Effect of Zn substitution on the structural and magnetic properties of nanocrystalline NiFe2O4 ferrites

Nanocrystalline Ni_1?xZn_xFe₂O₄ (where, x = 0.0, 0.2, 0.4, 0.6, 0.8 & 1) samples were synthesized through solution combustion technique using oxylyl de-hydrazide (ODH) as a fuel and the effect of dopant and its concentration on the structural and magnetic properties was investigated. As-prepared samples were characterized using different characterization techniques such as, XRD, SEM-EDS, TEM and Raman spectroscopy for their phase-purity, crystallinity, surface morphology and elemental composition; also magnetic properties were investigated through EPR, Mossbauer spectroscopy and vibrating sample magnetometer (VSM). Rietveld fitted XRD and Raman studies confirm the formation of cubic spinel structured ferrites and substitution of Zn ion at Ni site without formation of impurity phases. No other structural changes were observed and the structure remains in cubic phase with increase of Zn concentration. SEM and TEM micrographs reveal that the particles are agglomerated and the particles size were found in the nano range. Also good stoichiometric composition was observed in all the compositions of Zn substituted Ni ferrite samples. Magnetic measurements (VSM) reveal that pure Ni ferrites exhibits soft magnetic behaviour. Further the ferromagnetic behaviour suppressed with the substitution of diamagnetic Zn ion and with increase of its concentration in Ni ferrites, which was further evidenced in the Mossbauer spectroscopic results. At room temperature, electronic paramagnetic resonance spectra exhibits a broad resonance signal with Lande's g factor varies from 2.23 to 1.95 with increase in Zn content, which is attributed to spin exchange interactions between Fe³⁺, Ni²⁺ and Zn²⁺ ions also asymmetric EPR spectra was observed. The investigated results show that, Zn substitution has greater impact on the magnetic properties of Ni ferrites due to the diamagnetic nature of Zn, which inturn alters the cationic distribution and the exchange interactions between Ni-Fe and Fe-Fe.     

Room temperature ferromagnetism in hydrothermally grown Ni and Cu co-doped ZnO nanorods

The fabrication and magnetic characteristics of Ni and Cu co-doped ZnO nanorods are reported. The Ni and Cu co-doped ZnO nanorods are synthesized by a facile hydrothermal method. Structural characterizations reveal that Ni and Cu ions enter into ZnO lattices without any secondary phases. SEM images show that they are regular nanorods. Magnetic measurements indicate that the obtained rod arrays exhibit room-temperature ferromagnetic behaviors. The exchange interaction between free delocalized carriers (holes from valence band) and the localized d spins of Ni and Cu ions is considered as the cause of the room-temperature ferromagnetism.     

Comprehensive study of ferromagnetic MgNd2X4 (X = S,Se) spinels for spintronic and solar cells device applications

Full potential LAPW + lo method is used for exploring electronic, structural and thermoelectric properties for MgNd₂X₄ (X = S, Se) spinels that are found to show ferromagnetic-semiconductor behaviour in the spinel structure. The investigated negative value of formation energy and positive value of phonon spectra computed using PBEsol GGA indicates the energetic and dynamical stability of the studied cubic ferromagnetic-semiconductors. We have used TB-mBJ potential functional for electronic and magnetic properties, which lead to a reliable account of electronic structure, demonstrating band occupancy in the spinels along with a clear explanation of density of states. The stability of ferromagnetic state in the studied materials is because of the exchange splitting of Nd cations based on p-d hybridization which is in accordance with the results obtained for electronic band structure and density of states. The exchange splitting of bands can be justified by the spin magnetic moment between anions and cations, and sharing of charge. The computed values of dielectric constant and their associated optical parameters are used to explain the optical active behaviour of the spinels under investigation; indicating that the two spinels studied in the present work are suitable for solar cells device applications. The calculation of thermoelectric properties is very useful for determining a material's potential use in waste energy recovery systems and many other innovative applications.     

