resolution, where the in-plane component of the magnetic field is zero. This could be the Bloch point of the 3D vortex DW, which has been predicted from micromagnetic simulations before.18,19,34 Another interesting feature is a small asymmetry of the stray fields around the DW, indicated by the dashed vertical lines in Figure 5a,b. A reason for this could be the strong uniaxial magnetocrystalline anisotropy of hcp Co, which is almost perpendicular to the NW. The micromagnetic simulation of the 3D vortex DW shown in the Figure 5b is in excellent agreement with the experimental observations, including the mentioned asymmetry. D

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are generated by interfaces between materials of different magnetic anisotropies, that is, hcp Co and fcc Ni. 3D ordered arrays of multisegmented Co/Ni cylindrical NWs with sharp atomic interfaces between segments have been prepared by a simple electrochemical deposition method. We showed both theoretically and experimentally that the interfaces, at which strong stay fields are created, act as very efficient pining sites for DW propagation. Using an original modification of the standard DPC imaging method, we visualized 3D vortex DWs with a Bloch point at the Co/Ni interfaces with a resolution better than 4 nm. The presented approach is an efficient way to solve a crucial issue related to current driven DW motion in cylindrical NWs, which is a promising solution for data storage devices.

METHODS Growth of Multisegmented Nanowires. AOTs with highly ordered hexagonal and self-assembled nanopore arrays were prepared by a two-step anodization process in oxalic acid. Prior to anodization, high-purity (99.999%) aluminum disks were degreased in acetone with ultrasound and cleaned by electropolishing in a mixture of perchloric acid and ethanol (HClO4:C2H5OH = 1:4 in volumetric ration) for 2 min at 6 °C under vigorous stirring. Afterward, samples were rinsed in an ethanol solution and dried. The first anodization procedure was performed using a 0.3 M oxalic acid solution as electrolyte at 3 °C temperature and an anodization voltage of 40 V. Following the first anodization, the sample was immersed in chromic acid/phosphoric acid mixture at room temperature until the oxide layer was dissolved. The second anodization was performed for 20 h resulting in the pores reaching a depth of 40 μm. The pores diameter was increased to 50 and 80 nm with a time controlled treatment in phosphoric acid. Then, the nonoxidized Al layer at the bottom of the disk was chemically removed as well as the alumina bottom layer. A thin Au layer was then sputtered onto the open backside of the membrane to serve as an electrode for the subsequent Co electroplating. Co/Ni nanowires were grown at room temperature by using two aqueous solutions of 250 g/ L CoSO4 and 40 g/L H3BO3 and 300g/L NiSO4, 46 g/L NiCl2, and 40 g/L H3BO3. An Ag/AgCl reference electrode was combined into a three-electrode system in which a platinum electrode served as a counter electrode to conduct potentiostatic direct current (DC) electrodeposition. Electroplating was performed at −1 V. The solution pH value was kept 3.5. The electroplating time was tuned to reach a specific value of the length of the segments from 300 nm to 4 μm. Structural Characterization. An X’Pert PRO X-ray diffractometer was employed for the characterization of the crystal structure of the NW’s array. The θ−2θ scans were performed with the scattering vector parallel to the nanowire axes (perpendicular to the plane of AOT). Before the XRD measurements, the Au metallic contact layer was removed using ion milling. As prepared membranes were broken and the sharp cross sections have been used for characterization by scanning electron microscopy (SEM). The cross sections of arrays for TEM studies were prepared by focused ion beam (FIB) protocol. Electron microscopy studies were carried on transmission electron microscope (TEM) Titan G2 60-300 (FEI, Netherlands), operated at 300 kV. For studying crystal structure of individual NWs the AOT membranes were dissolved in Cr2O3/H3PO4−H2O solution at 40 °C and the NWs were dispersed in ethanol. Magnetic Characterization. The magnetic properties of the nanowire arrays were studied using a vibrating sample magnetometer. The magnetization curves were measured under magnetic fields up to 20 kOe applied parallel (∥) and perpendicular (⊥) with respect to the nanowire’s axis. LM-STEM images for the VBF-DPC were acquired on a Titan 60300 electron microscope (FEI Co) equipped with a high brightness electron gun (x-FEG), and Fischione high angle annual dark field STEM detector. The microscope was operated at 200 keV acceleration voltage. The LM-STEM was intentionally tuned for OL = 0%

Figure 5. (a) VBF-DPC image of the vortex DW pinned at the Co/ Ni interface. The arrows show the direction of the magnetic field, which is constructed from the raw data of two orthogonal components of the magnetic field. The dashed lines show the position of the Co/Ni interface (purple) and DW (red). (b) Micromagnetic simulation of the DW structure (color cones show magnetization direction; black arrows are calculated stray fields).

