Magnetization Ratchet in Cylindrical Nanowires - ACS Publications

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Magnetization Ratchet in Cylindrical Nanowires Cristina Bran,*,† Eider Berganza,† Jose A. Fernandez-Roldan,† Ester M. Palmero,†,⊥ Jessica Meier,† Esther Calle,† Miriam Jaafar,† Michael Foerster,‡ Lucia Aballe,‡ Arantxa Fraile Rodriguez,§,∥ Rafael P. del Real,† Agustina Asenjo,† Oksana Chubykalo-Fesenko,† and Manuel Vazquez*,† †

Institute of Materials Science of Madrid, CSIC, 28049 Madrid, Spain ALBA Synchrotron Light Facility, CELLS, 08290 Barcelona, Spain § Departament de Física de la Matèria Condensada and ∥Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, 08028 Barcelona, Spain

ACS Nano 2018.12:5932-5939. Downloaded from pubs.acs.org by UNIV OF FINDLAY on 09/11/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The unidirectional motion of information carriers such as domain walls in magnetic nanostrips is a key feature for many future spintronic applications based on shift registers. This magnetic ratchet effect has so far been achieved in a limited number of complex nanomagnetic structures, for example, by lithographically engineered pinning sites. Here we report on a simple remagnetization ratchet originated in the asymmetric potential from the designed increasing lengths of magnetostatically coupled ferromagnetic segments in FeCo/Cu cylindrical nanowires. The magnetization reversal in neighboring segments propagates sequentially in steps starting from the shorter segments, irrespective of the applied field direction. This natural and efficient ratchet offers alternatives for the design of three-dimensional advanced storage and logic devices. KEYWORDS: multisegmented nanowires, remagnetization ratchet, magnetic domains, vortex domain walls, dipolar coupling asymmetry in their speed with respect to magnetic fields for 2D data storage systems.12,13 Different alternatives for the pinning/ depinning of a DW in nanostrips have been attempted recently by various techniques, among others by geometrical notches and corners or under the action of local stray fields generated by neighboring tips or wires.14,15 The DW ratchet effect was suggested for the DW motion in nanostrips with asymmetric notches, resulting in an asymmetric pinning depending on the DW motion direction.16 On the other hand, the cylindrical geometry of nanowires leads to interesting properties in the local domain structure and motion of individual DWs. Thus, cylindrical self-standing nanowires are promising nano-objects for applications in nanotechnology areas such as new-generation spintronicbased magnetic recording, energy technologies, logic and microwave devices, biomagnetics, robotics, and thermomagneto-electric devices.17−24 The circular symmetry promotes the spontaneous development of vortex domains with magnetic moments following a circumferential path at the surface but staying longitudinal in the core.25 Cylindrical nanowires offer

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agnetic domain wall, DW, manipulation either by the application of magnetic fields or by the injection of electric current is commonly accomplished in planar patterned ferromagnetic nanostrips. Their unidirectional motion is a key concept underlying next-generation domainwall-mediated data storage devices and shift registers without mechanically moving parts: a magnetic ratchet device allows linear or rotary motion in only one direction, preventing it in the opposite one, and originates in the asymmetric energy barrier or pinning sites. Many fascinating prototypes for magnetic ratchet effects are attracting attention, from fundamental to engineering functionality points of view. Magnetic quantum ratchets have been recently reported in artificially asymmetric graphene1−3 or in superconducting systems where vortices are pinned at asymmetric substrates or by designed antidot arrays.4,5 Nanopatterned magnetic films with asymmetrical holes or dots can also show a ratchet effect.6,7 Skyrmion ratchets represent an ac current-based method for controlling the skyrmion position and motion for spintronic applications.8 Shift registers based on DW ratchets have been proposed in planar nanostrips where DWs experience an energy landscape engineered to favor unidirectional ratchetlike propagation.9,10 The unidirectional motion of DWs has been proposed in memory devices based on in-plane field-controlled DW pinning11 and in chiral DWs exhibiting © 2018 American Chemical Society