Hydrothermal synthesis of uniform Fe-doped Gd(OH)3 nanorods and their magnetic properties: Phase conversion from paramagnetism to ferromagnetism

High quality pure and Fe-doped Gd(OH)₃ nanorods were fabricated through a template-free hydrothermal method for the first time. Analysis of XRD indicates that Fe³⁺ was incorporating in the interstitial sites rather than occupying the substitutional sites, forming pure hexagonal structure of Gd(OH)₃ without any other impurity phase. TEM characterizations show that all the samples perform uniform rod-like morphologies with similar diameter and length, which suggests that the Fe doping has little influence on the morphologies of samples. ICP and XPS spectra suggest that the dopant Fe³⁺ is incorporated into the inner body sites, not on the surface of nanorods. Magnetic studies show that the magnetic phase can be converted from paramagnetism to room-temperature ferromagnetism by doping Fe³⁺ ions into the Gd(OH)₃ nanorods. The saturation magnetization (Ms) is sensitive to the amount of Fe dopants, and the Ms for Fe_0.03Gd_0.97(OH)₃ nanorods reaches the maximum value of 0.184 emu/g. It is considered that the ferromagnetic ordering is possibly originated from the exchange interaction of Fe³⁺ through the oxygen vacancies, leading to the formation of point defect-mediated bound magnetic polarons (BMPs). Ruling out the affect of morphologies and secondary magnetic phase on the magnetic properties, the ferromagnetic ordering in uniform Fe-doped Gd(OH)₃ nanorods, in which the dopant Fe³⁺ is incorporated into the inner body sites of nanorods, are of great importance to deeply understand the rare earth-based DMS/DMD systems and have potential applications in spintronic devices.     

Structural,Thermal and Magnetic Properties of Thin Metal Films and Adsorbate–Substrate Systems Studied by XAFS and XMCD

Thin metal films often exhibit interesting properties that are essentially different from the bulk ones. XAFS (X-ray absorption fine structure) and XMCD (X-ray magnetic circular dichroism) techniques are quite suitable to investigate structural, thermal and magnetic properties of thin metal films. In this proceeding, we will present following two topics concerning structural and magnetic properties of adsorbates on thin metal films. The first one is the adsorption geometry of SO₂ on a 1-monolayer (ML) Pd thin film grown on a Ni(111) single crystal. It was found by S K-edge XAFS that SO₂ is lying flat on 1-ML Pd/Ni(111). This result is not similar to the bulk Pd surface but to the bulk Ni one. This finding indicates significant modification of the electronic structure of the 1-ML Pd film compared to the bulk one. The second topic is the magnetic moment induced on CO adsorbed on Ni epitaxial films grown on Cu(001). The O K-edge XMCD results revealed that in the perpendicularly magnetized 10-ML Ni film the orbital moment of CO is parallel to the substrate Ni magnetization, while it is antiparallel in the in-planar magnetized 6-ML and thick (>100 ML) films. The origin of the induced orbital moment at CO is discussed.     

Synthesis of High Coercivity Core–Shell Nanorods Based on Nickel and Cobalt and Their Magnetic Properties

Hybrid magnetic nanostructures with high coercivity have immense application potential in various fields. Nickel (Ni) electrodeposited inside Cobalt (Co) nanotubes (a new system named Ni @ Co nanorods) were fabricated using a two-step potentiostatic electrodeposition method. Ni @ Co nanorods were crystalline, and they have an average diameter of 150 nm and length of ~15 μm. The X-ray diffraction studies revealed the existence of two separate phases corresponding to Ni and Co. Ni @ Co nanorods exhibited a very high longitudinal coercivity. The general mobility-assisted growth mechanism proposed for the growth of one-dimensional nanostructures inside nano porous alumina during potentiostatic electrodeposition is found to be valid in this case too.     

Transition metal atoms encapsulated in adamantane molecules

We performed a first principles total energy investigation on the structural, electronic, and magnetic properties of 3d-transition metal-encapsulated adamantane molecules (TM@C₁₀H₁₆, with TM = Cr, Mn, Fe, Co, and Ni). We find that the C-C interactions are strong enough to maintain the molecular rigidity upon TM incorporation, although outward relaxations and formation energies are large. We built a microscopic model that explains the electronic structure of those molecules.     

