Debashish Das
@iitbbs.ac.in
Research Associate
IIT Bhubaneswar
RESEARCH INTERESTS
Magnetism
Monte Carlo simulation
Machine Learning
Molecular Dynamics
Study the properties for industrial applications
(a) Binding energy
(b) Elastic constant and elasticity
(c) Corrosion
(d) Quantum capacitance
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Prashant Singh, Debashish Das, Duane D Johnson, Raymundo Arroyave, and Aftab Alam
IOP Publishing
Abstract We present a systematic study of the effect of Pd-alloying on phase stability, electronic structure, and elastic properties in L10 Fe–Ni using density-functional theory. Being from the same group of the periodic table, Pd is the best candidate for chemical alloying. The Fe–Ni/Fe–Pd/Ni–Pd bond-length increases with increasing Pd-concentration, which weakens the hybridization between low lying energy states below Fermi-level. The reduced hybridization decreases the relative thermodynamic stability of L10 Fe(Ni1−x Pd x ) until x = 0.75. Beyond this concentration, the relative stability gets enhanced, which is attributed to a unique change in the lattice distortion (c/a). The elastic properties show a non-monotonous behavior as a function of x, which is again due to a specific change-over in the uniaxial strain. We found that Pd alloying increases the local Fe moment and structural anisotropy of L10 FeNi, which are important for applications such as microwave absorption, refrigeration systems, recording devices, imaging and sensors. We believe that the present study for the chemical alloying effect can provide critical insights toward the understanding of electronic-structure and elastic behavior of other technologically important materials.
Sayandeep Ghosh, Sobhit Singh, Debashish Das, Subhradip Ghosh, Pankaj Kumar Mishra, and Subhash Thota
IOP Publishing
Abstract We report the electronic structure and magnetic properties of Co2Ti1−x Ge x O4 (0 ⩽ x ⩽ 1) spinel by means of the first-principle methods of density functional theory involving generalized gradient approximation along with the on-site Coulomb interaction (U eff) in the exchange-correlation energy functional. Special emphasis has been given to explore the site occupancy of Ge atoms in the spinel lattice by introducing the cationic disorder parameter (y) which is done in such a way that one can tailor the pyrochlore geometry and determine the electronic/magnetic structure quantitatively. For all the compositions (x), the system exhibits weak tetragonal distortion (c/a ≠ 1) due to the non-degenerate d z 2 and d x 2 − y 2 states (e g orbitals) of the B-site Co. We observe large exchange splitting (ΔEX ∼ 9 eV) between the up and down spin bands of t 2g and e g states, respectively, of tetrahedral and octahedral Co2+ (4A2(g)(F)) and moderate crystal-field splitting (ΔCF ∼ 4 eV) and the Jahn–Teller distortion (ΔJT ∼ 0.9 eV). These features indicate the strong intra-atomic interaction which is also responsible for the alteration of energy band-gap (1.7 eV ⩽ E g ⩽ 3.3 eV). The exchange interaction (J BB ∼ −4.8 meV, for (x, y) = (0.25, 0)) between the Co2+ dominates the overall antiferromagnetic behaviour of the system for all ‘x’ as compared to J AA (∼−2.2 meV, for (x, y) = (0.25, 0)) and J AB (∼−1.8 meV, for (x, y) = (0.25, 0)). For all the compositions without any disorderness in the system, the net ferrimagnetic moment (Δμ) remains constant, however, increases progressively with increasing x due to the imbalance of Co spins between the A- and B-sites.
Debashish Das and Aftab Alam
American Physical Society (APS)
Debjit Manna, Jiban Kangsabanik, Tapan Kumar Das, Debashish Das, Aftab Alam, and Aswani Yella
American Chemical Society (ACS)
Recently, lead free all-inorganic double perovskites have revolutionized the photovoltaic research, showing promising light emitting efficiency and tunability via modification of inherent structural and chemical properties. Here, we report a combined experimental and theoretical study on the variation of carrier-lattice interaction and optoelectronic properties of Cs2AgIn1-xBixCl6 double perovskite nanocrystals with varying alloying concentrations. Our UV-vis study confirms the parity allowed first direct transition for x ≤ 0.25. Using a careful analysis of Raman spectra assisted with first principles simulations, we assign the possible three types of active modes to intrinsic atomic vibrations; 2 T2g modes (one for translational motion of 'Cs' and other for octahedral breathing), 1 Eg and 1 A1g mode for various stretching of Ag-Cl octahedra. Ab-initio simulation reveals dominant carrier-phonon scattering via Fröhlich mechanism near room temperature, with longitudinal optical phonons being effectively activated around 230 K. We observe a noticeable increase in hole mobility (~4 times) with small Bi alloying, attributed to valence band ( VB) maxima acquiring Bi-s orbital characteristics, thus resulting in dispersive VB. We believe that, our results should help to gain a better understanding of the intrinsic electronic and lattice dynamical properties of these compounds and provide a base towards improving the overall performance of double perovskite nanocrystals.
Debashish Das and Subhradip Ghosh
Wiley
Recent experiments on Fe substituted spinel CoCr2O4 have discovered multiple functional properties in the system such as temperature and composition dependent magnetic compensation, and tunable exchange bias. These properties are attributed to the renormalization of the inter‐atomic magnetic exchange interactions arising due to the non‐regular site occupancies of the magnetic cations. We perform ab initio electronic structure calculations by DFT + U method and combine with a generalized thermodynamic model to compute the site occupancy patterns of magnetic cations, structural properties and magnetic exchange interactions of Co (Cr1−xFex)2 O4 for the entire composition range 0 < x < 1. We find that the substituting Fe atoms prefer to occupy the tetrahedral sites of the spinel structure for the entire range of x , in agreement with the experimental inferences. By computing the variations of the various inter‐atomic magnetic exchange interactions as a function of site occupancy, we provide a microscopic picture of the evolution of a collinear structure from a non‐collinear one due to substitution of Fe in CoCr2O4. The computed results are analyzed using the electronic structures. This work is a systematic study exploring the interrelations of site occupancy, composition, electronic structure and magnetic interactions in Fe‐substituted CoCr2O4 which helps gain fundamental knowledge of this material.