Thus, at remanence a multisegmented NW in a single domain state has north and south magnetic poles at the ends plus periodic poles at the interfaces. Once a DW is nucleated and moves toward the interface, the interaction between the stray fields from the DW itself and the interface (they produce the same sign magnetic poles) causes the appearance of a repulsive force which, we assume, pins the DW near the interface (similar to the interaction between two permanent magnets which are facing each other). In order to realize efficient memory devices, it is important to understand the dynamics of 3D vortex DWs. For cylindrical nanowires, the DW dynamics has so far only been studied in theory. According to those calculations, the speed of a 3D vortex DW is around 600 m/s at the depinning field and can be increased to thousands of meters per second by increasing the applied magnetic field.19 Together with the predicted absence of the Walker breakdown in cylindrical nanowires18,19 3D vortex DWs are expected to have very attractive dynamic properties. Magnetic imaging based on TEM or MFM methods does not have sufficient temporal resolution to track the realtime motion of DWs. Hence, the experimental study of the DW dynamics in cylindrical nanowires requires ultrafast methods such as dynamic X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) and scanning transmission X-ray microscopy (STXM)35 or emerging methods for fast imaging in electron microscopy (not applied to study DW motion so far).

CONCLUSIONS We have introduced an elegant approach to create a periodic potential for DW pinning in cylindrical NWs. The pinning sites E

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ACS Nano condition to allow zero magnetic field environment. To confirm that the magnetic field from the objective or condenser lenses did not influence the initial magnetic state of the materials during experiments, the specimens were rotated horizontally in the specimen holder and flipped over. For highest sensitivity of the VBF-DPC method, the case of the semiconvergence angle α is bigger or equal to collection angle β the geometrical considerations suggest to apply an overlap γ between the zero-field diffraction disc and VBF detector such that γ = β, that is, the edge of the diffraction goes through the center of VBF detector. The convergence semiangle of the STEM probe was measured to be α = 0.6 mrad. The VBF detector was produced with an SA aperture of 40 μm, resulting in the collection semiangle β = 0.2 mrad. Holograms of the remanent magnetic states of the NWs were acquired using the Lorentz mode of the microscope, which allows the specimen to be imaged in a field free environment with the main objective lens of the microscope being switched off. Off-axis electron holograms were acquired with an electron biprism operated typically at around +160 V. Phase shift reconstruction has been done using a reference image. Two phase images of swapped samples were subtracted to eliminate the electrostatic contribution to the phase shift. For the construction of magnetic induction maps, the cosine function was applied to the magnetic contribution to the phase shift to produce magnetic phase contours. The in-plane component of the magnetic induction has been calculated from the holograms. MFM images have been recorded in lift-off mode (100 nm distance) by MFP-3D-Bio from Asylum Research. Standard atomic force microscopy nanosensor probes with a magnetic coating have been used. A drop of ethanol containing NWs was placed on a clean Si wafer and then dried. A single NW was selected by SEM and its position was marked by FIB. MFM measurements were carried out at remanent state after applying variable external magnetic fields 0- 5 kOe parallel to the NW’s axis. Simulations. The magnetization process of the Co, Ni, and Co/Ni NWs has been simulated by MAGPAR package with finite element discretization.36 The parameters used for the simulations were reported elsewhere.30 The average finite element discretization size was chosen to be 2 nm. The direction of the magnetocrystalline anisotropy was considered in agreement with TEM data (at 85° with respect to the NW axis for the Co segment and {220} texture for Ni segments).

