B.A. Hamad

Investigation of the structural, electronic, and magnetic properties of Ni-based Heusler alloys from first principles

Density functional theory (DFT) calculations are performed to investigate the structural, electronic, magnetic, and elastic properties of Ni2MnZ (Z = B, Al, Ga, In) and Ni2FeZ (Z = Al, Ga) full Heusler alloys. The alloys are found to be metallic ferromagnets with total magnetic moments of about 4μB/f.u. and 3μB/f.u for Ni2MnZ and Ni2FeZ alloys, respectively. The Ni2MnAl and Ni2MnIn alloys are found to be stable at L21 phase, while the other alloys are more stable in the tetragonal phase with c/a ratios of 1.38 and 1.27 for Ni2MnB and Ni2MnGa, respectively and 1.35 for both Ni2FeAl and Ni2FeGa. The Ni2MnB alloy exhibits the highest electron spin polarization in its tetragonal phase, which is about 88% greater than that of L21 structure. However, the Ni2MnGa, Ni2FeAl, and Ni2FeGa alloys exhibit lower spin polarizations in their tetragonal phase than those at the L21. The most contribution of the total magnetic moments comes from Mn or Fe atoms, whereas Ni atoms exhibit much smaller magnetic moments. However...

The electronic structure and spin polarization of Fe3−xMnxSi and Fe3−yMnSiy alloys

First principle calculations using supercell approach and coherent potential approximation (CPA) are performed to investigate the electronic and magnetic structures of Fe3−xMnxSi and Fe3−yMnSiy alloys, where x,y=0, 0.25, 0.50, 0.75, 1.00, 1.25, 1.5, 1.75, and 2.25. Using supercell calculations we obtained a metallic behavior for x=0, 0.25, and 0.5 in Fe3−xMnxSi alloys with spin polarizations of 24%, 39%, and 93%, respectively. The behavior starts to be half-metallic at x=0.75 with a small direct band gap that increases for higher concentrations of Mn. Among the half-metallic systems, only those of L21 structure at x=1 and 2 possess indirect band gaps along Γ-X symmetry line. The change of Si concentration in Fe3−yMnSiy structures retrieve the metallic behavior for all concentrations except y=1.25 that shows a half-metallic behavior with a direct band gap of 0.27 eV. We obtained a good agreement between supercell and CPA calculations for the values of the magnetic moment and the trends of the formation ene...

Magnetism in vanadium-molybdenum systems

The magnetism of relaxed and nonrelaxed vanadium-molybdenum (Vn/Mo) systems is investigated for n = 1-3 layers in the (001) and (111) orientations. This study is carried out using the the real-space self-consistent tight-binding recursion method in the Hartree-Fock approximation with Hubbard Hamiltonian. The magnetic moment of a V monolayer epitaxially grown on a semi-infinite Mo substrate is found to be larger in the (001) orientation than that in the (111). The magnetisms in bilayer and three layer V systems become comparable in the (001) and the (111) orientations in the nonrelaxed case, but are larger in the (001) for the relaxed systems. The magnetic moments are found to decrease in the relaxed systems for both cases.

Spin polarization of Cr/V vicinal structures

Abstract The magnetism is investigated for chromium (Cr) overlayers adsorbed on vanadium (V) vicinal substrates. The calculations of the magnetic moments are carried out by the real-space self-consistent tight-binding (TB) recursion method in the Hartree–Fock approximation. These structures exhibit the largest magnetic moment at the edge atom and the least at the kink. Appreciable magnetic moments are induced on V sublayers antiferromagnetically coupled with the surface moments where this coupling is frustrated at kink atoms. The net magnetization increases as a function of the step length and persists for few layers below the surface.

Ab-initio calculations of the electronic and magnetic structures of Co2Cr1−xMnxAl alloys

Density functional theory based on full-potential linearized augmented plane-wave method is used to investigate the structural, electronic, and magnetic properties of Co2Cr1−xMnxAl (x = 0, 0.25, 0.5, 0.75, 1) alloys. The parent full Heusler alloy (Co2CrAl) exhibits the least total magnetic moment of 3 μB/f.u. For the alloyed structures, this value is found to increase as a function of Mn concentration. The magnitudes of Co2Cr1−xMnxAl total magnetic moment show a trend consistent with the Slater–Pauling behavior. Co2CrAl full Heusler alloy reveals a half-metallic behavior with an indirect band gap along the Γ-X symmetry line. For the non-stoichiometric structures, the energy levels of the valence band shift to higher energies until they cross the Fermi level for x = 0.75 and 1.