Received: March 22, 2018 Accepted: May 29, 2018 Published: May 29, 2018 5932

DOI: 10.1021/acsnano.8b02153 ACS Nano 2018, 12, 5932−5939

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Cite This: ACS Nano 2018, 12, 5932−5939

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the other to record the evolution of the local hysteresis loops with the segment length. Several nanowires were measured, and although specific peculiarities may occasionally be found, the local hysteresis loops presented here are representative of their general behavior, characterized by several individual Barkhausen jumps. Figure 2a−f shows a series of MOKE hysteresis loops at selected positions (as indicated in Figure 2g) along the nanowire, going from the shortest (a) to the longest (f) segments, respectively. The shortest (a) segment shows a single magnetization jump, ΔM1, at Hc,1 ≈ 190 Oe. As we move toward longer segments, two new jumps, ΔM2 (b, c) and ΔM3 (d−f), are observed at higher fields, Hc,2 ≈ 265 Oe and Hc,3 ≈ 365 Oe, respectively. This indicates that magnetization reversal is unidirectional and takes place in several steps starting from the shortest segments. The profile of the relative amplitude of the individual jumps is depicted in Figure 2g (the broken lines are guides for the eye), where we observe that the main contributions ΔM1, ΔM2, and ΔM3 come from the shorter, medium, and longer segments, respectively. Figure 3a presents the MFM image in the middle region of a nanowire, schematically shown in Figure 3b. The so-called 3D imaging MFM-based technique43 has been used to study the magnetization process and to obtain a hysteresis loop. In this nonstandard MFM mode, the tip performs successive scans along the same region of the nanowire while the in-plane magnetic field varies between ±700 Oe. Along the first magnetic field sweep between −700 Oe and +700 Oe (Figure 3c), the magnetization reversal proceeds several steps from the nanowire’s left end and is pinned at marked intermediate segments (red dotted arrows), denoted in the picture by a dark contrast (see (2), (3), and (4)), until a nearly saturated state is achieved (5). Notice that the dark contrast corresponds to a magnetic configuration similar to the head-to-head domain wall. A sketch of the reversal process is shown in Figure 3d. Similarly, under an opposite sweep field (from +700 Oe to −700 Oe), DWs between segments of opposite magnetization (identified by a bright contrast) are observed at nearly the same segment locations under comparable field amplitudes (Figure 3e). In this case, the MFM contrast is opposite to the previous one since the stray field distribution at these DWs is similar to a tail-to-tail configuration. The red and blue dotted arrows in Figure 3c,e indicate the magnetization direction of the growing magnetic domains. It is worth mentioning that in both MFM experiments the reversal magnetization process starts at the nanowire’s left end, i.e., where the FeCo segments are shorter. Figure 3f shows the hysteresis loop (in blue) reconstructed from the MFM data in Figure 3c,e, as well as the reconstructed loop (in dashed red) from the subsequent MFM-based field scanning (Supporting Information). Note that only slight differences can be observed between consecutive loops. To gain further insight into the magnetization distribution, X-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM) measurements were carried out at the ALBA Synchrotron.44 This technique allows the determination of the magnetization distribution both at the surface and in the inner part of the nanowires.30,31,33 A description of the contrast formation can be found in the Supporting Information. The results in Figure 4 correspond to a nearly 20-μm-long nanowire. Above each stack of XMCDPEEM images, we show the direct X-ray absorption spectroscopy (XAS) image at the Co L3 absorption edge for the chemical identification of the Cu segments. The XMCD-PEEM