Debashish Das and Subhradip Ghosh
IOP Publishing
Cation disorder over different crystallographic sites in spinel oxides is known to affect their properties. Recent experiments on Mn doped multiferroic CoCr2O4 indicate that a possible distribution of Mn atoms among tetrahedrally and octahedrally coordinated sites in the spinel lattice give rise to different variations in the structural parameters and saturation magnetisations in different concentration regimes of Mn atoms substituting the Cr. A composition dependent magnetic compensation behaviour points to the role conversions of the magnetic constituents. In this work, we have investigated the thermodynamics of cation disorder in Co(Cr1−xMnx)2O4 system and its consequences on the structural, electronic and magnetic properties, using results from first-principles electronic structure calculations. We have computed the variations in the cation-disorder as a function of Mn concentration and the temperature and found that at the annealing temperature of the experiment many of the systems exhibit cation disorder. Our results support the interpretations of the experimental results regarding the qualitative variations in the sub-lattice occupancies and the associated magnetisation behaviour, with composition. We have analysed the variations in structural, magnetic and electronic properties of this system with variations in the compositions and the degree of cation disorder from the variations in their electronic structures and by using the ideas from crystal field theory. Our study provides a complete microscopic picture of the effects that are responsible for composition dependent behavioural differences of the properties of this system. This work lays down a general framework, based upon results from first-principles calculations, to understand and analyse the substitutional magnetic spinel oxides A(B1−xCx)2O4 in presence of cation disorder.
Debashish Das, Shreemoyee Ganguly, Biplab Sanyal, and Subhradip Ghosh
IOP Publishing
CoCr2O4 has attracted significant attention recently due to several interesting properties such as magnetostriction, magnetoelectricity etc. More recent experiments on Fe substituted CoCr2O4 observed a variety of novel phenomena such as the magnetic compensation accompanied by the occurrence of exchange bias, which reverses its sign. Understanding of such phenomena may lead to control the properties of these material in an efficient way to enhance its potential for multifunctional applications. In this paper, we study the fundamental understanding of Fe doping in modifying the structural and magnetic properties of CoCr2O4 with varying composition and substitution of Fe at different sublattices by first-principles density functional calculations. We have analysed in detail the effect of Fe substitution on crystal field and exchange splittings, magnetic moments and interatomic exchange parameters. It is also observed that with increasing concentration of Fe impurity, the system has a tendency towards forming an ‘inverse spinel’ structure as observed in experiments. Such tendencies are crucial to understand this system as it would lead to modifications in the magnetic exchange interactions associated with sites with different symmetry finally affecting the magnetic structure and the multiferrocity in turn.
Debashish Das, Rajkumar Biswas, and Subhradip Ghosh
IOP Publishing
The structural and magnetic properties of spinel compounds CoB2O4 (B = Cr, Mn and Fe) are studied using the DFT+U method and generalized gradient approximation. We concentrate on understanding the trends in the properties of these materials as the B cation changes, in terms of relative strengths of crystal fields and exchange fields through an analysis of their electronic densities of states. We find that the electron–electron correlation plays a significant role in obtaining the correct structural and electronic ground states. Significant structural distortion in CoMn2O4 and ‘inverted’ sublattice occupancy in CoFe2O4 affects the magnetic exchange interactions substantially. The trends in the magnetic exchange interactions are analysed in terms of the structural parameters and the features in their electronic structures. We find that the Fe states in CoFe2O4 are extremely localised, irrespective of the symmetry of the site, which makes it very different from the features of the states of the B cations in two other compounds. These results provide useful insights into the trends in the properties of CoB2O4 compounds with variation of B cation, which would help in understanding the results of recent experiments on doping of Mn and Fe in multiferroic CoCr2O4.
Ram Kumar, R. Padam, Debashish Das, S. Rayaprol, V. Siruguri, and D. Pal
Royal Society of Chemistry (RSC)
We report the magnetic structures of an Fe substituted cobalt chromite system, Co(Cr1−xFex)2O4 (x = 0.05 and 0.075), determined from the analysis of temperature dependent neutron diffraction measurements.
Debashish Das and Subhradip Ghosh
IOP Publishing
Using the DFT+U method and generalized gradient approximation (GGA) we perform the first systematic study of the chromite series ACr2O4 (A = Mn, Fe, Co, Ni) by computing their structural and magnetic properties. The results are analyzed by their electronic structures. We find that in spite of varying structural distortions, the electronic structures are very similar across the series. Such similarities are responsible for qualitative uniformities in their magnetic phases at low temperatures, as observed in the experiments. We find that the strong electron–electron correlation, along with competing magnetic exchange splitting and the crystal field splitting, are responsible for their electronic properties such as the electronic band gaps. Our results regarding the magnetic exchange parameters are in good agreement with the available results and show the relative importance of the pairwise exchange interactions in each of the compounds. The ground state magnetic spin structures and the ferrimagnetic transition temperatures obtained from these exchange parameters, in combination with a phenomenological theory, qualitatively agree with the experiments and other theoretical results.