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Here, we report room temperature deposition of nickel oxide (NiO) thin films on silicon and glass substrates by direct current (dc) magnetron sputtering using a metallic nickel target. The effect of sputtering power (20 W, 25 W, and 30 W) on the thin films, their surface scaling, fractal dimension, and optical properties are extensively investigated. Autocorrelation and height–height correlation functions were applied to AFM images to extract deep insights about the thin films' surfaces. Fractal dimension (D f) was extracted through the power spectral density (PSD) function. Various scaling exponents, including α, β, and 1/z, of the NiO film's surface were independently observed. The local roughness exponent, α, was approximately 0.81 for films deposited with 20 W sputtering power and decreased to 0.71 with higher sputtering power. The interface width (σ) scales with sputtering power (Sp) as ∼ Spβ, with a growth exponent (β) value of 1.17. The lateral correlation length (ξ) follows as ∼ Sp1/z with a 1/z value of 0.703. Additionally, optical parameters were recorded through UV–Vis. optical spectroscopy, and an attempt was made to correlate them with fractal parameters (D f & α). Optical absorption (reflection) increased (decreased) with increasing D f values. The minimum (maximum) reflection (absorption) was observed on the roughest surface (D f = 2.29). The calculated band gap decreased with increasing fractal dimension. This investigation suggests that sputtered surfaces with minimal reflectivity, band gap, and enhanced light-absorbing capacity could potentially be used as active solar layers for advanced optoelectronic devices. [ABSTRACT FROM AUTHOR]

Ordered arrays of magnetic nanotubes are prepared by combining a porous template (anodic alumina) with a self-limiting gas-solid chemical reaction (atomic layer deposition). The geometric parameters can thus be tuned accurately (tube length of 1–50 μm, diameter of 20–150 nm, and wall thickness of 1–40 nm), which enables one to systematically study how confinement and anisotropy effects affect the magnetic properties. In particular, the wall thickness of such ordered Fe3O4 nanotubes has a nonmonotonic influence on their coercive field. Theoretical models reproduce the size effects that are experimentally observed and interpret them as originating from a crossover between two distinct modes of magnetization reversal. [ABSTRACT FROM AUTHOR]

The magnetic dynamic susceptibility of cobalt dots hosting a skyrmionic bubble has been obtained by means of micromagnetic simulations by exciting the system with a magnetic field pulse. When the pulse is applied parallel to the dot axis, a single resonance peak is obtained, whereas when the pulse is applied perpendicular to the dot axis, multiple peaks are obtained. The origin of each of these peaks and their evolution as a function of uniaxial anisotropy are studied in detail. The results presented in this work are important in view of the possible realization of tunable frequency devices, since we showed that it is possible to adjust a set of frequencies of skyrmionic bubbles by controlling the uniaxial anisotropy constant of the dot and the direction in which the magnetic pulse is applied. [ABSTRACT FROM AUTHOR]

Iron germanide (FeGe) emerges as a promising magnetic alloy for spintronics and high-density data storage, owing to its distinctive magnetic properties and compatibility with existing fabrication techniques. This compatibility enables the synthesis of customized FeGe nanocylinders characterized by chirality, where their magnetization asymmetrically twists. Within specific size parameters, these nanocylinders can accommodate skyrmions—swirling magnetic structures with significant implications for information storage and processing technologies. This study investigates the response of FeGe nanocylinders to external magnetic fields, focusing on how their magnetic properties vary with dimensions (diameter and length). Specifically, we analyze the impact of length on the pseudo-static properties of short FeGe nanocylinders and examine the average topological charge and remanence states across different aspect ratios. Our investigation underscores the relationship between chirality and diverse magnetization states in four types of nanocylinders with varying aspect ratios. This comprehensive analysis elucidates the connection between nanocylinder magnetic states and the average topological charge—a critical factor in advancing ultra-low-energy data storage and logic devices. [ABSTRACT FROM AUTHOR]

• Wire-tube nanostructures. • Hysteresis loops. • Barkhausen jumps. We have studied the magnetostatic interactions between two parallel cylindrical nanostructures with wire-tube morphology as a function of their interaxial separation and the magnetic field applied using micromagnetic simulations. We find that the hysteresis curves of the system can exhibit one, two or three Barkhausen jumps as a function of the separation between the nanostructures and the angle at which the external magnetic field is applied, which are related to the reversal of the magnetic moments of certain segments of the nanostructures. In addition, by studying the averages of magnetization we obtain that for angles less than or equal to 60 degrees, the nanostructures reverse their magnetization through the nucleation and propagation of domain walls (vortex for the tube segment and transverse for the wire segment), while for higher angles the nanostructures revert their magnetization by quasi-coherent rotation. [ABSTRACT FROM AUTHOR]

