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The mechanical properties of metallic nanowires and nanotubes were investigated using atomistic molecular dynamics simulations on Ni polycrystalline structures, similar to those experimentally obtained by Atomic Layer Deposition. We studied the response of nanostructures under uniaxial deformations with different thickness, geometry, and crystalline degree. Plastic deformation is due to stacking fault and coherent twin boundary formation, and to grain boundary activity. Different fracture processes are obtained from these systems, being the thin nanotubes failing thanks to a mix of brittle failure by grain boundary decohesion and ductile fracture due to significantly more twins than with a thicker nanotube and nanowire during the ductile fracture. The stress-strain curves, atomic displacements, and defects formation were analyzed, finding that nanotubes with a fraction of the volumetric mass have practically the same Young modulus and ultimate tensile stress, while fracture strain is slightly larger for nanowire. From all these studied cases, it is remarkable the result where ultra-thin nanotubes can withstand a 21% of tensile stress-strain with a similar yield strength than nanowires, but with a volumetric mass reduction of 60%, offering a lightweight alternative to design mechanical nanodevices with minimal loss of mechanical performance.

Magnetic nanorings have been widely studied due to their potential applications in spintronic and magnonic devices. In this work, by means of analytical calculations and micromagnetic simulations we have analyzed the magnetic energy of nanorings with variable anisotropy along their radius. Four magnetic states, including two new magnetic configurations, here called meron and knot-like states, are considered, looking to the relative lower energy states as a function of anisotropy. Phase diagrams with this states are presented., 6 pages, 4 figures

Oxygen absorption and the thermal stability of Al147 nanoparticles were studied by means of classical molecular dynamics simulations and Monte Carlo methods. The results suggest that for the studied sizes, oxygen incorporation yields an Al2O3 nanoparticle with a Janus-like morphology, contrary to the expected core–shell nanostructure observed in simulations and experiments of nanometer-size nanoparticles. A simulated annealing, introduced to support this assumption, shows that the Janus-like morphology has a lower energy than that of Al@Al2O3 with a core@shell conformation. Also, the thermal behavior of a Janus-like Al/Al2O3 nanoparticle as a function of oxygen concentration was investigated. It is observed that the partial oxidation reduces the nanoparticle melting temperature because the number of pure aluminum atoms is reduced. In fact, the melting point can be as low as 400 K for an Al147O30 nanoparticle. The melting process leads to a solid alumina region that coexists with liquid-like aluminum nanoparticles. The oxide never adopts a protective shell covering configuration of the aluminum nanoparticle.

Mechanical properties of nanomaterials, such as nanowires and nanotubes, are an important feature for the design of novel electromechanical nano-architectures. Since grain boundary structures and surface modifications can be used as a route to modify nanostructured materials, it is of interest to understand how they affect material strength and plasticity. We report large-scale atomistic simulations to determine the mechanical response of nickel nanowires and nanotubes subject to uniaxial compression. Our results suggest that the incorporation of nanocrystalline structure allows completely flexible deformation, in sharp contrast with single crystals. While crystalline structures at high compression are dominated by dislocation pinning and the multiplication of highly localized shear regions, in nanocrystalline systems the dislocation distribution is significantly more homogeneous. Therefore, for large compressions (large strains) coiling instead of bulging is the dominant deformation mode. Additionally, it is observed that nanotubes with only 70% of the nanowire mass but of the same diameter, exhibit similar mechanical behavior up to 0.3 strain. Our results are useful for the design of new flexible and light-weight metamaterials, when highly deformable struts are required.

