Speakers and talks

Electronic structure of metal-metalloid interfaces: Co-Mn-O; Fe-Mn-O; Co-Si

The electronic structure of metal-rich metal-metalloid interfaces is investigated within classical ab initio codes: TB-LMTO, SIESTA, DACAPO. We have performed calculations concerning one monolayer of Mn on Co, or Fe. The magnetic map of these systems will be presented. The effect of added O_2 at the surface is described within TB-LMTO for Co-Mn and within SIESTA for Fe-Mn. Additionnal calculations with DACAPO are presently on the way concerning the Co-Mn-O nanostructures. Some results concerning bulk Co-Si, Co_2-Si, Co_3-Si and Co_4-Si within DACAPO will be presented. Films of metastable B2 Co-Si present magnetism whereas B20 Co-Si are not magnetic. Magnetization of Si-terminated and Co-terminated B2 Co-Si films will be given.

Lattice dynamics of an anharmonic crystal: Evidence for interactions between atomic vibrations at high temperatures

In studying the lattice dynamics of strongly anharmonic crystal, we solve a set of nonlinear Langevin equations for interacting oscillators while a multiwell potential is calculated in the displacive limit from the first principles. The model applied to the peculiar vibrations of bcc-Zr along [111] allows us to analyse all contributions to the spectral density and their influence on each other. We predict the effect of induced anharmonicity for quick vibrations due to their interaction with intrinsically anharmonic slow vibrations. This effect results in the broadband distribution in energy of inelastic neutron scattering known as the symmetry-forbidden phonon-branch splitting.

Temperature dependent magnetic anisotropy in metallic magnets from an ab-initio band structure theory: FePt

On the basis of a first-principles, relativistic electronic structure theory of finite temperature metallic magnetism, we investigate the variation of magnetic anisotropy, K, with magnetisation, M, in metallic ferromagnets. We apply the theory to the high magnetic anisotropy material, L1_0-ordered FePt, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pt layering for all M and to be proportional to the second power of M for a broad range of values of M. For small M, near the Curie temperature, the calculations pick out the easy axis for the onset of magnetic order. Our results are in good agreement with recent experiments on this important magnetic material.

Transport properties of ultrathin magnetic films

We present a model for the description of the transport properties of thin metallic films based on the solution of the linearized Boltzmann equation with a quite simple and at the same time reasonable assumption for the relaxation time. This approximation for the relaxation time derived from the microscopic treatment of the scattering processes is valid in the limits of very thin as well as very thick films. Both classical and quantum size effects, as well as surface states manifest in our results.

Hydrogen Adsorption on Low-Index Surfaces of Ti- and Pd-based Alloys

Conventional total-energy calculations based on full-potential all-electron techniques allow one to understand the microscopic origin of different phenomena in bulk and at surface. They could be the basis for the "atomistic engineering" of materials with desired properties. The modification of surface electron properties by hydrogen adsorption and the effect of surface reactivity on hydrogen capacity also remain open question. The explanation of physical-chemical nature of hydrogen adsorption at the metal alloy surfaces, prediction of their properties, and possible reaction pathways still remain a challenge.
The aim of this work is the investigation of hydrogen adsorption at low-index surfaces in the set of titanium and palladium alloys (TiNi, TiFe, TiPd, PdTa). We also study the hydrogen adsorption at clean Pd surfaces as well as the influence of Pd coating on the hydrogen adsorption properties in Ti-based alloys.
The full potential linearized-augmented-plane-wave (FLAPW) method was applied for surface electronic structure calculations. The supercell technique was used for the surface simulation of 5-9 layer slabs separated by vacuum regions. The atomic positions were relaxed by minimizing the forces acting on atoms. To determine the equilibrium adsorption geometry we have allowed a minimization of the total energy with respect to the distance of the adsorbed atom from the surface layer. The chemical bond between surface states of substrate and hydrogen is analyzed in details. The hydrogen adsorption on the Ti-terminated B2-TiMe (001) surface and Ta-terminated PdTa (001) surface is found to be preferable. Among four considered adsorbate sites Me-bridge position on TiMe (110) surface is more stable. In addition on PdTa (110) surface three-fold position for hydrogen was considered and it found to be preferable. We establish that H-Me (Me=Fe, Ni, Pd) interaction is more strongly then H-Ti one. H absorption can cause a weakening of interatomic Me-Me bond and as result leads to phase instability. The adsorption properties on Pd-covered TiMe surfaces are analyzed in respect to substrate composition. The present calculations allow to predict the preferable surface orientation and composition for hydrogen adsorption.

