Local Spin Density Calculations Research Articles

Stereoelectronic Properties of 1,2,4-Triazole-Derived N-heterocyclic Carbenes - A Theoretical Study

Stereoelectronic Properties of 1,2,4-Triazole-Derived N-heterocyclic Carbenes - A Theoretical Study

Accurate Homogeneous Electron Gas Exchange-Correlation Free Energy for Local Spin-Density Calculations

An accurate analytical parametrization for the exchange-correlation free energy of the homogeneous electron gas, including interpolation for partial spin polarization, is derived via thermodynamic analysis of recent restricted path integral MonteCarlo (RPIMC) data. This parametrization constitutes the local spin density approximation (LSDA) for the exchange-correlation functional in density functional theory. The new finite-temperature LSDA reproduces the RPIMC data well, satisfies the correct high-density and low- and high-T asymptotic limits, and is well behaved beyond the range of the RPIMC data, suggestive of broad utility.

First principles study on spin and orbital magnetism in CuFeSb and CuCoSb alloys with C1 b structure

In this work the effect of spin–orbit coupling and orbital polarization (OP) corrections on the spin and orbital magnetism of two half-Heusler alloys (CuFeSb and CuCoSb) is investigated by means of local spin density calculations. It is demonstrated that OP corrections predict large orbital moments for Fe and Co individual atoms in the considered compounds. It is found that the Heusler alloys with C1 b structure are potential candidates to show large orbital magnetism.

Self-interaction corrected local spin density calculations of actinides

We use the self-interaction corrected local spin-density approximation in order to describe localization-delocalization phenomena in the strongly correlated actinide materials. Based on total energy considerations, the methodology enables us to predict the ground-state valency configuration of the actinide ions in these compounds from first principles. Here we review a number of applications, ranging from electronic structure calculations of actinide metals, nitrides and carbides to the behaviour under pressure of intermetallics, and O vacancies in PuO2.

Magnetic Circular Dichroism in Two-Photon Photoemission

We report the observation of magnetic circular dichroism (MCD) in two-photon photoemission (2PPE). The Heusler alloys Ni2MnGa and Co2FeSi were investigated by excitation with femtosecond laser light, showing MCD asymmetries of A=(3.5+/-0.5)x10;{-3} for Ni2MnGa and of A=(2.1+/-1.0)x10;{-3} for Co2FeSi, respectively. A theoretical explanation is provided based on local spin-density calculations for the magnetic dichroic response; the computed 2PPE MCD agrees well with the experiment. The observed 2PPE magnetic contrast represents an interesting alternative for future time-resolved photoemission studies on surface magnetism practicable in the laboratory.

Ferrimagnetism inEuFe4Sb12due to the Interplay off-Electron Moments and a Nearly Ferromagnetic Host

We combine x-ray magnetic circular dichroism spectroscopy at Fe L2,3 edges, at Eu M4,5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu0.95Fe4Sb12 is a ferrimagnet in which the Fe 3d moment and the Eu2+ 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L3 edge XAS, we find that about 13% of the Eu have a formal valence of 3+. We ascribe the origin of ferrimagnetism at a relatively high transition temperature TC of 85 K in Eu0.95Fe4Sb12 to f-electron interaction with the nearly ferromagnetic [Fe4Sb12]2.2- host lattice.

Spin and orbital magnetism in full Heusler alloys: A density functional theory study ofCo2YZ(Y=Mn,Fe;Z=Al,Si,Ga,Ge)

The effect of spin-orbit coupling and orbital polarization (OP) corrections on the spin and orbital magnetism of full Heusler alloys is investigated by means of local spin-density calculations. It is demonstrated that OP corrections are needed to explain the experimental orbital moments ${M}_{l}$. Model calculations employing one ligand field parameter yield the correct order of magnitude of ${M}_{l}$, but do not account for its quantitative composition dependence. Spin-orbit coupling reduces the degree of spin polarization of the density of states at Fermi level by a few percent. We provide arguments that ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Ga}$ and the ${\mathrm{Co}}_{2}YZ$ compounds with $Y=\mathrm{Fe}$ might not be half metals as suggested by recent experiments for $Z=\mathrm{Si}$.