Here we report, the room temperature deposition of silver (Ag) thin films on silicon, glass and silicon solar cell substrates by magnetron sputtering. The effect of thickness on the deposited thin films and their morphological, fractal, optical and solar cell parameters have been extensively investigated. AFM results show that the Ag thin films are composed of different-size of nano-grains and the grain size increases with increasing thickness. Mono-fractal parameters, namely fractal dimension (Df) and Hurst exponent (H), were determined from the power spectral density function. X-ray photoelectron spectroscopy studies indicate that the deposited samples exhibit the presence of oxidation state of Ag (0) in thin films, confirming the stable structure. The optical analysis shows that the transmittance of the deposited thin films decreases with increasing thickness, on the other hand, the silver thin film with thickness of 20 nm shows surface Plasmon resonance (SPR) at 436 nm. Current-voltage (I–V) curves were measured in dark and under illumination mode at standard testing conditions, i.e., 300K, 1000 W/m2 and 1.5 AM global using a solar simulator. These measurement studies indicate that thicknesses are effectively impacts on photovoltaic performance. The calculated values of change in solar efficiency (Δ η) are 0.26, −0.20, −0.76 and −6.88 % for thin films with nominal thickness 0.5, 1, 2 and 20 nm respectively. • Ag thin films were deposited by magnetron sputtering technique. • The influence of ultrathin layers on the fractal dimensions of Ag films was studied. • The influence of ultrathin layers on the optical properties of Ag films was studied. • The influence of ultrathin layers on the solar cell performance have been explored. • The influence of optical parameters viz. reflection, plasmonic on solar cell efficiency have been studied. [ABSTRACT FROM AUTHOR]

In this study, we investigate the pseudo-static magnetic properties of Permalloy conical frustum nanowires using micromagnetic simulations. We thoroughly examine how both the major and minor radii influence the magnetic reversal mechanism when an external magnetic field is applied parallel to the nanowire axis. The obtained results show that under specific geometrical conditions, magnetization reverts though a Bloch point-type domain wall. In these cases, hysteresis curves exhibit two Barkhausen jumps during magnetization reversal, forming a plateau field range in which a Bloch point domain wall nucleates and propagates until its annihilation after the second Barkhausen jump. The nucleation of a Bloch point domain wall in a frustum conical nanowire geometry is reported. These findings highlight the significance of this geometry in nucleating these attractive topological defects for promising applications. [ABSTRACT FROM AUTHOR]

Abstract Magnetic hysteresis in Ni nanowire arrays grown by electrodeposition inside the pores of anodic alumina templates is studied as a function of temperature in the range between 5 K and 300 K. Nanowires with different diameters, aspect ratios, inter-wire distance in the array and surface condition (smooth and rough) are synthesized. These microstructure parameters are linked to the different free magnetic energy contributions determining coercivity and the controlling magnetization reversal mechanisms. Coercivity increases with temperature in arrays of nanowires with rough surfaces and small diameters ─33 nm and 65 nm─ when measured without removing the alumina template and/or the Al substrate. For thicker wires ─200 nm in diameter and relatively smooth surfaces─ measured without the Al substrate, coercivity decreases as temperature rises. These temperature dependences of magnetic hysteresis are described in terms of an effective magnetic anisotropy K a , resulting from the interplay of magnetocrystalline, magnetoelastic and shape anisotropies, together with the magnetostatic interaction energy density between nanowires in the array. The experimentally determined coercive fields are compared with results of micromagnetic calculations, performed considering the magnetization reversal mode acting in each studied array and microstructure parameters. A method is proposed to roughly estimate the value of K a experimentally, from the hysteresis loops measured at different temperatures. These measured values are in agreement with theoretical calculations. The observed temperature dependence of coercivity does not arise from an intrinsic property of pure Ni but from the nanowires surface roughness and the way the array is measured, with or without the alumina template and/or the aluminum support. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

