Cylindrical Magnetic Nanowires Research Articles

Dynamics of chiral domain walls in bent cylindrical magnetic nanowires

Cylindrical magnetic nanowires (NWs) constitute a viable component of 3D nanoscale magnetic devices and engineering their response to external stimuli is necessary for their future functionalization. Here, by means of micromagnetic simulations, we study the dynamical response of vortex–antivortex and Bloch point domain walls under the action of an applied magnetic field in curved arc-shaped NWs varying the saturation magnetization value. Our results provide evidence that, in the range considered in this work, the curvature has no influence on the critical diameters, delimiting different domain wall types. However, it has a relevant effect on the domain wall dynamics. Specifically, the vortex–antivortex domain wall oscillates back and forth while rotating around the nanowire, and the frequency and amplitude can be tuned by curvature and applied field. On the contrary, Bloch point domain wall dynamics does not show any oscillatory behavior, and the domain wall is rapidly expelled from the nanowire with velocities similar to that of the straight cylindrical nanowires. These results allow engineering magnetic response of cylindrical nanowires with curvature.

Giant supermagnonic Bloch point velocities in cylindrical ferromagnetic nanowires.

Achieving high velocities of magnetic domain walls is a crucial factor for their use as information carriers in modern nanoelectronic applications. In nanomagnetism and spintronics, these velocities are often limited either by internal domain wall instabilities, known as the Walker breakdown phenomenon, or by spin wave emission, known as the magnonic regime. In the rigid domain wall model, the maximum magnon velocity acts as an effective "speed of light", providing a relativistic analogy for the domain wall speed limitation. Cylindrical magnetic nanowires are an example of systems without the Walker breakdown phenomenon. Here we demonstrate that the magnonic limit could be outstandingly surpassed in cylindrical nanowires with high magnetization, such as iron. Our numerical modeling shows the Bloch point domain wall velocities as high as 14 km s-1, well above the magnonic limit estimated in the interval 1.7-2.0 km s-1. The key ingredient is the three-dimensional conical shape of the domain wall, which elongates and breaks during the dynamics, expelling backwards pairs of Bloch points. This leads to domain wall acceleration, the effect, which resembles the "jet propulsion". This effect will be very important for three-dimensional networks based on cylindrical magnetic nanowires.

Probing geometry-induced magnetic defects in cylindrical modulated nanowires with optically detected spin resonance in nitrogen-vacancy center in diamond.

Cylindrical magnetic nanowires (NWs) have gained significant interest as building-blocks of spintronics devices and magnetic sensors thanks to their geometry-tunable magnetic properties and anisotropy. While the synthesis and compositional control of NWs have seen major improvements in recent years, considerable challenges remain for the characterization of local magnetic features at the nanoscale. Here, we demonstrate non-perturbative field distribution mapping and minimally invasive magnetic imaging with scanning nitrogen-vacancy magnetometry. This enables a sensitivity down to 3 μT Hz-1/2 used to localize ultra-scaled magnetic defects with lateral dimensions below 50 nm. The imaging reveals the presence of magnetic inhomogeneities in correspondence of periodical geometrical modulations/anti-notches in axial magnetized nanowires that are largely undetectable with standard metrology. The features induce local fluctuations of the NWs' magnetization orientation that are sensed by SNVM and compared with magnetic force microscopy. Finally, the strong magnetic field confinement in the nanowires is leveraged to study the interaction between the stray magnetic field and the fluorescence generated by two nitrogen-vacancies contained in the probe sensor, thus clarifying the contrast formation mechanisms.

A Material View on Extrinsic Magnetic Domain Wall Pinning in Cylindrical CoNi Nanowires

Speed and reliability of magnetic domain wall (DW) motion are key parameters that must be controlled to realize the full potential of DW-based magnetic devices for logic and memory applications. A major hindrance to this is extrinsic DW pinning at specific sites related to shape and material defects, which may be present even if the sample synthesis is well controlled. Understanding the origin of DW pinning and reducing it are especially desirable in electrochemically deposited cylindrical magnetic nanowires (NWs), for which measurements of the fascinating physics predicted by theoretical computation have been inhibited by significant pinning. We experimentally investigate DW pinning in CoxNi100–x NWs by applying quasi-static magnetic fields. Wire compositions were varied with x = 20, 30, and 40, while the microstructure was changed by annealing or by varying the pH of the electrolyte for deposition. We conclude that pinning due to grain boundaries is the dominant mechanism, decreasing inversely with both the spontaneous magnetization and grain size. Second-order effects include inhomogeneities in lattice strain and the residual magnetocrystalline anisotropy. Surface roughness, dislocations, and impurities are not expected to play a significant role in DW pinning in these wire samples.

