Magnetization Ratchet in Cylindrical Nanowires - ACS Nano (ACS

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Magnetization Ratchet in Cylindrical Nanowires Cristina Bran, Eider Berganza, Jose Angel Fernandez-Roldan, Ester M. Palmero, Jessica Meier, Esther Calle, Miriam Jaafar, Michael Foerster, Lucia Aballe, Arantxa Fraile Rodríguez, Rafael P. Del Real, Agustina Asenjo, Oksana Chubykalo-Fesenko, and Manuel Vazquez ACS Nano, Just Accepted Manuscript • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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Magnetization Ratchet in Cylindrical Nanowires Cristina Bran*1, Eider Berganza1, Jose A. Fernandez-Roldan1, Ester M. Palmero1†, Jessica Meier1, Esther Calle1, Miriam Jaafar1, Michael Foerster2, Lucia Aballe2, Arantxa Fraile Rodriguez3,4, Rafael P. del Real1, Agustina Asenjo1, Oksana ChubykaloFesenko1 and Manuel Vazquez*1 1

Institute of Materials Science of Madrid, CSIC. 28049 Madrid. Spain

2

ALBA Synchrotron Light Facility, CELLS. 08290 Barcelona. Spain

3

Departament de Física de la Matèria Condensada, Universitat de Barcelona.

08028 Barcelona. Spain 4

Institut de Nanociència i Nanotecnologia (IN2UB). Universitat de Barcelona

08028 Barcelona. Spain

KEYWORDS. multisegmented nanowires; remagnetization ratchet; magnetic domains; vortex domain walls; dipolar coupling

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.

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This magnetic ratchet effect has been so far achieved in limited number of complex nanomagnetic structures for example by lithographically engineered pinning sites. Here we report on 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.

Magnetic 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 of domain-wall-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 functionalities points of view. Magnetic quantum ratchets have been recently reported in artificially asymmetric graphene,1-3 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,

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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 a unidirectional ratchet-like propagation.9,


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The unidirectional

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motion of DWs has been proposed in memory devices based on in-plane fieldcontrolled DW pinning,11 and chiral DWs exhibiting 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 DWs 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 spintronic-based magnetic recording, energy technologies, logic and microwave devices, biomagnetics, robotics or for thermo-magneto-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 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 includes typically Co in order to introduce crystalline anisotropy,

promoting

the

formation

of

vortex29 or transverse domains.30

Micromagnetic modelling 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 DW de-pin 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


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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 anti-notches33,34) or in composition, containing alternating segments of ferromagnetic/ferromagnetic, FM/FM,

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or ferromagnetic/non-magnetic, FM/NM36,37 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 a way to provoke its stepped unidirectional propagation irrespective of the applied field direction.

Figure 1. FeCo/Cu multisegmented nanowires nanowire with variable length of FeCo segments increasing from the left’s end. (a) SEM image of an 8.5 µm long nanowire with indication of the length of selected FeCo segments. (b) magnetic image of a 19.7 µm long nanowire at remanence reconstructed from 3 MFM images.


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RESULTS AND DISCUSSION. Figure 1(a) shows the scanning electron microscopy (SEM) image of a Fe35Co65/Cu nanowire where both FeCo segments with increasing length and Cu layers (constant in length) are visible. The FeCo nanowires show a bcc (110) symmetry as determined by X-ray diffraction (XRD). The magnetic force microscopy (MFM) image in Figure 1(b) 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.

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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 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 here presented are representative of their general behavior, characterized by several individual Barkhausen jumps. Figure 2(a-f) shows a series of MOKE hysteresis loops at selected positions (as indicated in Fig. 2(g)) along the nanowire, going from the shortest (a) to the longest (f) segments, respectively. The shortest (a) segment shows a single magnetization jump, ∆M1, at Hc,1 ~ 190 Oe. As we move towards longer segments, two new jumps, ∆M2 (b)-(c)), and ∆M3 ((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


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relative amplitude of the individual jumps is depicted in Fig. 2(g) (the broken lines are guidelines to the eye) where we observe that the main contributions ∆M1, ∆M2 and ∆M3 come respectively from the shorter, the medium and the longer segments.

Figure 2. (a to 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 the magnetization jumps ∆M1, ∆M2 and ∆M3 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 segments regions, respectively.