specific advantages such as the possibility to tailor the DW shape,26 to adjust their stability during propagation, or to suppress the Walker breakdown.27,28 Their tailored composition typically includes Co in order to introduce crystalline anisotropy, promoting the formation of vortex29 or transverse domains.30 Micromagnetic modeling unveils the remagnetization process in cylindrical nanowires as taking place essentially by the nucleation of complex magnetic closure structures at the nanowire ends from which DWs depin and propagate.26,31 There are two main reversal modes defined by a characteristic DW: either a transverse DW similar to that found in nanostrips or vortex DW with a singularity at the axis so that magnetization reversal takes place by the propagation of a Bloch point DW at a critical field. The synthesis of cylindrical NWs inside porous templates with designed composition and geometry is achieved by an inexpensive electrochemical route.32 This method allows for the tailoring of specific geometrical and compositional profiles with periodical modulations in diameter (e.g., notches and antinotches33,34) or in composition, containing alternating segments of ferromagnetic/ferromagnetic, FM/FM,35−37 or ferromagnetic/nonmagnetic, FM/NM,36−39 materials for various sensor applications.40,41 Here we report the realization of a remagnetization ratchet in cylindrical FM/NM nanowires by modifying the magnetic segment lengths during the electrochemical synthesis. Our final objective is to manipulate the magnetization reversal in such a way as to provoke its stepped unidirectional propagation irrespective of the applied field direction.

RESULTS AND DISCUSSION Figure 1a shows the scanning electron microscopy (SEM) image of a Fe35Co65/Cu nanowire where both FeCo segments

Figure 1. FeCo/Cu multisegmented nanowires with a variable length of FeCo segments increasing from the left end. (a) SEM image of an 8.5-μm-long nanowire with an indication of the length of selected FeCo segments. (b) Magnetic image of a 19.7-μm-long nanowire at remanence reconstructed from three MFM images.

with increasing length and Cu layers (constant in length) are visible. The FeCo segments show bcc (110) symmetry as determined by X-ray diffraction (XRD). The magnetic force microscopy (MFM) image in Figure 1b taken at remanence indicates the overall axial magnetization in the nanowire. Moreover, the alternating dark−bright contrast observed along the nanowire reproduces its multisegmented character and denotes that magnetic charges accumulate at the FeCo interfaces with the Cu layers. Surface local hysteresis loops were measured using a magneto-optical Kerr effect (MOKE) magnetometer under a maximum magnetic field of ±500 Oe applied parallel to the nanowire axis.42 Considering the nominal width of the laser spot of about 3 μm, each raw loop contains the average contribution from various segments. At each spot, 1000 raw loops were averaged in order to reduce the signal-to-noise ratio. The spot was moved in 1 μm steps from one nanowire end to 5933


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Figure 2. (a−f) Local MOKE hysteresis loops for the FeCo/Cu multilayer nanowire at different positions schematically shown in (g), where the profile of the normalized amplitude of magnetization jumps ΔM1, ΔM2, and ΔM3 at Hc,1, Hc,2, and Hc,3, respectively, is also depicted. Note the larger contributions of the jumps at Hc,1 and Hc,3 at the short and long segment regions, respectively.

Figure 3. (a) MFM image at remanence of the central part of the nanowire where the measurement under an applied field parallel to the nanowire is carried out. (b) Schematic view of the multisegmented nanowire. (c, e) MFM-based images of the nanowire subjected to subsequent field sweeping from a nearly saturated field in opposite directions. (d) Schematic view of the magnetic configuration under increasing applied field in (c). (f) Hysteresis loop (blue) reconstructed from images in (c) and (e) and the subsequent loop (dashed red) obtained after two successive field scans (Supporting Information). Red and blue dotted lines in (c) and (e) account for the positive and negative magnetization directions of the nanowire segments.

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Figure 4. Sequences of XAS (above, circled in red, with an indication of the beam direction, white arrow) and PEEM images of a multisegmented FeCo/Cu nanowire, previously magnetized under the maximum field along rightward (a) and leftward (b) polarity, where Cu segments can be identified. Selected PEEM images under an increasing applied field along leftward (a) and rightward (b) polarity. The inset in (b) shows the reconstructed hysteresis loop.

