The atomic structure, electronic, and magnetic properties of thin Mn films epitaxially grown on Cu(100) substrates have been investigated by ab initio density-functional studies. Because the local-density approximation leads to a rather poor description of the magnetostructural properties of bulk Mn, a detailed study of the effect of generalized gradient corrections (GGC) to the exchange-correlation functional on the structure and magnetism of Mn in three and two dimensions has been performed. For the bulk we find that the GGC's lift the almost-degeneracy between the competing magnetic configurations and lead to a large magnetovolume effect, in much better agreement with experiment. For free-standing Mn monolayers the effect of the GGC's is even more pronounced: the relative stability of square and hexagonal layers is inverted, antiferromagnetic ordering leads to a large increase of the equilibrium distances. Therefore all investigations of Mn films on Cu substrates have been performed in the generalized gradient approximation. The results demonstrate that homogeneous Mn overlayers are unstable against interdiffusion and the formation of ordered surface alloys. At a coverage of $\ensuremath{\Theta}=0.5$ Mn monolayers, an ordered ferromagnetic $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface alloy is formed. The same atomic structure is assumed at a coverage of $\ensuremath{\Theta}=1$ and leads to an antiferromagnetic coupling between the CuMn alloy layers. In both homogeneous alloy layers and in the surface alloys, Mn is in a high-spin state with a magnetic moment close to $4{\ensuremath{\mu}}_{B}.$ The large atomic volume of magnetic Mn leads an outward relaxation of the Mn atoms and a pronounced buckling of the surface. Detailed comparisons of the calculated atomic structure with low-energy electron diffraction and photoelectron diffraction experiments and of the electronic structure with photoemission and inverse photoemission spectroscopies are reported.

The atomic and magnetic structure of FCC-Fe layers on a Cu(100) substrate is investigated by ab initio calculations as a function of the thickness of the film. Fe films with a thickness varying between 1 and 3ML stabilize a ferromagnetic (FM) ground state. For thicker films with an even number of Fe layers (4, 6 and 8ML) a bilayer AF magnetic structure develops, while for an odd number of Fe layers (5, 7 and 9ML) various spin structures coexist. The magnetic properties of Fe thin layers are compared to those of bulk FCC-Fe biaxially strained to the lattice constant of the Cu substrate and the dependence of magnetic energies in the form of the exchange-correlation functional is discussed.

The structure and magnetic properties of Mn and CuMn overlayers on Cu(001) are investigated within the generalized-gradient approximation of density-functional theory via ab-initio calculations based on the ultra-soft pseudo-potential method. For a one monolayer (1ML) Mn film, we predict in-plane c(2×2) antiferromagnetism with large moments, as slight outward relaxation and almost no buckling. For a 2ML Mn film, we find layered antiferromagnetism, a modest outward relaxation of both overlayers and again no buckling. For a c(2×2) 1ML CuMn surface alloy, we find a high-moment (mMn=4.09μB) ferromagnetic ground-state with a pronounced buckling of 0.26Å. A c(2×2) 2ML CuMn alloy film is antiferromagnetic (mMn1=4.13μB and mMn2=−3.66μB in the first and second layer, respectively), with strong outward relaxation and buckling of the top layer. The surface alloy is found to be energetically more favourable than an overlayer. The predicted relaxation and buckling are shown to be driven by magnetic effects. It is shown that the strong enhancement of the magnetic moments in the surface alloy is also reflected in a large exchange splitting in the Mn-d band.

Phase stability, structural, and electronic properties of iron silicides in the Fe3Si, FeSi, and FeSi2 compositions are investigated by first-principle density-functional calculations based on ultrasoft pseudopotentials and all-electron methods. Structural stabilization versus spin-polarization effects are discussed at the Fe3Si composition, while for epsilon-FeSi and beta-FeSi2 we investigate their structural properties and the corresponding semiconducting band properties. All the computed results are analyzed and compared to available experimental data. The stability of the bulk phases, the lattice parameters, the cohesive energies and magnetic properties are found to be in good agreement with experiment when using the generalized gradient approximations for the exchange-correlation functional. Density-functional calculations are unable to account for the small bulk modulus of epsilon-FeSi despite that the computed lattice constant and internal atomic positions coincide with the experimental results. Both full-potential and ultrasoft-pseudopotential methods confirm for beta-FeSi2 the indirect nature of the fundamental gap, which is attributed to a transition between Y to 0.6X Lambda being 30% smaller than the experimental gap. Ultrasoft pseudopotential calculations of Fe-Si magnetic phases and of various nonequilibrium metallic phases at the FeSi and FeSi2 composition are presented. These calculations provide nb initio information concerning the stabilization of metallic pseudomorphic phases via high pressures or epitaxy. [S0163-1829(99)05419-3].

The epitaxy of face-centred-cubic (fcc) Fe layers on a Cu(001) substrate has been investigated by means of ab initio local spin-density calculations (including generalized gradient corrections) as a function of the thickness of the film. Fe films with a thickness varying between one and three monolayers (ML) adopt a ferromagnetic (FM) ground state. For thicker films with even numbers of Fe layers (4 ML, 6 ML and 8 ML), bilayer antiferromagnetic (AF) structures develop, while for odd numbers of Fe layers (5 ML, 7 ML and 9 ML), a variety of energetically almost degenerate spin structures each with a ferromagnetically coupled bilayer at the free surface are found. The magnetic structures of the films are strongly coupled to their crystal structures: the interlayer distance between ferromagnetically coupled layers is expanded, while that between antiferromagnetically coupled layers is reduced compared to the layer distance in the substrate and in ideal fcc Fe films. Our results explain the observed change from a tetragonally distorted structure in the ferromagnetic regime to thicker films that are almost fcc (on average) due to the antiferromagnetism in the deeper layers.