Properties of electroless and electroplated Ni–P and its application in microgalvanics

Composition, microstructure, surface morphology, mechanical properties and electrochemical behaviour of electroless (el) and electroplated (ep) Ni–P deposits are studied using XPS, SEM–EDX, AFM, nanoindentation measurements, cyclic voltammetry and capacitance measurements. Ni–P layers were compared with ep Ni films and bulk Ni. Ni–P layers prepared by both techniques contain 12–14 wt% phosphorus, present in oxidation states of P⁰ and P³⁻. El and ep Ni–P deposits are amorphous and are characterised by a relatively low average surface roughness (2 and 4 nm, respectively). The ep layers possess a rhythmic-lamellar microstructure indicating a periodic change of electrodeposition conditions. The el Ni–P layers do not show such laminated structure but exhibit small surface pores, which are absent in the ep layers. Comparable values for the hardness and the reduced elasticity modulus of el and ep coatings are determined from the nanoindentation data. The observed small differences indicate that the mechanical properties of Ni–P deposits depend not only on the phosphorus content but also on the deposit microstructure. Microelectrochemical measurements with the so-called droplet cell show that the electrochemical behaviour of both el and ep Ni–P coatings is practically identical and does not depend on the location on the sample surface. Evolution of O₂ and H₂ on Ni–P are similar to pure Ni (ep and bulk), but the corrosion resistance in acid solution is much better. The very similar properties and electrochemical behaviour of el and ep Ni–P deposits suggest that both materials are suitable for various applications in microsystem technology. For different substrates and microstructures of different size and geometry, deposition conditions have still to be optimised.     

The passive behaviour of AISI 316 in alkaline media and the effect of pH: A combined electrochemical and analytical study

In this paper, thin surface films were formed on AISI 316 immersed in alkaline solutions of pH ranging from 13 to 9 under open circuit potential conditions. The electrochemical behaviour of the films formed under these conditions was evaluated by different electrochemical techniques: d.c. potentiodynamic polarisation, capacitance measurements (Mott-Schottky approach) and electrochemical impedance spectroscopy. The chemical composition was studied by X-ray photoelectron spectroscopy (XPS).The results reveal a good agreement between the film composition, the electronic properties and the electrochemical behaviour.The analytical results showed that the composition of the surface film changes with the pH. The films become enriched in Cr(III) and Fe(III) species and depleted in Fe(II) and Ni species, as the pH decreases from 13 to 9. The results are consistent with the formation of a film, presenting a bilayer structure, composed of an outer oxide/hydroxide layer, enriched in iron, and an inner anhydrous layer, rich in chromium and nickel oxides.Impedance measurements reveal that the resistance of the surface film increases during immersion and that the pH influences the evolution of the charge transfer resistance and the evolution of the film capacitance.Capacitance measurements based on the Mott-Schottky approach show that the concentration of electronic defects decreases with pH and that the films formed at pH 13 present the highest density of donors and, therefore, a more conductive behaviour.     

Correlation between structure,dielectric and multiferroic properties of lead free Ni modified BaTiO3 solid solution

Present study highlights the influence of Ni doping on the structural, optical, dielectric, ferroelectric and magnetic properties of nanocrystalline BaTi_1-xNi_xO₃ (0 ≤ x ≤ 0.06) ceramics synthesized by sol-gel auto combustion process. Phase identification and crystal structure are examined through Rietveld refinement analysis that ensures mono-phase nature with tetragonal crystal structure in P4mm space group. The observed variation in the structural parameters viz. lattice constants, unit cell volume, bond lengths (Ti-O) and bond angles (Ti-O-Ti) are the direct evidence of distortion produce in the unit cell under the effect of Ni doping. The FTIR studies confirm perovskite structure and presence of stretching/bending vibrations of the various bands present in the samples. The optical properties divulge a minor alteration in optical bandgap under the influence of Ni content. Dielectric studies reveal higher value of the dielectric constant for pristine sample in the low frequency region, but its value decreases on Ni doping. The dielectric response, analyzed through UDR model, exhibits deviation from linear behavior at higher frequencies. Ferroelectric measurements demonstrate that the pristine sample has higher values of remnant polarization (P_r) and maximum polarization (P_m) which decrease linearly with the increase in Ni doping. Magnetic hysteresis loops at room temperature establish a weak ferromagnetic nature of the samples that arises due to the carrier-mediated exchange interactions with smaller values of magnetic parameters. These investigations ensure that the ferromagnetism can be induced in BaTiO₃ by appropriate doping of Ni ions, which may find their potential use in the field of multiferroics.     