High-temperature anti-Invar behavior ofγ-Fe precipitates inFexCu100−xsolid solutions: Ferromagnetic phases

High-temperature magnetization and neutron diffraction measurements on metastable ${\mathrm{Fe}}_{x}{\mathrm{Cu}}_{100\ensuremath{-}x}$ solid solutions have recently shown to imply that $\ensuremath{\gamma}$-Fe precipitates present ferromagnetic anti-Invar behavior. For this reason, we have studied the ferromagnetic phases of $\ensuremath{\gamma}$-Fe in moment-volume parameter space, using the general potential linearized-augmented plane-wave method and the fixed spin moment procedure in order to calculate the corresponding total energy. We find that only two ferromagnetic phases (one related to a low-spin state and the other to a high-spin state) can exist and even coexist in limited volume ranges (3.55-3.59 \AA{}). Hence, our results provide a ``revisited'' version of the local spin density calculations used in the early article by Moruzzi et al. [Phys. Rev. B 34, 1784 (1986)]. In addition, the fixed spin moment method---using an energy-moment-volume space representation---allows us to conclude that the high-spin state is the ground state of the $\ensuremath{\gamma}$-Fe precipitates, as the anti-Invar behavior is an intrinsic property of these states. This simple scenario seems to adequately describe the perplexing phenomenology recently observed on ${\mathrm{Fe}}_{x}{\mathrm{Cu}}_{100\ensuremath{-}x}$ solid solutions.

Spin-polarized surface state of MnSb(0 0 0 1)

Knowledge of the spin-dependent electronic structure at surfaces and interfaces plays an increasingly important role when assessing possible use of novel magnetic materials for spintronic applications. It is shown that spin- and angle-resolved photoelectron spectroscopy together with ab initio electronic structure methods provides a full characterization of the surface electronic structure of ferromagnetic MnSb(0 0 0 1). Two different surface reconstructions have been compared in spin- and angle-resolved valence-band photoemission. For annealing at elevated temperatures, the (1 × 1)-structure transforms into 2 × 2 and a majority-spin peak appears at −1.7 eV inside a majority-spin bulk band gap at the surface Brillouin zone centre. Its sensitivity to oxygen supports an interpretation as magnetic compound surface state. Local spin density calculations predict at the same energy (−1.75 eV) a prominent d surface state of majority spin for (1 × 1)-Mn terminated MnSb(0 0 0 1) but no such feature for (1 × 1)-Sb termination. The calculation shows that neither the bulk nor the surface is half-metallic, in agreement with the expectation for the hexagonal NiAs structure.

Spin polarization and band alignments at NiMnSb/GaAs interface

The half-metallic NiMnSb compound is recently attracting renewed interest for its application in devices making use of spin polarized charges injected into conventional semiconductor. For technological purposes it is desirable to design interfaces where the peculiar half-metallic character of NiMnSb is not degraded. We focus here on the NiMnSb/GaAs(0 0 1) heterojunction that we investigate by accurate pseudopotential density functional—local spin density calculations. We find in general that the half-metallicity is lost when NiMnSb is joined with the semiconductor, even at the interface with a mixed (Mn,As) plane which is one of the most promising for maintaining the desired half-metallicity. In this case, however, the effect is localized just on the interfacial atoms. Electronic and magnetic bulk properties are recovered within a couple of atomic planes far from the interface, so that the band alignments are well defined.

