Chemical ordering and composition fluctuations at the (001) surface of the Fe-Ni Invar alloy

We report on a study of (001) oriented fcc Fe-Ni alloy surfaces which combines first-principles calculations and low-temperature STM experiments. Density functional theory calculations show that Fe-Ni alloy surfaces are buckled with the Fe atoms slightly shifted outwards and the Ni atoms inwards. This is consistent with the observation that the atoms in the surface layer can be chemically distinguished in the STM image: brighter spots (corrugation maxima with increased apparent height) indicate iron atoms, darker ones nickel atoms. This chemical contrast reveals a c2x2 chemical order (50% Fe) with frequent Fe-rich defects on Invar alloy surface. The calculations also indicate that subsurface composition fluctuations may additionally modulate the apparent height of the surface atoms. The STM images show that this effect is pronounced compared to the surfaces of other disordered alloys, which suggests that some chemical order and corresponding concentration fluctuations exist also in the subsurface layers of Invar alloy. In addition, detailed electronic structure calculations allow us to identify the nature of a distinct peak below the Fermi level observed in the tunneling spectra. This peak corresponds to a surface resonance band which is particularly pronounced in iron-rich surface regions and provides a second type of chemical contrast with less spatial resolution but one that is essentially independent of the subsurface composition.Comment: 7 pages, 5 figure

Potential, core-level and d band shifts at transition metal surfaces

We have extended the validity of the correlation between the surface 3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d transition metal series and to the neighboring elements Sr and Ag via accurate first-principles calculations. We find that the correlation is quasilinear and robust with respect to the differencies both between initial and final-state calculations of the SCLS's and two distinct measures of the SDBS's. We show that despite the complex spatial dependence of the surface potential shift (SPS) and the location of the 3d and 4d orbitals in different regions of space, the correlation exists because the sampling of the SPS by the 3d and 4d orbitals remains similar. We show further that the sign change of the SCLS's across the transition series does indeed arise from the d band-narrowing mechanism previously proposed. However, while in the heavier transition metals the predicted increase of d electrons in the surface layer relative to the bulk arises primarily from transfers from s and p states to d states within the surface layer, in the lighter transition metals the predicted decrease of surface d electrons arises primarily from flow out into the vacuum.Comment: RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.

Transmission of correlated electrons through sharp domain walls in magnetic nanowires: a renormalization group approach

The transmission of correlated electrons through a domain wall in a ferromagnetic one dimensional system is studied theoretically in the limit of a domain wall width smaller or comparable to the electron Fermi wavelength. The domain wall gives rise to both potential and spin dependent scattering of the charge carriers. Using a poor man's renormalization group approach for the electron-electron interactions, we obtain the low temperature behavior of the reflection and transmission coefficients. The results show that the low-temperature conductance is governed by the electron correlations, which may suppress charge transport without suppressing spin current. The results may account for a huge magnetoresistance associated with a domain wall in ballistic nanocontacs.Comment: 13 pages, 6 figure

Electron Transport through Disordered Domain Walls: Coherent and Incoherent Regimes

We study electron transport through a domain wall in a ferromagnetic nanowire subject to spin-dependent scattering. A scattering matrix formalism is developed to address both coherent and incoherent transport properties. The coherent case corresponds to elastic scattering by static defects, which is dominant at low temperatures, while the incoherent case provides a phenomenological description of the inelastic scattering present in real physical systems at room temperature. It is found that disorder scattering increases the amount of spin-mixing of transmitted electrons, reducing the adiabaticity. This leads, in the incoherent case, to a reduction of conductance through the domain wall as compared to a uniformly magnetized region which is similar to the giant magnetoresistance effect. In the coherent case, a reduction of weak localization, together with a suppression of spin-reversing scattering amplitudes, leads to an enhancement of conductance due to the domain wall in the regime of strong disorder. The total effect of a domain wall on the conductance of a nanowire is studied by incorporating the disordered regions on either side of the wall. It is found that spin-dependent scattering in these regions increases the domain wall magnetoconductance as compared to the effect found by considering only the scattering inside the wall. This increase is most dramatic in the narrow wall limit, but remains significant for wide walls.Comment: 23 pages, 12 figure

Adlayer core-level shifts of random metal overlayers on transition-metal substrates

We calculate the difference of the ionization energies of a core-electron of a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using density-functional-theory. We analyze the initial-state contributions and the screening effects induced by the core hole, and study the influence of the alloy composition for a number of noble metal-transition metal systems. Data are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x) Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our analysis clearly indicates the importance of final-state screening effects for the interpretation of measured core-level shifts. Calculated deviations from the initial-state trends are explained in terms of the change of inter- and intra-atomic screening upon alloying. A possible role of alloying on the chemical reactivity of metal surfaces is discussed.Comment: 4 pages, 2 figures, Phys. Rev. Letters, to appear in Feb. 199

Theory of interlayer exchange interactions in magnetic multilayers

This paper presents a review of the phenomenon of interlayer exchange coupling in magnetic multilayers. The emphasis is put on a pedagogical presentation of the mechanism of the phenomenon, which has been successfully explained in terms of a spin-dependent quantum confinement effect. The theoretical predictions are discussed in connection with corresponding experimental investigations.Comment: 18 pages, 4 PS figures, LaTeX with IOP package; v2: ref. added. Further (p)reprints available from http://www.mpi-halle.de/~theory

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 parameterization and the full potential LDA+U calculations give a very similar picture of states near the Fermi energy which reside in an exchange-split sp-d hybridized valence band with dominant orbital character of the host semiconductor; this 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.Comment: 29 pages, 25 figure

Dynamical correlations in multiorbital Hubbard models: Fluctuation-exchange approximations

We study the two band degenerate Hubbard model using the Fluctuation Exchange approximation (FLEX) method and compare the results with Quantum Monte-Carlo calculations. Both the self-consistent and the non-self-consistent versions of the FLEX scheme are investigated. We find that, contrary to the one band case, in the multiband case, good agreement with the Quantum Monte-Carlo results is obtained within the electron-electron T-matrix approximation using the full renormalization of the one-particle propagators. The crossover to strong coupling and the formation of satellites is more clearly visible in the non-self-consistent scheme. Finally we discuss the behavior of the FLEX for higher orbital degeneracy.Comment: 18 pages with 12 PS figure

Disorder effects in diluted ferromagnetic semiconductors

Carrier induced ferromagnetism in diluted III-V semi-conductor is analyzed within a two step approach. First, within a single site CPA formalism, we calculate the element resolved averaged Green's function of the itinerant carrier. Then using a generalized RKKY formula we evaluate the Mn-Mn long-range exchange integrals and the Curie temperature as a function of the exchange parameter, magnetic impurity concentration and carrier density. The effect of the disorder (impurity scattering) appears to play a crucial role. The standard RKKY calculation (no scattering processes), strongly underestimate the Curie temperature and is inappropriate to describe magnetism in diluted magnetic semi-conductors. It is also shown that an antiferromagnetic exchange favors higher Curie temperature.Comment: tex file + 4 .eps figures are included. submited to PR