An analytical expression for the energy of Neel skyrmions in ultra-thin nanodots considering exchange, uniaxial anisotropy, Dzyaloshinskii-Moriya, and dipolar contributions has been obtained. In particular, we have proposed for the Neel skyrmion, a general ansatz for the magnetization component perpendicular to the dot, given by m z ( r ) = [ 1 - ( r / R s ) n ] / [ 1 + ( r / R s ) n ] , where R s is the radius of the skyrmion and n is an even integer number. As proof of concept, we calculate the energy of a Neel skyrmion in an ultra-thin Co/Pt cylinder, and we find that the dipolar contribution cannot be neglected and that both Dzyaloshinskii-Moriya interaction and anisotropy play an important role to stabilize the skyrmion. Additionally, we have obtained a good agreement between our analytical calculations and previously published micromagnetic simulations for n = 10 . For this reliable value of n , we have obtained that for a Dzyaloshinski Moriya constant D = 5.5 (mJ/m 2 ), it is possible to stabilize a Neel skyrmion for K u in the range 0.4 (MJ/m 3 ) < K u < 1.3 (MJ/m 3 ), whereas for K u = 0.8 ( MJ / m 3 ) , the skyrmion stabilizes for 5.0 (mJ/m 2 ) < D < 6.0 (mJ/m 2 ). Thus, this analytical equation can be widely used to predict stability ranges for the Neel skyrmion in spintronic devices. [ABSTRACT FROM AUTHOR]

Ni 81 Fe 19 O y thin films with different thicknesses were obtained by atomic layer deposition. After a thermal reduction process, Ni 81 Fe 19 O y films were converted to their strong Ni 81 Fe 19 ferromagnetic phase, usually known as Permalloy. The different thicknesses were obtained by considering 500, 1000, 1500 and 2000 supercycles, which are defined as a cycle of NiCp 2 /O 3 followed by another cycle of FeCp 2 /O 3 . The samples were characterized before and after the thermal reduction process with respect to their crystallinity, morphology and magnetism. In particular, the thermally reduced samples exhibit holes whose sizes increase significantly as the thickness of the samples also increases, affecting their magnetic behaviour. As Permalloy is widely used in many applications, its synthesis through atomic layer deposition is expected to introduce comparative advantages over other synthesis processes, opening up new possibilities for nanoelectronic devices. [ABSTRACT FROM AUTHOR]

The magnetization reversal mechanisms in soft/hard multisegmented nanowires have been investigated using numerical simulations. In most of the studied systems the magnetization reversal process starts at the ends. However in short soft/hard/soft nanowires, the magnetization reversal process starts at the center of the wire, generating an unusual behavior of the coercivity as a function of the length. [ABSTRACT FROM AUTHOR]

In this study, we investigate the magnetic properties of interconnected permalloy nanowire networks using micromagnetic simulations. The effects of interconnectivity on the hysteresis curves, coercivity, and remanence of the nanowire networks are analyzed. Our results reveal intriguing characteristics of the hysteresis curves, including nonmonotonic behaviors of coercivity as a function of the position of horizontal nanowires relative to vertical nanowires. By introducing horizontal nanowires at specific positions, the coercivity of the nanowire networks can be enhanced without altering the material composition. The normalized remanence remains relatively constant regardless of the position of the horizontal wires, although it is lower in the interconnected nanowire arrays compared to nonconnected arrays. These findings provide valuable insights into the design and optimization of nanowire networks for applications requiring tailored magnetic properties. [ABSTRACT FROM AUTHOR]

In this paper, our objective was to investigate the static and dynamic magnetic properties of Fe3O4 nanotubes that are 1000 nm long, by varying the external radius and the thickness of the tube wall. We performed a detailed numerical analysis by simulating hysteresis curves with an external magnetic field applied parallel to the axis of the tubes (along the z-axis). Our findings indicate that nanotubes with an external radius of 30 nm exhibit non-monotonic behavior in their coercivity due to a change in the magnetization reversal mechanism, which was not observed in nanotubes with external radii of 80 nm. Additionally, we explored the dynamic susceptibility of these nanotubes and found that the position and number of resonance peaks can be controlled by manipulating the nanotube geometry. Overall, our study provides valuable insights into the behavior of Fe3O4 nanotubes, which can aid in the design and improvement in pseudo-one-dimensional technological devices. [ABSTRACT FROM AUTHOR]

Ferromagnetic films down to thicknesses of tens of nanometers and composed by polycrystalline Fe and Fe2O3 nanopillars are grown in large areas by glancing angle deposition with magnetron sputtering (MS-GLAD). The morphological features of these films strongly depend on the growth conditions. Vertical or tilted nanopillars have been fabricated depending on whether the substrate is kept rotating azimuthally during deposition or not, respectively. The magnetic properties of these nanopillars films, such as hysteresis loops squareness, adjustable switching fields, magnetic anisotropy and coercivity, can be tuned with the specific morphology. In particular, the growth performed through a collimator mask mounted onto a not rotating azimuthally substrate produces almost isolated well-defined tilted nanopillars that exhibit a magnetic hardening. The first-order reversal curves diagrams and micromagnetic simulations revealed that a growth-induced uniaxial anisotropy, associated with an anisotropic surface morphology produced by the glancing angle deposition in the direction perpendicular to the atomic flux, plays an important role in the observed magnetic signatures. These results demonstrate the potential of the MS-GLAD method to fabricate nanostructured films in large area with tailored structural and magnetic properties for technological applications. [ABSTRACT FROM AUTHOR]

