J. Berakdar

Multiferroic oxides-based flash memory and spin-field-effect transistor

We propose a modified spin-field-effect transistor fabricated in a two dimensional electron gas (2DEG) formed at the surface of multiferroic oxides with a transverse helical magnetic order. The topology of the oxide local magnetic moments induces a resonant momentum-dependent effective spin-orbit interaction acting on 2DEG. We show that spin polarization dephasing is strongly suppressed, which is crucial for functionality. The carrier spin precession phase depends linearly on the magnetic spiral helicity. The latter is electrically controllable by virtue of the magneto-electric effect. We also suggest a flash-memory device based on this structure.

Local Control of Ultrafast Dynamics of Magnetic Nanoparticles

Using the local control theory we derive analytical expressions for magnetic field pulses that steer the magnetization of a monodomain magnetic nanoparticle to a predefined state. Finite-temperature full numerical simulations confirm the analytical results and show that a magnetization switching or freezing is achievable within few precessional periods and that the scheme is exploitable for fast thermal switching.

Magnetoresistance of a semiconducting magnetic wire with a domain wall

We investigate theoretically the influence of the spin-orbit interaction of Rashba type on the magnetoresistance of a semiconducting ferromagnetic nanostructure with a laterally constrained domain wall. The domain wall is assumed sharp (on the scale of the Fermi wavelength of the charge carriers). It is shown that the magnetoresistance in such a case can be considerably large, which is in qualitative agreement with recent experimental observations. It is also shown that spin-orbit interaction may result in an increase of the magnetoresistance. The role of localization corrections is also briefly discussed.

Quantum Otto heat engine based on a multiferroic chain working substance

We study a quantum Otto engine operating on the basis of a helical spin-1/2 multiferroic chain with strongly coupled magnetic and ferroelectric order parameters. The presence of a finite spin chirality in the working substance enables steering of the cycle by an external electric field that couples to the electric polarization. We observe a direct connection between the chirality, the entanglement and the efficiency of the engine. An electric-field dependent threshold temperature is identified, above which the pair correlations in the system, as quantified by the thermal entanglement, diminish. In contrast to the pair correlations, the collective many-body thermal entanglement is less sensitive to the electric field, and in the high temperature limit converges to a constant value. We also discuss the correlations between the threshold temperature of the pair entanglement, the spin chirality and the minimum of the fidelities in relation to the electric and magnetic fields. The efficiency of the quantum Otto cycle shows a saturation plateau with increasing electric field amplitude.

Temperature-dependent magnetization dynamics of magnetic nanoparticles

Recent experimental and theoretical studies show that the switching behavior of magnetic nanoparticles can be controlled well by external time-dependent magnetic fields. In this work, we inspect theoretically the influence of temperature and magnetic anisotropy on the spin dynamics and switching properties of single domain magnetic nanoparticles (Stoner particles). Our theoretical tools are the Landau–Lifshitz–Gilbert equation extended to deal with finite temperatures within a Langevin framework. Physical quantities of interest are the minimum field amplitudes required for switching and the corresponding reversal times of the nanoparticles magnetic moment. In particular, we contrast the cases of static and time-dependent external fields and analyze the influence of damping for a uniaxial and a cubic anisotropy.

Charge-transfer polaron induced negative differential resistance and giant magnetoresistance in organic spin-valve systems

Based on the static polaron Su–Schrieffer–Heeger model and the nonequilibrium Greens function formalism, we investigate the negative differential resistance (NDR) effect in organic spin-valve systems at low temperature and interpret it with a self-doping picture. A giant negative magnetoresistance exceeding 300% is theoretically predicted as the results of the NDR effects.

The electron-impact double ionization of atoms: an insight into the four-body Coulomb scattering dynamics

Abstract Over the past two decades impressive progress has been made in the theoretical and the experimental study of the multiple excitation and of the complete fragmentation of four-body Coulomb systems. The double ionization of atoms by charged particle impact is employed routinely to prepare and to explore the Coulomb four-body excited states (the two ionized electrons and the scattered charged projectile moving in the field of the residual ion). The spectrum of this four-body system can be determined experimentally by resolving simultaneously the momentum vectors of all particles. Such a multi-coincidence measurement entails however low counting rates which makes the experimental realization a challenging task. This work gives a brief overview on recent achievements in multi-detection techniques and outlines the various methods to carry out the double ionization experiments induced by electron impact. The advantages and the limits of the various experimental approaches are pointed out. On the theoretical side, serious difficulties are encountered which are prototypical for the theoretical treatment of many-body correlated systems: (A) With increasing number of interacting particles (and hence of degrees of freedom) a direct numerical evaluation of the four-body Greens function, which encompasses the entire spectrum of the system, becomes a challenge. (B) Due to the non-integrable character of interacting many particle systems, an analytical approach can only be approximate. In this report we discuss in details the various methods that have been put forward to deal with the four-body problem, including: perturbative many-body treatments (first and second order theories) and non-perturbative methods as well as pure numerical approaches. Due to the complicated structure of the four-particle continuum spectrum we present and discuss simple qualitative arguments to explain the main features (peaks and dips) that are observed in the experiments. The limitations of these simple methods are illustrated by contrasting the predictions with full numerical calculations and with experimental data. Future directions and possible applications are also discussed.

Electric tuning of magnetization dynamics and electric field-induced negative magnetic permeability in nanoscale composite multiferroics

Steering magnetism by electric fields upon interfacing ferromagnetic (FM) and ferroelectric (FE) materials to achieve an emergent multiferroic response bears a great potential for nano-scale devices with novel functionalities. FM/FE heterostructures allow, for instance, the electrical manipulation of magnetic anisotropy via interfacial magnetoelectric (ME) couplings. A charge-mediated ME effect is believed to be generally weak and active in only a few angstroms. Here we present an experimental evidence uncovering a new magnon-driven, strong ME effect acting on the nanometer range. For Co92Zr8 (20 nm) film deposited on ferroelectric PMN-PT we show via ferromagnetic resonance (FMR) that this type of linear ME allows for electrical control of simultaneously the magnetization precession and its damping, both of which are key elements for magnetic switching and spintronics. The experiments unravel further an electric-field-induced negative magnetic permeability effect.

Theory of two-electron photoemission from surfaces

A general theoretical approach to double photoemission from solid surfaces is formulated in terms of two-electron Green functions and two-electron states. By incorporating the screened Coulomb interaction between the two outgoing electrons in a dynamically screened effective one-electron potential, approximate expressions for the two-photoelectron current are derived, which essentially consist of elements well-known from one-electron photoemission theory. q 2000 Elsevier Science Ltd. All rights reserved.

Probing the Spin Polarization in Ferromagnets

The emission of correlated electrons from an itinerant ferromagnet following the impact of a polarized electron beam is analyzed in terms of irreducible tensorial parameters that can be measured. Under favorable conditions, specified in this work, these parameters are related to the spin polarization in the ferromagnet. The formal results are illustrated by numerical studies of the polarized electron pair emission from a Fe(110) surface and a novel technique for the investigation of magnetic properties of ferromagnets is suggested.