Controlled evolution of three-dimensional magnetic states in strongly coupled cylindrical nanowire pairs

Cylindrical magnetic nanowires are promising systems for the development of three-dimensional spintronic devices. Here, we simulate the evolution of magnetic states during fabrication of strongly-coupled cylindrical nanowires with varying degrees of overlap. By varying the separation between wires, the relative strength of exchange and magnetostatic coupling can be tuned. Hence, we observe the formation of six fundamental states as a function of both inter-wire separation and wire height. In particular, two complex three-dimensional magnetic states, a 3D Landau Pattern and a Helical domain wall, are observed to emerge for intermediate overlap. These two emergent states show complex spin configurations, including a modulated domain wall with both Néel and Bloch character. The competition of magnetic interactions and the parallel growth scheme we follow (growing both wires at the same time) favours the formation of these anti-parallel metastable states. This works shows how the engineering of strongly coupled 3D nanostructures with competing interactions can be used to create complex spin textures.

Dynamics of chiral domain walls under applied current in cylindrical magnetic nanowires

The dynamics of two types of chiral magnetic domain walls in magnetic cylindrical nanowires under spin-polarized current are investigated by means of micromagnetic simulations. We show that Bloch point domain walls with chirality identical to that of the Oersted field can propagate without dynamical instabilities with velocities ∼300 m/s. The domain wall width is shown to widen at larger current densities limiting the velocity increase. For domain walls with opposite chirality, we observed a new pinning mechanism created by the action of the Oersted field, limiting their propagation distance even after chirality switching. Vortex–antivortex domain walls transform into Bloch point domain walls, and after that they can unexpectedly propagate either along or against the direction of the current. Our findings demonstrate that domain wall dynamics under current in cylindrical magnetic nanowires can result in a plethora of different behaviors that will have important implications for future 3D spintronic devices.

Area law for magnetic domain walls in bent cylindrical nanowires

The dynamics of several systems in nature occurs under some constraints and symmetries that ensure the appearance of constants of motion. In this work, we discuss the dynamics of the magnetic domain wall (DW) under the Walker regime (i.e., when its position oscillates as a function of time) in bent cylindrical magnetic nanowires (NWs) with constant curvatures. It is shown that the DW position sweeps, in relation to the curvature center, the same area for different NW curvatures. This phenomenon appears due to an exchange-driven curvature-induced interaction. The translational DW motion is accompanied by its rotation around the NW axis, leading to a periodic curvature-independent angular momentum, from which one obtains an area's law for the DW motion.

Electrodeposited Magnetic Nanowires with Radial Modulation of Composition.

In the last few years, magnetic nanowires have gained attention due to their potential implementation as building blocks in spintronics applications and, in particular, in domain-wall- based devices. In these devices, the control of the magnetic properties is a must. Cylindrical magnetic nanowires can be synthesized rather easily by electrodeposition and the control of their magnetic properties can be achieved by modulating the composition of the nanowire along the axial direction. In this work, we report the possibility of introducing changes in the composition along the radial direction, increasing the degrees of freedom to harness the magnetization. In particular, we report the synthesis, using template-assisted deposition, of FeNi (or Co) magnetic nanowires, coated with a Au/Co (Au/FeNi) bilayer. The diameter of the nanowire as well as the thickness of both layers can be tuned at will. In addition to a detailed structural characterization, we report a preliminary study on the magnetic properties, establishing the role of each layer in the global collective behavior of the system.

Asymmetric dual Bloch point domain walls in cylindrical magnetic nanowires

Cylindrical magnetic nanowires have been studied extensively over the past ten years due to the presence of domain walls with novel topology and outstanding dynamic properties. In soft magnetic systems, where shape anisotropy forces the magnetization along the wire axis, and for radii above 50 nm, two topologically distinct walls have been previously identified. The Bloch point wall (BPW) has a circulating magnetization texture around the circumference and contains a single Bloch point within the center of the wire cross section. In contrast, asymmetric transverse walls (ATWs) have a circulating magnetization structure on the surface and contain two topological defects, a vortex and an anti-vortex on opposing sides. These surface defects are connected via a vortex tube that penetrates the volume. In this study, we have numerically investigated the domain wall magnetization textures for nickel nanowires of radii 50–120 nm. Beyond reproducing the known BPW and ATW topology, we discover a new domain wall type that contains aspects of both. This new domain wall type, which we call asymmetric dual Bloch point wall (ADBPW), has surface vortices similar to an ATW and two Bloch-point textures adjacent to the internal vortex tube. Time-resolved simulations investigating the stability of ADBPW show its field-driven transformation into a BPW via the ejection of a single Bloch point at the surface and subsequent annihilation of surface vortices.

