Magnetostatic dipolar anisotropy energy and anisotropy constants in arrays of ferromagnetic nanowires as a function of their radius and interwall distance

Producción CientíficaMagnetostatic dipolar anisotropy energy and the total dipolar anisotropy constant, ${K}_{{total}}$, in periodic arrays of ferromagnetic nanowires have been calculated as a function of the nanowire radius, the interwall distance of the nanowires in the arrays and the geometry of the array (square or hexagonal), by using a realistic atomistic model and the Ewald method. The simulated nanowires have a radius size up to 175 Å that corresponds to 31 500 atoms, and the simulated nanowire arrays have interwall distances between 35 and 3000 Å. The dependence of total magnetostatic dipolar anisotropy constant on the nanowire radius, their interwall distance and the type of array symmetry has been analyzed. The total dipolar anisotropy constant, which is the sum of the intrananowire dipolar anisotropy constant, ${K}_{{intra}}$, due to the dipolar interactions inside an isolated nanowire and the main responsible of the shape anisotropy, and of the internanowire dipolar anisotropy constant, ${K}_{{inter}}$, due to the magnetostatic dipolar interactions among nanowires in the array, have been calculated and compared with the magnetocrystalline anisotropy constant for three nanowire compositions and their crystalline structures. The simulations of the nanowire arrays with large interwall distances have been used to calculate the intrananowire anisotropy constant, ${K}_{{intra}}$, and to analyze the competition between the intrananowire, internanowire and magnetocrystalline anisotropies. According to some magnetic theories, the ratio $| {K}_{{inter}}/{K}_{{intra}}|$ equals to the areal filling fraction of a nanowire array. Present calculations indicate that the equation for the areal filling fraction matches perfectly for any interwall distance and radius of Ni and Co nanowire arrays. This first equation is used to write a general equation that relates the radius and interwall distance of nanowire arrays with the intrananowire, internanowire and magnetocrystalline anisotropies. This general equation allows to design the geometry of nanowire arrays with the desired orientation of the easy magnetization axis.Ministerio de Economía, Industria y Competitividad (Grants MAT2014–54378-R, MAT2016–76824-C3-3-R and PGC2018–093745-B-I00)Junta de Castilla y León (Ref. project VA124G18

Magnetothermopower and magnetoresistance of single Co-Ni/Cu multilayered nanowires

The magnetothermopower and the magnetoresistance of single Co Ni/Cu multilayered nanowires with various thicknesses of the Cu spacer are investigated. Both kinds of measure-ment have been performed as a function of temperature (50 K to 325 K) and under applied mag-netic fields perpendicular to the nanowire axis, with magnitudes up to 15 % at room tempera-ture. A linear relation between thermopower S and electrical conductivity σ of the nanowires is found, with the magnetic field as an implicit variable. Combining the linear behavior of the S vs. σ and the Mott formula, the energy derivative of the resistivity has been determined. In order to extract the true nanowire materials parameters from the measured thermopower, a simple model based on the Mott formula is employed to distinguish the individual thermopower contributions of the sample. By assuming that the non-diffusive thermopower contributions of the nanowire can be neglected, it was found that the magnetic field induced changes of thermopower and re-sistivity are equivalent. The main emphasis in the present paper is put on a comparison of the magnetoresistance and magnetothermopower results and it was found that the same correlation is valid between the two sets of data for all samples, irrespective of the relative importance of the giant magnetoresistance or anisotropic magnetoresistance contributions in the various indi-vidual nanowires

Influence of Anodic Conditions on Self-ordered Growth of Highly Aligned Titanium Oxide Nanopores

Self-aligned nanoporous TiO2templates synthesized via dc current electrochemical anodization have been carefully analyzed. The influence of environmental temperature during the anodization, ranging from 2 °C to ambient, on the structure and morphology of the nanoporous oxide formation has been investigated, as well as that of the HF electrolyte chemical composition, its concentration and their mixtures with other acids employed for the anodization. Arrays of self-assembled titania nanopores with inner pores diameter ranging between 50 and 100 nm, wall thickness around 20–60 nm and 300 nm in length, are grown in amorphous phase, vertical to the Ti substrate, parallel aligned to each other and uniformly disordering distributed over all the sample surface. Additional remarks about the photoluminiscence properties of the titania nanoporous templates and the magnetic behavior of the Ni filled nanoporous semiconductor Ti oxide template are also included

Tailoring of magnetocaloric response in nanostructured materials: Role of anisotropy

The magnetocaloric response of an ensemble of oriented uniaxial magnetic objects, perpendicularly magne- tized to their easy axes, for temperatures close to the blocking temperature is calculated with the aim of demonstrating that the control of the sample’s microstructure makes up an effective way to tailor its magne- tocaloric response. Coexisting positive and negative magnetocaloric effect (MCE) is found for a model mate- rial with a single magnetic phase transition. Both MCE regimes are controlled by the magnitude of the applied magnetic field. As a proof of concept, experimental results for arrays of self-assembled ferromagnetic nano- wires embedded into highly ordered nanoporous anodic alumina templates are shown, suggesting the validity of the numerical calculations

Temperature gradient-induced magnetization reversal of single ferromagnetic nanowires

