M. N. Singh

Large dielectric permittivity and possible correlation between magnetic and dielectric properties in bulk BaFeO3−δ

We report structural, magnetic, and dielectric properties of oxygen deficient hexagonal BaFeO3−δ. A large dielectric permittivity comparable to that of other semiconducting oxides is observed in BaFeO3−δ. Magnetization measurements indicate magnetic inhomogeneity and the system shows a paramagnetic to antiferromagnetic transition at ∼160 K. Remarkably, the temperature, at which paramagnetic to antiferromagnetic transition occurs, around this temperature, a huge drop in the dissipation factor takes place and resistivity shoots up; this indicates the possible correlation among magnetic and dielectric properties. First principle simulations reveal that some of these behaviors may be explained in terms of many body electron correlation effect in the presence of oxygen vacancy present in BaFeO3−δ indicating its importance in both fundamental science as well as in applications.

Air annealing effects on lattice structure, charge state distribution of cations, and room temperature ferrimagnetism in the ferrite composition Co2.25Fe0.75O4

The ferrite composition Co2.25Fe0.75O4 has been prepared by chemical coprecipitation route. The as-prepared material has been annealed at different temperatures to investigate thermal activated changes in structural phase, charge states of cations, and room temperature magnetic properties. Synchrotron x-ray diffraction (SXRD) patterns have shown splitting of cubic spinel structure into Fe-rich and Co-rich phases for annealing temperature up to 800 ?C. Single phase cubic spinel structure has been stabilized at annealing temperature 900 ?C. The existence of Fe4+ ions, as confirmed from x-ray absorption near edge structure spectroscopy (XANES), is the new information for spinel ferrite. Raman spectra indicated normal spinel structure. The results of SXRD, XANES and Raman spectra have been used to estimate distribution of Co and Fe ions in spinel structure. The variation in population of Co and Fe ions and phase instability has affected the magnetic properties. The sample annealed at 800 ?C shows maximum coercivity (?567 Oe) and squareness (?0.38), whereas the single-phased sample showed lowest values of ferrimagnetic parameters. The tuning of magnetic parameters by thermal activated structural phase variation of a hetero-structured magnetic system appears to be a new technique for the development of magnetic materials.

Structural investigations on Co3-xMnxTeO6; (0 < x ≤ 2); High temperature ferromagnetism and enhanced low temperature anti-ferromagnetism

In the quest of materials with high temperature ferromagnetism and low temperature anti-ferromagnetism, we prepare Co3-xMnxTeO6; (0 < x ≤ 2) solid solutions. Room temperature structural investigations on these solid solutions as a function of Mn concentration using Synchrotron X-ray diffraction (SXRD) and X-ray absorption near edge structure measurements in corroboration with magnetism are presented. Phase diagram obtained from Rietveld Refinement on SXRD data as a function of Mn concentration indicates doping disproportionate mixing of both monoclinic (C2/c) and rhombohedral (R 3¯) structure for x < 0.5, while only R 3¯ structure for x ≥ 0.5. Further, it shows increase in both lattice parameters as well as average transition metal-oxygen (Co/Mn-O) bond lengths for x ≥ 0.5. Co and Mn K-edge XANES spectra reveal that both Co and Mn are in mixed oxidation state, Co2+/Mn2+ and Co3+/Mn3+. Relative ratios of Co3+/Co2+ and Mn3+/Mn2+ obtained using Linear combination fit decrease with increasing x (for x ≥ 0.5)....

Structural investigations of pulsed laser-deposited NiO epitaxial layers under different fluence values

The epitaxial NiO layers deposited with higher fluence values are found to be strained, and the strain increases with the fluence values. The X-ray diffraction (XRD) profile taken from the synchrotron beam shows the presence of relaxed grains of NiO in addition to the strained grains, where the fraction of relaxed grains gradually increases with the fluence values. The presence of Pendellosung fringes in the XRD profile for the layers deposited at lower fluence values confirms good interfacial and crystalline qualities. As the fluence value is increased, the Pendellosung fringes start merging indicating relatively poor interfacial and crystalline qualities. The NiO layers are of epitaxial nature and grown along [111] direction with two domain structures that are in-plane rotated by 60° with respect to each other. The analysis of local structures from extended X-ray absorption fine structure measurements also indicates that the NiO lattice is strained at higher fluence values. The Ni–O bond distance does not change with the fluence values; however, Ni–Ni bond distance increases with the fluence values in corroboration with XRD results. The surface topography shows island growth of NiO at lower fluence values giving larger roughness, and these islands start merging with an increase in the fluence values leading to relatively smoother layers.

Determination of transition metal ion distribution in cubic spinel Co1.5Fe1.5O4 using anomalous x-ray diffraction

We report anomalous x-ray diffraction studies on Co ferrite with composition Co1.5Fe1.5O4 to obtain the distribution of transition metal ions in tetrahedral and octahedral sites. We synthesize spinel oxide (Co1.5Fe1.5O4) through co-precipitation and subsequent annealing route. The imaginary part (absorption) of the energy dependent anomalous form factor is measured and the real part is calculated theoretically through Kramers–Kronig transformation to analyze anomalous x-ray diffraction peak intensities. Fe and Co K-edge x-ray absorption near edge structure (XANES) spectra are used to estimate charge states of transition metals. Our analysis, within experimental errors, suggests 44% of the tetrahedral sites contain Co in +2 oxidation state and the rest 56% sites contain Fe in +2 and +3 oxidation states. Similarly, 47% of the octahedral sites contain Fe in +3 oxidation states, whereas, the rest of the sites contain Co in +2 and +3 oxidation states. While a distinct pre-edge feature in the Fe K-edge XANES is...

Chemical and Microstructural Analysis of a Tin Coin of Sangam Period

An ancient coin from the Tamirabharani river bed was analysed for microstructure and chemical composition with an objective of understanding the nature of manufacture, and for corroborating information on the trade routes between Rome and South India. Advanced synchrotron X-ray and electron microscopy techniques were employed to evaluate the phases and their crystal structures, microstructure and chemistry. The study helped to identify the process route for the manufacture of the coin.