BACKGROUNDCOVID-19 has spread rapidly, and there are still many characteristics of this new disease to be unveiled. We propose a simple method to calculate a “figure of merit” FC to provide an early characterization of the disease status in country C.METHODSWe use mathematical tools to adjust a Gaussian function to the daily increase of infected patients. Maximum value and full width half maximum of the Gaussian are characteristics of the development of the development of the pandemic in each country. These parameters are supplemented by the testing volume and the mortality rate to produce just one characterizing parameter: FC. In addition, the stability of the Gaussian fits was calculated within an entire week towards the end of the study period. Seventeen different countries were fully considered, while others are considered when discussing particular properties. Data employed is publically available.FINDINGSFitted Gaussian functions render effective information about the development of COVID-19. The number of critical days vary between 11 (South Korea) and 52 (Mexico). FC varies between 1 (Australia) and 899 (Mexico). The epidemic appears stabilized in some countries and unstable in others. Some large countries are experiencing fast development of the propagation of the disease with high FC. A correlation between low (high) values of the mortality rate (and to some extent FC) and the presence (absence) of BCG vaccination is exposed.INTERPRETATIONThe adjustment of a Gaussian to daily data of COVID-19 in each country reveals the different propagation dynamics, properly characterized by the parameters proposed here. Testing plays a clear role to control the spread of the disease. Mortality rate spans more than one order of magnitude and is somewhat related to permanent massive BCG vaccination. The figure of merit, FC, introduced here spans more than 2 orders of magnitude which makes it a useful indicator to quickly find out the status of the pandemics in each territory. Geography plays a role: low population density and isolated countries can be efficient in controlling the spread of the disease.HIGHLIGHTSAn easy-to-evaluate parameter is defined to rapidly establish the status of the evolution of COVID-19 in any given territory. 17 countries in 4 continents are chosen to apply it and compare different evolutions.The parameter or figure of merit (FC) is dynamic: it combines testing, mortality rate, and characteristics of the Gaussian function that fits the new daily contagions.FC spans more than two orders of magnitude which makes it a very sensitive indicator to promptly detect second waves or local outbreaks.It is found that countries with massive BCG vaccination present low COVID-19 mortality rates and low values for FC.

Magnonic excitations in the two-dimensional (2D) van der Waals (vdW) ferromagnet chromium triiodide (${\mathrm{CrI}}_{3}$) are studied. We find that bulk magnons exhibit a nontrivial topological band structure without the need for Dzyaloshinskii-Moriya interaction. This is shown in vdW heterostructures, consisting of single-layer ${\mathrm{CrI}}_{3}$ on different 2D materials such as ${\mathrm{MoTe}}_{2}$, ${\mathrm{HfS}}_{2}$, and ${\mathrm{WSe}}_{2}$. We find numerically that the proposed substrates substantially modify the out-of-plane magnetic anisotropy on each sublattice of the ${\mathrm{CrI}}_{3}$ subsystem. The induced staggered anisotropy, combined with a proper band inversion, leads to the opening of a topological gap of the magnon spectrum. Since the gap is opened nonsymmetrically at the ${\mathbf{K}}^{+}$ and ${\mathbf{K}}^{\ensuremath{-}}$ points of the Brillouin zone, an imbalance in the magnon population between these two valleys can be created under a driving force. This phenomenon has a close analogy to the so-called valley Hall effect and is thus termed the magnon valley Hall effect. In linear response to a temperature gradient, we quantify this effect by the evaluation of the temperature dependence of the magnon thermal Hall effect. These findings open a different avenue by adding the valley degrees of freedom besides the spin in the study of magnons.

A detailed analytic and numerical analysis of the interaction between two bubble skyrmions has been carried out. Results from micromagnetic calculations show a strong dependence of the parameters of the skyrmion magnetic profile as a function of the magnetostatic interaction. The magnetic core and edge-width sizes of the skyrmion increase or decrease depending on the relative position between the skyrmions and the uniaxial perpendicular anisotropy. In particular, when a magnetic disk is over another, there is a transition from a Bloch-like skyrmion configuration to a N\'{e}el-like skyrmion configuration as the distance between the disks decreases. This transition is due to the magnetostatic interaction between them. Therefore, it is possible to stabilize a bubble skyrmion with a N\'{e}el configuration without the Dzyaloshinskii-Moriya interaction. Thus, these results can be used for the parameters control of the skyrmions in magnetic spintronic devices that need to use these configurations.

Arrays of magnetic nanowires in porous alumina can be used to store information inscribed on the system by orienting the magnetization of selected wires pointing in a desired direction, so symbols can be read as ferromagnetic sectors. However, this information is subject to aging and the stored information could be gradually lost. We investigate here two mechanisms proposed to improve the prevalence of the stored information: opposite ferromagnetic band at the center of the symbol and bi-segmented nanowires acting as two layers of nanowires storing the same information. Both mechanisms prove to increase resistance to the action of external magnetic fields for the case of Ni wires in a geometry compatible with actually grown nanowires. Advantages and disadvantages of these mechanisms are discussed.