Figure 3(a) presents the MFM image at the middle region of a nanowire, schematically shown in Fig. 3(b). The so-called 3D imaging MFM–based technique43 has been used to study the magnetization process and to obtain a hysteresis loop. In this non-standard 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 (Fig. 3c) the magnetization reversal proceeds at several steps from the nanowire’s left end, and is pinned at marked


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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 head-to-head domain wall. A sketch of the reversal process is shown in Fig. 3(d). Similarly, under 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 (Fig. 3(e)). In this case, the MFM contrast is opposite to the previous one since the stray field distribution at these DWs is similar to a tale-to-tale configuration. The red and blue dotted arrows in Fig. 3(c) and 3(e) indicate the magnetization direction of the growing magnetic domains. It is worth mentioning that in both MFM experiments the reversal magnetization process starts in the nanowire´s left end i.e. where the FeCo segments are shorter. Figure 3(f) shows the hysteresis loop (in blue) reconstructed from the MFM data in Figs. 3(c) and 3(e), as well as the reconstructed loop (in dashed red) from the subsequent MFM-based field scanning (see Supplementary Information). Note that only slight differences can be observed between consecutive loops.


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Figure 3. (a) MFM image at remanence of the central part of the nanowire where the measurement under applied field parallel to the nanowire are carried out. (b) Schematic view of the multisegmented nanowire. In (c) and (e), MFM-based images of the nanowire submitted to subsequent field sweeping from nearly saturating field in opposite directions. (d) Schematic view of the magnetic configuration under increasing applied field in (c). (f) The hysteresis loop (blue) reconstructed from images in (c) and (e) and the subsequent loop (dashed red) obtained after two successive field scanning (see Supplementary Information). Red and blue dotted lines in (c) and (e) account for the positive or negative magnetization direction of the nanowire segments

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 ALBA Synchrotron.44 This technique allows determining the magnetization distribution both at the surface and at the inner part of


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nanowires.30,31,33 A description of the contrast formation can be found in the Supplementary Information. The results in Fig. 4 correspond to a nearly 20 µm long nanowire. Above each stack of XMCD-PEEM images, we show the direct X-ray Absorption Spectroscopy (XAS) image at the Co L3 absorption edge for chemical identification of the Cu segments. The XMCD-PEEM 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 in situ application of in-plane magnetic fields up to ±800 Oe.45 The magnetic field is applied along the nanowire axis as indicated by a white arrow in Fig. 4. It is difficult to obtain XMCD images with the required spatial resolution in 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 with a black arrow in Fig. 4) appear white/black in the NW (black/white in the shadow) in Figs. 4(a) and (b). In addition, the homogeneous contrast of opposite orientation between the NW and shadow regions observed in all 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 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 initiates at the end with shorter segments and proceeds


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unidirectionally by individual jumps, with specific de-pinning fields at each Cu segment position. This is highlighted in the schematic loop inserted in Fig. 4(b).

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

Micromagnetic simulations were performed in an individual FeCo/Cu multilayer nanowire using mumax3 code,46 using 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


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neighboring segments, but spaced enough to increase 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 Fig. 5 presents the evolution of the total and internal (collective contribution of exchange, dipolar energies and magnetocrystalline anisotropy) energies as a function of the axial magnetization component. The latter in the ratchet-like picture is proportional to the DW position although the reality is somewhat different. The energies clearly display a ratchet-like potential created by the increasing shape anisotropy of longer segments, the exchange energy and the pinning sites. Careful inspection of the magnetization configurations, see examples in the right panel of Fig. 5, reveals that the demagnetization proceeds by 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 the 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 vortex-like structures which minimize the stray field at the ends of each segment. These structures propagate towards 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 (see Fig. 5(a)). 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 modelled case occurs in short segments only. This is also detected as small Barkhausen jumps in the modelled hysteresis loop (see Supplementary information).


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The configurations b-d, in the right panel of Fig. 5 although corresponding to the abrupt change in the slope of the energy landscape, are dynamical and do not correspond to true minima of the total energy. However, in real experimental those configurations may be stabilized by the presence of defects.

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.

The reported experimental data reflect 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. Such kind of randomness also reminds the creep DW motion observed in magnetic materials under the action of sub-threshold forces usually thermally activated. In fact, the creep motion of elastic interfaces shares the same physics for a wide variety of objects. Particularly, magnetic DWs have served as prototypical examples of the above phenomenon as a


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result of their interaction with the applied forces in the presence of the disorder in the system. An interest has been activated by recent studies of creep motion induced by current rather than 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.


