Paper & Patents

We propose transition metal substituted Fe2P as a new promising material for spin-transfer torque magnetic random-access memory (STT-MRAM) application. Using first-principles calculations based on density functional theory and Monte Carlo simulations, we demonstrate that this material can be used as a ferromagnetic electrode in the magnetic tunnel junction (MT) of STT-MRAM due to its moderate perpendicular magnetic anisotropy, high ferromagnetic transition temperature, and large tunnel magnetoresistance. This work is expected to provide a basis for the development of a new class of Fe2P-based electrode materials for STT-MRAM devices.

First-principles theoretical analysis of the catalytic activity of van der Waals hetero-structures of 1HMoS2 and graphene substituted with three chemical types of nitrogen species (1) Graphitic (G). (1) Pyridinic (Pn) and (iii) Pyrrolic (Pr), for application in catalysis of hydrogen evolution reaction (HER) has been presented. Graphitic and pyrrolic N substituents result in n-type electronic structure, whereas substitution of pyridinic Nimparts p-type electronic character to the hetero-structure. Work functions (v ) of the hetero-structures suggest that graphitic N-graphene:MoS2 hetero-structure (p = 3,8 eV) is expected to be effective in catalysing the reduction of Ht to evolve H2. 1H-MoS2 monolayer in the hetero-structure contributes by enabling increased H20 adsorption and offsetting the band edge energies optimal for the catalytic activity. Near optimum Gibbs free energy of H-adsorption (AG) were obtained for graphitic (AGH 0.29 eV) and pyrrolic (AGH -0.2 eV) N-graphene:MoS2 hetero structures. Our work showcases how catalytic and electronic properties of the N-doped graphene:MoS2 hetero-structure depends on the chemical identity of N-sites and uncovers a route to 2D heterostructures with high catalytic activity.

Iron sulfides are fascinating catalytic phases because these include S-modified Fe δ+ (δ ≤ 2) species functioning as H 2 O 2 activators to form [rad] OH used for oxidatively degrading aqueous contaminants (e.g., phenol). As an initial step for locating S-modified metal species (M δ+ ) that outperform Fe δ+ in catalytic H 2 O 2 cleavage, hexagonal metal sulfides (MS) were synthesized using Mn, Fe, Co, Ni, and Cu to understand electric potential-assisted H 2 O 2 scission kinetics on M δ+ species. Ni δ+ species were found to show the greatest [rad] OH productivity among all M δ+ species studied, mainly resulting from the Lewis acidic nature of Ni δ+ species adequate to expedite the liberation of [rad] OH species. This was partially evidenced by H 2 O 2 activation/phenol degradation runs on M δ+ species, wherein initial H 2 O 2 activation rate (-r H2O2,0 ) or initial phenol degradation rate (-r PHENOL,0 ) of Ni δ+ species was 3–9 times those of the other M δ+ species. Ni δ+ species, therefore, were located in the middle of the volcano-shaped curve plotting -r H2O2,0 (or -r PHENOL,0 ) versus the type of M δ+ . Kinetic assessment of M δ+ species under fine-tuned reaction environments also showed that regardless of varying H 2 O 2 concentrations, M δ+ species were found to retain their -r H2O2,0 values in the absence of electric potentials. Conversely, M δ+ species could enhance -r PHENOL,0 values at larger electric potentials, where greater energies were likely exerted on M δ+ species. This indeed corroborated that [rad] OH desorption from M δ+ species was the rate-determining step to direct catalytic H 2 O 2 scission. In addition to heterogeneous catalytic nature of Ni δ+ species in fragmenting H 2 O 2 , outstanding H 2 O 2 scission ability provided by Ni δ+ species could also compensate for their moderate catalytic stability at pH-neutral condition.

Magnetic exchange interactions in pure and vanadium (V)-doped Fe16N2 are studied within the framework of density functional theory (DFT). The Curie temperatures were obtained via both mean field approximation (MFA) and Monte Carlo (MC) calculations based on interactions that were obtained through DFT. The Curie temperature (TC) for pure Fe16N2 that was obtained under MFA is substantially larger than the experimental value, suggesting the importance of thermal fluctuations. At zero field, the calculated magnetic susceptibility shows a sharp peak at T = TC that corresponds to the presence of localized d-states. From the nature of the exchange interactions, we have determined the reason for the occurrence of the giant magnetic moment in this material, which remained a mystery for decades. Finally, we posit that Fe16N2 can also act as a satisfactory spin injector for III–V semiconductors, in addition to its application as a permanent magnet, since it has very high spin polarization (compared to elemental ferromagnets) and smaller lattice mismatch (compared to half-metallic Heusler alloys) with conventional III–V semiconductors such as GaAs and InGaAs. We demonstrate this application in the case of Fe16N2(001)/InGaAs(001) hetero-structures, which exhibit substantial spin polarization in the semiconductor (InGaAs) region. PACS number: 82.65.My, 82.20.Pm, 82.30.Lp, 82.65.Jv.

We present a detailed ab-initio study of semi-classical transport in n-ZnSe using Rode's iterative method. Inclusion of ionized impurity, piezoelectric, acoustic deformation and polar optical phonon scattering and their relative importance at low and room temperature for various n-ZnSe samples are discussed in depth. We have clearly noted that inelastic polar optical phonon scattering is the most dominant scattering mechanism over most of the temperature region. Our results are in good agreement with the experimental data for the mobility and conductivity obtained at different doping concentrations over a wider range of temperatures. Also we compare these results with the ones obtained with relaxation time approximation (RTA) which clearly demonstrate the superiority of the iterative method over RTA.

