• Anjali

Articles written in Pramana – Journal of Physics

• Lattice sum calculations and a Mössbauer study on electric field gradient in phlogopite

Lattice sum contributions have been calculated at the two octahedral sites in a phlogopite mica assuming a systematic distribution of octahedral cationic charges. This, unlike the case of a random distribution of charges, is able to reproduce broad features of quadrupole doublet spectra in Mössbauer experiments suggesting that the doublet assignments in terms of the two structural sites,M1 (trans) andM2(cis), are quite valid. Angle dependence of Mössbauer spectra has also been studied for a ferric-rich phlogopite mica sheet to determine the orientation of the EFG principal axis component and the sign of the quadrupole coupling constant. Quadrupole splitting values and the positive sign of coupling constants match the theoretical predictions quite well but the model fails to predict the observed EFG orientation.

• On preparation of fine particles of MnFe2O4 and their magnetic properties

Influence of the heat treatment, following a wet chemical process, has been examined on the sizes of the obtained particles of MnFe2O4. X-ray diffraction, magnetization and Mössbauer measurements have been used. The average sizes of the obtained particles are in the range of ∼ 100 Å–300Å with the upper size limit being ∼ 450 Å and more. The average size as well as size distribution is strongly influenced by the nature of the heat treatment followed for preparation of the sample. Prolonged hydrothermal heating resulted in larger average size than did dry heating at 400°C of the precipitate obtained from the initial wet process. Further dry heating following hydrothermal treatment did not lead to any major growth. Chemical nature of the starting materials also affects the sizes.

• Magnetic structure of (Fe0.97Cr0.03)2P

Magnetic behaviour of di-metal iron phosphide with a small substitution of iron by chromium, (Fe0.97Cr0.03)2P, has been studied using SQUID magnetometry and powder neutron diffraction. It is paramagnetic at temperatures above ∼180 K with persisting short range ferromagnetic (FM) order. At lower temperatures three different regions of magnetic behaviour are identified. FM order evolves in the region 180 K-120 K but much more slowly and with much less magnetic moments than in Fe2P. In the region 120 K-50 K negative exchange interactions gain some importance leading to a loss of FM order. Below 50 K FM interactions again dominate. Pinning centres influence the behaviour at low temperature up to ∼100 K.

• Magnetic behaviour of nano-particles of Fe2.9Zn0.1O4

DC magnetization measurements are reported in the temperature range 20–100 K on a poly-disperse nano-particle sample of the spinel ferrite Fe2.9Zn0.1O4 with a log-normal size distribution of median diameter 43.6 Å and standard deviation 0.58. Outside a core of ordered spins, moments in surface layer are disordered. Results also show some similarities with conventional spin glasses. Blocking temperature exhibits a near linear variation with two-third power of the applied magnetic field and magnetizationM evolves nearly linearly with logarithm of timet. Magnetic anisotropy has been estimated by analysing theM-logt curve. Anisotropy values show a large increase over that of bulk particle samples. Major contribution to this enhancement comes from the disordered surface spins.

• Magnetization and neutron diffraction studies on FeCrP

Crystal structure and magnetic behaviour of FeCrP have been investigated using magnetization and neutron diffraction measurements. FeCrP crystallizes in orthorhombic FeZrP type structure (Pnma space group,Z = 4) in which Cr atoms occupy the pyramidal site and Fe atoms occupy the tetrahedral site with total preference. Structural parameters including positional parameters have been refined. The refined values of positional parameters for Fe and Cr are quite different from those in FeZrP. The nature of magnetization-temperature curve is suggestive of antiferromagnetic nature withTN = 280 (±10) K. Preliminary analysis of neutron diffraction pattern at 13 K is indicative of a rather complicated magnetic structure.

• Magnetic behaviour of (Fe0.85Cr0.15)2As

Magnetization and neutron diffraction measurements have been made on the title pseudo-binary of tetragonal anti-ferromagnets Fe2 As and Cr2 As. In this system antiferromagnetic (AFM) ordering appears below 310 K. The moments are confined in theab plane but unlike in the end members they are tilted off thea-axis. In addition to the AFM structure a weak ferromagnetic behaviour shows up below∼80 K with a rather low moment of ∼0.07 μB per formula unit at 5 K and under a field of 3 T.

