Showing papers by "Arthur J Freeman published in 1997"

Relativistic spin-polarized theory of magnetoelastic coupling and magnetic anisotropy strain dependence: Application to Co/Cu(001)

Abstract: A self-consistent relativistic spin-polarized version of the total-energy full-potential linearized-augmented-plane-wave (FLAPW) method is developed on the basis of a second-variation treatment of the spin-orbit coupling (SOC). As illustration, the method is applied to determine the magnetoelastic coupling, orbital magnetic moment anisotropy and magnetic anisotropy energy (MAE) of a Co overlayer on Cu(001). The MAE (-0.36 meV) calculated at the equilibrium overlayer/substrate distance is in good agreement with experiment. As discovered earlier by Wu and Freeman, we find a linear dependence of the MAE on the overlayer/substrate distance. The calculated positive effective magnetoelastic coupling coefficient (1.13 meV) is caused by a positive surface magnetoelastic anisotropy (0.23 meV). This causes a negative magnetostriction coefficient ${\ensuremath{\lambda}}_{001}=\ensuremath{-}5.20\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and an isotropic magnetostriction coefficient ${\ensuremath{\lambda}}_{s}=\ensuremath{-}5.65\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ that is in very good agreement with previous studies based on a perturbative SOC treatment.

Dynamical and geometrical aspects of NO chemisorption on transition metals: Rh, Pd, and Pt

Abstract: The technical relevance of Rh, Pd, and Pt as catalysts used to reduce ${\mathrm{NO}}_{\mathrm{x}}$ pollutants has stimulated great interest in a detailed understanding of the chemisorption process of these systems. While experiment indicates Rh to be a better catalyst than the other transition metals, theory still does not give a satisfying explanation for this behavior. We have examined the c(2\ifmmode\times\else\texttimes\fi{}2) atop chemisorption of NO on Rh, Pd, and Pt substrates with our full-potential linearized augmented-plane-wave method for thin films. Simultaneous relaxation of the NO bond length and the distance between the metal and N, as well as surface relaxation of the metal, was performed. Various vibration frequencies were determined from the dynamical matrix of the system. The analysis of the dynamical matrix shows stronger bonding of NO to Pd and Pt compared to the Rh surface. We find the metal surface to be strongly affected by NO chemisorption, including a buckling that is about 50% larger on Pd and Pt than on the Rh surface. While the calculated electronic properties, such as density of states, give very similar results for the three systems the geometric and dynamical properties may explain the observed higher efficiency of Rh as a catalyst.

Structural and electronic properties of transition-metal/batio3(001) interfaces

Abstract: Electronic and structural properties of transition-metal/BaTiO{sub 3}(001) interfaces are studied by first-principles local-density full-potential linearized augmented plane-wave calculations with slab models. Equilibrium interlayer separations between metal overlayers (for the 5d metals Ta, W, Ir, and Pt) and the BaTiO{sub 3} substrate are calculated by total-energy determinations. It is found that the preferred adsorption site for metal atoms on the BaTiO{sub 3} surface is above the O site and the metal-oxygen distance increases from Ta to Pt while the binding energy decreases. Significant hybridization is found between metal d states and the O 2p{endash}Ti 3d states. The Fermi levels of the metals lie in the gap of BaTiO{sub 3} and metal-induced gap states, as suggested by Heine{close_quote}s theory [Proc. Phys. Soc. London {bold 81}, 300 (1962); Surf. Sci. {bold 2}, 1 (1964); Phys. Rev. {bold 138}, A1689 (1965)], are observed. The Schottky barrier in the interfaces is calculated by the position of E{sub F} in the gap and the dependence of the barrier height on the metal work function is different from either Schottky and Mott{close_quote}s or Bardeen{close_quote}s [Phys. Rev. {bold 71}, 717 (1947)] speculation. {copyright} {ital 1997} {ital The American Physical Society}

LDA simulations of pressure-induced anomalies in c/a and electric-field gradients for Zn and Cd

