Half Metals

We study the electronic and optical properties of pure and Ta-doped anatase TiO2 structures using a plane-wave-based first-principles calculation. The pure anatase TiO2 has an indirect band gap of 2.76 eV, while the Ta-doped anatase TiO2 is a metal. Zero-energy dielectric constants of 4.02 and 3.48 were found for E || a and E || c, respectively, in the pure anatase TiO2. Based on the calculated imaginary parts of the dielectric function (ε 2), the pure anatase TiO2 has anisotropic interband transition edges of 2.75 eV and 3.55 eV for E || a and E || c respectively, which show an optical dichroism. The Ta-doped anatase TiO2 is found to have metallic behaviour, which is clearly observable through the significantly high electronic absorption in ε 2 at the zero energy level. However, the Ta-doped TiO2 also shows anisotropic interband transition edges of 2.55 eV and 3.45 eV for E || a and E || c, respectively, based on ε 2. Based on these results, Ta dopant plays an important role for the semiconductor-to-metallic transformation in anatase TiO2. At the same time, Ta dopant also promotes the red-shift of the interband transition edges and enhances the dichroism. This study presents the significant modification in the optical properties of anatase TiO2 due to the presence of Ta dopant based on the dielectric functions.

In this report, structural, electronic, magnetic and transport properties of quaternary Heusler alloys CoRuMnGe and CoRuVZ (Z = Al, Ga) are investigated. All the three alloys are found to crystallize in cubic structure. CoRuMnGe exhibits L2 1 structure whereas, the other two alloys have B2-type disorder. For CoRuMnGe and CoRuVGa, the experimental magnetic moments are in close agreement with the theory as well as those predicted by the Slater-Pauling rule, while for CoRuVAl, a relatively large deviation is seen. The reduction in the moment in case of CoRuVAl possibly arises due to the anti-site disorder between Co and Ru sites as well as V and Al sites. Among these alloys, CoRuMnGe has the highest T C of 560 K. Resistivity variation with temperature reflects the half-metallic nature in CoRuMnGe alloy. CoRuVAl shows metallic character in both paramagnetic and ferro-magnetic states, whereas the temperature dependence of resistivity for CoRuVGa is quite unusual. In the last system, ρ vs. T curve shows an anomaly in the form of a maximum and a region of negative temperature coefficient of resistivity (TCR) in the magnetically ordered state. The ab initio calculations predict nearly half-metallic ferromagnetic state with high spin polarization of 91, 89 and 93% for CoRuMnGe, CoRuVAl and CoRuVGa respectively. In the case of CoRuMnGe, the XRD analysis reveals that the Co and Ru sites are equally probable. Hence, to investigate the electronic properties of the experimentally observed structure, the Co-Ru swap disordered structures of CoRuMnGe alloy are also simulated and it is found that the disordered structures retain half-metallic nature, high spin polarization with almost same magnetic moment as in the ideal structure. Nearly half-metallic character, high T C and high spin polarization make CoRuMnGe alloy promising for room temperature spintronic applications.

In this study, we have explored the structural,
electronic, and magnetic properties of V-doped zincblende
MgSe andMgTe compounds using density functional calculations.
The Wu-Cohen generalized gradient approximation
is used for optimizing the structural properties, while the
modified Becke and Johnson local (spin) density approximation
functional has been employed to compute the
electronic and magnetic properties. The spin dependent
band structures, electronic density of state, and magnetic
moments calculated for V-dopedMgSe and MgTe semiconductors
exhibit occurrence of 100 % spin polarization at
the Fermi level which confirms stable half-metallic ferromagnetism
in these materials. The spin-down gaps and thehalf-metallic gaps are analyzed in terms of V-3d and Se-4p
(Te-5p) hybridization, where it is observed that the V-3d
states play a key role in generating spin polarization and the
magnetic moment in these compounds. The exchange constants
N0αand N0β have been calculated to demonstrate the
effects resulting from exchange splitting process. Furthermore,
spin-polarized charge density calculation is presented
for elucidating the bonding nature, while pressure dependence
of total magnetic moment for three concentrations of
V-doped MgSe and MgTe are also discussed.