Bimetallic nanoparticles (NPs) can be tailored by varying the concentration of their constituent elements, resulting in novel structures and/or configurations, leading to interesting electronic, mechanical, and chemical properties. In this paper, by means of molecular dynamics calculations, we study the morphology of bimetallic FeCu NPs as a function of the Cu concentration. Our results evidence a core-shell (CS) structure for low Cu concentrations and a Janus (JN)-like morphology for high Cu content. Structural and energy characterizations were performed to determine the atomic-scale behavior of the NPs. Using a continuous model to describe immiscible components, we obtain a stability transition curve between CS and JN-like structures for several NP sizes and concentrations. Results from both methods are compared with experimental data obtained for NPs with low and high Cu content, evidencing a good agreement among the three approaches. Fil: Rojas Nunez, Javier. Centro para el desarrollo de Nanociencia y Nanotecnología; Chile. Universidad de Santiago de Chile; Chile Fil: Gonzalez Valdes, Rafael Ignacio. Centro para el desarrollo de Nanociencia y Nanotecnología; Chile. Universidad Mayor; Chile Fil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Allende, Sebastian. Universidad de Santiago de Chile; Chile. Centro para el desarrollo de Nanociencia y Nanotecnología; Chile Fil: Sepúlveda, Pamela. Centro para el desarrollo de Nanociencia y Nanotecnología; Chile. Universidad de Santiago de Chile; Chile Fil: Arancibia Miranda, Nicolás. Centro para el desarrollo de Nanociencia y Nanotecnología; Chile. Universidad de Santiago de Chile; Chile Fil: Baltazar, Samuel E.. Universidad de Santiago de Chile; Chile. Centro para el desarrollo de Nanociencia y Nanotecnología; Chile

The magnetic stability of a multilayer system formed by N slabs separated by nonmagnetic spacers has been studied, emphasizing the role of dipolar interactions among magnetic slabs. By means of numerical and analytical calculations, we have identified a regime in which the magnetic slabs behave independently, i.e., a critical thickness of the spacer at which dipolar interactions can be safely disregarded. The methodology used to establish this critical distance can be used for other geometries, as it is a useful tool to identify regimes of non-interacting magnetic slabs.

The interplay between curvature and magnetization is a cornerstone in magnetism. Thus, in this work, we address the role of curvature on the propagation of magnetic skyrmions along a curved race track. Using a micromagnetic approach and simulations, we observed the creation of a curvature-induced force (CIF) that acts on the skyrmion while traveling along the track. We also show that curvature can stop the skyrmion if the current density is below a critical value. Above this limit, the skyrmion goes through the defect, a direct consequence of the CIF. Our results allow a better definition of the electric current needed for moving skyrmions along curved tracks.

A detailed analytical and numerical analysis of the skyrmion core size dependence as a function of the uniaxial perpendicular anisotropy and radius in magnetic nanodots has been carried out. Results from micromagnetic calculations show a non-monotonic behavior between the skyrmion core size and the uniaxial perpendicular anisotropy. The increment of the radius reduces the skyrmion core size at constant uniaxial perpendicular anisotropy. Thus, these results can be used for the control of the core sizes in magnetic artificial skyrmion crystals or spintronic devices that need to use a skyrmion configuration at room temperature.

Magnetic nanotubes have been widely studied because they are promising candidates to be part in devices based on spintronic and magnonic technologies. However, the experimental techniques used to prepare these elements could not guarantee to have perfect nanostructures. Therefore, some geometric imperfections can appear. In this direction, the bent of a nanotube could play an essential role in the magnetic properties of a device. In this work, we analyze the influence of curvature on the magnetic properties of a bent nanotube, a topic scarcely studied, and that can have a strong impact on applications., 17 pages

Imogolite is a fascinating inorganic nanotube that is found in nature or synthesized in a laboratory. The synthesis process is carried out in liquid media, and leads to the formation of almost monodisperse diameter nanotubes. Here we investigate, employing classical molecular dynamics simulations, the interaction of water and imogolite for nanotubes of several radii. We established that water penetrates the pores of N = 9 and larger nanotubes, and adopts a coaxial arrangement in it. Also, while water molecules can diffuse along the center of the nanotube, the molecules next to the inner imogolite walls have very low mobility. At the outer nanotube wall, an increase of water density is observed, this effect extends up to 1 nm, beyond which water properties are bulk-like. Both phenomena are affected by the imogolite curvature.