We have investigated hysteresis curves and magnetization reversal mechanisms in permalloy wire-tube nanostructures when an external magnetic field is applied perpendicular to the axis of the nanostructure, in the x direction. Our results show that the nanotube reverses its magnetization through the wave reversal mechanism, while the nanowire does so through pseudo-coherent rotation, according to the literature. The wire-tube nanostructure exhibits a more complex reversal mechanism, where each segment begins to revert depending on whether it is a wire or tube segment, but quickly the entire nanostructure adopts the wire segment reversal mechanism as its own, considerably reducing the area within of the hysteresis curve exhibited by the nanostructure. The possibility to alter the magnetization reversal mode in such nanostructures provides an attractive way to control the motion of the magnetic domain walls. [ABSTRACT FROM AUTHOR]

A long piece of magnetic material shaped as a central cylindrical wire (diameter d = 50 nm) with two wider coaxial cylindrical portions (diameter D = 90 nm and thickness t = 100 nm) defines a bimodulated nanowire. Micromagnetism is invoked to study the equilibrium energy of the system under the variations of the positions of the modulations along the wire. The system can be thought of as composed of five independent elements (3 segments and 2 modulations) leading to 2 5 = 32 possible different magnetic configurations, which will be later simplified to 4. We investigate the stability of the configurations depending on the positions of the modulations. The relative chirality of the modulations has negligible contributions to the energy and they have no effect on the stability of the stored configuration. However, the modulations are extremely important in pinning the domain walls that lead to consider each segment as independent from the rest. A phase diagram reporting the stability of the inscribed magnetic configurations is produced. The stability of the system was then tested under the action of external magnetic fields and it was found that more than 50 mT are necessary to alter the inscribed information. The main purpose of this paper is to find whether a prototype like this can be complemented to be used as a magnetic key or to store information in the form of firmware. Present results indicate that both possibilities are feasible. [ABSTRACT FROM AUTHOR]

• Magnetic GO and rGO-based composites decorated with Py NWs were obtained. • Electrochemical results unveils that Py NWs play a remarkable role in the increased conductivity of both composites. • Relative conductivity increase of Py NWs/GO is significantly higher than that of Py NWs/rGO. Low dimensional carbon allotropes are the most used materials in composites. In addition, when magnetic nanostructures are assembled with carbon-based materials, a versatile material for building sensitive electrodes is achieved. Here, we propose a route to obtain magnetic composites based on both graphene oxide and reduced graphene oxide with permalloy nanowires. A comparative analysis of the electrochemical performance for such composites reveals that a relatively small amount of magnetic nanowires leads to an increased conductivity with respect to that of the pristine matrix, being such effect more pronounced for the graphene oxide-based composite. Therefore, the presence of permalloy nanostructures plays a determinant role in the conductive properties of graphene oxide-based materials, providing an attractive alternative for sensing purposes. [ABSTRACT FROM AUTHOR]

Carbon–metal-based composites arise as advanced materials in the frontiers with nanotechnology, since the properties inherent to each component are multiplexed into a new material with potential applications. In this work, a novel composite consisting of randomly oriented permalloy nanowires (Py NWs) intercalated among the sheets of multi-layered graphene oxide (GO) was performed. Py NWs were synthesized by electrodeposition inside mesoporous alumina templates, while GO sheets were separated by means of sonication. Sequential deposition steps of Py NWs and GO flakes allowed to reach a reproducible and stable graphene oxide-based magnetic assembly. Microscopic and spectroscopic results indicate that Py NWs are anchored on the surface as well as around the edges of the multi-layered GO, promoted by the presence of chemical groups, while magnetic characterization affords additional support to our hypothesis regarding the parallel orientation of the Py NWs with respect to the GO film, and also hints the parallel stacking of GO sheets with respect to the substrate. The most striking result remains on the electrochemical performance achieved by the composite that evidences an enhanced conductive behaviour compared to a standard electrode. Such effect provides an approach to the development of permalloy nanowires/graphene oxide-based electrodes as attractive candidates for molecular sensing devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Anodic alumina membranes with modulated pore diameters serve as template for the preparation of magnetic nanowires. Filling the pores with Ni by electrodeposition delivers wires replicating the variation in modulation in pore diameter from 80 to 160 nm. Such structures are of interest for the observation and control of magnetic domain wall motion. Single-object characterization utilizing the magneto-optical Kerr effect magnetometry evidences a strong correlation between geometric parameters and magnetic properties. Ensemble magnetization measurements with a superconducting quantum interference device show the effect of dipolar interactions. Analytical models can reproduce the lowering of coercivity due to the presence of enhanced stray fields within the array. Magnetic force microscopy at individual wires indicates the presence of a strong stray field in the vicinity of the diameter change. The preparation technique demonstrates a mass production method of nano-objects with designed geometric irregularities, which could be used to control the motions of magnetic domain walls. [ABSTRACT FROM AUTHOR]