Figure 5. (Left) Total and internal (with the exception of the Zeeman one) magnetic energies in the nanowire as a function of the longitudinal magnetization component evaluated by the micromagnetic simulations during the ascending branch of the hysteresis loop. (Right) Selected magnetization configurations showing the sequential reversal during the reversal process corresponding to the internal energy minima marked on the left figure.

images present contrast both in the wire itself (dotted lines labeled NW) and in the shadow due to the photoemission from the substrate after transmission through the wire core (wider region of uniform opposite contrast). For observations of the magnetic reversal, we used a sample holder which permits the in situ application of in-plane magnetic fields of up to ±800 Oe.45 The magnetic field is applied along the nanowire axis as indicated by the white arrows in Figure 4. It is difficult to obtain XMCD images with the required spatial resolution in the applied field which disturbs the low-energy electrons in the PEEM. Therefore, we restricted our observations to the remanent states, i.e., after reducing the applied field to zero. Figure 4 shows a sequence of selected XMCD images of a nanowire extracted from the positive (a) and negative (b) branches of the remanent hysteresis loops. Magnetic domains with magnetic moments pointing parallel or antiparallel to the polarization vector (indicated by a white arrow in Figure 4) appear white/black in the NW (black/white in the shadow) in Figure 4a,b. In addition, the homogeneous contrast of opposite orientation between the NW and shadow regions observed in all of the individual segments indicates a uniform axial magnetization at the NW surface and core. At the maximum applied field (800 Oe), both inner and surface magnetizations are uniform across

the whole wire and fully reversed under the opposite magnetic field. The XMCD images unambiguously confirm that irrespective of the field direction, both in the surface and the bulk of the nanowire, the reversal is initiated at the end with shorter segments and proceeds unidirectionally by individual jumps, with specific depinning fields at each Cu segment position. This is highlighted in the schematic loop inserted into Figure 4b. Micromagnetic simulations were performed in an individual FeCo/Cu multilayer nanowire using the mumax3 code,46 with typical micromagnetic parameters for this composition,47,48 for a deeper understanding of the experimental results. The simulations start with a saturated state along the nanowire length, and the magnetization configuration is followed under an increasing field applied in the opposite direction. Generally, the shorter segments store larger magnetostatic energy so that they will remagnetize first, which is the origin of the ratchet potential. An important remark is that the FeCo segments are close to ensure a strong magnetostatic interaction between neighboring segments but spaced enough to promote the sequential nature of the magnetization reversal. The simulations confirm that the demagnetization starts at the nanowire end with shorter segments, independently of the applied field direction. The left panel of Figure 5 presents the evolution of 5935


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by MFM and detected by MOKE) as well as in the whole nanowire segments (seen in XMCD-PEEM measurements and simulations). (b) The reversal process always propagates unidirectionally, irrespectively of the external field direction, being initiated at the end of segments with shorter lengths. Such a ratchet effect originates from the broken symmetry induced by the increasing length of the FeCo segments, and as in a domino effect, it is promoted by the magnetostatic coupling between adjacent segments. (c) The reversal is often pinned at specific locations associated with Cu layers. The number and position of those pinning centers may vary from scan to scan (as well as from nanowire to nanowire), although often preferred sites are identified. Regarding the quantitative values, all data indicate that switching occurs at around 200 Oe applied field in the shorter segments and increases up to 300 to 500 Oe as one moves toward longer segments. (d) The micromagnetic simulations reveal that rather than the propagation of one domain wall, a more complex process takes place. Although the switching is sequential from one segment to another, the magnetization process inside each segment takes place by the formation of vortices and skyrmion tube states, followed by the final collapse of the internal core. The formation of skyrmion tubes with opposite chiralities and strong topological protection may constitute the origin of pinned magnetic states. Although statically large segments seem to demagnetize simultaneously, dynamically the propagation is also sequential. Here, the structural defects may produce pinning. The analysis of the energy landscape proves the existence of a ratchetlike potential. (e) The reported remagnetization ratchet is induced by the variable length of the segments. That suggests that any other cause of asymmetry breaking at one end of the wire should result in a similar remagnetization ratchet. The observed ratchet effect in multisegmented cylindrical nanowires constitutes a promising and simple route toward the control of magnetic carrier information in future threedimensional magnetic memories and shift registers.

