M. E. Elzain

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.

Studies on Al Kidirate and Kapoeta meteorites

Mössbauer spectroscopy (20–300 K), magnetic susceptibility measurements (77–350 K), scanning electron microscopy and X-ray diffraction experiments have been performed on two meteorite samples: one from an old fall (Kapoeta) and another from a very recent fall (Al Kidirate). The two specimens differ in their mineralogy. Chondrules appear to be absent in Kapoeta and it is probably a pyroxene-plagioclase achondrite with ferrohypersthene as the most abundant mineral. On the other hand, the Al Kidirate meteorite is an ordinary chondrite and the specimen consists of olivine, orthopyroxene, troilite and kamacite. The Mössbauer measurements confirm the above characterization, showing a paramagnetic doublet for the Kapoeta sample and at least two paramagnetic doublets and magnetic sextets for the Al Kidirate specimens. The former were assigned to Fe in pyroxene sites, while the latter was assigned to Fe in pyroxene, olivine, Fe-S and Fe-Ni alloys. The difference in the mineralogy of the two meteorites has also been reflected in the temperature-dependent magnetic susceptibility. The magnetization and the hyperfine interaction parameters will be discussed in relation to the mineralogy.

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.

Magnetic properties of Mn-implanted 6H-SiC single crystal

The electronic and magnetic structures of Mn-doped 6H-SiC have been investigated using ab initio calculations. Various configurations of Mn sites and vacancy types have been considered. The calculations showed that a substitutional Mn atom at either Si or C sites possesses a magnetic moment. The Mn atom at Si site possesses larger magnetic moment than Mn atom at C site. The magnetic properties of ferromagnetically and antiferromagnetically coupled pair of Mn atoms in the presence of vacancies have also been explored. Our calculations show that antiferromagnetically coupled pair of Mn atoms at Si sites with neighboring C vacancy is magnetically more stable. Relaxation effects were also studied. The results are correlated to the measured magnetic properties obtained for Mn-implanted 6H-SiC for various Mn concentrations.

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.

Ferromagnetic iron-chromium alloys

The spin polarized discrete variational method in the local density approximation is used to calculate the electronic structure of 15-atom clusters representing Fe-Cr alloys. The Fe local moment at the central site is found to depend on the orientation (relative to that of Fe) of the Cr moment in the neighbouring shell. For parallel orientation, the Fe moment decreases with increasing number of Cr atoms while it increases slightly in the antiparallel case. The Cr moment dependence on Cr in the NN shell is also determined. These results are incorporated in an approximate relation leading to computation of average moments, and comparison to experimental data is made. It is concluded that there is ferromagnetic coupling between Fe and Cr in the Cr-rich region.

Magnetism in Fe/V superlattices and alloys

Abstract The first principles discrete variational Xα method was used to calculate the electronic structures, magnetic moments and the conduction electron contact spin densites at a central V site in a number of 15 atom clusters representing [110]-interfaces in Fe/V superlattices and alloys. It was found that interactions between the spin-up and spin-down 3d electrons behave differently as the number of Fe atoms, P, in the first coordination shell of a central V atom changes. The 3d magnetic moments at the V sites were found to vary strongly with P, and their variation with respect to changes in the number of Fe atoms in the second shell, Q, was found to depend on P. The sign of 4s and 4p moments was found to change from positive to negative as the average 3d magnetic moment per atom at the first shell sites goes from negative to positive. The conduction electron contact spin densities follow the pattern of the 4s moment as P varies. The results of previously calculated Fe-centered and the present V-centered clusters are combined to study the average magnetic moment in iron-vanadium superlattices and alloys.

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.