V. Suresh Babu

Modeling of disorder and X-ray diffraction in coal-based graphitic carbons

Abstract In this work, X-ray diffractograms of about 20 coal-based graphites and carbons, with different degrees of graphitic order, have been modeled using the recent computational model of Shi et al. This model relates the crystalline parameters c (2d002) and a and the crystallite sizes Lc and La, to the parameters of the disorder present in carbons. These disorder parameters include the probability P for a random displacement δ between adjacent layers, the probability Pt for finding a 3R stacking fault, the strain parameters, and the preferred orientation. Plots among d002, Lc, and La lead us to conclude that a step transition exists in graphites near d 002 - = 3.37 A where a two-fold increase in Lc is observed for a fixed La. This observation of a step at 3.37 A is consistent with the recent observation of Bragg et al. in a graphite where disorder was produced by successive grinding, thermal treatments and neutron irradiation. A theoretical interpretation of this step-transition is however not yet available. Plots of d002 against 〈δ2〉, 2P, (2P + Pt) are discussed in terms of the disorder models proposed by Franklin and by Ruland. The preferred orientation evaluated from the fit varies systematically with the ratio L c L a and the experimental X-ray anisotropy ratio.

The linear temperature dependence of the paramagnetic resonance linewidth in the manganate perovskites and

The temperature dependence of the static magnetic susceptibility (5 to 700 K) and that of the electron paramagnetic resonance linewidth (300 to 700 K) measured at 9.25 GHz is reported for bulk polycrystalline samples of and with negative giant magnetoresistance. For both systems, and the product increase linearly with temperature above . Following the analysis of Huber and Seehra for , it is argued that this linear temperature dependence of is most probably due to contributions from spin - phonon interaction. This result supports the recent suggestions of the importance of lattice effects in the magnetism of these oxides.

High-temperature thermal expansion of binary alloys of Ni with Cr, Mo and Re: a comparison with molecular dynamics simulations

In this paper, thermal expansion studies of the fcc binary alloys (M = Cr, Mo, Re) for from ambient temperature to 900 K are reported. The results of the changes in the lattice constant and the coefficient of thermal expansion with x are compared with the recent molecular dynamics (MD) simulations of Mei, Cooper and co-workers providing the first detailed comparison of this type between experiment and theory over an extended temperature and composition range. For NiMo, good agreement is obtained between experiment and theory for the increase in with x, the decrease of with x, and the temperature variation of . For NiCr, the agreement is considered to be fair for the increase of with x, the decrease of with x, and the versus temperature data. For the NiRe alloys, the observed decrease of with x (say near 900 K) is the opposite to the MD simulation prediction of an increase of with x in this system, although the theoretical predictions at higher temperatures (say 1500 K) tend to be in line with the experimental observations at 900 K. It is argued that effects of magnetic ordering are not responsible for the observed departure from the MD simulation prediction in the NiRe system. A rule-of-mixtures equation gives a linear decrease of with x for the alloys, in qualitative agreement with the experimental results.

Magnetic properties of radio frequency sputtered thin films of La–Pb–Mn oxides

We report the synthesis by radio frequency sputtering and characterization of thin films of La0.74Pb0.26MnOz on Si, the substrate of choice for device integration. Textured films with (110) orientation were grown on (100) Si and the films are ferromagnetic below 325 K. Resistivity ρ data show a metal‐to‐semiconductor transition at Tms=250 K. The ferromagnetic resonance linewidth shows a discontinuity at Tms. Data on magnetoresistance MR=[ρ(0)−ρ(H)]/ρ(0) versus temperature indicate a maximum value of 22% at Tms for a static magnetic field of 2 T and MR values of 15%–22% are observed over the entire temperature range 5–300 K.

Magnetic properties of amorphous BiFeO3‐PbZrO3 sputtered films

Amorphous thin films of (1−x)BiFeO3‐xPbZrO3 for x=0.1–0.9 have been prepared by the technique of rf sputtering and the effects of sputtering conditions and annealing at high temperatures on magnetic properties of such films have been studied through magnetization measurements. A sintered polycrystalline target was used to sputter 0.2–1.4‐μm‐thick films in oxygen, argon, and mixed oxygen and argon atmospheres onto glass and silicon substrates. As‐sputtered films are x‐ray amorphous and show paramagnetism at room temperature. A hard‐ferromagnetlike character with remanence is observed in films annealed at temperatures Ta above 500 °C. The room‐temperature magnetization increases with further increase in Ta and shows a maximum for Ta = 650 °C. A partial crystallization occurs for Ta ≳ 650 °C and is accompanied by a decrease in the magnetization. The magnetization for annealed samples shows a dependence on x, with a minimum of 0.6 emu/g for x=0.9 and a maximum of 40 emu/g for x=0.5.