Theoretical investigations of the structural, electronic and optical properties of Hg1−xCdxTe alloys

The structural, electronic and optical properties of mercury cadmium telluride (Hg1−xCdxTe; x = 0.0, 0.25, 0.5, 0.75) alloys are studied using density functional theory within full-potential linearized augmented plane wave method. We used the local density approximation (LDA), generalized gradient approximation (GGA), hybrid potentials, the modified Becke–Johnson (LDA/GGA)-mjb and Hubbard-corrected functionals (GGA/LDA + U), for the exchange-correlation potential (Eex). We found that LDA functional predicts better lattice constants than GGA functional, whereas, both functionals fail to predict the correct electronic structure. However, the hybrid functionals were more successful. For the case of HgTe binary alloy, the GGA + U functional predicted a semi-metallic behaviour with an inverted band gap of −0.539 eV, which is closest to the experimental value (−0.30 eV). Ternary alloys, however, are found to be semiconductors with direct band gaps. For the x = 0.25 and 0.50, the best band gaps are found to be 0.39 and 0.81 eV using LDA-mbj functional, whereas, the GGA-mbj functional predicted the best band gap of 1.09 eV for Hg0.25Cd0.75Te alloy, which is in a very good agreement with the experimental value (1.061 eV). The optical properties of the alloys are obtained by calculating the dielectric function ɛ(ω). The peaks of the optical dielectric functions are consistent with the electronic gap energies of the alloys.

Investigations of the electronic and magnetic structures of Co2YGa (Y=Cr, Mn) Heusler alloys and their (100) surfaces

Density functional theory calculations are performed to investigate the structural, electronic, and magnetic properties of bulk structures of Co2YGa (Y = Cr, Mn) Heusler alloys and the surfaces along the (100) orientation. The bulk structures of both alloys show a ferromagnetic behavior with total magnetic moments of 3.03μB and 4.09μB and high spin polarizations of 99% and 67% for Co2CrGa and Co2MnGa, respectively. The surfaces are found to exhibit corrugations due to different relaxations of the surface atoms. For the case of Co2CrGa, two surfaces preserve the half metallicity, namely those with Cr-Ga and Ga– terminations with high spin polarizations above 90%, whereas it dropped to about 50% for the other surfaces. However, the spin polarizations of Co-Co and Mn-Ga terminated surfaces remain close to that of bulk Co2MnGa alloy, whereas it is suppressed down to 17% for Co– termination. The highest local magnetic moments are found to be 3.26 μB and 4.11 μB for Cr and Mn surface atoms in Cr-Ga and Mn– term...

Substitutional alloyed overlayers on Pd(0 0 1) substrates

Abstract The magnetism is investigated for substitutional c(2×2) alloyed (V, Cr, Mn, Fe and Mo) structures adsorbed on Pd semi-infinite surfaces using a simple tight-binding method. The local magnetic moments of V, Cr, Mn, Fe and Mo are enhanced from those of pseudomorphic overlayes. They also exhibit ferrimagnetic configurations at the surface that leads to appreciable net surface magnetization.

Anisotropy Effects on the Thermoelectric Electronic Transport Coefficients

Abstract Engineering thermoelectric (TE) materials for applications in power generation and cooling requires an understanding of how anisotropy influences the TE properties. In this paper we use an angular-dependent, multivalley formalism to model the band structure and to explore the effect of anisotropy on the Seebeck and conductivity tensors. Specifically we explore the effect of degeneracy and relative orientation of the effective mass ellipsoids near critical points on the shape of these tensors. Examples of these types of anisotropic effects are explored within the above formalism and with the full band structures of two materials: half-Heusler ZrNiSn and (Sr,Ba)Nb2O6 with the tetragonal tungsten bronze structure.

A first principle study for the adsorption and absorption of carbon atom and the CO dissociation on Ir(100) surface

We employ density functional theory to examine the adsorption and absorption of carbon atom as well as the dissociation of carbon monoxide on Ir(100) surface. We find that carbon atoms bind strongly with Ir(100) surface and prefer the high coordination hollow site for all coverages. In the case of 0.75 ML coverage of carbon, we obtain a bridging metal structure due to the balance between Ir-C and Ir-Ir interactions. In the subsurface region, the carbon atom prefers the octahedral site of Ir(100) surface. We find large diffusion barrier for carbon atom into Ir(100) surface (2.70 eV) due to the strong bonding between carbon atom and Ir(100) surface, whereas we find a very small segregation barrier (0.22 eV) from subsurface to the surface. The minimum energy path and energy barrier for the dissociation of CO on Ir(100) surface are obtained by using climbing image nudge elastic band. The energy barrier of CO dissociation on Ir(100) surface is found to be 3.01 eV, which is appreciably larger than the association energy (1.61 eV) of this molecule.