It is shown that the interplay between curvature and interfacial Dzyalonshinsky-Moriya interaction (DMI) is a pathway to ultrafast domain wall (DW) dynamics in ferromagnetic nanotubes. In this work, we theoretically study the effect that interfacial DMI has on the average velocity of a vortex DW in thin ferromagnetic nanotubes grown around a core composed of heavy atoms. Our main result shows that by delaying the Walker breakdown instability, the DW average velocity is of the order of 103 m s-1, which is greater than usual values for these systems. The remarkable velocities achieved through this configuration could greatly benefit the development of spintronic devices.

The magnetization dynamics of nano-oscillators may be excited by both magnetic fields and spin-polarized currents. While the dynamics of single oscillators has been well characterized, the synchronization of several ones is not fully understood yet. An analytical and numerical study of the nonlinear dynamics of two magnetostatically coupled spin valves driven by spin-transfer torques is presented under the macrospin approximation. The oscillators interact via magnetostatic fields and exhibit a robust synchronized magnetization motion. We describe the magnetization dynamics of the system using the Landau-Lifshitz-Gilbert-Slonczewski equation. Using a modal decomposition technique, we describe the dynamics, synchronization, and competition of oscillatory modes as a function of the current density, and the geometrical parameters of the setup. Simulations of the Landau-Lifshitz-Gilbert-Slonczewski equation show good agreement with an approximate analytic solution.

Using a simple method we produce polyethylene film with embedded chains of cobalt nanoparticles. The crystalline magnetic nanoparticles were synthesized through a simple chemical reduction method at room temperature using ultrasonic assistance. These particles were incorporated in a polyethylene matrix using a solution blending mechanism under an external magnetic field. The morphology and magnetic properties of Co nanoparticle aggregates were studied experimentally and by means of Monte Carlo simulations, showing a chain-like structures due to the strong dipolar interaction between aggregates. The hysteresis loops reveal typical ferromagnetic behavior at room temperature and magnetic anisotropy associated to a linear ordering of particles into the polymeric matrix. Numerical results confirmed the chain-like character of the aggregates and that an external magnetic field aligns them along its direction. The low-cost of the fabrication process of these polymeric magnetic films give them a strong potential for industrial and technological applications.

A detailed analytical and numerical study of the spin wave modes of the free layer of a nanopillar spin torque nano-oscillator (STNO) has been performed as a function of the magnetostatic interaction between the free and the fixed magnetic layers. Results for higher frequency normal modes show that the magnetostatic interaction does not appreciably affect the spin wave frequencies and the critical current densities, due to more relevance of the exchange interaction in these modes. For lower frequency normal modes we observe a decrease in frequency and in the critical current density for auto-oscillations when the strength of the magnetostatic interaction between the layers increases, an effect that may be appreciable.

International audience; Segmented magnetic nanowires are a promising route for the development of three dimensional data storage techniques. Such devices require a control of the coercive field and the coupling mechanisms between individual magnetic elements. In our study, we investigate electrodeposited nanomagnets within host templates using vibrating sample magnetometry and observe a strong dependence between nanowire length and coercive field (25 nm to 5 µm) and diameter (25 nm to 45 nm). A transition from a magnetization reversal through coherent rotation to domain wall propagation is observed at an aspect ratio of approximately 2. Our results are further reinforced via micromagnetic simulations and angle dependent hysteresis loops. The found behavior is exploited to create nanowires consisting of a fixed and a free segment in a spin-valve like structure. The wires are released from the membrane and electrically contacted, displaying a giant magnetoresistance effect that is attributed to individual switching of the coupled nanomagnets. We develop a simple analytical model to describe the observed switching phenomena and to predict stable and unstable regimes in coupled nanomagnets of certain geometries.

A detailed analytical and numerical study of the spin wave modes of two nanopillar spin torque nano-oscillators coupled by magnetostatic interactions is presented under the macrospin approximation. Results show that the normal modes of the system oscillate with the magnetizations in-phase or anti-phase in both disks. The frequencies and critical current densities necessary to induce auto-oscillations of the spin wave modes of the coupled system depend on the relative position of the nanopillars and the applied magnetic field. If the oscillators are identical, these modes are degenerate at a certain relative position of the nanopillars, while if the oscillators are non-identical, such degeneracy is removed. Then, we can conclude that the magnetostatic coupling between two spin transfer torque nano-oscillators is a powerful mechanism to control the spin wave modes of these systems.