Temperature dependence of the infrared spectra of C60: orientational transition and freezing

Abstract The temperature dependence (8–350 K) of the position and intensity of the IR modes of C 60 near 526, 576, 1182 and 1430 cm −1 is reported. Data indicate a phase transition T 0 near 260 K and support the model of a glassy state below ≈ 100 K predicted recently by molecular dynamic calculations. The first-order nature of T 0 is smeared by impurities in the sample.

Site selectivity of Mn atoms in γ–TiAl alloys determined by x-ray scattering

The relative variations of the intensities of the superlattice peaks in the x-ray scattering of Ti 50−x Mn x Al 50 alloys (x=0,0.77,1.85, and 3.30) are used to determine the site occupancy by Mn atoms. Up to x=1.85, Mn occupies the Ti sites whereas for x=3.30, some Al sites may also be occupied in the L1 0 structure of γ-TiAl alloys

Rapid loss of magnetic order in Ni on alloying with Cr, Mo, Re, and Si

In this article, the variation of the Curie temperature Tc and the magnetic moment μ of Ni in the fcc alloys Ni1−xMx (M=Cr, Mo, Re, and Si) are reported for concentrations x between 0 and 15 at. %. The decrease of Tc and μ with x reported earlier for NiCr alloys are also observed for the NiMo, NiRe, and NiSi alloys. The initial rates of decreases are about 60 K/at. % for Tc and 0.05 μB/at. % for μ, leading to critical concentration xc≂13% above which ferromagnetism disappears. Qualitatively, these decreases may be understood in terms of transfer of outer shell electrons of M atoms to the 3d↓ band of Ni in the alloys. However, a quantitative calculation of xc is not yet available.

Magnetic and magnetoresistance studies on radio frequency sputtered La–Pb–Mn–O films

Results on structural and magnetic characterization of radio frequency sputtered thin films of La0.74Pb0.26MnOz on (100) Si are presented. Stoichiometric films with a thickness of 0.6 μm showing (110) textured growth were deposited in a mixed argon–oxygen atmosphere. The low temperature magnetization is 10% smaller than the expected value for a collinear ferromagnetic order in the oxide. Zero‐field resistivity data show a metal‐to‐semiconductor transition centered at Tms=250 K. Data on magnetoresistance (MR)=[ρ(0)−ρ(H)]/ρ(0) versus temperature reveal the following important features: (i) a maximum value of 22% for H=2 T at the metal‐to‐insulator transition temperature, (ii) MR value of 15%–22% over the entire temperature range 4.2–300 K, and (iii) a substantial MR, ∼10% even at temperatures as high as 380 K, well above the Curie temperature. Data on the temperature dependence of ferromagnetic resonance linewidth at 9.4 GHz show a discontinuity at Tms.

Measurements and control of anisotropy in ten coal-based graphites

Abstract Using a solvent extraction process to produce high-purity extracts from untreated and hydrogenated coal, ten coal-based graphites with different levels of anisotropy have been produced. Anisotropy of these graphites and that of H-451 (a petrocoke-based commercial graphite) has been assessed by means of the intensity of the (002) line in X-ray diffraction (XRD), by magnetic susceptibility (χ), by the coefficient of thermal expansion (α), and by electrical conductivity (σ) studies. Measurements were made along directions longitudinal ( l ) and transverse (t) to the extrusion direction. The XRD ratio R for the (002) line varies between 0.23 for the most anisotropic and 0.94 for the least anisotropic of the graphites. χ l χ t and α l α t for various graphites increase systematically as R increases, although the variations are not linear over the extended range. It is argued that measurements of R , χ l χ t , and α l α t provide reliable techniques for the measurements of anisotropy in graphites. Examination of the relationship of the anisotropy to the nature of the precursors used in producing graphites shows that hydrogenation of coal prior to the extraction process produces anisotropic graphites, the anisotropy being controlled by the degree of hydrogenation. Furthermore, it is shown that by blending appropriate amounts of hydrogenated and nonhydrogenated coal extracts, graphites of desired anisotropy can be produced.