the total and internal (collective contribution of exchange, dipolar, and magnetocrystalline anisotropy) energies as a function of the axial magnetization component. The latter in the ratchetlike picture is proportional to the DW position although the reality is somewhat different. The energies clearly display a ratchetlike potential created by the increasing shape anisotropy of longer segments, the exchange energy, and the pinning sites. A careful inspection of the magnetization configurations (see examples in the right panel of Figure 5) reveals that the demagnetization proceeds by the sequential switching of different segments starting from the left side (with the exception of the first two short segments which start almost simultaneously but are partially magnetized until full demagnetization). Although the switching is mostly sequential from one segment to the next, the reversal inside each segment starts by the nucleation of the open vortexlike structures which minimize the stray field at the ends of each segment. These structures propagate toward the segment center to form recently discussed skyrmion tubes49 with a nanowire surface shell magnetized parallel to the field and a small core antiparallel to it. When such structures nucleate at the ends of each segment with opposite chiralities and meet in the center, they create a complex topologically protected 3D wall49 which is one of the origins of the magnetization pinning (Figure 5a). As the remagnetization proceeds, the skyrmion core decreases in diameter and disappears. Furthermore, the analysis of the energy landscape indicates that the true magnetization pinning in the idealized modeled case occurs in short segments only. This is also detected as small Barkhausen jumps in the modeled hysteresis loop (Supporting Information). Configurations b−d in the right panel of Figure 5, although corresponding to the abrupt change in the slope of the energy landscape, are dynamic and do not correspond to true minima of the total energy. However, in real experiment those configurations may be stabilized by the presence of defects. The reported experimental data reflect a certain degree of randomness in the location of remagnetization pinning, seemingly related to local defects at Cu interfaces which are not fully accounted for by more idealistic micromagnetic simulations. This kind of randomness also reminds us of the creep DW motion observed in magnetic materials under the action of subthreshold forces that are usually thermally activated. In fact, the creep motion of elastic interfaces shares the same physics for a wide variety of objects. In particular, magnetic DWs have served as prototypical examples of the above phenomenon as a result of their interaction with the applied forces in the presence of the disorder in the system. Interest has been activated by recent studies of creep motion induced by current rather than a magnetic field.50 In our case, the disordered interface is represented by the imperfect Cu layers, and subsequent statistical studies should be conducted to unveil in this case the role of temperature activation for potential uses of the unidirectional remagnetization ratchet.

EXPERIMENTAL DETAILS Multisegmented cylindrical magnetic nanowires were grown by electrodeposition inside the pores of anodic alumina templates previously synthesized by hard anodization.32,33,51 The electrodeposition rate decreased in time to result in FeCo magnetic segments with variable length from one end to the opposite one. Nanowires are 120 nm in diameter with a total length of about 20 μm. The FeCo segments are around 250 nm long at one end and gradually increase length with an average incremental step of about 15 nm between neighboring segments. The length of the Cu segments is kept constant at 30 nm. For the investigation of individual nanowires, they are released from the template by chemical etching, followed by subsequent cleaning that sometimes breaks the nanowires into shorter pieces. The Fe35Co65 composition of magnetic segments was determined by energy-dispersive X-ray spectroscopy (EDX), and their bcc (110) cubic crystallographic structure was confirmed by Xray diffraction (XRD) using a PANalytical X’pert Pro X-ray diffractometer in Bragg−Brentano geometry. The chemical composition is selected because of its high saturation magnetization (μ0Ms = 2 T). Experimental data obtained by different techniques were taken in a number of nanowires with different total lengths. A number of nanowires were taken for the different experimental measurements, and although specific peculiarities were occasionally found, the