Abstract: We present results of {ital ab initio} simulations of the effect of hydrostatic pressure on the electronic structure, lattice parameters, and electric-field gradients (EFG) for hcp Zn and Cd using the full-potential linear muffin-tin orbital method in conjunction with the new Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) to the density functional for exchange correlation. Theoretical equilibrium volumes for Zn and Cd are found to be in excellent agreement with experiment (whereas non-GGA corrected local density approximation underestimates them by as much as 10{percent}). We find an anomaly in the pressure dependence of c/a at reduced unit cell volumes (at V/V{sub 0}{approx_equal}0.89 for Zn and in a broad region from V/V{sub 0}=0.92 to 0.85 for Cd) and a similar anomaly in the EFG tensor. At the same time we do not find the electronic topological transition due to the destruction of a giant Kohn anomaly which was previously thought to be responsible for the lattice anomalies in Zn. {copyright} {ital 1997} {ital The American Physical Society}

Electronic structure of TlBa 2 CaCu 2 O 7-δ

Abstract: The core levels of ${\mathrm{TlBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (Tl-1212) epitaxial films have been measured with x-ray photoelectron spectroscopy (XPS). The valence electronic structure has been determined using the full-potential linear muffin-tin-orbital band-structure method and measured with XPS. The calculations show that a van Hove singularity (VHS) lies above the Fermi level (${\mathrm{E}}_{\mathrm{F}}$) for the stoichiometric compound (\ensuremath{\delta}=0), while for 50% oxygen vacancies in the Tl-O layer (\ensuremath{\delta}=0.5) ${\mathrm{E}}_{\mathrm{F}}$ is in close proximity to the VHS. Samples annealed in nitrogen (to reduce the hole overdoping by the removal of oxygen) exhibit higher core-level binding energies and a higher ${\mathrm{T}}_{\mathrm{c}}$, consistent with a shift of ${\mathrm{E}}_{\mathrm{F}}$ closer to the VHS. Comparisons are made to the core levels and valence bands of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Tl-2212) and ${\mathrm{HgBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ (Hg-1212). The similarity of the Cu ${2\mathrm{p}}_{3\mathrm{/}2}$ spectra for Tl-1212 and Tl-2212 indicates that the number of Tl-O layers has little effect on the Cu-O bonding. However, the Tl-1212 and Hg-1212 Cu ${2\mathrm{p}}_{3\mathrm{/}2}$ signals exhibit differences which suggest that the replacement of ${\mathrm{Tl}}^{3+}$ with ${\mathrm{Hg}}^{2+}$ results in a decrease in the O 2p\ensuremath{\rightarrow}Cu 3d charge-transfer energy and differences in the probabilities of planar vs apical oxygen charge transfer and/or Zhang-Rice singlet-state formation. Differences between the Tl-1212 and the Tl-2212 and Hg-1212 measured valence bands are consistent with the calculated Cu 3d and (Tl,Hg) 6s/5d partial densities of states.

Strain-induced magnetocrystalline anisotropy in Ni on Cu(001)

Abstract: The origin of the observed perpendicular magnetic anisotropy (PMA) in Ni/Cu(001) for thick Ni layers is investigated using the ab initio full potential linearized augmented plane wave method. With the aid of the state tracking and torque approaches, very stable results are obtained for the magnetocrystalline anisotropy (MCA) energies for both the Ni/Cu(001) overlayer systems and the distorted bulk fct Ni. We find that the PMA is due mainly to the strain induced bulk contribution rather than to interfacial hybridization. The calculated value of the bulk MCA energy, 65 μeV/atom, is very close to recent experimental data extrapolated to zero temperature, 70 μeV/atom.

Effect of lattice distortions on the competition between the double and superexchange mechanisms in LaMnO3

Abstract: Double exchange (DE), superexchange (SE), and higher-order contributions to the exchange interaction energy between Mn layers in ${\mathrm{LaMnO}}_{3}$ are studied using the linear-muffin-tin-orbital method as generalized to treat noncollinear magnetic configurations. The degree of the internal lattice distortion $(f)$ and the angle $(\ensuremath{\theta})$ between ferromagnetically ordered Mn layers are considered as simulation parameters. We find that both global and internal lattice distortions dramatically influence the character of the exchange interactions: global distortions associated with variations of the apical Mn-O bond length promotes the DE contribution; the bending of the Mn-O bonds in the $(a\ensuremath{-}b)$ plane suppresses the DE and promotes the antiferromagnetic SE contribution to the interlayer exchange energy. Overall, the character of exchange interactions is determined by SE contributions: in hypothetical ferromagnetic phases DE interactions mediated by itinerant electrons is present but still rather small compared with SE; in antiferromagnetic phases this DE is negligible and non-Heisenberg terms are small compared with SE. We find further that a metal-to-semiconductor transition (in the sense of a band-gap opening) occurs in the range of parameters of magnetic and lattice distortion variations given by $\ensuremath{\theta}g~120\ifmmode^\circ\else\textdegree\fi{}$ and $fg~$ 0.7.

