Size distribution and frustrated antiferromagnetic coupling effects on the magnetic behavior of ultrafine akaganéite (β-FeOOH) nanoparticles

The magnetic properties of low dimensional materials of several iron oxyhydroxide phases, such as akaganéite (β-FeOOH) or lepidocrocite (γ-FeO(OH)), remain poorly explored, probably due to their specific preparation as single crystalline phase requires special conditions owing to their structural instability. In the present work, ultrafine akaganéite nanoparticles were prepared by the hydrolysis of FeCl3 solutions at room temperature induced by the presence of NaOH. The resulting product was characterized by several analytical techniques. Structural investigations using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) revealed that the sample was mainly constituted by rather-equiaxial akaganéite nanocrystals with mean diameter of 3.3 ± 0.5 nm. In addition, a small amount of rodlike akaganéite particles with 23 ± 5 nm in length and 5 ± 1 nm in width was also detected. The study of the respective dependences of the dc magnetization and the ac susceptibility on temperature and exciting magnetic field revealed complex magnetic relaxation processes, high coercivity values at low temperature, and exchange bias effect. These results have been tentatively explained considering size distribution effects and the presence of superparamagnetic and spin glass-like contributions arising from the frustration of the antiferromagnetic order owing to surface effects and an insufficient filling of the akaganéite channels with Cl- anions.Financial support from the Mexican Council of Science and Technology (CONACYT) and Universidad Autonoma de Nuevo León under research projects CB12-179486 and PAICYT-CE793-11, respectively, is acknowledged. Spanish funding under the MINECO research project MAT2010-20798-C05-04 and FEDER is also acknowledged.Peer Reviewe

Ni-Co Alloy and Multisegmented Ni/Co Nanowire Arrays Modulated in Composition: Structural Characterization and Magnetic Properties

Design of novel multisegmented magnetic nanowires can pave the way for the next generation of data storage media and logical devices, magnonic crystals, or in magneto-plasmonics, among other energy conversion, recovery, and storage technological applications. In this work, we present a detailed study on the synthesis, morphology, structural, and magnetic properties of Ni, Co, and Ni-Co alloy and multisegmented Ni/Co nanowire

Optical characterization of ALD-coated nanoporous alumina structures: effect of sample geometry or coated layer material

Optical characterization of nanoporous alumina-based structures (NPA-bSs), obtained by ALD deposition of a thin conformal SiO2 layer on two alumina nanosupports with different geometrical parameters (pore size and interpore distance), was performed by two noninvasive and nondestructive techniques such as spectroscopic ellipsometry (SE) and photoluminescence (Ph) spectra. SE measurements allow us to estimate the refraction index and extinction coefficient for the studied samples and their dependence with wavelength for the 250–1700 nm interval, showing the effect of sample geometry and cover-layer material (SiO2, TiO2, or Fe2O3), which significantly affect the oscillatory character of both parameters, as well as changes associated with the light incidence angle, which are attributed to surface impurities and inhomogeneity. Photoluminescence curves exhibit a similar shape independently of sample pore-size/porosity, but they seem to affect intensity values. This analysis shows the potential application of these NPA-bSs platforms to nanophotonics, optical sensing, or biosensing.Partial funding for open access charge: Universidad de Málag

Pseudo-monocrystalline properties of cylindrical nanowires confinedly grown by electrodeposition in nanoporous alumina templates

Four different cylindrical nanowire systems (nickel, cobalt, Co23Cu77, and multisegmented Co58Ni42/Co83Ni17 nanowires) with single-crystal-like properties were characterized by transmission electron microscopy and selected-area electron diffraction (SAED) under different tilting angles. Although these nanowires have different chemical compositions, crystalline structures and/or diameters, they exhibit similar behaviors, which are unexpected for ideal single-crystals. All the samples presented SAED patterns that did not experience changes from one zone axis to another when the nanowire was tilted in a wide range of angles, exhibiting related apparent interplanar distances that are dependent on the nanowire inclination, yielding deformed patterns that can be unrecognizable. Moreover, face-centered cubic nanowires presented classically forbidden reflections. These behaviors were explained by considering the characteristics of the measurement technique and the confined template-assisted growth, which force the atoms to be accommodated in a cylindrical volume with nanoscale dimensions, yielding the frustrated formation of stable facets and right angles in the nanowire radial directions, together with the formation of stacking faults

Optical and Electrochemical Characterization of Nanoporous Alumina Structures: Pore Size, Porosity, and Structure Effect

Three nanoporous alumina structures (NPASs) obtained by the two-step anodization method were optically and electrochemically characterized. Two of the structures were symmetric (NPAS-Sf and NPAS-Ph) and one was asymmetric (NPAS-And); pore size ranged from 10 nm to 100 nm and porosity was 12% in the case of the symmetrical NPAS and 23% and 30% for each surface of the asymmetric structure NPAS-And(A) and (B), respectively. Optical parameters of the studied samples (refraction index and extinction coefficient) were obtained from ellypsometric spectroscopy measurements carried out for wavelengths ranging between 250 nm and 1700 nm (visible and near infrared regions), with the total average refraction indices being 1.54, 1.52, 1.14, and 1.05 for NPAS-Sf, NPAS-Ph, NPAS-And(A), and NPAS-And(B), respectively, which indicates porosity control of refraction index values. Electrochemical characterizations (concentration potential and impedance spectroscopy measurements) were performed with NaCl solutions, and they allowed us to estimate samples of effective fixed charge concentration (1.22 × 10−2 M, 1.13 × 10−3 M, and 1.15 × 10−3 M), ion transport numbers, permselectivity (33.0%, 3.1%, and 9.6%), and the electrical resistance of each solution/sample system as well as the interfacial effects associated to solution concentration–polarization, which seems to be mainly controlled by pore size and sample symmetry