We have calculated the dynamic susceptibility of modulated permalloy nanowires of 1-µm long and 50 nm diameter using micromagnetic simulations. The resonance modes obtained from these simulations are investigated as functions of both the position of the modulation along the nanowire as well as the size of it. The results presented in this work are important in view of the possible realization of tunable frequency magnonic devices, since we showed that it is possible to adjust a set of frequencies by controlling the geometric parameters of the system. [ABSTRACT FROM AUTHOR]

The oxides LiMn 2 O 4 , LiFe 0.75 Zn 0.25 MnO 4 , and LiFe 0.25 Zn 0.25 Mn 1.5 O 4 , referred to as LMO, LFZMO 0.75–0.25, and LFZMO 0.25–0.25, were synthesized at 800 °C using ultrasound-assisted thermal decomposition of nitrates (UA-TDN). X-ray diffraction (XRD) analyses confirmed the spinel structure (Fd3m space group) for all materials. Raman spectroscopy validated the characteristic absorption bands of the spinel phase. Energy-dispersive spectroscopy (EDS) verified the uniform distribution of Fe, Zn, Mn, and O in all samples and proved the expected stoichiometry for the different oxides. Scanning electron microscopy (SEM) images showed irregular polyhedral agglomerates in the synthesized oxides, with grain sizes ranging from approximately 600 to 800 nm. Electrochemical characterization was performed under N 2 and O 2 atmospheres, revealing electron-withdrawing effects with increasing iron content in the oxide composition. LFZMO 0.75–0.25, with a higher iron content, displayed better performance than LMO and LFZMO 0.25–0.25 in terms of discharge capacity and cycling performance at various rates. The greater discharge capacity of LFZMO 0.75–0.25 (47 mA h g −1) was in line with the cyclic voltammetry findings, wherein lower energy (∆G Li + / Li 0 = 55.5 kJ mol−1) for lithium insertion correlated with an increased discharge capacity. Electrochemical Impedance Spectroscopy (EIS) was employed to determine diffusion constants, providing insights into Li+ insertion kinetics in the cathodic electrode and highlighting the significant influence of Zn and Fe incorporation on the lithium manganese oxide. Our findings are intriguing as the diffusion coefficient of D Li + in LFZMO 0.75–0.25 (2.3 ×10−12 cm2 S−1) surpasses that of LMO (8.5 ×10−14 cm2 S−1) and LFZMO 0.25–0.25 (9.4 ×10−18 cm2 S−1). This observation further elucidates the more positive formal potential for Li+ insertion in LFZMO 0.75–0.25. [Display omitted] • The presence of Fe and Zn in LiMn 2 O 4 affects the charge/discharge capacity. • Cycling voltammetry approaches the energy required for lithium insertion in the charging process. • The diffusion rate of lithium-ion is enhanced by the Zn atom in LFZMO 0.75-0.25, giving a more electronegative environment. • At High Fe concentration Mn4+/Mn3+ dismutation is avoided improving LFZMO 0.75-0.25 performance. [ABSTRACT FROM AUTHOR]

We performed density-functional-theoretical calculations of the silanediol-like molecules adsorbed on (TiO 2) 20 tetrahedral-like clusters. We study the adsorption on the triangular face of the (TiO 2) 20 clusters in two different sites (C e n t e r and E d g e). Our results suggest that the band gap of the (TiO 2) 20 nano-clusters can be controlled (tune or adjust) changing the length of the molecule as well as the terminal element within the molecule chain. The adsorption energy or functionalization energy of the cluster is almost independent of the number of atoms, presenting only a slight increase with the number of atoms in the adsorbed molecular chain. We also analyze the dependence of the gap with iso-electronic final monomer for a few selected cases. • DFT theoretical calculations of Transition Metal Oxides with adsorbed molecules. • Possible way to control the gap of Transition Metal clusters by molecules. • Dependence of the gap on the ending monomer of the adsorbed molecules. [ABSTRACT FROM AUTHOR]