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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 proved to proceed in few irreversible jumps at which magnetization reverses at the surface (as observed 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 propagates always unidirectionally, irrespectively of the external field direction, initiating at the end of segments with shorter length. Such ratchet effect originates in the broken symmetry induced by the increasing length of the FeCo segments and, like in a domino effect it is promoted by the magnetostatic coupling between adjacent segments. c) The reversal is often pinned at specific locations associated to 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 towards 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 on 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


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produce pinning. The analysis of the energy landscape proves the existence of ratchet-like potential. e) The reported remagnetization ratchet is induced by the variable length of segments. That suggests that any other cause of asymmetry breaking at one end of the wire should result in the similar remagnetization ratchet. The observed ratchet effect in multisegmented cylindrical nanowires constitutes a promising and simple route towards control of magnetic carrier information in future three-dimensional magnetic memories and shift registers.

EXPERIMENTAL DETAILS. Multisegmented cylindrical magnetic nanowires were grown by electrodeposition inside the pores of anodic alumina templates previously synthesized by hard anodization32,33,51. 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 increases 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 a 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 a 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


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nanowires with different total length. A number of nanowires were taken for the different experimental measurements, and although specific peculiarities were occasionally found, the selected results presented along this manuscript are representative for all 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 of 658 nm and power 7.5mW) under a maximum applied field of ±500 Oe. A Variable Field Magnetic Force Microscope (VF-MFM) from Nanotec Electronica S.L.52 has been also used to image the magnetic state of the surface, working with amplitude modulation and the two passes method. The phase-locked loop (PLL) was enabled to track the resonance frequency of the oscillating cantilever and hence, the frequency shift signal was recorded to obtain the MFM images. All the measurements were performed with microchips Multi75M-G, from Budget Sensors (nominal values, resonance frequency 75 kHz and force constant 3 N/m). The oscillation amplitude of the cantilever was set in 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 energy analyzer. The samples are illuminated with circularly polarized X-rays at a grazing angle of 16o with respect to the surface, at the resonant Co L3 absorption edge (778 eV). The emitted photoelectrons (low energy secondary electron with ca. 1 eV 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 pixel-wise asymmetry of two PEEM images sequentially recorded with left-and right-handed circular polarization (for more details see Supplementary Information).29


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Micromagnetic simulations were performed in an individual FeCo/Cu multilayer nanowire using mumax3 code.45 The nanowire, roughly 3 µm in length and 80 nm in diameter, is formed by 8 single crystal bcc FeCo segments with increasing length from 200 nm 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 in the same range. The material parameters used in calculations are typical values in 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 Supplementary information.

ASSOCIATED CONTENT Supplementary Information. The Supporting Information is available free of charge on the ACS Publications website http://pubs.acs.org. Additional information about the samples preparation, MFM and XMCD-PEEM experiments and micromagnetic simulations. AUTHOR INFORMATION Corresponding Authors: *Email: [email protected] [email protected] Present Addresses † Division of Permanent Magnets and Applications, IMDEA Nanoscience, 28049 Madrid, Spain


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Author Contributions M.V., C.B., A.A., R.P. and O.C.-F. conceived the idea of the manuscript. C.B. synthesized and structurally characterized the nanowires. J.M. and E.P. conducted MOKE measurements. E.B conducted MFM measurements under supervision of A.A. and M.J.. C.B., R.P., A.FR., M.F., L.A., E.P., E.B, E.C., A.A. and M.V. conducted the XMCD-PEEM measurements. J.A.F.-R. conducted the micromagnetic simulations under 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. with significant contributions by L.A., M.F., A.FR., A.A., J.A.F.R., M.J. and E.B. All authors reviewed and approved the final version of the manuscript.

ACKNOWLEDGEMENTS The work has been supported by the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) under the grants MAT2016-76824-C3-1-R and FIS2016-78591-C33-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. Meier acknowledges the DAAD support during her stay at ICMM/CSIC. M. Jaafar acknowledges funding grant from Spanish MINECO under MAT2015-73775-JIN research project. J.A. Fernandez-Roldan acknowledges support from Spanish MINECO and FSE though the fellowship BES-2014-068789.


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