CONCLUSIONS From the set of experimental data and micromagnetic simulations, we can summarize the following: (a) The remagnetization of individual cylindrical FeCo/Cu nanowires with tailored increasing length segments has been proven to proceed in a few irreversible jumps at which magnetization reverses at the surface (as observed 5936


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ACS Nano selected results presented in this article are representative of all of the measurements. The magnetic hysteresis loops of individual nanowires were measured using a Kerr effect magnetometer (NanoMOKE 2, nominal laser spot of 3 μm, wavelength 658 nm, and power 7.5 mW) under a maximum applied field of ±500 Oe. A variable field magnetic force microscope (VF-MFM) from Nanotec Electronica S.L.52 has also been used to image the magnetic state of the surface, working with amplitude modulation and the twopass method. The phase-locked loop (PLL) was enabled to track the resonance frequency of the oscillating cantilever, so the frequency shift signal was recorded to obtain the MFM images. All measurements were made with Multi75M-G microchips from Budget Sensors (nominal values, resonance frequency 75 kHz, force constant 3 N/ m). The oscillation amplitude of the cantilever was set at 10−15 nm, and a typical retrace distance of 70 nm was chosen. The XMCD−PEEM measurements were performed at the CIRCE beamline of the ALBA Synchrotron Light Facility (Barcelona, Spain) using an ELMITEC LEEM III instrument with an energy analyzer. The samples are illuminated with circularly polarized X-rays at a grazing angle of 16° with respect to the surface at the resonant Co L3 absorption edge (778 eV). The emitted photoelectrons (low-energy secondary electrons with ca. 1 eV of kinetic energy) used to form the surface image are proportional to the X-ray absorption coefficient, and thus the element-specific magnetic domain configuration is given by the pixelwise asymmetry of two PEEM images sequentially recorded with left-and right-handed circular polarization. (For more details, see the Supporting Information.29) Micromagnetic simulations were performed in an individual FeCo/ Cu multilayer nanowire using the mumax3 code.45 The nanowire, roughly 3 μm in length and 80 nm in diameter, is formed by eight single-crystal bcc FeCo segments with increasing length from 200 to 600 nm (increasing 50 nm between adjacent magnetic segments). A 30 nm empty space between adjacent segments is introduced to represent the Cu spacer segments. The dimensions are smaller than for the experimentally measured nanowires in order to facilitate calculations but are in the same range. The material parameters used in calculations are typical values from the literature: saturation magnetic polarization 2 T, exchange stiffness 25 pJ/m, and magnetocrystalline anisotropy constant 104 J/m3.47,48 Further details of the simulations are presented in the Supporting Information.