For dissipative dynamical systems described by a system of ordinary differential equations, we address the question of how the probability of chaotic dynamics increases with the dimensionality of the phase space. We find that for a system of d globally coupled ODE’s with quadratic and cubic non-linearities with randomly chosen coefficients and initial conditions, the probability of a trajectory to be chaotic increases universally from ~10−5 − 10−4 for d = 3 to essentially one for d ~ 50. In the limit of large d, the invariant measure of the dynamical systems exhibits universal scaling that depends on the degree of non-linearity, but not on the choice of coefficients and the largest Lyapunov exponent converges to a universal scaling limit. Using statistical arguments, we provide analytical explanations for the observed scaling, universality and for the probability of chaos.

We study the equilibrium and non-equilibrium physics of two qubits interacting through an ultrastrong coupled qubit-cavity system. By tuning the qubits energy gap while keeping the ultrastrong coupling system to its ground state, we demonstrate a strong two-qubit interaction as well as an enhanced excitation transfer between the two qubits. Our proposal has twofold implications: a means to attain multipurpose parity-protected quantum information tasks in superconducting circuits, and a building block for ultrastrong coupled cavity-enhanced exciton transport in disordered media., Comment: Accepted by Quantum Science and Technology, 19 pages, 10 figures

The magnetization reversal of three-segment cylindrical nanoelements comprising of alternating nanowire and nanotube sections is investigated by means of Monte Carlo simulations. Such nanoelements may feature a three-state behaviour with an intermediate plateau in the hysteresis curve due to a metastable pinning of the domain walls at the wire-tube interfaces. It turns out that vortex as well as transverse domain walls contribute to the magnetization reversal. By varying the geometric parameters, the sequence, or the material of the elements the nucleation location of domain walls, as well as their nucleation field, can be tailored. Especially interesting is the novel possibility to drive domain walls coherently in the same or in opposite directions by changing the geometry of the hybrid nanoelement. This important feature adds additional flexibility to the construction of logical devices based on domain wall movement. Another prominent outcome is that domain walls can be nucleated near the centre of the element and then traverse to the outer tips of the cylindrical structure when the applied field is increased, which also opens the possibility to use these three-segment nanoelements for the field induced delivery of domain walls as substitutes for large nucleation pads., 17 pages (1.5 spacing), 8 Figures, 1 Table

The magnetization reversal mechanisms of small rings have been investigated as a function of the geometry. Stepped and non stepped hysteresis loops were obtained and four different reversal mechanisms were identified. In spite the important information that is possible to get from the hysteresis loops, in some cases it is not possible to identify from them the reversal mechanisms. However, susceptibility curves deliver enough information to determine precisely the reversal mode, making these curves a valuable tool for the study of the magnetization reversal in rings.

The possibility of a three-state nanoelement, composed by a wire and a tube, is investigated by means of Monte Carlo simulations. The desired behavior may be identified by a step or plateau in the hysteresis curve, corresponding to a partial pinning of the domain wall at the interface between wire and tube sections. This step may be augmented in segmented nanoelements with large coercivity difference between the sections. Different possibilities, such as geometry and choice of materials, are explored.

The magnetic vortex structure, that is present in several nanoscopic systems, is stable and can be manipulated through the application of a magnetic field or a spin polarized current. The size and shape of the core are strongly affected by the anisotropy, however its role on the core behavior has not yet been clarified. In the present work we investigate the influence of a perpendicular anisotropy on the annihilation and shape of magnetic vortex cores in permalloy disks. We have used both micromagnetic simulations with the OOMMF code, and an analytical model that assumes that the shape of the core does not change during the hysteresis cycle, known as the rigid core model, to calculate the annihilation fields. In both cases we found that the annihilation fields decrease with increasing perpendicular anisotropy for almost all the structures investigated. The simulations show that for increasing anisotropy or dot thickness, or both, the vortex core profile changes its shape, becoming elongated. For every dot thickness, this change does not depend on the dot radius, but on the relative distance of the core from the center of the dot., 7 pages and 9 figures