First principles studies were performed in order to find out the possibility of inducing half-metallicity in Heusler Compound CoFeMnSb, by means of alloying it with 3d-transition metal elements. Proper alloying element is selected through the calculations of formation energies. These calculations were tested with different concentrations of alloying elements at different atomic sites. Among the selected transition metal elements Sc and Ti are proposed to be excellent alloying elements particularly at Mn site. By using these alloying elements complete half metallic behaviour is obtained in CoFeMn_0.25 Sc_0.75 Sb, CoFeMn_0.75Ti_0.25Sb, CoFeMn_0.625Ti_0.375Sb, CoFeMn_0.50Ti_0.50Sb, CoFeMn_0.25Ti_0.75Sb and CoFeTiSb alloys. Shifting of Co-Fe d-states towards lower energy region leads to zero density of states at Fermi level for the spin minority channel. Alloying effects on the electronic structure and magnetization are discussed in details. Thermodynamical stability of these new alloys are major part of this study. The Curie temperatures of CoFeMn_0.25Sc_0.75Sb and CoFeMn_0.75 Ti_0.25Sb were found to be 324.5 K and 682 K; respectively, showing good candidature for spintronics applications. For understanding the bonding nature of constituent atom of CoFeMnSb, crystal orbital Hamiltonian populations have been analysed.

A detailed study on the inter-metallic alloy, Zr 2 TiAl, has been carried out using first principle electronic structure calculations. We found that a small value of bi-axial strain/stress cause a phase change from anti-ferromagnetic (AFM) to ferromagnetic (FM) with a structural transition from face center cubic (fcc) to body center tetragonal (bct). Calculated electronic band structures show that all strained structures are metallic in nature with Zr-d and Ti-d orbital dominated energy bands near the Fermi level (E F). The stability of FM phase is confirmed with phonon dispersion calculations by using density functional perturbation theory (DFPT). It has been observed that AFM state with both positive and negative bi-axial stress exhibits unstable modes while corresponding FM state shows no such instability. This clearly indicates the existence of large spin phonon coupling in this material.

Chemical reactions are often associated with an energy barrier along the reaction pathway which hinders the spontaneity of the reaction. Changing the energy barrier along the reaction pathway allows one to modulate the performance of a reaction. We present a module, Python Algorithms for Searching Transition stAtes (PASTA), to calculate the energy barrier and locate the transition state of a reaction efficiently. The module is written in python and can perform nudged elastic band, climbing image nudged elastic band and automated nudged elastic band calculations. These methods require the knowledge of the potential energy surface (and its gradient along some direction). This module is written such that it works in conjunction with density functional theory (DFT) codes to obtain this information. Presently it is interfaced with three well known DFT packages: Vienna Ab initio Simulation Package (VASP), Quantum Espresso and Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA). This module is easily extendable and can be interfaced with other DFT, force-field or empirical potential based codes. The uniqueness of the module lies in its user-friendliness. For users with limited computing resources, this module will be an effective tool as it allows to perform the calculations image by image. On the other hand, users with plentiful computing resources (such as users in a high performance computing environment) can perform the calculations for large number of images simultaneously. This module gives users complete flexibility, thereby enabling them to perform calculations on large systems making the best use of the available resources. Program summary: Program Title: PASTA Program files doi: http://dx.doi.org/10.17632/rv7fdm5gkf.1 Licensing provisions: BSD 3-clause Programming language: Python External routines/libraries: numpy, matplotlib Nature of problem: Most of the reactions have an energy barrier on their reaction pathway. This energy barrier affects the progress of the reaction. Solution method: We implement the NEB, CI-NEB and AutoNEB method to locate transition state and estimate the energy barrier. Our module works with density functional theory codes: VASP, SIESTA and Quantum Espresso presently.

In this article, we propose an antisite defect tolerant ternary Heusler alloy, Co2MnSb, which is highly efficient for spintronic applications. Using ab-initio Density Functional Theory (DFT), we study the effect of experimentally observed intrinsic point defect (antisite defect) and lattice constant (Lc) on the halfmetallic characteristics, mechanical stability and magnetic properties of Co2MnSb. Ab-initio simulation predicts halfmetallic ferromagnetic characteristics with a high value of total magnetic moment, 6.00 (5.92) μB/f.u. and large Curie temperature (TC), 1109 K (1094 K) at relaxed (experimental) Lc. Halfmetallic characteristics and mechanical stability are sensitive to Lc variation. Experimentally, it has been observed that intrinsic defects in Heusler alloys always degrades the halfmetallic characteristics and spin polarization. Hence, all the possible binary and ternary kind of antisite defects between the transition metals and the non-magnetic p-block element of Co2MnSb have been simulated upto the disorder concentration (’x’) of 11.1%. Our theoretical analysis reveals that even in presence of antisite disorder, the alloy preserves its halfmetallic characteristics specially at lower disorder concentrations. However, electronic density of states and total magnetic moment are affected significantly in presence of disorder. In some disordered alloys, total magnetic moment exceeds beyond 6.00 μB, indicating towards the higher value of TC with respect to the parent compound. Formation energy of particular disordered alloys compete with the formation energy of parent alloy, makes the compound defect tolerant material. Halfmetallic characteristics, high magnetic moment and large TC make the defect tolerant Co2MnSb alloy highly efficient for spintronic applications.