Fe 2 Mn 1-x Cr x Si (x ¼ 0, 0.1, and 0.2) alloys were investigated for their magnetic and transport properties in view of the expected half metallicity. It is found that Cr substitution suppresses the antiferromagnetic phase present in parent Fe 2 MnSi, which completely disappears for x ¼ 0.2. Curie temperature of the alloys increases from 230 K to 299 K as x is increased from 0 to 0.2. The value of the Rhodes-Wohlfarth ratio indicates that the system shows iterant magnetism. Resistivity measurements also show absence of antiferromagnetic phase for x ¼ 0.2. Resistivity data have been fitted by considering the electron-phonon and electron-magnon scattering contributions, which indicates the presence of half metallicity in these compounds. Temperature dependence of resistivity data shows magnetoresistance of À3% and À2.5% at Curie temperature with applied field of 50 kOe for x ¼ 0.1 and 0.2, respectively. V C 2014 AIP Publishing LLC. [http://dx.

The structural, electronic and magnetic properties of the low index (111), (110) and (001) surfaces of the
rocksalt Potassium Sulfide (rs-KS) are investigated by using pseudo-potential calculations in the framework
of spin density functional theory. The results show that the bulk half-metallicity of this compound is well preserved
on its low index surfaces. The spin flip gap at the studied surfaces is close to the bulk value except for
the potassium terminated (111) surface which exhibits about half of the bulk spin flip gap. The calculated
surface energies in the framework of ab-initio atomistic thermodynamics indicate that the potassium terminated
(111) surface of KS has the lowest energy over entire allowed range of the chemical potentials, while
the surface energy of the (001) and (110) surfaces approaches the surface energy of the stable surface at the
sulfur rich environment. In the framework of the Heisenberg model, it is argued that the surface effects have
small effects on the interlayer exchange interaction in the stable K-terminated (111) surface, compared with
the bulk KS.

We theoretically study a finite size SF1N F2 spin valve, where a normal metal (N) insert separates a thin standard ferromagnet (F1) and a thick half-metallic ferromagnet (F2). For sufficiently thin superconductor (S) widths close to the coherence length ξ0, we find that changes to the relative magnetization orientations in the ferromagnets can result in substantial variations in the transition temperature Tc, consistent with experiment [Singh et al., Phys. Rev. X 5, 021019 (2015)]. Our results demonstrate that, in good agreement with the experiment, the variations are largest in the case where F2 is in a half-metallic phase and thus supports only one spin direction. To pinpoint the origins of this strong spin-valve effect, both the equal-spin f1 and opposite-spin f0 triplet correlations are calculated using a self-consistent microscopic technique. We find that when the magnetization in F1 is tilted slightly out-of-plane, the f1 component can be the dominant triplet component in the superconductor. The coupling between the two ferromagnets is discussed in terms of the underlying spin currents present in the system. We go further and show that the zero energy peaks of the local density of states probed on the S side of the valve can be another signature of the presence of superconducting triplet correlations. Our findings reveal that for sufficiently thin S layers, the zero energy peak at the S side can be larger than its counterpart in the F2 side.

Local atomic magnetic moments in crystalline Fe are perturbed by the presence of dislocations. The effects are most pronounced near the dislocation core and decay slowly as the strain field of the dislocation decreases with distance. We have calculated local moments using the locally self-consistent multiple scattering (LSMS) method for a supercell containing a screw-dislocation quadrupole. Finite size effects are found to be significant indicating that dislocation cores affect the electronic structure and magnetic moments of neighboring dislocations. The influence of neighboring dislocations points to a need to study individual dislocations from first principles just as they appear amid surrounding atoms in large-scale classical force field simulations. An approach for the use of the LSMS to calculate local moments in subvolumes of large atomic configurations generated in the course of classical molecular dynamics simulation of dislocation dynamics is discussed.