Ordered arrays of Co${}_{x}$Ni${}_{1\ensuremath{-}x}$ nanowires ($0lxl1$) were fabricated by a template-assisted method using electrodeposition into anodic aluminum oxide membranes. Tuning of the Co-alloy composition by changing the Ni content enables control of the effective anisotropy axis, which is determined by the balance between the hcp and fcc magnetocrystalline and shape anisotropies. We report on the nanowires' structural and magnetic properties (e.g., hysteresis curves and their parameters as well as first-order reversal curve analysis), paying particular attention to their angular dependence. It is confirmed that the crystal phase of nanowires with length 2.5 $\ensuremath{\mu}$m and diameter 35 nm shifts from hcp to fcc as the Ni content increases. That results in a significant modification of the magnetization process and, accordingly, of the magnetic properties of the array. Analytical calculations of the angular dependence of the coercivity allow us to confirm that the magnetization reversal is mostly ascribed to the propagation of a transverse domain wall. Fitting of the experiment to these calculations indicates the presence of a transverse crystalline anisotropy (ascribed to the hcp phase) in Co wires, while this changes to an axial anisotropy (fcc phase) as the Ni content increases.

We investigate the effect of the dipolar magnetic interaction between magnetic layers mediated by non-magnetic ones in multisegmented nanowires. Multiple symmetric and non-symmetric segments are considered. We observe that due to the dipolar interaction, the magnetization reversal can follow two possible routes, a serial, or a parallel reversal process. We show that by using asymmetrical multi-segmented wires it is possible to stop the magnetization reversal of the system at any particular point.

The possibility of confining magnetic nanoparticles inside multisegmented nanotubes by using strong field gradients is considered by means of Monte Carlo simulations. The problem is reduced to the random walk performed by the nanoparticle on the energy landscape produced by the tube’s magnetic field. The role of tube material, number of segments, and spacer thickness in the amount of time spent by the particle inside the tube is examined, concluding that it is possible to control the encapsulation time by using different architectures.

We have used Monte Carlo simulations to investigate the magnetic properties of asymmetric dots as a function of their geometry. The asymmetry of round dots is produced by cutting off a fraction of the dot and is characterized by an asymmetry parameter $\alpha$. This shape asymmetry has interesting effects on the coercivity ($H_{c}$), remanence ($M_{r}$), and barrier for vortex and C- state formation. The dependences of $H_{c}$ and $M_{r}$ are non monotonic as a function of $\alpha$ with a well defined minima in these parameters. The vortex enters the most asymmetric part and exits through the symmetric portion of the dot. With increasing $\alpha $ the vortex formation starts with a C-state which persists for longer fields and the barrier for vortex exit diminishes with increasing asymmetry, thus providing control over the magnetic chirality. This implies interesting, naively-unexpected, magnetic behavior as a function of geometry and magnetic field., Comment: 6 pages, 6 figures

During the last years great attention has been given to the encapsulation of magnetic nanoparticles. In this work we investigated the stability of small magnetic particles inside magnetic nanotubes. Multisegmented nanotubes were tested in order to optimize the stability of the particle inside the nanotubes. Our results evidenced that multisegmented nanotubes are more efficient to entrap the particles at temperatures up to hundreds of kelvins., 4 pages, 4 figures

The hysteresis curves of multilayer microwires consisting of a soft magnetic nucleus, intermediate non-magnetic layers, and an external hard magnetic layer are investigated. The magnetostatic interaction between magnetic layers is proved to give rise to an antiferromagnetic-like coupling resulting in a magnetostatic bias in the hysteresis curves of the soft nucleus. This magnetostatic biasing effect is investigated in terms of the microwire geometry. The experimental results are interpreted considering an analytical model taking into account the magnetostatic interaction between the magnetic layers., Comment: 6 pages, 7 figures

We report on the experimental and theoretical investigation of the magnetization reversal in magnetic nanotubes that have been synthesized by a combination of glancing angle and atomic layer deposition. Using superconducting quantum interference device magnetometry the angular dependence of the coercive fields is determined and reveals a nonmonotonic behavior. Analytical calculations predict the crossover between two magnetization reversal modes, namely, the movement of different types of domain boundaries (vortex wall and transverse wall). This transition, already known in the geometrical dependences of the magnetization reversal in various nanotubes, is found within one type of tube in the angular dependence and is experimentally confirmed in this work.