Collective behaviour of magnetic nanoparticles: pseudo-exchange, Devil`s staircase, frustrations

I am going to discuss some features of magnetostatic interaction in chains of ferromagnetic nanoparticles. First, the inhomogeneous states (solitons) in a single chain of classical dipoles are studied numerically and analytically. An analytical solution is based on long-wave approximation for dipole sums. Second, analytical calculations of the hysteresis curve for a system of interacted nanoparticle chains are provided. It is shown that magnetisation curve has self-similar character (complete "devil`s staircase"). A simple method for experimentally distinguish the influence of interaction from coercitivity dispersion on the magnetisation curve is proposed. Third, frustrations in chain of ferromagnetic nanodisks are discussed.

Multiple metamagnetic transitions in uniaxial Fe/Cr multilayers

The superlattices [Fe/Cr]₁₂ with uniaxial in-plain anisotropy and stairs-like dependence of magnetization and giant magnetoresistance were grown on special (211)MgO substrates by MBE. It was found that "steps" on the M(H) and ?R(H)/R dependences are connecting with multiple metamagnetic transitions - consecutive 180-degree "rotation" of an individual Fe layers magnetization ("spin-flip") in external magnetic field applied in film plain along easy axis. The number of these transitions depends on mainly by the number of ferromagnetic layers and by the ratio of exchange coupling energy and uniaxial anisotropy energy, determined by means of Cr and Fe layer thickness respectively. It was shown that the real magnetization scenario of the superlattices [Fe/Cr]₁₂ with different Cr layer thickness is very sensitive to the Cr layers thickness. The physical reasons explaining the existence of multiple metamagnetic transitions in the superlattices were analyzed.

Kondo-like effect and GMR in Fe/Cr cluster-layer nanostructures

Magnetic and magnetoresistive properties of multilayer nanostructures Fe/Cr with a different average Fe layers thickness (t_Fe= 0.3 - 20A) were investigated at low temperatures (down to 2 K) and magnetic field (up to 9T). Nanostructures were grown in ultrahigh vacuum by MBE on single crystal substrates MgO and Al₂O₃. It was shown that Fe layers in nanostructures with t_Fe< 2A are not continuous, but consist of separate ferromagnetic clusters. Such kind nanostructures exhibit superparamagnetic properties and have anomalous temperature dependence of electrical resistivity. Anomalous behavior here means following: in the beginning the resistance goes down with a temperature decreasing, then, starting from some temperature, which is depends on quantity of Fe clusters in a cluster layer, the resistivity increases like in alloys with Kondo-effect. The anomaly of electrical resistivity reduces in applied magnetic field. It was found that GMR value increases more considerably then in usual superlattices with continuous ferromagnetic layers.

The nature of spin-density-wave (SDW) state in the bulk Chromium

Ab initio calculations of the bulk Cr magnetic structure, performed last years, have shown that traditional theory based on the idea of Fermi-surface nesting failed in attempts to explain SDW ground state. The ground state of bulk Cr was found to be antiferromagnetic (AF) in all ab initio theories. Using model Hamiltonian approach with very restricted set of phenomenological parameters, we developed the method, which allow self-consistent calculation of SDW with different wavelength L. The profile of magnetic moment and the energy of the system agree with ab initio results and the ground state proves to be AF. However when L exceeds 51 monolayers (ML), the threshold energy for transition between different SDW states was found to be lower then threshold energy for SDW-AF transition. It is the reason, which leads to the stability of SDW. Nodes of SDW can be considered as one-dimensional quasiparticles with repulsive interaction. When the distances between these quasiparticle became less than definite value (32 ML) they annihilate similar to the gas of soap bubbles. It explains qualitatively the temperature dependence of SDW wavelength in the bulk Cr.