Candidates for epitaxially stabilized structures of Fe, Co, and Ni silicides are searched by ab initio calculations. We find that the pseudomorphic phases of FeSi2 in the C1 structure and CoSi and NiSi in the B2 structure soften dramatically under compressive biaxial strain induced by epitaxy on a (100) substrate. This supersoft effect is reflected by zero strain energy, constant volume, and constant bond energies.

Temperature-dependent band-structure calculations are done for fcc Ce at different volumes. Large temperature and volume variations of the density of states, the bulk modulus, and the magnetic moments are found to give a qualitative description, within the density functional theory, of the Ce alpha-gamma phase transition and the thermodynamic properties of the gamma phase.

We present a study of the accuracy, transferability, and plane-wave convergence properties of ultrasoft Vanderbilt-type pseudopotentials for Fe, Co, and Ni in the context of atomic, molecular, and solid calculations. Special attention has been given to the magnetic properties of these systems. To go beyond the local-spin-density-approximation, generalized gradient approximations for the exchange-correlation functional have been included. All calculations have been performed using a plane-wave basis set, and we show that ultrasoft pseudopotentials allow - as expected - for a considerably lower cutoff energy than standard soft norm-conserving pseudopotentials. Lattice properties show very good agreement with all-electron calculations and experiment, while larger discrepancies exist for magnetic structural energy differences (which however remain smaller than 2 mRy/atom). These differences can be traced back to the frozen core approximation which is implicitly assumed in the construction of the pseudopotentials. More accurate results for the magnetization energies of atomic configurations can be obtained by treating the 3p semicore states as valence states.

The electronic and vibrational free energies of some hcp and bcc transition metals are computed {ital ab} {ital initio}. The vibrational part is obtained from a total-energy calculation over lattices with atoms randomly displaced according to a Gaussian distribution. The relative importance of electronic and vibrational excitations in the stabilization of the high-temperature bcc structure is clarified. {copyright} {ital 1996 The American Physical Society.}

The electronic and vibrational free energies of the b.c.c., f.c.c. and h.c.p. phases of iron are calculated by ab initio band structure methods. The electronic and pV contributions to Gibbs energy show a non-linear dependence with temperature and play an important role for the h.c.p.-b.c.c. phase stability. The b.c.c. phase is found to be stable at low pressure. For large pressure, an upper limit of the magnetic entropy is estimated. It lowers Gibbs energy of the b.c.c. phase, but with all energy terms, h.c.p. included, remains stable at all temperatures.

Selfconsistent band structure methods based on the LMTO method are developed and applied for calculations of free energy properties of structurally disordered materials. We study temperature variations of phonon related properties in some 3d, 4d and 5d metals of different structure, by the average scattering method, within the quasi-harmonic approximation. As shown earlier for 4d metals [4], the effect of vibrations on the electronic structure is well described by the average scattering around one single atom. Results for phonon moments and some temperature dependent properties like phase stability are presented for several metals showing polymorphism with temperature and at ordinary pressure. The role of electronic free energies for stabilizing the high temperature bcc phase in Ti and Zr metals and related alloys is analysed.

The effects of short-range order for bcc-based Fe-V, Fe-Cr and Fe-Co alloys have been investigated by supercell calculations and approximate self-consistent Linear Muffin Tin Orbital for substitutional disorder. This has allowed us to study the stability and electronic energy properties of ordered superlattices and of random solutions. We analyse the influence of local environment effects on the equilibrium and magnetic properties of the respective alloys. Total energy results show that short range order is important for Fe-Cr and Fe-V so that certain configurations are prefered, while in the Fe-Co system the total energies of several configurations are very close. It also implies that the density-of-states of the FeCo system can be described by the method for substitutional alloys. On the other hand differences in the short range order of FeV or FeCr, have large effects on density-of-states and magnetic order.

We present calculations of magnetovolume properties for different strong paramagnetic systems such as ZrV2, TiBe2, and ZrZn2 in the C15 structure. These materials show indications of anti-Invar behavior, i.e., enhanced magnetostriction, and thermal expansion.

The linear muffin tin orbital method is applied to compute the energy properties of several ordered binary Fe-Ni, Fe-Co, and ternary Fe-Ni-Co compounds near the bcc-fcc transition, from first principles. The calculations use the local spin density and the Perdew–Wang gradient corrected approximation to treat exchange and correlation. We address the importance of the gradient corrected approximation for the calculation of the fcc-bcc energetics for these 3D Fe-based systems near the Fe rich region. By adding Co to the Fe-Ni alloy it is possible to reduce the volume expansion accompanying the fcc to bcc transition. To study these effects we have considered the energetics in several ordered ternary systems with a fcc and bcc lattice. When increasing the e/a ratio from 8.5 (Fe3Ni) to 8.75 (NiFe2Co) and to 9 (Co or FeNi) one finds increased stability of the FM fcc phase compared to the bcc phase. We observe also that by increasing the e/a ratio the magnetic moment of the fcc lattice becomes more stable; this induces an augmentation of the spontaneous magnetostriction and a decrease of the volume change accompanying the fcc-bcc transition. A comparison of the different computed magnetic and ground-state properties with experimental data is provided.