I. A. Al-Omari

Mechanism of ac conduction in nanostructured manganese zinc mixed ferrites

Mn1−xZnxFe2O4 nanoparticles (x = 0 to 1) were synthesized by the wet chemical co-precipitation technique. X-ray diffraction and transmission electron microscopy and high resolution transmission electron microscopy were effectively utilized to investigate the different structural parameters. The ac conductivity of nanosized Mn1−xZnxFe2O4 were investigated as a function of frequency, temperature and composition. The frequency dependence of ac conductivity is analysed by the power law σ(ω)ac = Bωn which is typical for charge transport by hopping or tunnelling processes. The temperature dependence of frequency exponent n was investigated to understand the conduction mechanism in different compositions. The conduction mechanisms are mainly based on polaron hopping conduction.

Coordination versatility of tridentate pyridyl aroylhydrazones towards iron: tracking down the elusive aroylhydrazono-based ferric spin-crossover molecular materials

The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL(1)) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL(2)) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe(II)(HL)(2)](ClO(4))(2), [Fe(II)L(2)] and [Fe(III)L(2)]ClO(4) (HL = HL(1) or HL(2)) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe(III)(L(1))(2)]ClO(4) is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe(III)(L(1))(2)]ClO(4) and [Fe(III)(L(2))(2)]ClO(4) are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the (6)A(1)↔(2)T(2) transition curve is sigmoidal with T(½)∼280 K and the SCO is virtually complete. As regards [Fe(III)(L(1))(2)]ClO(4), Mössbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state.

Template-Assisted Synthesis and Characterization of Passivated Nickel Nanoparticles

Potential applications of nickel nanoparticles demand the synthesis of self-protected nickel nanoparticles by different synthesis techniques. A novel and simple technique for the synthesis of self-protected nickel nanoparticles is realized by the inter-matrix synthesis of nickel nanoparticles by cation exchange reduction in two types of resins. Two different polymer templates namely strongly acidic cation exchange resins and weakly acidic cation exchange resins provided with cation exchange sites which can anchor metal cations by the ion exchange process are used. The nickel ions which are held at the cation exchange sites by ion fixation can be subsequently reduced to metal nanoparticles by using sodium borohydride as the reducing agent. The composites are cycled repeating the loading reduction cycle involved in the synthesis procedure. X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron microscopy, Energy Dispersive Spectrum, and Inductively Coupled Plasma Analysis are effectively utilized to investigate the different structural characteristics of the nanocomposites. The hysteresis loop parameters namely saturation magnetization and coercivity are measured using Vibrating Sample Magnetometer. The thermomagnetization study is also conducted to evaluate the Curie temperature values of the composites. The effect of cycling on the structural and magnetic characteristics of the two composites are dealt in detail. A comparison between the different characteristics of the two nanocomposites is also provided.

On the dielectric dispersion and absorption in nanosized manganese zinc mixed ferrites

The temperature and frequency dependence of dielectric permittivity and dielectric loss of nanosized Mn(1-x)Zn(x)Fe(2)O(4) (for x = 0, 0.2, 0.4, 0.6, 0.8, 1) were investigated. The impact of zinc substitution on the dielectric properties of the mixed ferrite is elucidated. Strong dielectric dispersion and broad relaxation were exhibited by Mn(1-x)Zn(x)Fe(2)O(4). The variation of dielectric relaxation time with temperature suggests the involvement of multiple relaxation processes. Cole-Cole plots were employed as an effective tool for studying the observed phenomenon. The activation energies were calculated from relaxation peaks and Cole-Cole plots and found to be consistent with each other and indicative of a polaron conduction.

Influence of substrate topography on the growth and magnetic properties of obliquely deposited amorphous nanocolumns of Fe-Ni

We investigated the influence of substrate surface roughness on the structural and magnetic properties of obliquely deposited amorphous nanocolumns of Fe–Ni. Experiments showed that the surface roughness of the substrate greatly determines the morphology of the columnar structures and this in turn has a profound influence on the magnetic properties. Nucleation of Fe–Ni nanocolumns on a smooth silicon substrate was at random, while that on a rough glass substrate was defined by the irregularities on the substrate surface. It has been found that magnetic interaction between the nanocolumns prepared on a silicon substrate was due to their small inter-column separation. Well separated nanocolumns on a glass substrate resulted in exchange isolated magnetic domains. The size, shape and the distribution of nanocolumns can be tailored by appropriately choosing the surface roughness of the substrate. This will find potential applications in thin film magnetism.