Adv. Mater. 2010, 22, 2435–2439 2010 WILEY-VCH Verlag G The size-dependent properties of pseudo-one-dimensional nano-objects have been abundantly documented for single-phase rods or wires. Elongated nanostructures that coaxially combine several phases of distinct physical properties could generate additional effects—spintronic, multiferroic, magnetoplasmonic, to name a few. Such core/shell wires have already been prepared and investigated: they combine two materials of a common class (epitaxial semiconductors to enhance confinement), they utilize a purely structural core as substrate for a functional tube, or they derive from a post-synthetic chemical reaction at a wire surface. To date, however, multiphase one-dimensional nanostructures incorporating several functionalities in a single object are rare. Indeed, a general preparative method is still missing to generate core/shell wires with the following four advantageous characteristics: (i) ability to combine two chemically and physically very different materials; (ii) possibility to introduce an inert layer (insulator, diffusion barrier, spacer); (iii) tunability of each geometric parameter (core radius, shell thickness, and separation between them); (iv) scalability. We propose the combination of atomic layer deposition (ALD) and electrodeposition in an ordered nanoporous template as one such preparative strategy. The template defines the order of the material and the diameter of the wires, ALD is used to conformally deposit one or several shells (including the inert layers), whereas electrodeposition furnishes the core. In this manner, the materials of core and shell can be chosen independently of each other and the thickness of every individual layer is accurately tunable. We demonstrate this method by synthesizing ordered nanostructures embedded in an alumina matrix and consisting of a nickel core and an iron oxide shell separated by a silica spacer layer. This combination of two coaxial nanomagnets is of interest for increasing data storage densities: it could either shield each object and decouple it from its neighbors, or store more than one bit of information per object. However, the occurrence of several distinct switching events in coaxially arranged magnetic phases has not been evidenced experimentally to date (except in larger microwires of diameter >10mm). This situation contrasts with the variety and practical importance of the effects described in multilayered magnetic films (in particular GMR and TMR). We start with porous alumina membranes prepared electrochemically as templates featuring hexagonally arranged pores of diameter (150 15) nm and length 20mm. The subsequent preparative steps are displayed in Figure 1. First, a thin acid-resistant SiO2 layer is coated onto the inner pore walls (Fig. 1a). After dissolution of the aluminum substrate, a long H3PO4 etch is performed to ensure complete dissolution of the barrier layer (Fig. 1b), without risk of pore widening. Then reactive ion etching, RIE (Fig. 1c), removes the exposed SiO2 tips and achieves a clean opening of the pores for the electrodeposition step. In the ALD processes that follow for Fe2O3 and SiO2 (Fig. 1d), near-perfect conformality of the tubes is ensured by long purge times, which prevent any undesired CVD side-reaction. The Ni cores are electrodeposited inside the multilayer nanotube array after definition of a gold electrode on one side of the sample (Fig. 1e). Here, the inner silica tube also serves as electrical insulator. Finally, gold is removed and Fe2O3 is converted to Fe3O4 by dihydrogen (Fig. 1f). [22] The thicknesses of the Fe3O4 shell and the silica spacer can be varied at will: this in turn defines the diameter of the Ni core. The quality and versatility of this preparative strategy are revealed from the microscopic investigations of the samples. Scanning electron microscopy (SEM) images taken in top view (Fig. 2a–d) evidence the hexagonal order of the tubes and the homogeneity of their diameters after various steps of elaboration. A clean opening is obtained after RIE (Fig. 2a) and remains after the ALD steps (Fig. 2b). Thus, electrodeposition results in complete filling of the pores (Fig. 2c,d). The uniform SEM contrast observed from the side (Fig. 2d) proves the homogeneous Ni deposition inside the nanotubes of the whole sample and along their whole length. Isolated tubes and core/shell

The angular dependence of the transverse and vortex modes in a magnetic nanotube is investigated as a function of the tube geometry, by means of analytical calculations and numerical simulations. A critical radius defining the transition between vortex and transverse reversal modes is determined, leading to low or high coercivity modes just by varying the direction of the external field, in a fixed nanotube.

The field driven nucleation and propagation of transverse domain walls in a uniform Ni nanowire is examined by means of Monte Carlo simulations combined with a scaling technique. Simulations show a nonlinear behavior followed by the simultaneous nucleation of several domain walls. Also we study the angular dependence of the propagation of the transverse domain wall with external field. Results are analyzed in terms of the classical relaxation model for interface dynamics.