Electric fields and valence-band offsets at strained [111] heterojunctions

Abstract: Ab initio full-potential linearized augmented plane wave (FLAPW) [H. J. F. Jansen and A. J. Freeman, Phys. Rev. B 30, 561 (1984); M. Weinert, H. Krakauer, E. Wimmer, and A. J. Freeman, ibid. 24, 864 (1981)] calculations have been performed for the [111] ordered common atom strained layer superlattices (in particular, the common-anion GaSb/InSb system and the common-cation InAs/InSb system). We have focused our attention on the potential line up at the two sides of the homopolar isovalent heterojunctions considered, and, in particular, on its dependence on the strain conditions and on the strain induced electric fields. We propose a procedure to locate the interface plane, where the band alignment could be evaluated; furthermore, we suggest that the polarization charges, due to piezoelectric effects, are approximately confined to a narrow region close to the interface and do not affect the potential discontinuity. We find that the interface contribution to the valence band offset is substantially unaffected by strain conditions, whereas the total band line up is highly tunable as a function of the strain conditions. Finally, we compare our results with those obtained for the [001] heterojunctions.

Effects of interfacial relaxation on the magnetic coupling of Mn/Fe(111)

Abstract: The structural, electronic, and magnetic properties of monolayer Mn on an Fe(111) substrate are determined using the local spin density total energy full potential linearized augmented plane wave method and atomic force approach. A strong interplay between magnetism and atomic structure is found, e.g., the interfacial Fe layer undergoes a large downward relaxation by 0.59 a.u. from the ideal bcc position. As was found previously for bulk fcc Mn and Mn/Fe(001), the equilibrium Mn–Fe bond length is larger in the ferromagnetic state (4.47 a.u.) than that in the antiferromagnetic (AFM) state (4.31 a.u.). The AFM coupling between Mn and Fe is the ground state. The interplay between the interfacial magnetic coupling in the Mn/Fe(111) and the multilayer relaxation is discussed.

Induced magnetism of 4d transition metals: Rh and Ru/Fe(001) overlayers and sandwiches

Abstract: The structural, electronic, and magnetic properties of mono and bilayer Rh and Ru transition metals deposited on a Fe(001) substrate or sandwiched by an additional Fe layer have been determined by using the highly precise local density full-potential linearized augmented plane-wave method. Significantly, the atomic force determinations demonstrate that a large interfacial relaxation plays an important role in their electronic and magnetic properties. Both mono and bilayer Ru and Rh in the systems considered are found to be able to retain large spin magnetic moments. The ground state of bilayer Rh is found to have its moments coupled antiferromagnetically. In good agreement with x-ray magnetic circular dichroism (MCD) measurements, the deposition of Rh layers on Fe(001) is found to enhance the Fe spin magnetic moments in the interface. Calculated values of the spin and orbital parts of the magnetic moments and the MCD spectra are presented, and the role of overlayer-substrate hybridization is discussed.

Relativistic spin - polarized theory of magnetoelastic coupling and magnetic anisotropy strain dependence: application to Co/Cu(001).

Abstract: A self-consistent relativistic spin-polarized version of the total-energy full-potential linearized-augmented-plane-wave (FLAPW) method is developed on the basis of a second-variation treatment of the spin-orbit coupling (SOC). As illustration, the method is applied to determine the magnetoelastic coupling, orbital magnetic moment anisotropy and magnetic anisotropy energy (MAE) of a Co overlayer on Cu(001). The MAE (-0.36 meV) calculated at the equilibrium overlayer/substrate distance is in good agreement with experiment. As discovered earlier by Wu and Freeman, we find a linear dependence of the MAE on the overlayer/substrate distance. The calculated positive effective magnetoelastic coupling coefficient (1.13 meV) is caused by a positive surface magnetoelastic anisotropy (0.23 meV). This causes a negative magnetostriction coefficient ${\ensuremath{\lambda}}_{001}=\ensuremath{-}5.20\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and an isotropic magnetostriction coefficient ${\ensuremath{\lambda}}_{s}=\ensuremath{-}5.65\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ that is in very good agreement with previous studies based on a perturbative SOC treatment.