In this study, we investigate the temperature- and temperature gradient-dependent magnetization reversal process of individual, single-domain Co39Ni61 and Fe15Ni85 ferromagnetic nanowires via the magneto-optical Kerr effect and magnetoresistance measurements. While the coercive fields (HC) and therefore the magnetic switching fields (HSW) generally decrease under isothermal conditions at elevated base temperatures (Tbase), temperature gradients (ΔT) along the nanowires lead to an increased switching field of up to 15% for ΔT  = 300 K in Co39Ni61 nanowires. This enhancement is attributed to a stress-induced, magneto-elastic anisotropy term due to an applied temperature gradient along the nanowire that counteracts the thermally assisted magnetization reversal process. Our results demonstrate that a careful distinction between locally elevated temperatures and temperature gradients has to be made in future heat-assisted magnetic recording devices

Thermoelectric power factor enhancement by spin-polarized currents – a nanowire case study

In this work, thermoelectric (TE) measurements have been performed on the workhorses of today’s data storage devices, namely nanostructured materials exhibiting either the giant or the anisotropic magnetoresistance effect (GMR and AMR). In particular, the temperature-dependent (50 K - 300 K) and magnetic field-dependent (up to 1 T) TE power factor (PF) has been determined for several Co-Ni alloy nanowires with varying Co:Ni ratios as well as for Co-Ni/Cu multilayered nanowires with various Cu layer thicknesses, which were all synthesized via a template-assisted electrodeposition process. A systematic investigation of the resistivity, (rho), as well as the Seebeck coefficient, S, was performed for Co-Ni alloy nanowires exhibiting AMR and Co-Ni/Cu multilayered nanowires exhibiting GMR. At room temperature, measured values of TE PFs up to 3.6 mWK-2m-1 for AMR samples and 2.0 mWK-2m-1 for GMR nanowires were obtained. Furthermore, the TE PF was found to increase by up to 13.1 % for AMR Co-Ni alloy nanowires and by up to 52 % for GMR Co-Ni/Cu samples in an external applied magnetic field. According to these measurements, the magnetic nanowires exhibit TE PFs that are of the same order of magnitude as TE PFs of Bi-Sb-Se-Te based thermoelectric materials and, additionally, give the opportunity to adjust the TE power output to changing loads and hot spots through external magnetic fields

Special Issue “ALD Technique for Functional Coatings of Nanostructured Materials”

Atomic layer deposition (ALD) is a vapor-phase technique that consists of the alternation of separated self-limiting surface reactions, which enable film thickness to be accurately controlled at the angstrom level, based on the former atomic layer epitaxy method [...

Electroplating and magnetostructural characterization of multisegmented Co54

Highly hexagonally ordered hard anodic aluminum oxide membranes, which have been modified by a thin cover layer of SiO2 deposited by atomic layer deposition method, were used as templates for the synthesis of electrodeposited magnetic Co-Ni nanowire arrays having diameters of around 180 to 200 nm and made of tens of segments with alternating compositions of Co54Ni46 and Co85Ni15. Each Co-Ni single segment has a mean length of around 290 nm for the Co54Ni46 alloy, whereas the length of the Co85Ni15 segments was around 430 nm. The composition and crystalline structure of each Co-Ni nanowire segment were determined by transmission electron microscopy and selected area electron diffraction techniques. The employed single-bath electrochemical nanowire growth method allows for tuning both the composition and crystalline structure of each individual Co-Ni segment. The room temperature magnetic behavior of the multisegmented Co-Ni nanowire arrays is also studied and correlated with their structural and morphological properties

High-Performance 3D Nanostructured Silver Electrode for Micro-Supercapacitor Application

In the current energy crisis scenario, the development ofrenewable energy forms such as energy storage systems among the super-capacitors is an urgent need as a tool for environmental protection againstincreasing pollution. In this work, we have designed a novel 3D nanostructuredsilver electrode through an antireplica/replica template-assisted procedure. Thechemical surface and electrochemical properties of this novel 3D electrode havebeen studied in a 5 M KOH electrolyte. Microstructural characterization andcompositional analysis were studied by SEM, energy-dispersive X-ray spectros-copy, XRD technique, and Kripton adsorption at −198 °C, together with cyclicvoltammetry and galvanostatic charge−discharge cycling measurements,Coulombic efficiency, cycle stability, and their leakage current drops, in additionto the self-discharge and electrochromoactive behavior, were performed to fullycharacterize the 3D nanostructured electrode. Large areal capacitance value of 0.5F/cm2 and Coulombic efficiency of 97.5% are obtained at a current density of 6.4 mA/cm2 for a voltage window of 1.2 V (between−0.5 and 0.8 V). The 3D nanostructured silver electrode exhibits excellent capacitance retention (95%) during more than 2600cycles, indicating a good cyclic stability. Additionally, the electrode delivers a high energy density of around 385.87 μWh/cm2 and apower density value of 3.82 μW/cm2 and also displays an electrochromoactive behavior. These experimental results strongly supportthat this versatile combined fabrication procedure is a suitable strategy for improving the electrochemical performances of 3Dnanostructured silver electrodes for applications as micro-supercapacitors or in electrochemical devicesMinisterio de Ciencia, Innovación y Universidades (MICINN). España PID2019-108075RB-C32/AEI/10.13039/501100011033Principado de Asturias SV-PA-21-AYUD/2021/5139