The search for new antimicrobial substances has increased in recent years. Antimicrobial nanostructures are one of the most promising alternatives. In this work, titanium dioxide nanotubes were obtained by an atomic layer deposition (ALD) process over electrospun polyvinyl alcohol nanofibers (PVN) at different temperatures with the purpose of obtaining antimicrobial nanostructures with a high specific area. Electrospinning and ALD parameters were studied in order to obtain PVN with smallest diameter and highest deposition rate, respectively. Chamber temperature was a key factor during ALD process and an appropriate titanium dioxide deposition performance was achieved at 200 °C. Subsequently, thermal and morphological analysis by SEM and TEM microscopies revealed hollow nanotubes were obtained after calcination process at 600 °C. This temperature allowed complete polymer removal and influenced the resulting anatase crystallographic structure of titanium dioxide that positively affected their antimicrobial activities. X-ray analysis confirmed the change of titanium dioxide crystallographic structure from amorphous phase of deposited PVN to anatase crystalline structure of nanotubes. These new nanostructures with very large surface areas resulted in interesting antimicrobial properties against Gram-positive and Gram-negative bacteria. Titanium dioxide nanotubes presented the highest activity against Escherichia coli with 5 log cycles reduction at 200 μg/mL concentration. [ABSTRACT FROM AUTHOR]

Nickel oxide thin films with different thicknesses were synthesized by atomic layer deposition. After a thermal reduction process, under a controlled atmosphere of hydrogen, it was possible to convert nickel oxide to metallic nickel. The different thicknesses were obtained for 250 to 2000 cycles of NiCp 2 /O 3 . The Ni thin films were characterized by X-ray diffraction, scanning electron microscopy, and by Magneto-Optical Kerr effect measurements. Micromagnetic simulations have been performed to investigate the mechanism of magnetization reversion of these films. Interestingly, the thermal reduction of films synthesized by ALD allows the generation of both randomly oriented, sized and spaced Ni islands and antidot arrays from thin films. [ABSTRACT FROM AUTHOR]

In this paper we investigate by micromagnetic simulations the magnetic properties of Permalloy double-dot structures as a function of the center-to-center distance between the dots. Generally, a well-defined neck appears in the hysteresis curve, evidencing that the process of magnetization reversal occurs by nucleation and propagation of vortices. However, although the processes of magnetization reversal are similar for different distances between the dots, the coercivities are quite different, primarily due to the different energy contributions that dominate the system. The contribution of the shape anisotropy is important for Permalloy double-dot structures where the distance between the dots is less than their diameters, while the magnetostatic interaction between the two dots predominates if the distance between the dots is greater than their diameters. Finally, there is a critical distance for which the influence of magnetostatic interaction decreases sufficiently so that the interacting system can be considered as an isolated system. [ABSTRACT FROM AUTHOR]

We have fabricated Co–Pt cylindrical nanostructures comprised of a pair of nanotube and nanowire segments via direct electroplating into anodic alumina (AAO) membranes. The fabrication of such nanostructures is possible due to the penetration of sputtered gold (Au) nanoparticles inside the template, which serve as nucleation spots. The current transient monitored during the deposition process allowed us to distinguish between the nanotube and nanowire formation regime during the electrodeposition. In this paper, we report on the systematic changes in the angular dependence of the coercivity behavior due to the change in the length of the nanowire segment of these nanostructures, while keeping the nanotube segment constant. Considering the range of parameters studied, we found that the tube segment always reverses its magnetization through a vortex domain wall, whereas the nanowire segment reverses its magnetization through a transverse domain wall. Our results are in good agreement with literature. The possibility to alter the magnetization reversal mode in such nanostructures provides an attractive way to control the motion of the magnetic domain walls. [ABSTRACT FROM AUTHOR]

• Fe 2 O 3 thin films can be obtained at 200 °C by atomic layer deposition. • Fe 2 O 3 films can be reduced to Fe 3 O 4 films by annealing in H 2 /Ar at 400 °C. • Different thicknesses were obtained considering from 500 to 2000 cycles of FeCp 2 /O 3. • Controlling the thickness of Fe 3 O 4 films allows adjusting their magnetic properties. Hematite (α-Fe 2 O 3) thin films with different thicknesses between 4 and 16 nm were synthesized by atomic layer deposition, employing 500 to 2000 cycles of FeCp 2 /O 3. After a thermal reduction process, under a controlled atmosphere of hydrogen, it was possible to convert hematite to magnetite (Fe 3 O 4). SEM images of the iron oxide thin films showed that they are macroscopically homogeneous, although the magnetic measurements by MOKE indicated that the samples should present holes that are responsible for pinning the domain walls, raising the coercivity to values ​​that vary between 0.3 and 0.7 kOe. XRD revealed the presence of the Fe 3 O 4 phase by identifying the main peaks. The signal obtained by Raman spectroscopy was identified at 667 cm−1, which suggests the presence of the phonon mode A1g of Fe 3 O 4 in the reduced sample. Controlling the thickness of Fe 3 O 4 thin films allows adjusting their magnetic properties, coercivity and remanence, so that they can be used in potential technological applications. [ABSTRACT FROM AUTHOR]

