D. Stoeffler

Electronic structure, magnetism and growth of ultrathin films of transition metals

The conditions which determine the growth mode (Frank-van der Merwe, Stranski-Krastanov, Volmer-Weber) on a perfect surface are obtained in terms of the electronic structure of ultrathin films and interfaces. The various contributions (elastic, structural, chemical, magnetic, etc.) to the spreading energy and to the interface energy are obtained at zero Kelvin for overlayers of a transition metal A on a transition, metal B. We use a tight-binding model and the recursion method to get general trends for all transition metal pairs (A, B) but the electronic structure derived from this simple model agrees when available with the ab-initio (LSDA) calculations. The spreading energy results from a delicate balance between its contributions but the wetting of the substrate by a monolayer (λ1 < 0) is generally obtained when the thermodynamical criterion λA< λB is satisfied. Finally the stability of an overlayer with respect to the formation of two-dimensional ordered compounds is investigated.

Theoretical investigations of the magnetic behaviour of Cr monolayers deposited on a Fe(001) substrate: Role of a mono-atomic step

Abstract In this paper we present a theoretical study of the magnetic properties of n Cr monolayers (ML) deposited on a Fe(001) substrate. We use the real space recursion method in a tight binding framework to determine the electronic structure of non-periodic systems involving a large number of inequivalent atoms. The aim of this work is to investigate the possibility to obtain a magnetic defect in the Cr layer by frustrating the plane to plane antiferromagnetic (AF) order. First, we study the magnetism of n perfect Cr monolayers (1 ≤ n ≤ 10) deposited on a perfect (001) Fe substrate and we show that a magnetic defect can be obtained for n ≥ 6 ML but it is clearly less stable than the solution without defect. We show also that, after the deposition of 3 Fe monolayers onto the Cr, the magnetic defect is obtained for much smaller Cr thicknesses ( n ≥ 2). Then, we consider a Fe substrate presenting a mono-atomic step. We study the magnetic moments distributions during the growth process of Cr monolayers assuming that the growth starts at the step. We show that (i) the perturbation due to the step is limited to the vicinity of the step and has a small extension in the plane of the Cr layers, and (ii) a wall is generated in the Cr layer by the step and splits clearly the Cr layer into two domains. Finally, we determine the structure of collinear walls in chromium and we discuss their role on the magnetic properties.

Magnetic diode effect in double-barrier tunnel junctions

A quantum-statistical theory of spin-dependent tunneling through asymmetric magnetic double-barrier junctions is presented which describes both ballistic and diffuse tunneling by a single analytical expression. It is evidenced that the key parameter for the transition between these two tunneling regimes is the electron scattering. For these junctions a strong asymmetric behaviour in the I-V characteristics and the tunnel magnetoresistance (TMR) is predicted which can be controlled by an applied magnetic field. This phenomenon relates to the quantum well states in the middle metallic layer. The corresponding resonances in the current and the TMR are drastically phase-shifted under positive and negative voltage.

First principles study of the electric polarization and of its switching in the multiferroic GaFeO3 system

The electric polarization in the multiferroic GaFeO(3) system is determined from its electronic structure using first principles methods and the modern theory of polarization. By carefully following the electric polarization on a path connecting the polar and centrosymmetric structures, it is found to be -25 μC cm(-2), which is ten times larger than a previous estimation given in the literature a few years ago and two times smaller than the value obtained in a recent similar study. The switching of this electric polarization through a centrosymmetric structure is discussed in terms of the total energy barrier. It is exhibited that such a switching is particularly difficult to achieve in relation to the tetrahedral environment of half of the Ga atoms. The switching via domain wall motion is also discussed.