Distinguishing Local Demagnetization Contribution to the Magnetization Process in Multisegmented Nanowires.

Cylindrical magnetic nanowires are promising materials that have the potential to be used in a wide range of applications. The versatility of these nanostructures is based on the tunability of their magnetic properties, which is achieved by appropriately selecting their composition and morphology. In addition, stochastic behavior has attracted attention in the development of neuromorphic devices relying on probabilistic magnetization switching. Here, we present a study of the magnetization reversal process in multisegmented CoNi/Cu nanowires. Nonstandard 2D magnetic maps, recorded under an in-plane magnetic field, produce datasets that correlate with magnetoresistance measurements and micromagnetic simulations. From this process, the contribution of the individual segments to the demagnetization process can be distinguished. The results show that the magnetization reversal in these nanowires does not occur through a single Barkhausen jump, but rather by multistep switching, as individual CoNi segments in the NW undergo a magnetization reversal. The existence of vortex states is confirmed by their footprint in the magnetoresistance and 2D MFM maps. In addition, the stochasticity of the magnetization reversal is analysed. On the one hand, we observe different switching fields among the segments due to a slight variation in geometrical parameters or magnetic anisotropy. On the other hand, the stochasticity is observed in a series of repetitions of the magnetization reversal processes for the same NW under the same conditions.

Cylindrical nanowire arrays: From advanced fabrication to static and microwave magnetic properties

This article reviews information on arrays of cylindrical magnetic nanowires grown electrochemically inside ordered nanoporous alumina membranes, summarizing some of the most relevant aspects about the advanced synthesis of nanowires and their overall static and microwave magnetic properties. The electrochemical details for the preparation of templates with well-defined hexagonal ordering are addressed focusing on the different anodization methods including mild, hard, pulsed or anodization of aluminum rods. A section follows on the growth of metallic nanowires, either as single element (i.e., Fe, Ni, Co), alloys (i.e., CoFe, CoNi, FeNi) or modulated in composition (i.e., Ferromagnetic/Metal). The static magnetic properties as anisotropy, coercivity and magnetostatic interactions are overviewed with particular emphasis on the influence of magnetocrystalline anisotropy and on the temperature dependence of those properties. The last section is devoted to the ferromagnetic resonance of nanowire arrays of different composition. The dependence of microwave absorption on the orientation of applied field is also a useful tool to determine the magnetic anisotropy parameters and the significance of magnetostatic interactions. The magnetization easy axis can be switched from the nanowire direction to the plane of the membrane as the nanowire filling factor increases or at a compensation temperature, in both cases it derives from the energetic balance of anisotropy and magnetostatic interactions among nanowires. A final subsection is devoted to the spin waves resonance spectra, and the perspectives of applications making use of microwave properties. We expect the article can be useful for researchers interested on the topic and provide a wide overview on the matter to those starting a new research line.

The microstructure and magnetic behaviors of Pr Fe B/Fe7Co3 dual phase nanowires: As a perpendicular magnetic recording candidate

Cylindrical magnetic nanowires, possessing high aspect ratio and outstanding perpendicular shape anisotropy, have been an excellent candidate for next generation perpendicular magnetic recording. Due to the scarcity of heavy rare earth elements (HREs), it is of great scientific significance and potential application value to add trace HREs to improve the magnetism of nanowires and prepare new magnetic materials with special applications. Here, under the condition of saving the use of rare earth resources, a novel type of hard/soft PrFeB/Fe7Co3 magnetic nanowires was fabricated via electrodeposition method, which is not inferior to that of large amount of HREs doping. Present study reports the phase composition, phase transformation, microstructure and magnetic properties in highly-ordered arrays of PrFeB/Fe7Co3 nanowires embedded in anodic aluminum oxide (AAO) template. The results show that after 903 K-annealing process, Pr2Co17 and FeB phases preferentially nucleate, followed by Fe7Co3 + Pr2Fe14B + Pr2Co14B + PrB4 phases, and finally Pr2Fe17 phase nucleate. It is found that the annealed PrFeB/Fe7Co3 nanowires have excellent magnetic properties, and the improvement of its magnetic properties is attributed to location pinning, special coherent interface and exchange coupling.