Structural and electronic properties of GaN/Al interfaces

Abstract: The structural and electronic properties of the GaN/Al interface are determined from first principles local density full potential linearized augmented plane wave (FLAPW) calculations. The charge distribution of the gap states as a function of the distance from the interface shows that the gap states induced into the semiconductor by the presence of Al are strongly localized in the junction region. Furthermore, we find that Al does not provide good ohmic contacts on the clean nitrides considered, in contrast with experimental results on chemically treated GaN, but in agreement with recent measurements on the clean surface[1]. We also study some auxiliary systems (all grown on a GaN substrate), i.e. the Al/AlN interface, the GaN/AlN heterojunction and the GaN/Al with an AlN intralayer (GaN-AlN/Al). The transitivity rule for the GaN/Al, AlN/Al and GaN/AlN interfaces is fairly well satisfied and small differences must be ascribed to differences in the interface morphology. Finally, we find that the AIN intralayer does not significantly affect the p-type Schottky barrier height of the GaN/Al interface.

Exchange coupling and electronic structure of La1−xCaxMnO3: Effect of lattice distortion, canting, and doping (abstract)

Abstract: The influence of lattice distortion, canting of magnetic moments, and doping on the exchange coupling and the electronic structure of La1−xCaxMnO3 was studied using a generalization of the linear muffin-tin orbital (LMTO) method for noncollinear magnetic ordering1 and the LMTO–Green function technique in single-site approximation.2 Jahn–Teller type lattice distortions (f), including rotation and tilting of the oxygen octahedra, were varied from the ideal orthorhombic phase (f=0) to the observed structure3 (f=1). The canting angle (Θ) is considered also as a simulation parameter which is varied from Θ=0° ferromagnetic (FM) phase to Θ=180° antiferromagnetic (AFM). The effect of doping was simulated for x=1.,0.5,0 ordered alloys. We found that (i) doping and change of the magnetic ordering (from FM to AFM) promotes the competition between positive eg and negative t2g contributions to the exchange coupling between ferromagnetic sheets (J⊥); (ii) the J⊥ exchange is strongly of non-Heisenberg type, the bilinear...

Interactions of point and extended defects in structural intermetallics: real-space LMTO-recursion calculations

Abstract: A real-space TB-LMTO-recursion method for electronic structure calculations is applied to the study of interacting extended and point defects in NiAl. Results of calculations for the pure intermetallic and with ternary additions (within a supercell model) show good agreement with band structure results. Further, electronic structure and total energy calculations of point (single impurity, M=Ti, V, Cr, Mn, Fe and Co) and planar defects such as anti-phase boundaries (APB) were carried out and the interaction between them was determined. We found that for the ½〈111〉{110} APB in NiAl, ternary additions occupy exclusively the 3d-metal sublattice and decrease the APB energy (except for Co). Finally, we employ TB-LMTO-REC to study the electronic structure of the most complex extended defect, a dislocation. We demonstrate for the 〈100〉{010} edge dislocation in NiAl that: (i) quasi-localized states may exist as a result of specific lattice distortions in the dislocation core with a type of “broken” bonds; (ii) the electronic structure changes appreciably in the process of dislocation motion; (iii) van-Hove singularities present in the ideal crystal may be shifted to E;r as a result of the dipolar character of the deformations in the dislocation core.

Magnetic Effects at Surfaces and Interfaces (Including Grain Boundaries)

Abstract: First-principles electronic structure studies based on local spin density functional theory and performed on extremely complex simulations of ever increasingly realistic systems, play a very important role in explaining and predicting surface and interface magnetism. This has led to solving even more challenging problems like the embrittlement of the Fe grain boundary, discussed here. Now, a major issue for first-principles theory is the treatment of the weak spin-orbit coupling (SOC) in magnetic transition metals and their alloys and its subsequent effects: (i) A major breakthrough in eliminating the numerical randomness for the determination of the magneto-crystalline anisotropy was made with the state-tracking and torque approaches. This now enables us to treat magnetostriction and its inverse effect, strain-induced magnetic anisotropy in transition metal bulk, thin films and alloys, (ii) The magneto-optical Kerr effects and x-ray magnetic circular dichroism are now directly calculated and compared with experiment. In all this work, and more recently, on the first-principles calculations of giant magneto-resistance in multilayers, extensive first-principles calculations and model analyses provide simple physical insights and guidelines to search for new magnetic recording and sensor materials.