Influence of Y-doped induced defects on the optical and magnetic properties of ZnO nanorod arrays prepared by low-temperature hydrothermal process

ABSTRACT: One-dimensional pure zinc oxide (ZnO) and Y-doped ZnO nanorod arrays have been successfully fabricated on the silicon substrate for comparison by a simple hydrothermal process at the low temperature of 90°C. The Y-doped nanorods exhibit the same c-axis-oriented wurtzite hexagonal structure as pure ZnO nanorods. Based on the results of photoluminescence, an enhancement of defect-induced green-yellow visible emission is observed for the Y-doped ZnO nanorods. The decrease of E2(H) mode intensity and increase of E1(LO) mode intensity examined by the Raman spectrum also indicate the increase of defects for the Y-doped ZnO nanorods. As compared to pure ZnO nanorods, Y-doped ZnO nanorods show a remarked increase of saturation magnetization. The combination of visible photoluminescence and ferromagnetism measurement results indicates the increase of oxygen defects due to the Y doping which plays a crucial role in the optical and magnetic performances of the ZnO nanorods.     

In situ-electron spin resonance: a useful tool for the investigation of vanadium phosphate catalysts (VPO) under working conditions

Differently prepared (VO)₂P₂O₇ phases and an amorphous V³⁺PO catalyst were investigated under conditions of the selective n-butane oxidation and the toluene ammoxidation, respectively, using a self-constructed in situ-ESR flow reactor in the X-band. By examining the temperature dependence and the line shape of the ESR signals exchange integrals as well as the 2nd and the 4th moment were obtained. These parameters characterize the spin—spin exchange behaviour and, thus, structural and electronic disorder of the catalysts. Increasing structural disorder was found to improve the catalytic performance in the n-butane oxidation. For both catalytic processes a significant reversible alteration of the ESR line shape was observed under working conditions which is discussed in terms of a perturbation of exchange interactions between neighbouring vanadyl centres near the surface the oxidation state of which is assumed to fluctuate between +4 and +5 during the catalytic reaction.     

Synchrotron based x-ray absorption spectroscopy investigation and temperature dependent ferroelectric properties of Ni doped BaTiO3 nanostructures

In this report, the local structure geometry of BaTi_1-xNi_xO₃ (0.0 ≤ x ≤ 0.06) ceramics has been studied through synchrotron based x-ray-absorption spectroscopy, measured at the Ti K, Ba L₃ and Ni K-edges at room temperature. The x-ray absorption near edge spectroscopy (XANES) at Ti K-edge confirms the TiO₆ octahedron geometry of Ti atom due hybridization between Ti(3d)-O(2p) orbitals and signifies the non-centrosymmetric nature of BaTiO₃ samples. However, Ti–O off-center displacement (non-centrosymmetric) is disturbed under the effect of Ni doping as a result of oxygen vacancies formation. More interestingly, XANES studies at Ni K-edge ensure successful substitution of Ni in BaTiO₃ as Ni²⁺ ions. Extended x-ray absorption fine structure (EXAFS) data at Ni K-edge have been fitted to estimate pertinent local structural parameters of the Ni–O, Ni–Ba, Ni–Ti and Ni–Ni shells (viz. bond lengths and disorder parameters) and it also reveal that the structural disorders around the Ti sites in the doped BaTiO₃ expand with Ni doping. The mixed-valence states of titanium ion, i.e., Ti⁴⁺ and Ti³⁺ in the doped samples were established with the x-ray photoelectron spectroscopy (XPS). Moreover, XPS divulges the creation of oxygen vacancies due to Ni doping in BaTiO₃ matrix. The complementary information about the lattice vibration is analyzed through Raman studies that approve the softening of the transverse optical (TO) mode present at 515 cm^?1. The temperature dependent ferroelectric studies affirm that the ferroelectricity vanishes in the doped samples due to the decrement in the off-center displacement between the Ti⁴⁺ and O^2? ions in the TiO₆ octahedral geometry. Differential thermal analysis (DTA) exhibits a ferroelectric tetragonal to para-electric cubic phase transition in the pristine as well as Ni doped BaTiO₃ samples.     