• Magnetic behaviour of nano-particles of Fe2.8Zn0.2O4

Magnetization measurements are reported on a nano-particle sample of Znsubstituted spinel ferrite Fe2.8Zn0.2O4 in the temperature range 20–300 K. Analysis of small-angle neutron scattering data shows the sample to have a log-normal particle size distribution of median diameter 64.4 Å and standard deviation 0.38. Magnetization evolves over a long period of timet going nearly linearly with logt. Magnetic anisotropy, estimated by fitting M-logt curve, shows many fold increase over that of bulk particle sample. Major enhancement owes to disordered moments in surface layer. In the nano-particle state as well increasing amount of Zn causes anisotropy to decrease.

• Bianchi type-I bulk viscous fluid string dust magnetized cosmological model in general relativity

Bianchi Type-I magnetized bulk viscous fluid string dust cosmological model is investigated. To get a determinate model, we have assumed the conditions σ ∝θ andζθ = constant where σ is the shear,θ the expansion in the model andζ the coefficient of bulk viscosity. The behaviour of the model in the presence and absence of magnetic field together with physical and geometrical aspects of the model are also discussed.

• A fragment-based approach towards $\it{ab-initio}$ treatment of polymeric materials

The broad range of applications of $\pi$-conjugated polymeric materials in industries such as automobiles, textiles, packaging, medical etc. have led to their extensive studies in both academic and industrial fields. Predicting the structure of these polymers is important for the study of their properties. The present work uses a ‘divide and conquer’-type approach for the $\it{ab-initio}$ studies of these polymeric systems. The method employs a fragmentation technique with independent fragment optimization for obtaining optimized geometries of the oligomers of various polymeric materials such as polyfuran, polypyrrole, polythiophene and other such $\pi$-conjugated polymers. A few test calculations performed in the study provide fair concurrence between the energies and the HOMO–LUMO energy gaps obtained using the fragmentation-based approach with those obtained using the full optimization of the whole oligomer. Also, a significant reduction in time complexity occurs for the present fragment-based approach compared to the parent system optimization. The results are encouraging and prompt for studies of large polymeric materials.

• Theoretical study of 1p, 2p-halo nuclei formed via the decay of elements in the superheavy region with Z ranging from 115 to 120

The radioactive decay probabilities of various proton halo (p-halo) nuclei from parent isotopes in the superheavy (SH) region, Z =115–120, were analysed by taking the Coulomb and proximity potential as the interacting barrier. The nuclear decay half-lives (t$_{1/2}$), barrier penetrability and various other attributes for the decay of p-halo nuclei such as $^8$B, $^9$C, $^{11,12}$N, $^{17}$F, $^{17,18}$Ne, $^{23}$Al and $^{26,27,28}$P from the isotopes $^{261–278}$115, $^{264–279}$116, $^{267–284}$117, $^{271–283}$118, $^{274–288}$119 and $^{277–285}$120 are determined. From the determined decay lifetime values, it is understood that most of the p-halo decays are probable. The error bars in the halo radius are incorporated for the nuclear decay half-life for these p-halo nuclei. Moreover, the effects of shell closure in the daughter and the parent nuclei are also clear from the plots of calculated decay half-lives vs. neutron number of the daughter nuclei. Peak and dip in the plots show the closed shell effects of the parent and daughter nuclei respectively. We found the closed shell effect of the parent at Np ∼ 152 and closed shell effect of the daughter nuclei at Nd ∼ 132, 142. Further, we have analysed the Geiger–Nuttall (GN) plots of logarithmic half-life time vs. Q$^{−1/2}$ and the universal curve of logarithmic half-life time against negative logarithm of the barrier penetrability for the chosen p-halo decay from the SH parents, Z = 115–120 and are found to be linear. Also, we can find that the addition of proximity potential does not make any notable variation in the behaviour of the Geiger–Nuttall plots. Hence GN law is also applicableto p-halo decay.

• # Pramana – Journal of Physics

Volume 96, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019