On magnetism of Fe₂/V₁₃/Fe₃/V₁₃ superlattice

A model for magnetic properties of Fe₂/V₁₃/Fe₃/V₁₃ superlattice is proposed. A single Fe₂ or Fe₃ layer is described by means of the classical 2D xy-model. The free energy of the layer is obtained by a high-temperature cumulant expansion. Intralayer and interlayer interactions sterm from a polarization of vanadium, and they are taken into account through mean-field terms. An in-plane anisotropy is also considered. The model describes satisfactorily the behaviour of Fe₂/V₁₃/Fe₃/V₁₃ superlattice at high and intermediate temperatures. In particular, it demonstrates two sequential transitions (the first order and the second order) in a magnetic field that was observed at 100 K.

Adsorbates on transition-metal based materials and local modifications of the magnetic properties

The magnetic properties of materials can be modified locally by chemisorption of selected elements or molecules. For instance the spin moment of a polarized transition metal can be drastically decreased by the presence of another non-polarized transition metal. Another intriguing case is observed for Co: CO adsorption quenches the local magnetic moment of Co neighbors, whereas NH₃ or light elements have much less influence. The variation of the magnetization direction in presence of adsorbates has been also experimentally observed. The case of the oxygen-driven magnetization reorientation in Fe is particularly spectacular. For those different situations, ab initio and semi empirical electronic structures calculations are performed. These results help to understand the subtle mechanisms and general trends are derived. Acknowledgements : This work has benefited of the Research Training Network « Computational Magnetoelectronics» (N° HPRN-CT-2000-00143) of the European Commission.

Magnetic stucture of the surface of non-magnetic metals

The free-electron like surface state on the (111) surface of gold shows a splitting into two parabolic subbands induced by the spin orbit interaction. Spin-resolved high-resolution photoemission experiments performed with a full three-dimensional spin polarimeter provide a detailed image of the resulting spin structure. In particular, spin-resolved momentum distribution maps show that the spin vector lies in the surface plane and is perpendicular to the momentum of the electrons as expected in a free-electron model. This method of measuring the spin structure of a two-dimensional electron gas allows the observation of the direction of electric fields as probed by the electrons. Although the energy splitting can only be understood as a consequence of strong atomic electric fields, no modulation of the spin direction due to these fields is detected.

Experimental aspects of spin density waves in thin Cr films

The spin-density wave (SDW) state in thin chromium films is well known to be strongly affected by strain and proximity effects from neighbouring layers. To date the main attention has been given to effects arising from the exchange interaction between ferromagnetic films and the SDW magnetism in Cr films via their common interface (for reviews see [1-3]). Recently we have investigated via neutron and synchrotron scattering studies the proximity effects in Cr/V films and multilayers, where the boundary condition is due to the hybridization of Cr with paramagnetic V at the interface. We find that the V/Cr interface has a strong and long-range effect on the polarization, propagation, period, and the Neel temperature of the SDW in rather thick Cr films [4]. Furthermore, we find below the spin-flip transition at TSF = 100K a single domain longitudinal SDW propagating in the film plane, and above TSF a transverse SDW also propagating in-plane with magnetic moments parallel to the interface. As soon as the Cr film is sandwiched between two V layers on either side, the longitudinal SDW starts to propagate normal to the film plane, and with decreasing Cr film thickness the longitudinal SDW is successively taken over by a commensurate SDW with moments remaining in the out-of-plane direction. This unusually strong interaction between the Cr-SDW and the paramagnetic V layer is unexpected and not predicted by theory.
1. D. T. Pierce, J. Unguris, R.J. Celotta, M.D. Stiles, J. Magn. Magn. Matter. 200, 290 (1999)
2. H. Zabel, J. Phys.: Condens. Matter 11, 9303 (1999)
3. R. S. Fishman, J. Phys.: Condens. Matter 13, R235 (2001)
4. E. Kravtsov, A. Nefedov, F. Radu A. Remhof, H. Zabel, B. Hjorvarsson, A. Hoser, S. B. Wilkins, Phys. Rev. B 70, 054425 (2004)