Detailed examination of domain wall types, their widths and critical diameters in cylindrical magnetic nanowires

Understanding and mastering the magnetic properties of domain walls in cylindrical nanowires is a fundamental pillar for developing novel 3D information technologies. For this purpose, a good comprehension of domain wall (DW) dynamical properties, which are strongly dependent on its type and size, is needed. In this work, by means of micromagnetic simulations, we focus on the accurate determination of DW types (transverse, vortex-antivortex, and the Bloch point) and DW widths and present a state diagram as a function of the diameter and the nanowire material. Using different initial states and trajectories we find a large region of metastability where the Bloch-point domain wall co-exists with either transverse or a more energetical vortex-antivortex one. We determine the domain wall width and its dependence on the nanowire diameter either for the transverse or the vortex-antivortex domain wall showing that it is always larger than the nanowire diameter. We also find simple expressions for the DW widths and the critical diameter for the transition between different DW types which agree well with direct simulations. Our results are useful for the experimental design of cylindrical nanowires for multiple applications.

Field tunable three-dimensional magnetic nanotextures in cobalt-nickel nanowires

Cylindrical magnetic nanowires with large transversal magnetocrystalline anisotropy have been shown to sustain non-trivial magnetic configurations resulting from the interplay of spatial confinement, exchange, and anisotropies. Exploiting these peculiar 3D spin configurations and their solitonic inhomogeneities are prospected to improve magnetization switching in future spintronics, such as power-saving magnetic memory and logic applications. Here we employ holographic vector field electron tomography to reconstruct the remanent magnetic states in CoNi nanowires with 10 nm resolution in 3D, with a particular focus on domain walls between remanent states and ubiquitous real-structure effects stemming from irregular morphology and anisotropy variations. By tuning the applied magnetic field direction, both longitudinal and transverse multi-vortex states of different chiralities and peculiar 3D features such as shifted vortex cores are stabilized. The chiral domain wall between the longitudinal vortices of opposite chiralities exhibits a complex 3D shape characterized by a push out of the central vortex line and a gain in exchange and anisotropy energy. A similar complex 3D texture, including bent vortex lines, forms at the domain boundary between transverse-vortex states and longitudinal configurations. Micromagnetic simulations allow an understanding of the origin of the observed complex magnetic states.

Magnetic reversal modes in cylindrical nanostructures: from disks to wires

Cylindrical magnetic nanowires are key elements of fast-recording and high-density 3D-storage devices. The accurate tuning of the magnetization processes at the nanoscale is crucial for the development of future nano-devices. Here, we analyzed the magnetization of Ni nanostructures with 15–100 nm in diameter and 12–230 nm in length and compared our results with experimental data for periodic arrays. Our modelling led to a phase diagram of the reversal modes where the presence of a critical diameter (d ≈ 30 nm) triggered the type of domain wall (DW) formed (transverse or vortex); while a critical length (L ≈ 100 nm) determined the number of DWs nucleated. Moreover, vortex-DWs originated from 3D skyrmion tubes, reported as one of the best configurations for storage devices. By increasing the diameter and aspect-ratio of nanowires with L > 100 nm, three reversal modes were observed: simultaneous propagation of two vortex-DWs; propagation of one vortex-DW; or spiral rotation of both DWs through “corkscrew” mechanism. Only for very low aspect-ratios (nanodisks), no skyrmion tubes were observed and reversal occurred by spiral rotation of one vortex-DW. The broad range of nanostructures studied allowed the creation of a complete phase diagram, highly important for future choice of nanoscaled dimensions in the development of novel nano-devices.

Time-resolved imaging of Œrsted field induced magnetization dynamics in cylindrical magnetic nanowires

Recent studies in three-dimensional spintronics propose that the Œrsted field plays a significant role in cylindrical nanowires. However, there is no direct report on its impact on magnetic textures. Here, we use time-resolved scanning transmission x-ray microscopy to image the dynamic response of magnetization in cylindrical Co30Ni70 nanowires subjected to nanosecond Œrsted field pulses. We observe the tilting of longitudinally magnetized domains toward the azimuthal Œrsted field direction and create a robust model to reproduce the differential magnetic contrasts and extract the angle of tilt. Furthermore, we report the compression and expansion, or breathing, of a Bloch-point domain wall that occurs when weak pulses with opposite signs are applied. We expect that this work lays the foundation for and provides an incentive to further studying complex and fascinating magnetization dynamics in nanowires, especially the predicted ultra-fast domain wall motion and associated spin wave emissions.