Electronic structure engineering of 2-D MoS2 sputtered thin films under ion beam irradiation: Induced by controlled defect generation

TMDs (Transition-metal dichalcogenides) come out in contemporary years as a remarkable class of two-dimensional (2D) materials and have allured enormous consideration. In the class of TMD materials, Molybdenum disulfide (MoS₂) has unveiled encouraging applications in the domain of photonics, electronics, electrochemistry and energy. Specifically, the defects originated in MoS₂ play a significant role in modifying the magnetic, electronic, catalytic and optical peculiarities of MoS₂, depicting an applicable way in modifying the efficiency of MoS₂ based devices. The course through which lattice defects influence the MoS₂ peculiarities are unresolved. In the present work, we present comprehensively how lattice defects impact the electronic structure of MoS_2. We have probed the prospect to employ swift heavy ion irradiation for nano-structuring of sputtered 2D MoS₂ thin films. Our extensive study of ion instigated structural, optical and morphological alterations in MoS₂ thin films manifests that resting on the parameters of irradiation profusion of defects can be established. Theoretically, the optical bandgap also has been determined using Density functional theory (DFT). We grow the thin films of MoS₂ by the sputtering method and induced the defect in MoS₂ thin films by ion irradiation. We evaluated how these defects impact the electronic structure of MoS₂ thin films by measurements taken from Raman, X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), UV–visible spectroscopy, Atomic force microscopy (AFM), X-ray diffraction (XRD) and Rutherford backscattering spectroscopy (RBS).     

Vertically oriented nickel nanorod/carbon nanofiber core/shell structures synthesized by plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition, without a nickel-containing gaseous precursor, was used to synthesize continuous nickel (Ni) nanorods inside the hollow cavity of carbon nanofibers (CNFs), thus forming vertically aligned Ni/CNF core/shell structures. Scanning and transmission electron microscopic images indicate that the elongated Ni nanorods originate from the catalyst particles at the tips of the CNFs and that their formation is due to the effect of extrusion induced by the compressive force of the graphene layers during growth. Different from previous work, each vertically-aligned core/shell structure reported is totally isolated from its neighbors. Continuous Ni nanorods are found to separate into smaller ones with increasing growth time, which was ascribed to (i) the limited amount of Ni available in the tip of the CNF, (ii) the polycrystalline nature of the Ni nanorods and (iii) the combined effects of the compressive stresses on the side of the Ni nanorods and of the tensile stress along their axis.     

Novel growth of carbon nanotubes on nickel nanowires

A novel growth phenomenon is presented in this paper where carbon nanotubes (CNT) were grown successfully on nickel (Ni) nanowire using chemical vapour deposition technique. The decomposed carbon from ethylene diffused through the surface of nanowires and precipitated into hollow cylindrical carbon structures. Nanotubes of various lengths are found to have grown along the length of the outer side of the nanowires and were firmly rooted to their walls. The presence of a thin layer of oxide (~ 3 nm) on the top surface of nanowires is believed to have promoted the growth of CNT. Raman, X-ray photoelectron (XPS) and electron energy loss spectroscopy (EELS) were conducted in order to understand the formation of nanotubes and verify their presence, their level of crystallinity and chemical bonding structure with nanowires. This hybrid nanostructure is also found to have ferromagnetic behaviour, which can be applied in devices such as magnetic sensors and spintronic devices that combine the unique properties of CNT and Ni nanowires.     