Size calibration in a model of strained epitaxy

The 1+1D model of strained epitaxy due to Ratsch and Zangwill [Surf. Sci. 293, 123 (1993)] was studied with the use of the kinetic Monte Carlo technique. Despite the fact that energetics of the model predicts the Ostwald ripening at the late stage of growth, in the simulations we observed size calibration both in the horizontal and vertical directions. Besides, the absence of coalescence and roughly linear correlation between the width and height of the growing islands were in qualitative agreement with experimental data on self-assembly of supported metallic Fe and Co clusters. This may mean that the self-assembled nanostructures are metastable objects with a very slow ripening kinetics.

Ab initio investigation of the electronic structure of Sr₂FeMoO₆ double perovskites presenting imperfections

We present ab initio calculations of the electronic structure of Sr2FeMoO6 double perovskites using the FLEUR implementation of the FLAPW method. By considering Fe and Mo antisite and oxygen vacancies in various supercells, we investigate
(i) the preservation of the half-metallic feature of this system in the presence of such imperfections which is essential for use of this materials as electrode in spintronic devices,
(ii) the occurence of anti-aligned Fe magnetic moments in order to discuss the observed magnetization reduction, and
(iii) the relation between the hyperfine fields of Fe and Mo atoms and their local magnetic neighborhood in relation with NMR and Moessbauer studies.
The calculations show that the half metallic feature is lost for all considered antisites, due to the large number of Fe-O-Mo bonds affected, whereas an oxygen vacancy preserves this feature. The results suggest also that oxygen vacancies can be at the origin of the magnetization reduction because it is found too small for antisite imperfections (the solutions with anti-aligned Fe magnetic moments are found highly metastable). Finally, we exhibit the nearly linear relation between the number of X-O-Y bonds (X, Y = Fe, Mo) and the values of the local magnetic moment and of the hyperfine field on a given atom.

Magnetism in small Pd clusters

We report a theoretical study of the magnetic behavior of selected free-standing PdN clusters N<=55 with fcc shapes. Particular attention is devoted here to the fcc structures taking into account recent experimental observations for Pd nanoparticles [Shinohara et al., Phys. Rev. Lett. 91, 197201 (2003); Sampedro et al., Phys. Rev. Lett. 91, 237203 (2003)].
The optimized geometrical structures were obtained from an uniform relaxation of the cubo-octahedral and the truncated-octahedral configurations using the Embedded Atom Method (EAM). The spin-polarized electronic structure and related magnetic properties of those optimized geometries were calculated by solving self-consistently a $spd$ tight-binding Hamiltonian.
We obtain very weak magnetic moments in a qualitative agreement with the early experiments, with those of Sampedro et al. and with most of the theoretical results concerning the negligible local magnetic moments in low-dimensional Pd systems . We explore the recently observed tendency of certain sites to be more magnetic than the rest of the cluster. Our results are discussed in the context of several experiments where some controversy still exists regarding the onset of magnetism and its magnitude.

Noncollinear magnetism caused by interdifussion in Cr films grown on Fe substrates

Using a self-consistent real space tight binding model parameterized to DFT calculations and without collinear restriction for the spin direction, we have investigated 50% Fe-Cr mixed layers deposited on the Fe(001) substrate with different arrangements. We demonstrate that the interdifussion at the Cr/Fe interface experimentally observed at the first stages of the Cr growth on Fe can originate non-collinear magnetic arrangements within the system and even domains in the Fe layers close to the interface when in adition, steps are present at the interface.

Thickness dependence of the magnetic polarization in FeGe₂ films

Femtomagnetism (30 min) (Zn,Co)O films studied by Kerr and XMCD (30 min)

Molecule-based magnets: the scaling approach

Controlling magnetic properties : from confined domains to self organized clusters and molecules

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