F. Máca

Magnetism without magnetic impurities in ZrO2 oxide

We present an ab initio study of the magnetism induced in ZrO2 dioxide by substitution of the cation by an impurity from the groups 1A or 2A of the Periodic Table (K and Ca). It is demonstrated that the K impurity leads to a robust induced magnetic moment on the surrounding O atoms in the cubic ZrO2 host whilst Ca impurity leads to a nonmagnetic groundstate. The estimated Curie temperature is above room temperature.

A self-consistent surface-Green-function (SSGF) method

Abstract We describe the basic aspects of a new, self-consistent Green-function method which allows to calculate the density of states, electron density, and related quantities for a localized perturbation (e.g. an isolated adsorbate or an intrinsic surface defect) at a crystal surface. The method is based on the density-functional theory and combines several ideas from recent theoretical developments, as, for example, from the layer-KKR Green-function method, from ab-initio pseudopotential theory, and from the self-consistent defect-Green-function method. Two applications of the method are presented (S on Pd(1 0 0) and Na on Al(1 0 0)) in order to demonstrate its efficiency and to address a recent controversial discussion concerning the nature of the bonding of alkali adsorbates on metals at very low coverage (Θ → 0).

Lattice constant in diluted magnetic semiconductors (Ga,Mn)As

Institute of Physics, AS CR, Na Slovance 2, CZ–182 21 Prague 8, Czech Republic(Dated: February 7, 2002)We use the density-functional calculations to investigate the compositional dependence of thelattice constant of (Ga,Mn)As containing various native defects. The lattice constant of perfectmixed crystals does not depend much on the concentration of Mn. The lattice parameter increasesif some Mn atoms occupy interstitial positions. The same happens if As antisite defects are present.A quantitative agreement with the observed compositional dependence is obtained for materialsclose to a complete compensation due to these two donors. The increase of the lattice constant of(Ga,Mn)As is correlated with the degree of compensation: the materials with low compensationshould have lattice constants close to the lattice constant of GaAs crystal.

The adsorption of sulphur on Pd(111). I: A LEED analysis of the (sqrt{2} x sqrt{3}) R 30° adsorbate structure

Abstract The adsorption of sulphur on the Pd(111) surface is studied by low energy electron diffraction (LEED). Four different adsorbate structures are identified. LEED intensity analyses are performed for the clean surface and for the ordered initial adlayer, i.e. the ( 3 × 3 ) R 30° S adsorption phase. It is found that the sulphur atoms occupy threefold-symmetric hollow sites, with a SPd chemisorption bond length of 0.222±0.003 nm.

Tetragonal phase of epitaxial room-temperature antiferromagnet CuMnAs

Recent studies have demonstrated the potential of antiferromagnets as the active component in spintronic devices. This is in contrast to their current passive role as pinning layers in hard disk read heads and magnetic memories. Here we report the epitaxial growth of a new high-temperature antiferromagnetic material, tetragonal CuMnAs, which exhibits excellent crystal quality, chemical order and compatibility with existing semiconductor technologies. We demonstrate its growth on the III-V semiconductors GaAs and GaP, and show that the structure is also lattice matched to Si. Neutron diffraction shows collinear antiferromagnetic order with a high Néel temperature. Combined with our demonstration of room-temperature-exchange coupling in a CuMnAs/Fe bilayer, we conclude that tetragonal CuMnAs films are suitable candidate materials for antiferromagnetic spintronics.

Microscopic Analysis of the Valence Band and Impurity Band Theories of (Ga,Mn)As

We analyze microscopically the valence and impurity band models of ferromagnetic (Ga,Mn)As. We find that the tight-binding Anderson approach with conventional parametrization and the full potential local-density approximation+U calculations give a very similar band structure whose microscopic spectral character is consistent with the physical premise of the k·p kinetic-exchange model. On the other hand, the various models with a band structure comprising an impurity band detached from the valence band assume mutually incompatible microscopic spectral character. By adapting the tight-binding Anderson calculations individually to each of the impurity band pictures in the single Mn impurity limit and then by exploring the entire doping range, we find that a detached impurity band does not persist in any of these models in ferromagnetic (Ga,Mn)As.

The Influence of Hydrogen Adsorption on Magnetic Properties of Ni/Cu(001) Surface

Ni/Cu(001) is known as a unique system showing the spin-reorientation transition from an in-plane to out-of-plane magnetization direction when the Ni-overlayer thickness is increased. We investigate different relaxed multilayer structures with a hydrogen adlayer using the full-potential linearized augmented plane-wave method. The relaxed geometries, determined by total energy and atomic force calculations, show that H-monolayer strongly influences the interlayer distance between the Ni-surface and sub-surface layers yielding the outward relaxation of Ni-layer at H/Ni interface. Furthermore, large decrease of local magnetic moments at the top surface area is found for the surface covered by H. The magneto-crystalline anisotropy energies are calculated for fully relaxed H/Ni-films. The spin-reorientation transition critical thickness of 4 ML is found in good quantitative agreement with the experiment.

Substrate-induced antiferromagnetism of a Fe monolayer on the Ir(001) surface

We present detailed ab initio study of structural and magnetic stability of a Fe-monolayer on the fcc(001) surface of iridium. The Fe-monolayer has a strong tendency to order antiferromagnetically for the true relaxed geometry. On the contrary an unrelaxed Fe/Ir(001) sample has a ferromagnetic ground state. The antiferromagnetism is thus stabilized by the decreased Fe-Ir layer spacing in striking contrast to the recently experimentally observed antiferromagnetism of the Fe/W(001) system which exists also for an ideal bulk-truncated, unrelaxed geometry. The calculated layer relaxations for Fe/Ir(001) agree reasonably well with recent experimental LEED data. The present study centers around the evaluation of pair exchange interactions between Fe-atoms in the Fe-overlayer as a function of the Fe/Ir interlayer distance which allows for a detailed understanding of the antiferromagnetism of a Fe/Ir(001) overlayer. Furthermore, our calculations indicate that the nature of the true ground state could be more complex and display a spin spiral-like rather than a c(2x2)-antiferromagnetic order. Finally, the magnetic stability of the Fe monolayer on the Ir(001) surface is compared to the closely related Fe/Rh(001) system.

Surface green's function for a rumpled crystal surface

Abstract The computer program allows to calculate the Greens function of a three-dimensional system with two-dimensional translation symmetry. Examples for its application are the crystal surface and the interface between two crystals. The crystal can be composed from rumpled atomic layers, that is, the atoms in the layer unit cell may have small differences in heights. The layer-by-layer KKR scheme is used. An overlayer with a coincidence structure of a substrate layer may be involved; the unit cell of any layer may be composed of up to four different atoms. The Greens function is evaluated in a spherical wave expansion, with basis functions centred at the atoms. It directly allows to calculate the surface electron charge density and the local and projected density of states.

Magnetic anisotropy of single 3 d spins on a CuN surface

First-principles calculations of the magnetic anisotropy energy for Mn- and Fe-atoms on CuN/Cu(001) surface are performed making use of the torque method. The easy magnetization direction is found to be different for Mn and Fe atoms in accord with the experiment. It is shown the magnetic anisotropy has a single-ion character and mainly originates from the local magnetic moment of Mn- and Fe-atoms. The uniaxial magnetic anisotropy constants are calculated in reasonable agreement with the experiment.