Structural,Optical, and Magnetic Properties of Nickel‐Doped Tin Dioxide Nanoparticles Synthesized by Solvothermal Method

We have successfully synthesized Sn_1?xNi_xO₂ (0.05 ≤ x ≤ 0.15) solid solutions in order to study their structural, optical, and magnetic properties at different Ni concentrations. X‐ray diffraction showed monophasic and crystalline tetragonal structure. The shifting of peaks toward higher angle is attributed to the incorporation of Ni²⁺ ions in SnO₂ host lattice. Particle growth restrained upon Ni‐doping and found to be in the range of 8–12 nm. Ni‐doped SnO₂ nanoparticles show blue shift in band gap studies, which is found to be in the range of 3.9–4.1 eV. High surface areas have been achieved for these solid solutions, which come out to be 130, 200, 457, 497, and 680 m²/g, respectively. The solid solutions exhibit paramagnetic behavior along with antiferromagnetic exchange coupling.     

Ultrasonic-assisted synthesis of Pd–Ni alloy catalysts supported on multi-walled carbon nanotubes for formic acid electrooxidation

Pd–Ni alloys with different compositions (i.e. Pd₂Ni, PdNi, PdNi₂) dispersed on multi-walled carbon nanotubes (MWCNTs) are prepared by ultrasonic-assisted chemical reduction. The X-ray diffraction (XRD) patterns indicate that all Pd and Pd–Ni nanoparticles exist as Pd face-centered cubic structure, while Ni alloys with Pd. The transmission electron microscopy (TEM) images show the addition of nickel decreases the particle size and improves the dispersion. The X-ray photoelectron spectroscopy (XPS) spectra demonstrate the electronic modification of Pd by nickel doping. The electrochemical measurements reveal that the PdNi catalysts have better catalytic activity and stability for formic acid electrooxidation, among them PdNi/MWCNTs is the best. The performance enhancement is ascribed to the increase of electroactive surface area (EASA) and nickel doping effect which might modify the electronic structure.     

Synthesis and characterization of Mn-doped ZnO nanorods grown in an ordered periodic honeycomb pattern using nanosphere lithography

We report a study of the structural, optical and magnetic properties of undoped and Mn-doped ZnO nanorods grown by chemical bath deposition in a periodic honeycomb lattice formation. Mn-doping is accomplished by a diffusion process at a constant time of 8 h for different temperatures of 500 °C, 600 °C and 700 °C. Undoped and Mn-doped ZnO nanorods had a hexagonal wurtzite structure with a (0 0 2) preferred orientation. From SEM results, it was seen that Mn-doped ZnO nanorods grew vertically in the honeycomb lattice with lengths of 0.8 μm. XPS results showed that Mn³⁺ ions was successfully incorporated in the ZnO matrix by substituting for Zn²⁺ ions and that Mn-doping increased the number of oxygen vacancies in ZnO compared to undoped ZnO. This result was also supported by photoluminescence data at 10 K. Magnetic data showed that all the samples exhibited ferromagnetic character. Although the origin of undoped ZnO is related to oxygen vacancy-induced d⁰ ferromagnetism, bound magnetic polarons are responsible from the ferromagnetism of Mn-doped ZnO samples which have T_c values above the room temperature.     

Local Density Approximation Total Energy Calculations for Silica and Titania Structure and Defects

Theoretical structural predictions are presented for three polymorphs of silica (α-quartz, α-cristobalite, and stishovite) and the rutile structure of titania. Several defects within the silica system (the oxygen vacancy, a puckering distortion in quartz with no oxygen vacancy, and titanium in silica) are also studied. These ab initio studies are conducted using the local density approximation for electronic structure and total energies and are implemented using some very recent advances in improved pseudopotentials and energy minimization algorithms. Results include two metastable conformations of the oxygen vacancy in silica, similar Si—O and Ti—O bond energies, and equilibrium structures of several defects.