ACKNOWLEDGMENTS Research reported in this publication was supported by the King Abdullah University of Science and Technology (KAUST). REFERENCES (1) Grobis, M. K.; Hellwig, O.; Hauet, T.; Dobisz, E.; Albrecht, T. R. High-Density Bit Patterned Media: Magnetic Design and Recording Performance. IEEE Trans. Magn. 2011, 47, 6−10. (2) Stipe, B. C.; Strand, T. C.; Poon, C. C.; Balamane, H.; Boone, T. D.; Katine, J. A.; Li, J.-L.; Rawat, V.; Nemoto, H.; Hirotsune, A.; Hellwig, O.; Ruiz, R.; Dobisz, E.; Kercher, D. S.; Robertson, N.; Albrecht, T. R.; Terris, B. D. Magnetic Recording at 1.5 Pb/m2 Using an Integrated Plasmonic Antenna. Nat. Photonics 2010, 4, 484−488. (3) Prince, B. Vertical 3D Memory Technologies, 1st ed.; John Wiley & Sons: New York, 2014. (4) Parkin, S. S. P.; Hayashi, M.; Thomas, L. Magnetic Domain-Wall Racetrack Memory. Science 2008, 320, 190−194. (5) Phung, T.; Pushp, A.; Thomas, L.; Rettner, C.; Yang, S.-H.; Ryu, K.-S.; Baglin, J.; Hughes, B.; Parkin, S. Highly Efficient In-Line Magnetic Domain Wall Injector. Nano Lett. 2015, 15, 835−841. (6) Fratila, R. M.; Rivera-Fernández, S.; de la Fuente, J. M. Shape Matters: Synthesis and Biomedical Applications of High Aspect Ratio Magnetic Nanomaterials. Nanoscale 2015, 7, 8233−8260. (7) Safi, M.; Yan, M.; Guedeau-Boudeville, M.-A.; Conjeaud, H.; Garnier-Thibaud, V.; Boggetto, N.; Baeza-Squiban, A.; Niedergang, F.; Averbeck, D.; Berret, J.-F. Interactions between Magnetic Nanowires and Living Cells: Uptake, Toxicity, and Degradation. ACS Nano 2011, 5, 5354−5364. (8) Hultgren, A.; Tanase, M.; Felton, E. J.; Bhadriraju, K.; Salem, A. K.; Chen, C. S.; Reich, D. H. Optimization of Yield in Magnetic Cell Separations Using Nickel Nanowires of Different Lengths. Biotechnol. Prog. 2005, 21, 509−515. (9) Song, M.-M.; Song, W.-J.; Bi, H.; Wang, J.; Wu, W.-L.; Sun, J.; Yu, M. Cytotoxicity and Cellular Uptake of Iron Nanowires. Biomaterials 2010, 31, 1509−1517. (10) Contreras, M. F.; Sougrat, R.; Zaher, A.; Ravasi, T.; Kosel, J. Non-chemotoxic Cancer Cell Death Induction Using Magnetic Nanowires. Int. J. Nanomed. 2015, 10, 2141−2153. (11) Alfadhel, A.; Li, B.; Zaher, A. O.; Yassine, O.; Kosel, J. A Magnetic Nanocomposite for Biomimetic Flow Sensing. Lab Chip 2014, 14, 4362−4369. (12) Alnassar, M. Y.; Alfadhel, A.; Ivanov, Y. P.; Kosel, J. Magnetoelectric Polymer Nanocomposite for Flexible Electronics. J. Appl. Phys. 2015, 117, 17D711. (13) Ivanov, Y. P.; Trabada, D. G.; Chuvilin, A.; Kosel, J.; ChubykaloFesenko, O.; Vázquez, M. Crystallographically Ddriven Magnetic Behaviour of Arrays of Monocrystalline Co Nanowires. Nanotechnology 2014, 25, 475702. (14) Vidal, E. V.; Ivanov, Y. P.; Mohammed, H.; Kosel, J. A Detailed Study of the Magnetization Reversal in Individual Ni Nanowires. Appl. Phys. Lett. 2015, 106, 032403. (15) Vivas, G.; Ivanov, Y. P.; Trabada, D. G.; Proenca, M. P.; Chubykalo-Fesenko, O.; Vázquez, M. Magnetic Properties of Co Nanopillar Arrays Prepared From Alumina Templates. Nanotechnology 2013, 24, 105703. (16) Bran, C.; Ivanov, Y. P.; García, J.; del Real, R. P.; Prida, V. M.; Chubykalo-Fesenko, O.; Vazquez, M. Tuning the Magnetization Reversal Process of FeCoCu Nanowire Aarrays by Thermal Annealing. J. Appl. Phys. 2013, 114, 043908. (17) Ivanov, Y. P.; Vivas, L. G.; Asenjo, A.; Chuvilin, A.; ChubykaloFesenko, O.; Vázquez, M. Magnetic Structure of a Single Crystal hcp Electrodeposited Cobalt Nanowire. Europhys. Lett. 2013, 102, 17009. (18) Hertel, R.; Yan, M.; Kakay, A.; Gliga, S. Beating the Walker Limit with Massless Domain Walls in Cylindrical Nanowires. Phys. Rev. Lett. 2010, 104, 057201. (19) Piao, H.-G.; Shim, J.-H.; Djuhana, D.; Kim, D.-H. Intrinsic Pinning Behavior and Propagation Onset of Three-Dimensional

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b01337. Additional information and figures on material characterizations, magnetic measurements and magnetic force microscopy study. (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

Y. I. and J.K. conceived the project. Y.I. fabricated the nanowires, designed, and performed TEM and MFM experiments, performed data analysis, and wrote the manuscript. A.C. and S.L. performed TEM experiments and data analysis and contributed to the manuscript writing. J.K. supported the experiments and contributed to the manuscript writing. All authors contributed to the discussions. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acsnano.6b01337 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.6b01337 ACS Nano XXXX, XXX, XXX−XXX