Effects of strain relaxation on the electronic properties of epitaxial Sr2FeMoO6 grown by pulsed laser deposition on SrTiO3 (001)

Thin films of Sr2FeMoO6 (SFMO) are grown epitaxially on (001) SrTiO3 substrates by pulsed laser deposition. The in-plane and out-of-plane SFMO lattice parameters are determined for different thicknesses, from 13 to 600 nm. Samples below a critical thickness of around 34 nm are fully strained, samples between 34 and 80 nm are relaxing, and samples between 80 and 600 nm are fully relaxed. Transmission electron microscopy reveals that the relaxation is done by stacking the faults perpendicular to the substrate. Ab initio calculations show first that the variation of SFMO lattice parameters due to strain has a limited impact on the magnetization, which is also observed experimentally. Second, ab initio calculations indicate that SFMO is half metallic only for thicknesses above 44 nm.

Magnetic couplings in Fe/Mn and Fe/Cr superlattices

Abstract In this paper, we point out the importance of the spacers magnetic order with regard to the properties of ferromagnetic/ antiferromagnetic superlattices at T = 0 K. We discuss (i) the magnetic properties of superlattices based on ferromagnetic (Fe) and antiferromagnetic (Cr and Mn) layers in the itinerant magnetism framework, (ii) the relation between the interfacial, interlayer and intralayer couplings and the magnetic moment distributions and (iii) the role of the magnetic frustrations. The interlayer magnetic couplings are found to oscillate with the parity of n but the magnitude of these oscillations decreases more rapidly with an Mn spacer than with a Cr spacer.

Electronic structure and interlayer magnetic couplings in metallic superlattices

Abstract The general trends for the magnetic interlayer couplings are calculated in a tight binding model from the electronic structure of perfect model superlattices A m B n (Fe-V, Co-Pd, Co-Ru and Fe-Cr). Different features are deduced for non-magnetic, nearly ferromagnetic and antiferromagnetic spacers B; they are discussed in relation with ab initio calculations and experimental results.

Structural defects in Sr2FeMoO6 double perovskite: Experimental versus theoretical approach

The lower than expected magnetization of imperfect Sr2FeMoO6 (SFMO) double perovskites is usually attributed to the presence of Fe at antisite positions that would be antiferromagnetically coupled to their regular neighbors. However, ab initio calculations suggest strongly that such defective Fe sites would be ferromagnetically coupled and, consequently, the magnetization reduction would originate from other kinds of defects. The magnetic, hyperfine, and structural properties of SFMO perovskites prepared by solid-state reaction under a variety of conditions are reported and correlated with ab initio calculations of the magnetic moments and hyperfine fields of Mo and Fe ions in different local environments (antisites, antisite neighbors, and neighbors of an oxygen vacancy). When plotted against the order parameter the experimental magnetization is found to decrease at a rate of about −7.6μB per Mo–Fe antisite pair as in other previous experiments, where the theoretical calculation predicts −6.56μB per anti...

Magnetic properties of Al-doped Zn0.95Co0.05O films: Experiment and theory

Al-doped ZnCoO films were grown by sol-gel process onto SiO2 and Si(100) substrates. Structural analysis shows that all samples have the hexagonal wurtzite structure with a slight preferential orientation along the c-axis. The insertion of Al and Co into the ZnO matrix has been experimentally evidenced by UV-visible spectroscopy and transmission electron microscopy. This is further supported by x-ray photoelectron spectroscopy which indicated that all Co is under ionic form and that the samples contain no metallic clusters. The creation of free carriers by Al doping was confirmed by Hall effect and resistance measurements. Although a weak ferromagnetism is observed in all films, no clear influence of Al doping on the magnetic properties is evidenced which is in agreement with electronic structure calculations. The calculations show clearly that there is almost no overlap between the Al and the Co states, suggesting no change in the magnetic properties of ZnCoO with the Al doping. The only major role of Al...

Quantum coherent transport versus diode-like effect in semiconductor-free metal/insulator structure

Quantum coherent transport in double barrier tunnel junctions has been exploited for building micrometric-size semiconductor-free diodes. At room temperature, we observe strongly asymmetric current–voltage characteristics with an asymmetry ratio increasing with the bias voltage, reaching a maximum of 20 at 1 V. Our experimental data can be perfectly explained using a theoretical model involving resonance-assisted tunneling. The coherent/resonant tunneling regime is achieved using metallic 3 nm diameter monodisperse Cu clusters, sandwiched between two Al2O3 barriers.