Adsorbate-induced demagnetization and restructuring of ultrathin magnetic films: CO chemisorbed onγ-Fe/Cu(100)

First-principles local-spin-density (LSD) investigations of the structural, magnetic, and electronic properties of clean and CO-adsorbed ultrathin $\ensuremath{\gamma}$-iron films epitaxially grown on Cu(100) surfaces demonstrate that both the geometrical and the magnetic structures of the films are profoundly modified by the adsorption of CO. The enhanced magnetic moments of the top-layer atoms are strongly quenched by the presence of the adsorbate. Due to the pronounced magnetovolume effect, this leads also to a correlated change in the interlayer relaxations. Strikingly, the adsorbate-induced demagnetization is primarily limited to those surface atoms directly bonded to the adsorbate. This leads to the formation of an in-plane magnetic pattern in a partially adsorbate-covered film. The comparison of the calculated vibrational eigenfrequencies of the CO adsorbate with experiment confirms the picture based on the LSD calculations.

First principles calculations of the electronic structure of Fe/sub 1-x/Co/sub x/Pt

The L1/sub 0/ alloys CoPt and FePt have strong uniaxial magneto-crystalline anisotropy as a consequence of the layering of Co or Fe and Pt planes. This makes these compounds of interest in permanent magnet applications. In this work we present the results of a study of the magnetic properties of the L1/sub 0/ alloys Fe/sub 1-x/Co/sub x/Pt for the range of compositions from FePt to CoPt. Local spin density calculations using the coherent potential approximation are used to follow trends in the magneto-crystalline anisotropy and magnetic moments across the composition range. Total energy calculations of ordered and disordered alloys are used to develop a pseudo-binary phase diagram of the Fe/Co layer predicting regions of phase separation in this layer at temperatures below 650/spl deg/C.

Ab initio local-spin-density study of the structural and magnetic properties of La 1− xCa xMnO 3 systems

Ab-initio local-spin density calculations of the structural and magnetic properties of mixed La 1− x Ca x MnO 3 perovskites showing colossal magnetoresistance are reported. For LaMnO 3 our calculations predict an orthorhombic (Pnma) distortion of the crystal, a layered-type antiferromagnetic order and a narrow insulating gap ( E g=162 meV). CaMnO 3 is predicted to be nearly cubic, insulating ( E g=452 meV) and shows rock-salt type antiferromagnetism. Substitution of La by Ca leads to a gradual reduction of the geometrical distortion. At a 50/50 ratio of Ca and La, the groundstate is collinear layered antiferromagnetic and metallic. In disordered alloys with x∼0.25 or x∼0.75 the substitutional disorder leads to a weakly noncollinear groundstate, the canting of the Mn-moments being larger in the La-rich regime.

Electronic states and magnetism of ultrathin Fe/Cu/Si(1 1 1) films

Epitaxial Fe/Cu/Si(1 1 1) ultrathin overlayers with Fe thicknesses ranging from 1 to 3 ML (monolayer) have been grown as well as a 30 ML thick iron film. The evolution of their electronic and magnetic properties next to the iron fcc–bcc structural transition has been investigated by using several UHV techniques and compared to experimental results obtained for a completely bcc Fe/Cu heterostructure. For fcc Fe coverages we found that at room temperature the magnetic moment, m, is nearly zero contrary to that obtained by local spin density calculations which reported for a surface iron layer an m value even higher than in the bulk. Our measurements of electronic density of states suggest that the ultrathin film exchange splitting is even lower than that of bulk iron contrary to what is expected by theory. The observed decrease of the exchange splitting is not sufficient to explain the macroscopic magnetic behaviour.

Ultrathin Mn films on Cu(111) substrates: Frustrated antiferromagnetic order

Using local-spin density calculations performed in the generalized-gradient approximation we have examined the possibility that Mn monolayers epitaxially grown on Cu(111) substrates represent a physical realization of the frustrated antiferromagnetic planar (AFP) model. Indeed, we find that the noncollinear ground state of the AFP model with $\ifmmode\pm\else\textpm\fi{}120\ifmmode^\circ\else\textdegree\fi{}$ orientations of the nearest-neighbors is also the ground state of Mn/Cu(111). However, due to the rather long-range antiferromagnetic interactions, the energetic preference over a raw-wise antiferromagnetic order is only very weak.