Identification of Corrosion Products Due to Seawater and Fresh Water

Mossbauer and X-ray diffraction (XRD) measurements were performed on corrosion products extracted from the inner surface of two different metal tubes used in a desalination plant in Oman. One of the tubes corroded due to the seawater while the second was corroded due to fresh water. The corrosion products thus resulted due to seawater were scrapped off in to two layers, the easily removable rust from the top is termed outer surface corrosion product and the strongly adhered rust as internal corrosion product. The Mossbauer spectra together with the XRD pattern of the outer surface showed the presence of magnetite (Fe3O4), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and hematite (Fe2O3). The inner surface however showed the presence of akaganite, goethite, and magnetite. On the other hand, the corrosion products due to the fresh water showed only the presence of goethite and magnetite. The mechanism of the corrosion process will be discussed based on the significant differences between the formation of the iron components of the corrosion products due to seawater and the fresh water.

Mössbauer and structural studies of Fe0.75Ni0.25−xCrx and Fe0.65Ni0.35−xCrx alloy systems

Abstract X-ray diffraction patterns for Fe 0.75 Ni 0.25− x Cr x show that the samples with x =0 and 0.25 form a single bcc-type phase with a lattice parameter of 2.870 A, while the samples with x =0.05 and 0.10 form the bcc-type phase with an additional γ-fcc phase, as a minor phase with a lattice parameter of 3.585 A. On the other hand all the Fe 0.65 Ni 0.35− x Cr x samples show a single γ-fcc phase. The lattice parameter for the Fe 0.65 Ni 0.35− x Cr x is found to increase linearly with increasing chromium concentration. Mossbauer spectra for Fe 0.75 Ni 0.25− x Cr x show that all the samples are magnetically ordered and some of them have a central paramagnetic line with a very low intensity (between 2 and 15%) for the samples with x =0, 0.05, and 0.10. However, Mossbauer spectra for Fe 0.65 Ni 0.35− x Cr x show that the sample with x =0 is magnetically ordered and the hyperfine field splitting starts to collapse at x =0.05 and disappears completely at x =0.10 and above. The average isomer shift of this system is negative and it decreases with increasing Cr concentration.

Hf Doping Effect on Hard Magnetism of Nanocrystalline Zr

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr18-xHfxCo82 ribbons (x = 0, 2, 4, and 6) have been studied. X-ray diffraction and thermomagnetic measurement results indicated that upon rapid solidification processing four magnetic phases occur: rhombohedral Zr2Co11, orthorhombic Zr2Co11, hcp Co, and cubic Zr6Co23 phases. Microstructure analysis results showed the reduction in the percentage of the soft-magnetic phase (Co) compared to the hard-magnetic phase (Zr2 Co11 (rhombohedral)) with the increase in the Hf concentration. All the samples under investigation have ferromagnetic nature, at 4.2 K and at room temperature. The coercive force (Hc) and the saturation magnetization are (Ms) found to linearly increases with x (x ≤ 2), then Hc slightly increases and Ms slightly decreases with increasing x. The maximum energy product (BH)max at room temperature is found to increases with increasing x reaching a maximum value for x = 4. The magnetocrystalline anisotropy parameter of these samples are calculated to be K = 1.1 MJ/m3 and independent of Hf concentration. The above results indicate that the replacement of Zr by Hf improves the hard-magnetic properties of this class of rear-earth-free nanocrystalline permanent magnet materials.

_{18\hbox{-}{\rm x}}

Abstract We describe a combination of Mossbauer spectroscopic and X-ray structural studies on the alloy system Fe0.7−xTixAl0.3 where x=0, 0.05, 0.1, 0.2, 0.3 and 0.35. The dependence of the Mossbauer hyperfine parameters and the lattice constant on x are determined, and a comparison is made to the known behavior of metallic Fe–Al–T alloys. X-ray diffraction patterns indicate that all the samples are single phase with body center cubic structure. The lattice constant increases linearly with increasing Ti concentration. Room temperature Mossbauer studies show magnetic ordering for small x and paramagnetic behavior for large x. The Mossbauer spectra were fitted by a distribution of magnetic hyperfine fields for small x and two singlets were added for large x. Analysis of the X-ray and Mossbauer results show that the Ti atoms preferentially occupy the Fe site. The average hyperfine field and isomer shift decreased with increasing Ti concentration. The results are related to local environment effects on the hyperfine interactions.

Hf

Spinel-related Sn-doped Fe3O4 has been ball milled for different times up to 35h. Milling was found to transform the material to corundum-related Sn-doped α-Fe2O3. The influence of the milling time, the crystallite size, and the cationic distribution on transformation process is being analyzed with x-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The relatively fast spinel-to-corundum structural transformation observed is associated with more Fe3+ ions being reduced to Fe2+ due to doping with Sn4+ ions.