Polycrystalline quaternary Co2MnAl1−x Snx films with x=0.25, 0.5, 0.75, 1 were prepared at room temperature using magnetron sputtering technique on SiO2 substrates and post-annealed at various temperatures. We investigated the crystal structures, magnetic properties, and magnetic damping constants (α) of the prepared films. Out-of-plane angular dependences of the resonance field and the linewidth of the ferromagnetic resonance spectra were measured and analyzed using the Landau–Lifshitz–Gilbert equation to determine the magnetic properties and damping constant. Co2MnAl0.75Sn0.25 and Co2MnSn films had A2 ordered crystal structure while Co2MnAl0.25Sn0.75 and Co2MnAl0.5Sn0.5 films had A2 ordering up to 400 °C and 300 °C annealing temperature, respectively, and they had B2 ordering for the remaining temperatures. Also the crystal structure deteriorated at 600 °C for all of the film systems. The saturation magnetization, M S , of films increased with annealing temperature till 400 °C except Co2MnAl0.5Sn0.5 in which M S increased till 500 °C, which is consistent with the structural analysis. The effective magnetization was obtained from the FMR spectra and it was found that it decreased with increasing Sn-concentration and reached a minimum value at Co2MnAl0.25Sn0.75 composition. Lastly, Co2MnAl1−x Snx films annealed at 500 °C showed the best crystal ordering. The lowest α value was 0.008 and obtained from Co2MnAl0.5Sn0.5 films annealed at 500 °C.

ABSTRACT In this work we studied the effect of partial Fe substitution for Ti on the structural and magnetic properties of the Co2TiAl. X-ray diffraction analysis indicates the presence of B2 type disorder for x > 0, (111) reflections are absent for x > 0 which is the characteristic of B2 type disorder. XRD analysis also shows presence of second phase. Magnetization measurements also confirm the presence of dual phase. Curie temperature of the alloys increases with increase in Fe concentration. Saturation magnetic moments agree very well with those calculated by Slater-Pauling rule.

In this article, we investigate the predictions of the first principles on structural stability, electronic and mechanical properties of 2D nanostructures: graphene, silicene, germanene and stenane. The electronic band structure and density of states in all these 2D materials are found to be generic in nature. A small band gap is generated in all the reported materials other than graphene. The linearity at the Dirac cone changes to quadratic, from graphene to stenane and a perfect semimetalicity is exhibited only by graphene. All other 2D structures tend to become semiconductors with an infinitesimal band gap. Bonding characteristics are revealed by density of states histogram, charge density contour, and Mulliken population analysis. Among all 2D materials graphene exhibits exotic mechanical properties. Analysis by born stability criteria and the calculation of formation enthalpies envisages the structural stability of all the structures in the 2D form. The calculated second order elastic stiffness tensor is used to determine the moduli of elasticity in turn to explore the mechanical properties of all these structures for the prolific use in engineering science. Graphene is found to be the strongest material but brittle in nature. Germanene and stenane exhibit ductile nature and hence could be easily incorporated with the existing technology in the semiconductor industry on substrates.

Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanoparticles (NPs). The NPs of size 23±10 nm are prepared using the co-precipitation method. X-ray and selected area electron diffraction patterns have confirmed the cubic Heusler phase of the NPs with the A2-disorder. These NPs are soft ferromagnetic, and exhibit a high saturation magnetization (Ms) along with a very high Curie temperature (Tc) of 1060 K. The observed Tc value matches closely with the theoretically calculated one following a model provided by Wurmehl et al. (2005). The high Ms and Tc make the present system a potential candidate for magnetically activated nano-devices working at high temperatures. The near-integral value 5.9 μB/f.u. of Ms at low temperatures indicates that the half-metallic ferromagnetism is preserved even in the particles even on the 20 nm length scale. Additionally, we have facilitated the existing HA-NP preparation method, which can be used in synthesizing other HA-NPs. The first-principles density functional theory computations complement the experimental results.

Abstract: The Mn L_2, L_3 x-ray spectra of the Cu_2MnAl and Co_2MnZ (Z= Al, Ga, Sn, Sb) Heusler alloys have been investigated by the Resonant X-ray Emission Spectroscopy (RXES) using linearly polarized monochromatic synchrotron radiation for. The interplay between the half-metallic character of the Mn 3d electronic structure in connection with the local magnetic moment $\ mu_ {Mn} $ and Mn $2 p\ to 3d $ x-ray emission spectra is discussed.