• Non-monotonic coercivity and remanence as a function of diameter for Py and Ni NTs. • Wave reversal mode could be identified by the area enclosed in the hysteresis curve. • New phase diagram allows determining the stability of the wave reversal mode. The wave reversal mode is a magnetization reversal mechanism that appears in ferromagnetic nanotubes of certain geometric parameters when an external magnetic field is applied perpendicular to their axes. The distinctive feature of this mode is that leads to well-defined S-shaped hysteresis curves. In order to gain insight into the stability of this latter effect, we have performed micromagnetic simulations for permalloy and nickel nanotubes obtaining a non-monotonic behavior for coercivity as well as for remanence as a function of nanotube diameter for both materials. Motivated on these latter intriguing results, we found that measuring the area that encloses the hysteresis curve is a novel and simple strategy to identify the appearance of the wave reversal mode. An additional contribution of this work is the proposal of a new magnetic phase diagram that allows determining the stability of this reversal mechanism as a function of the geometric and magnetic parameters of the tubes. [ABSTRACT FROM AUTHOR]

• Introducing a diameter modulation on a magnetic nanowire changes the coercive field. • Change the position of the nanowire modulation allows tuning the coercive field. • Modulated magnetic nanowires can be used for magnetic memories. Magnetic nanowires of diameter d with a concentric cylindrical modulation of larger diameter D can lead to the production of devices with a variety of coercive fields. Besides D , the modulation thickness t and its position z along the axis of the wire are varied in the vicinity of values usually reported in the experimental literature. It is shown that the system can be understood as the combination of a long wire (with its usual properties for large aspect ratios), a short wire with lower coercive field and a wider modulation with a tendency to produce an early vortex configuration that softens the whole system from the magnetic point of view. The triggering mechanism to achieve the reversal at different coercive fields is studied by means of snapshots of the configurations just prior to the reversal itself. Altogether, upon spanning the parameter space (D , t , and z) a variety of hysteresis curves are obtained and a large range of coercive fields are shown to be possible. Micromagnetic simulations by means of Mumax3 solving the Landau-Lifshitz-Gilbert equation provide the theoretical framework of this work. Possible extensions of this work are also mentioned. [ABSTRACT FROM AUTHOR]

In this work we study the oscillations of the skyrmion cores in a multilayer nanodot as a function of the number of skyrmions hosted in the system. When all the skyrmions in the nanodot have the same core radius, and after applying a perpendicular spin-polarized current, a relaxation process takes place towards an equilibrium configuration that is accompanied by coherent damped oscillations of the skyrmion cores, whose frequency depends on the number of skyrmions present in the nanodot. Additionally, we found that the oscillation frequency is directly related to the total energy of the system. [ABSTRACT FROM AUTHOR]

• Permalloy nanowires of 30 nm in diameter and 1 μm in length obtained by AC electrodeposition. • Coercivity and remanence decrease with increasing angles at which the external field is applied. • Magnetostatic interaction modifies the shape of hysteresis curves of simulated Py NW arrays. • Py NW arrays reverse magnetization by nucleation and propagation of transverse domain walls. • Jumps in magnetization observable in hysteresis curves are proportional to the number of simulated Py NWs. This work contributes to the understanding of angular dependence of the magnetic properties of Permalloy nanowire arrays by means of a comparative analysis between experiment and micromagnetic simulations. On the one hand, Permalloy nanowires of 30 nm in diameter and 1 μm in length were synthesized by AC electrodeposition inside mesoporous alumina templates. Magnetic experimental measurements show that both coercivity and remanence decrease monotonously as the angle, at which the external field is applied, increases. On the other hand, the comparison between the simulation of a nanowire array and that performed for one single nanowire reveals that the magnetostatic interactions between the nanowires strongly modifies not only the shape of the hysteresis curve but also the coercivity and remanence values. Besides, simulations also disclose that the nanowires reverse their magnetization through the nucleation and propagation of transverse domain walls. Moreover, the simulation of an array of nanowires also discloses that the hysteresis curve exhibits noticeable jumps in the magnetization that are proportional to the number of nanowires that reverse their magnetization in the array. This latter could provide an interesting hint to control the performance of magnetic nanodevices. [ABSTRACT FROM AUTHOR]