Cylindrical Magnetic Nanowires Applications

Cylindrical magnetic nanowires (NWs) feature unique properties, which make them attractive for fundamental research and novel applications. These 1-D structures introduce a pronounced shape anisotropy that together with material selection can strongly affect the magnetic properties and can be tuned by incorporating segments of different materials or diameters along the length. They attract a large interest in the scientific community, ranging from physicists to material scientists to bioengineers. Consequently, these NWs are developed for and used in very diverse applications in medicine, biology, data and energy storage, catalysis, or microwave electronics, among others. In this review, we investigate the most active emerging applications of cylindrical NWs grown in alumina templates by electrochemical deposition. This method has several key features, including low cost and a high level of control over the design. A fundamental property that distinguishes those applications is the operating frequency, which we chose to apply as an underlying structure for this review. With this, we attempt to provide a wide and organized view of applications based on cylindrical magnetic NWs with a focus on tailored physical and chemical properties.

Magnetic Configurations in Modulated Cylindrical Nanowires.

Cylindrical magnetic nanowires show great potential for 3D applications such as magnetic recording, shift registers, and logic gates, as well as in sensing architectures or biomedicine. Their cylindrical geometry leads to interesting properties of the local domain structure, leading to multifunctional responses to magnetic fields and electric currents, mechanical stresses, or thermal gradients. This review article is summarizing the work carried out in our group on the fabrication and magnetic characterization of cylindrical magnetic nanowires with modulated geometry and anisotropy. The nanowires are prepared by electrochemical methods allowing the fabrication of magnetic nanowires with precise control over geometry, morphology, and composition. Different routes to control the magnetization configuration and its dynamics through the geometry and magnetocrystalline anisotropy are presented. The diameter modulations change the typical single domain state present in cubic nanowires, providing the possibility to confine or pin circular domains or domain walls in each segment. The control and stabilization of domains and domain walls in cylindrical wires have been achieved in multisegmented structures by alternating magnetic segments of different magnetic properties (producing alternative anisotropy) or with non-magnetic layers. The results point out the relevance of the geometry and magnetocrystalline anisotropy to promote the occurrence of stable magnetochiral structures and provide further information for the design of cylindrical nanowires for multiple applications.

Unveiling the Origin of Multidomain Structures in Compositionally Modulated Cylindrical Magnetic Nanowires.

CoNi/Ni multisegmented cylindrical nanowires were synthesized via an electrochemical route. The wires are 140 nm in diameter, with 1000 nm long Ni segments and CoNi segments between 600 and 1400 nm in length. The magnetic configuration was imaged by XMCD-PEEM in the demagnetized state and at remanence after magnetizing axially and perpendicularly. Ni segments, with cubic crystal symmetry, show an axial magnetic configuration with a small curling component at the surface. In turn, CoNi segments, with hexagonal crystal symmetry and a strong magnetocrystalline anisotropy perpendicular to the nanowires, show a single vortex state in the shorter segments and multivortex or multitransverse magnetic configurations in medium and long segments, respectively. A detailed study by micromagnetic simulations reveals that the magnetic configuration is determined mainly by the coupling between soft Ni and harder CoNi segments. For short CoNi segments, Ni segments are magnetostatically coupled and the chirality of the single vortex formed in CoNi remains the same as that of the curling in neighboring Ni segments. For longer CoNi segments, the remanent state is either the multivortex or multitransverse state depending on whether the previously applied field was parallel or perpendicular to the magnetocrystalline axis. The results point out the relevance of the cylindrical geometry to promote the occurrence of complex magneto-chiral effects and provide key information for the design of cylindrical magnetic nanowires for multiple applications.

Electric current and field control of vortex structures in cylindrical magnetic nanowires

Magnetization dynamics in a cylindrical Permalloy nanowire under simultaneously applied electric current and field is investigated by means of micromagnetic simulations. The reversal process starts with the creation of open vortex structures with different rotation senses at the nanowire ends. Our results conclude that the current alone enlarges or reduces the size of these vortex structures according to the rotational sense of the associated Oersted field. Large current intensity creates a vortex structure which covers the whole nanowire surface. At the same time the magnetization in the nanowire core remains the same, i.e., no complete magnetization reversal is possible in the absence of external field. The simultaneous action of the current and field allows for the complete control of the vortex structures in terms of setting the polarity and vorticity. The state diagram for the minimum field and current required for the vorticity and axial magnetization switching is presented. This control is essential for future information technologies based on three-dimensional vertical structures, and the presented state diagram will become very useful for future experiments on current-induced domain wall dynamics in cylindrical magnetic nanowires.