A detailed numerical analysis of the magnetization reversal processes in multisegmented nanowire arrays was developed. The nanowires have a long aspect ratio and are formed by magnetic and non-magnetic sections alternately arranged in such a way that the array resembles magnetic layers separated by non-magnetic layers. Attention has been focused on the influence of magnetostatic interaction in the magnetic pattern formation of these magnetic nanostructures. Results from a magnetic correlation function among layers show that three different reversal modes can be detected depending on the number and distance between the magnetic segments. As a consequence of the different reversal modes, a non-monotonic behavior of the annihilation field in function of the distance between the layers is evidenced. Thus, these results are important for the production of magnetic devices with multisegmented nanowire arrays.

The occurrence of equilibrium magnetic configurations in ferromagnetic low-symmetry nanoparticles has been investigated. T-shaped nanoparticles, which present strong configurational anisotropy, have been considered. It has been found that such particles exhibit four equilibrium magnetic states, whose stability at room temperature was estimated. The presence of structural imperfections, such as surface roughness, in those systems was also considered, resulting in no significant effects on the magnetic behavior of the particles. The present results are expected to be of relevance to magnetic storage.

The magnetic switching of ferromagnetic nanotubes is investigated as a function of their geometry. Two independent methods are used: Numerical simulations and analytical calculations. It is found that for long tubes the reversal of magnetization is achieved by two mechanism: The propagation of a transverse or a vortex domain wall depending on the internal and external radii of the tube., 4 pages, 4 figures

The effect of dipolar interactions in hexagonal arrays of Ni nanowires has been investigated by means of Monte Carlo simulations combined with a scaling technique, which allows the investigation of the internal structure of the wires. A strong dependence of the coercivity and remanence on the distance between wires has been observed. At intermediate packing densities the coercivity exhibits a maximum, higher than the non-interacting value. This behavior, experimentally observed, has been explained on grounds of the interwire dipolar interactions. Also, different reversal modes of the magnetization have been identified., To appear in Phys. Rev. B

A detailed numerical analysis of the magnetization reversal processes in T-shaped nanoparticles has been carried out. Attention has been focused on the influence of the symmetry of the particle on the formation, propagation, and interaction of internal magnetic structures such as domain walls, vortices, and antivortices. Results show that the lower the degree of symmetry of the particle, the more complex the reversal process is. Thus, symmetry represents an additional ingredient to control the magnetic properties of ferromagnetic nanoparticles.

During the last years, cylindrical structures like dots, wires, and tubes have been intensively investigated. However, stadium-shaped particles have been scarcely investigated because of the complexity associated to the control of its geometry. In this paper, we used a scaled atomistic representation with Monte Carlo simulations to investigate systematically the magnetization reversal process in stadium-shaped magnetic structures as a function of the size of the central volume. Different reversal mechanisms have been identified and its existence has been explained. The reversal process for short, almost circular, stadium structures occurs via the formation of a single vortex, whereas for longer stadium structures the reversal process involves either single vortices or vortex pairs. For the longest structures investigated, the reversal always involves two vortices that generate a central diamond region or a central antivortex, depending on the region of the nucleation of the vortices. Our results provide guidelines for the use of stadium-shaped particles in the control of vortex motion when a magnetic field is applied.

The hysteresis of multilayer nanowires composed by a soft magnetic cylindrical wire, a non-magnetic spacer layer and an external hard magnetic shell is investigated. The external magnetic shell originates a non-homogeneous magnetic field on the inner wire, which is responsible for a displacement and a change of the width of the hysteresis curve of the wire. Moreover, different reversal modes occur at each branch of the hysteresis loop, which can be understood by analyzing the interaction magnetostatic field along the wire. Our results open the possibility of controlling two parameters of the hysteresis loop, the coercivity and the bias, providing an interesting system to be investigated.

The investigation of interactions between magnetic nanotubes is complex and often involves substantial simplifications. In this letter an analytical expression for the magnetostatic interaction, taking into account the geometry of the tubes, has been obtained. This expression allows for the definition of a critical vertical separation for relative magnetization between nanotubes and can be used for tailoring barcode-type nanostructures with prospective applications such as biological separation and transport., Comment: 4 pages, 5 figures