• We have performed micromagnetic simulations of Ni and Py nanotubes as a function of the external magnetic field. • A new striking reversal mechanism, which we have called wave reversal mode (W), has been identified. • This phenomenon describes unusual behaviors in nanotubes when the field is applied perpendicular to its axis. • A sharp increase in remanence is an unequivocal signal that the system reverses according to this mechanism. We have investigated the magnetic properties of 14 nm thick and 1 μm long nickel and permalloy nanotubes with external diameters of 40 and 100 nm as a function of the angle θ at which the external magnetic field is applied. Our results show that the coercivity of 40 nm diameter nickel nanotubes follows a non-monotonic behavior from θ = 0° up to θ = 60°, while that corresponding to permalloy displays an increasing monotonic trend at the same angular range. At θ = 90°, both materials evidence a sharp drop of the coercivity to zero, indicating that the reversal mechanism has changed to a pseudo-coherent rotation. On the other hand, nickel and permalloy nanotubes with 100 nm in diameter exhibit a similar angular dependence of the coercivity, reversing their magnetization through the nucleation and propagation of vortex domain walls for angles lower than 75°. For θ = 90°, a novel striking mechanism, the wave reversal mode (W), arises. This phenomenon leads to an unusual S-type shape in the hysteresis curves at those given parameters, which is until now an effect that has not been reported for these nanostructures. [ABSTRACT FROM AUTHOR]

An antimicrobial polymeric bilayer structure based on the application of an acrylic coating containing hollow zinc oxide nanotubes over a polymeric substrate was developed in this work. Firstly, zinc oxide nanotubes (ZnONT) were obtained by an atomic layer deposition (ALD) process over electrospun polyvinyl alcohol nanofibers followed by polymer removal through calcination with the purpose of obtaining antimicrobial nanostructures with a high specific area. Parameters of electrospinning, ALD, and calcination processes were set in order to obtain successfully hollow zinc oxide nanotubes. Morphological studies through scanning electron microscopy (SEM) and transmission electron microscopy (TEM) microscopies confirmed the morphological structure of ZnONT with an average diameter of 180 nm and thickness of approximately 60 nm. Thermal and X-ray diffraction (XRD) analyses provided evidence that calcination completely removed the polymer, resulting in a crystalline hexagonal wurtzite structure. Subsequently, ZnONT were incorporated into a polymeric coating over a polyethylene extruded film at two concentrations: 0.5 and 1 wt. % with respect to the polymer weight. An antimicrobial analysis of developed antimicrobial materials was performed following JIS Z2801 against Staphylococcus aureus and Escherichia coli. When compared to active materials containing commercial ZnO nanoparticles, materials containing ZnONT presented higher microbial inhibition principally against Gram-negative bacteria, whose reduction was total for films containing 1 wt. % ZnONT. Antiviral studies were also performed, but these materials did not present significant viral reduction. [ABSTRACT FROM AUTHOR]

In this work, we take a step toward nanometer resolution magnetic tomography, demonstrating the use of resonant magnetic x-ray scattering for three-dimensional (3D) characterization of Co/Pd multilayers. Trading coherence for x-ray flux, we measured magnetic scattering out to full period resolutions of 20 nm for incidence angles up to 75° from normal incidence, which was sufficient for a semiquantitative analysis of the 3D magnetic domain structure of such films. For the analysis of such patterns, we developed a scattering model based upon multiple plane propagation of the beam through the sample, taking into account the effects of multiple scattering not accounted for with Born or Rytov approximation based models. Our results demonstrate that 3D statistical characterization of nanoscale magnetic features is currently possible, and it is expected that the advent of high brilliance sources such as MAX IV in Sweden and SIRIUS in Brazil will provide the coherent flux necessary to permit the generalization of this work to nanometer resolution magnetic tomography. [ABSTRACT FROM AUTHOR]

Reversal modes and magnetostatic interactions of multilayered Fe3O4/ZrO2/Fe3O4 nanotubes consisting of a ferromagnetic internal tube, an intermediate non-magnetic spacer and an external magnetic shell are investigated as a function of their geometric parameters and compared with those produced inside the pores of anodic alumina membranes by atomic layer deposition. Based on a continuum approach we obtained analytical expressions that underline the first experimental results and support their interpretation that the system of multilayer tubes behaves as the reversal of two isolated systems. It is observed that the magnetostatic interaction between both phases depends on the magnetic configurations in each phase and also on the geometrical parameters considered. These structures have potential applications in novel spintronics devices, ultra-small magnetic media and other nano-devices. [ABSTRACT FROM AUTHOR]

Review: 60 refs. [ABSTRACT FROM AUTHOR]