Effect of Sharp Diameter Geometrical Modulation on the Magnetization Reversal of Bi-Segmented FeNi Nanowires

Controlling functional properties of matter and combining them for engineering a functional device is, nowadays, a common direction of the scientific community. For instance, heterogeneous magnetic nanostructures can make use of different types of geometrical and compositional modulations to achieve the control of the magnetization reversal along with the nano-entities and, thus, enable the fabrication of spintronic, magnetic data storage, and sensing devices, among others. In this work, diameter-modulated FeNi nanowires are fabricated paying special effort to obtain sharp transition regions between two segments of different diameters (from about 450 nm to 120 nm), enabling precise control over the magnetic behavior of the sample. Micromagnetic simulations performed on single bi-segmented nanowires predict a double step magnetization reversal where the wide segment magnetization switches near 16 kA/m through a vortex domain wall, while at 40 kA/m the magnetization of the narrow segment is reversed through a corkscrew-like mechanism. Finally, these results are confirmed with magneto-optic Kerr effect measurements at the transition of isolated bi-segmented nanowires. Furthermore, macroscopic vibrating sample magnetometry is used to demonstrate that the magnetic decoupling of nanowire segments is the main phenomenon occurring over the entire fabricated nanowires

Tuning Nanohole Sizes in Ni Hexagonal Antidot Arrays: Large Perpendicular Magnetic Anisotropy for Spintronic Applications

In order to determine the compressive and tensile strength of concrete under conditions of explosive loading, and to develop a methodological framework in this regard, three types of concrete have been investigated: concrete with fine-grained granite in the form of crushed stone having a static compressive strength of 47 MPa, the same concrete with the addition of steel fibers and also the same concrete reinforced by steel bars. The samples were rods of 50 and 100 mm diameter and five to ten diameters in length. The compressive fracture occurs at a relatively small distance of propagation of the load pulse along the rod and is accompanied by fast decay. The measurements of parameters of the compression pulse at the end of the fracture zone allowed us to determine the values of the dynamic compressive strength of the concrete while measurements of the free-surface velocity history at long distances were used for determining the dynamic tensile strength or spall strength values. The values of the dynamic compressive strength were found to be 2.5 times higher than the static strength. The steel fibers increased the dynamic compressive strength by about 10%. The obtained values of the dynamic tensile strength are 3–8 times higher than the values of the static tensile strength. The steel fibers increase the tensile strength by 20–50%. The reinforced sample has shown an increase of dynamic tensile strength by a factor of about 30

Influence of ALD Coating Layers on the Optical Properties of Nanoporous Alumina-Based Structures

Optical changes associated with the surface coating of different metal oxides and nanolayers by the ALD technique of a nanoporous alumina structure (NPAS) obtained by the two-step anodization method were analyzed. The NPASs were coated with: (i) a single layer (SiO2 or TiO2), and (ii) a double layer of SiO2 plus Al2O3 or aluminum doped ZnO (AZO) to estimate the effect of surface layer coverage material, geometrical parameters (pore-size/porosity), and number of layers on light transmission/reflection. Chemical surface characterization of the different NPASs was carried out by analyzing XPS spectra, which allowed us to obtain an estimation of the coating layer homogeneity. Transmittance and spectroscopic ellipsometry measurements were analyzed in order to detect changes in characteristic optical parameters such as band gap, refractive index, and extinction coefficients associated with the material and the characteristics of the single or double coating layers

Surface Modification of Nanoporous Anodic Alumina during Self-Catalytic Atomic Layer Deposition of Silicon Dioxide from (3-Aminopropyl)Triethoxysilane

Changes associated to atomic layer deposition (ALD) of SiO2 from 3-aminopropyl triethoxysilane (APTES) and O3, on a nanoporous alumina structure, obtained by two-step electrochemical anodization in oxalic acid electrolyte (Ox sample) are analysed. A reduction of 16% in pore size for the Ox sample, used as support, was determined by SEM analysis after its coverage by a SiO2 layer (Ox+SiO2 sample), independently of APTES or O3 modification (Ox+SiO2/APTES and Ox+SiO2/APTES/O3 samples). Chemical surface modification was determined by X-ray photoelectron spectroscopy (XPS) technique during the different stages of the ALD process, and differences induced at the surface level on the Ox nanoporous alumina substrate seem to affect interfacial effects of both samples when they are in contact with an electrolyte solution according to electrochemical impedance spectroscopy (EIS) measurements, or their refraction index as determined by spectroscopic ellipsometry (SE) technique. However, no substantial differences in properties related to the nanoporous structure of anodic alumina (photoluminescent (PL) character or geometrical parameters) were observed between Ox+SiO2/APTES and Ox+SiO2/APTES/O3 samples