A.A. and M.J. C.B., R.P.d.R., A.F.R., M.F., L.A., E.M.P., E.B., E.C., A.A., and M.V. conducted the XMCD-PEEM measurements. J.A.F.-R. conducted the micromagnetic simulations under the supervision of O.C.-F. The results were analyzed and discussed by all authors during the whole process. The main text was written by M.V., C.B., O.C.-F., and R.P.d.R. with significant contributions by L.A., M.F., A.F.R., A.A., J.A.F.-R., M.J., and E.B. All authors reviewed and approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has been supported by the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) under grants MAT2016-76824-C3-1-R and FIS2016-78591-C3-3-R and by the Regional Government of Madrid under project S2013/MIT2850 NANOFRONTMAG-CM. We acknowledge the service from the MiNa Laboratory at IMN and funding from CM (project SpaceTec, S2013/ICE2822), MINECO (project CSIC13-4E-1794), and EU (FEDER, FSE). J.M. acknowledges DAAD support during her stay at ICMM/CSIC. M.J. acknowledges a funding grant from Spanish MINECO under the MAT2015-73775-JIN research project. J.A.F.-R. acknowledges support from Spanish MINECO and FSE though fellowship BES-2014-068789. REFERENCES (1) Drexler, C.; Tarasenko, S. A.; Olbrich, P.; Karch, J.; Hirmer, M.; Müller, F.; Gmitra, M.; Fabian, J.; Yakimova, R.; Lara-Avila, S.; Kubatkin, S.; Wang, M.; Vajtai, R.; Ajayan, P. M.; Kono, J.; Ganichev, S. D. Magnetic Quantum Ratchet Effect in Graphene. Nat. Nanotechnol. 2013, 8, 104−107. (2) Kheirabadi, N.; McCann, E.; Fal’ko, V. I. Cyclotron Resonance of the Magnetic Ratchet Effect and Second Harmonic Generation in Bilayer Graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 97, 075415−075425. (3) Budkin, G. V.; Golub, L. E. Orbital Magnetic Ratchet Effect. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 125316. (4) Wördenweber, R.; Dymashevski, P.; Misko, V. R. Guidance of Vortices and the Vortex Ratchet Effect in High-Tc Superconducting Thin Films Obtained by Arrangement of Antidots. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 184504. (5) Rouco, V.; Palau, A.; Monton, C.; Del-Valle, N.; Navau, C.; Sanchez, A.; Obradors, X.; Puig, T. Geometrically Controlled Ratchet Effect with Collective Vortex Motion. New J. Phys. 2015, 17, 073022− 073027. (6) Pérez-Junquera, A.; Marconi, V. I.; Kolton, A. B.; Á lvarez-Prado, L. M.; Souche, Y.; Alija, A.; Vélez, M.; Anguita, J. V.; Alameda, J. M.; Martín, J. I.; Parrondo, J. M. R. Crossed-Ratchet Effects for Magnetic Domain Wall Motion. Phys. Rev. Lett. 2008, 100, 037203. (7) Jaafar, M.; Yanes, R.; Perez De Lara, D.; Chubykalo-Fesenko, O.; Asenjo, A.; Gonzalez, E. M.; Anguita, J. V.; Vazquez, M.; Vicent, J. L. Control of the Chirality and Polarity of Magnetic Vortices in Triangular Nanodots. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 054439. (8) Reichhardt, C.; Ray, D.; Reichhardt, C. J. O. Magnus-Induced Ratchet Effects for Skyrmions Interacting with Asymmetric Substrates. New J. Phys. 2015, 17, 073034−073041. (9) Franken, J. H.; Swagten, H. J. M.; Koopmans, B. Shift Registers Based on Magnetic Domain Wall Ratchets with Perpendicular Anisotropy. Nat. Nanotechnol. 2012, 7, 499−503. (10) Lavrijsen, R.; Lee, J. H.; Fernández-Pacheco, A.; Petit, D. C. M. C.; Mansell, R.; Cowburn, R. P. Magnetic Ratchet for ThreeDimensional Spintronic Memory and Logic. Nature 2013, 493, 647− 650.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b02153. Sample preparation, MFM and XMCD-PEEM experiments, and micromagnetic simulations (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Cristina Bran: 0000-0001-8571-5846 Jose A. Fernandez-Roldan: 0000-0002-8320-5268 Arantxa Fraile Rodriguez: 0000-0003-2722-0882 Present Address ⊥

Division of Permanent Magnets and Applications, IMDEA Nanoscience, 28049 Madrid, Spain. Author Contributions

M.V., C.B., A.A., R.P.d.R., and O.C.-F. conceived the idea of the manuscript. C.B. synthesized and structurally characterized the nanowires. J.M. and E.M.P. conducted MOKE measurements. E.B. conducted MFM measurements under the supervision of 5937


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