Subhradip Ghosh

First-principles investigations of the electronic structure and properties related to shape-memory behavior in Mn2NiX (X = Al,Ga,In,Sn) alloys

Using first-principles density functional theory based method we have performed a systematic investigations of the electronic structures, the structural and magnetic properties related to shape memory applications for Mn2NiAl,Mn2NiGa,Mn2NiSn and Mn2NiIn alloys. Our results confirm that all the alloys undergo a volume conserving martensitic transformation at low temperatures and that the low temperature stable phase is a non-modulated tetragonal one. The relative stabilities of the martensitic phases and the magnetic properties differ considerably for Mn2NiAl, Mn2NiGa and Mn2NiIn and Mn2NiSn. Details of the electronic structures suggest that the differences in hybridizations between the magnetic components are responsible for these trends. Quantitative estimates of the energetics and the magnetizations indicate that Mn2NiGa and Mn2NiAl are more promising candidates for shape memory applications.

First-principles prediction of shape memory behavior and ferrimagnetism in Mn2NiSn.

Using first-principles density functional theory, we show that, in Mn(2)NiSn, an energy lowering phase transition from the cubic to tetragonal phase occurs which indicates a martensitic phase transition. This structural phase transition is nearly volume-conserving, implying that this alloy can exhibit shape memory behavior. The magnetic ground state is a ferrimagnetic one with antiparallel Mn spin moments. The calculated moments with different electronic structure methods in the cubic phase compare well with each other but differ from the experimental values by more than 1 μ(B). The reason behind this discrepancy is explored by considering antisite disorder in our calculations, which indicates that the site ordering in this alloy can be quite complex.

New quaternary half-metallic ferromagnets with large Curie temperatures

New magnetic materials with high Curie temperatures for spintronic applications are perpetually sought for. In this paper, we present an ab initio study of the structural, electronic and magnetic properties of Quaternary Heusler compounds CoX′Y′Si where X′ is a transition metal with 4d electrons and Y′ is either Fe or Mn. We find five new half-metallic ferromagnets with spin polarisation nearly 100% with very high Curie temperatures. The variation of Curie temperatures as a function of valence electrons can be understood from the calculated inter-atomic exchange interaction parameters. We also identify a few other compounds, which could be potential half-metals with suitable application of pressure or with controlled doping. Our results reveal that the half-metallicity in these compounds is intricately related to the arrangements of the magnetic atoms in the Heusler lattice and hence, the interatomic exchange interactions between the moments. The trends in the atomic arrangements, total and local magnetic moments, interatomic magnetic exchange interactions and Curie temperatures are discussed with fundamental insights.

Electronic and magnetic properties of disordered Fe?Cr alloys using different electronic structure methods

In this paper we have studied the electronic structure of bcc FexCr1-x alloys in the ferromagnetic phase using three different techniques: augmented space recursion coupled with tight-binding linea ...

First-principles computation of structural, elastic and magnetic properties of Ni2FeGa across the martensitic transformation

The structural stabilities, elastic, electronic and magnetic properties of the Heusler-type shape memory alloy Ni(2)FeGa are calculated using density functional theory. The volume conserving tetragonal distortion of the austenite Ni(2)FeGa find an energy minimum at c/a = 1.33. Metastable behaviour of the high temperature cubic austenite phase is predicted due to elastic softening in the [110] direction. Calculations of the total and partial magnetic moments show a dominant contribution from Fe atoms of the alloy. The calculated density of states shows a depression in the minority spin channel of the cubic Ni(2)FeGa just above the Fermi level which gets partially filled up in the tetragonal phase. In contrast to Ni(2)MnGa, the transition metal spin-down states show partial hybridization in Ni(2)FeGa and there is a relatively high electron density of states near the Fermi level in both phases.

Dynamical stability and phase transition of BeO under pressure

We study the pressure induced phase transitions in BeO at zero and finite temperatures using ab initio density functional perturbation theory. The aim is to primarily focus on the vibrational properties of this alkaline earth oxide under pressure to understand the mechanism of phase transitions. Static calculations predict the B4 to B1 transition to occur at 112 GPa at 0 K. Zero-point vibrations lower this pressure by 7 GPa. Dynamically, the B4 phase is found to be stable up to a pressure of about 820 GPa. On the other hand, the material in its B1 phase cannot be decompressed below 35 GPa. The possible transition path as well as the associated transition mechanism has been investigated from phonon excitations. The P-T stability field of wurtzite (B4) and rocksalt (B1) structure phases of BeO has been discussed using quasiharmonic approximation.

Ab initio calculation of lattice dynamics in BeO

Ground state lattice vibrational properties of wurtzite–BeO are reported using an ab initio plane-wave pseudopotential method. The ab initio results for the phonon dispersion relations are in good agreement with the available experimental data. The only discrepancy observed between experiment and present data for the longitudinal optic frequency at the centre of the Brillouin zone for a displacement along the symmetry axis is expected to be due to the indirect measurement of that mode in the experiment. The dielectric constant, the Born effective charges and the elastic constants for the compound are computed from the lattice dynamics. All of them agree well with the experimental results. The elastic constants calculated using the phonon spectra agree reasonably well with the results from other first-principles calculations. The good agreement of the quantities calculated, with the experimental results pave the way for future studies on the contribution of lattice vibrations to the pressure-induced phase transition in this compound. We try to understand the features of the phonon spectra from the component-projected phonon densities of states and by analysing the contributions of each atom type towards each normal mode. We find that the phonon spectra of BeO contains features common to some of the members with the same crystal structure as well as to some of the members in the same alkaline earth oxide group.

A new first principles approach to calculate phonon spectra of disordered alloys

The lattice dynamics in substitutional disordered alloys with constituents having large size differences is driven by strong disorder in masses, inter-atomic force constants and local environments. In this paper, a new first principles approach based on special quasirandom structures and an itinerant coherent potential approximation to compute the phonon spectra of such alloys is proposed and applied to Ni₀.₅Pt₀.₅ alloy. The agreement between our results and experiments is found to be much better than for previous models of disorder due to an accurate treatment of the interplay of inter-atomic forces among various pairs of chemical species. This new formalism serves as a potential solution to the longstanding problem of a proper microscopic understanding of lattice dynamical behavior of disordered alloys.

Complex magnetic interactions in off-stoichiometric NiMnGa alloys.

Using first-principles density functional theory, the magnetic pair interactions between various pairs of chemical specie have been calculated and the trends in magnetism with varying compositions and chemical ordering are analyzed for three off-stoichiometric NiMnGa alloys in their austenite phases. The experimentally observed trend of decreasing magnetization with increasing Mn concentration is attributed to the antiferromagnetic interactions among Mn atoms occupying sublattices other than the original Mn one. The role of chemical ordering on magnetization is also analyzed by total energy results and exchange interactions. We are able to explain the recently published neutron scattering experiments with our theoretical analyses.

First-principles investigations into the thermodynamics of cation disorder and its impact on electronic structure and magnetic properties of spinel Co(Cr1–x Mn x )2O4

Cation disorder over different crystallographic sites in spinel oxides is known to affect their properties. Recent experiments on Mn doped multiferroic [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] in presence of cation disorder.