A. A. Yousif

A Mossbauer and X-ray diffraction investigation of Li-Ti ferrites

Li0.5+0.5xTixFe2.5-1.5xO4, x 0.3. Similar distribution profiles have been deduced using the latter model. An asymmetric distribution is observed for x<or=0.4, with the hyperfine fields derived from the Lorentzian fit falling inside the distribution. As x increases a peak at lower hyperfine fields develops and becomes more prominent as x increases. The influence of Li and Ti substitution on the hyperfine field is discussed.

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.

A Comparative Quantitative Study of an Omani Soil Using X-Ray Diffraction Technique

Two methods for the quantification of clay minerals using X-ray diffraction techniques: the Constant Mineral Standards Method and the Constant Clay Method were applied to soil samples from Al-Khod (Northern Oman). The first method was based on the addition of different concentrations of clay to constant mineral standards while the latter was based on the additions of known internal standards to the clay sample. The clay investigated in this study contained montmorillonite, palygorskite, illite and kaolinite. The areas of the reflection peaks were found to relate linearly with additions of different proportions of clay or mineral standards. Using the two methods, the concentrations of the different components and their experimental errors were determined. From the errors and self consistency of the results obtained, and the practicality of sample preparation, it is proposed to use the Constant Mineral Standards Method.

Mössbauer and magnetic investigation of Fe-Mn alloy

Mössbauer, X-ray, magnetization and susceptibility measurements were performed to study Fe100−xMnx,x=5, 15, 39, 50. The different phases of Fe-Mn were identified, and hyperfine interaction parameters and average magnetic moments of some samples were determined. The average hyperfine field and average magnetic moment decrease asx increases. The influence of the Mn neighbourhood on the derived parameters is discussed in the light of calculations using the first principle discrete variational method in the local density approximation.

Hyperfine field and isomer shift in ferromagnetic Fe−Cr alloys

The valence contact spin and charge densities at Fe sites in ferromagnetic Fe−Cr alloys are calculated using the discrete variational method. The hyperfine field at Fe nucleus is expressed as a linear sum of a core term, that is proportional to the local 3d moment, and a valence term, which is proportional to the valence spin density. The dependence of the hyperfine field, the contact charge density and the 4s magnetic moment on the number and orientation of chromium atoms in the first and second shells is studied. Comparison to experimental data is made.

Structural and magnetic analysis of the transformation of Sn-doped magnetite to Sn-doped hematite by mechanical milling

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.

The electronic and magnetic structure of bcc iron-nickel alloy

The first principles discrete variational method and the local density approximation are used to calculate the electronic and magnetic properties of clusters of iron and nickel atoms representing the bcc iron-nickel alloy with nickel concentration up to about 30%. It is found that the presence of nickel atoms at sites neighbouring iron increases the iron local magnetic moment but decreases the magnitude of the magnetic hyperfine field. The magnetic hyperfine field at the iron site increases with increasing number of nickel atoms at the next-nearest-neighbour sites. Consequently, the experimentally observed maximum in both magnetization and hyperfine field versus nickel concentration is attributed to different iron sites. The 3d local densities of states at iron and nickel sites are calculated. The average density of states is found to remain unaltered for the majority sub-band, whereas it exhibits large deformation for the minority sub-band. An energy diagram is deduced from the density of states and is used to explain the formation of magnetic moment at iron and nickel sites. We deduce the following from these calculations. The observed increase in the local magnetic moment in the iron site is attributed to the reduction in the weight of the bonding d states and the accompanying constancy of the weight of the antibonding states, which ar driven by sd interaction.

Lattice parameters and magnetic properties of F-Sb-Te alloys with nickel arsenide structure

Abstract Single phase Fe-Sb-Te alloys with NiAs-type structure have been prepared in the composition range from Fe 1.173 Sb to Fe 1.105 Te and characterized by X-ray diffraction. Magnetic susceptibility measurements have been taken on these alloys in the temperature range from 77 up to 500 K. The lattice parameters show a linear variation with the alloy composition in the Sb-rich side but an anomalous change for the binary Fe 1.105 Te. The effective magnetic moments extracted from the susceptibility variations with temperature show a similar anomaly at approximately the same composition. These are discussed in terms of a model of additive magnetic moments of different atomic species.

Mössbauer studies on Al−Co ferrites

Mössbauer spectroscopy studies have been performed on the spinel CoAlxFe2−xO4 (2<-x<-1.7) in the temperature range 77–750 K using either a liquid nitrogen bath cryostat or a furnace. The samples are magnetic at 77 K giving spectra that have magnetic sextets coexisting with a central line which increases in population with the Al-content indicating relaxation effects. The data shows that Al possesses no preference to either tetrahedral or octahedral sites of the ferrite over the whole range of concentration. The Mössbauer hyperfine interaction parameters and magnetic transition temperatures were determined. As expected the hyperfine field and Curie temperature decrease when the Al-content increases.

Magnetic properties of nanocrystalline FexCu1-x alloys prepared by ball milling

X-ray diffraction, Mossbauer and magnetization measurements were used to study FexCu1−x alloys prepared by ball-milling. The X-ray data show the formation of a nanocrystalline Fe—Cu solid solution. The samples with x ≥ 0.8 and x ≤ 0.5 exhibit bcc or fcc phase, respectively. Both the bcc and fcc phases are principally ferromagnetic for x ≥ 0.2, but the sample with x = 0.1 remains paramagnetic down to 78 K. The influence of the local environment on the hyperfine parameters and the local magnetic moment are discussed using calculations based on the discrete-variational method in the local density approximation.