Low-level DFT calculations II: molecules containing fluorine and second-row atoms

X α local spin density calculations were performed for a body of 112 molecules containing fluorine and second-row elements, silicon, phosphorus, sulphur and chlorine. The parameter α was treated as a variable and adapted to each molecule, depending upon the number and type of bonds involved. The X α atomization energies are in acceptable agreement with their experimental counterparts. The standard deviations, 5.6 and 6.2 kcal mol −1 for the X α(6-31G ∗∗) and X α(3-21G) methods, respectively, compare well with those obtained earlier for molecules containing only first-row elements. Compared to the sophisticated self-consistent hybrid B3LYP model, used here with a 6-311G(d,p) basis, our simplified approach permits considerable savings of computer time. It is shown that the 3-21G orbital basis generally offers a reasonable compromise between accuracy and economy.

Magnetic structure at copper–permalloy interfaces

First principles local spin density calculations of the magnetic structure of interdiffused Cu/Py multilayers were performed. It was possible to solve the Kohn–Sham [Phys. Rev. 140, A1133 (1965)] equations for both canted and collinear magnetic arrangements to a high level of convergence. We found multiple, metastable, noncollinear magnetic structures that showed a reduced contribution to the saturation magnetization consistent with roughly one “magnetic dead” layer per interface. These canted interface layers were stable with respect to collinear interfaces by about 2 mRy per atom at the interface. The calculated noncollinear total spin magnetic moments as a function of the number of Py layers were in good agreement with the experiment.

Silicon vacancy in SiC: A high-spin state defect

We report results from spin-polarized ab initio local spin-density calculations for the silicon vacancy (VSi) in 3C– and 2H–SiC in all its possible charge states. The calculated electronic structure for SiC reveals the presence of a stable spin-aligned electron-state t2 near the midgap. The neutral and doubly negative charge states of VSi in 3C–SiC are stabilized in a high-spin configuration with S=1 giving rise to a ground state, which is a many-electron orbital singlet T13. For the singly negative VSi, we find a high-spin ground-state A24 with S=3/2. In the high-spin configuration, VSi preserves the Td symmetry. These results indicate that in neutral, singly, and doubly negative charge states a strong exchange coupling, which prefers parallel electron spins, overcomes the Jahn–Teller energy. In other charge states, the ground state of VSi has a low-spin configuration.

Theory of hyperfine interactions for iron-acceptor pairs in silicon based on ab initio calculations

We present theoretical calculations of the Zeeman g-factor and of hyperfine interactions for trigonal and orthorhombic (-) pairs. Our calculations are based on magnetization densities obtained from ab initio local spin density calculations, making use of two fitting parameters for those values that cannot be extracted from our ab initio results. We show by a comparison with experimental magnetic resonance data that the large spread of absolute values and anisotropies of the experimental data can naturally be accounted for.

Extended Moment Formation and Second Neighbor Coupling inLi2CuO2

Comprised of ferromagnetic edge-sharing CuO$_2$ chains that order in antialigned fashion at T$_N$=9 K, Li$_2$CuO$_2$ is found from local spin density calculations to display several surprising characteristics: (1) the ordered moment/f.u. of 0.92 $\mu_B$ is the largest for any low dimensional cuprate system, in agreement with experiment, (2) 40% of this moment lies on the neighboring O ions, making it the largest oxygen moment yet reported, and (3) the second neighbors couplings are larger than nearest neighbors couplings. All of these phenomena arise naturally due to a well defined effective $d_{yz}$ type orbital that includes very strong O $p_{\sigma}$ character. We interpret the large moment as surviving reduction by quantum fluctuations due to extension caused by the $d-p$ hybridization.