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Jamal Berakdar

Publications and source records attributed to Jamal Berakdar.

At least 91 records · Page 5Linked to original sources
arXiv2011

Time-dependent magneto-transport in a driven graphene spin valve

Based on the time-dependent nonequilibrium Green's function method we investigate theoretically the time and spin-dependent transport through a graphene layer upon the application of a static bias voltage to the electrodes and a time-alternating gate voltage to graphene. The electrodes are magnetic with arbitrary mutual orientations of their magnetizations. We find features in the current that are governed by an interplay of the strength of the alternating field and the Dirac point in graphene: The influence of a weak alternating field on the zero bias conductance is strongly suppressed by the zero density of state at the Dirac point. In contrast, for a strong amplitude of the alternating field the current is dominated by several resonant peaks, in particular a marked peak appears at zero bias. This subtle competition results in a transition of the tunnel magnetoresistance from a broad peak to a sharp dip at a zero bias voltage applied to the electrodes. The dip amplitude can be manipulated by tuning the ac field frequency.

cond-mat.mes-hall↗
arXiv2011

Topological view on magnetic adatoms in graphene

We study theoretically the physical properties of a magnetic impurity in graphene. Within the Anderson model for a very strong Coulomb interaction on the impurity, we start from the Slave-Boson method and introduce a topological picture consisting of a degree of a map and a winding number (WN) to analyze the phase shift and the occupation on the impurity. The occupation is linked to WN. For a generic normal metal we find a fractional WN. In contrast, the winding is accelerated by the relativistic dispersion of graphene at half-filling leading to an integer occupation. We show that the renormalization parameter that shifts the impurity level is insufficient to invert the sign of the energy level. Consequently, the state at half-filling is stable unless a gate voltage is tuned such that the Fermi level touches the edge of the broadened impurity level. Only in this case the zero field susceptibility is finite and shows a pronounced peak structure with the gate voltage.

cond-mat.str-el↗
arXiv2011

Anisotropic thermoelectric effect in helimagnetic tunnel junctions

Thermoelectric transport across normal-metal/helical-multiferroic/ferromagnetic heterojunctions is theoretically investigated. We find a anisotropic charge and spin thermopower with a $C_{2v}$ symmetry. The angular dependence on the magnetization orientation of the ferromagnetic layer is substantiated by a phenomenological theory based on the symmetry of the effective spin-orbit interaction induced by the topology of the spiral magnetic order in the multiferroic barrier.

cond-mat.mes-hall↗
arXiv2011

Magnetic dynamics driven by the spin-current generated via spin-Seebeck effect

We consider the spin-current driven dynamics of a magnetic nanostructure in a conductive magnetic wire under a heat gradient in an open circuit, spin Seebeck effect geometry. It is shown that the spin-current scattering results in a spin-current torque acting on the nanostructure and leading to precession and displacement. The scattering leads also to a redistribution of the spin electrochemical potential along the wire resulting in a break of the polarity-reversal symmetry of the inverse spin Hall effect voltage with respect to the heat gradient inversion.

cond-mat.mes-hall↗
arXiv2011

Spin Density Waves and Domain Wall Interactions in Nanowires

We investigated how the dynamics of a domain wall are affected by the presence of spin density waves in a ferromagnetic wire. Domain walls and other scattering centres can cause coherent spin density waves to propagate through a wire when a current is applied. In some cases the spin torque due to these scattered electrons can be enhanced such that it is on a par with the exchange and anisotropy energies controlling the shape and dynamics of the domain wall. In such a case we find that the spin density waves enhance the current induced domain wall motion, allowing for domain wall motion with smaller current pulses. Here we consider a system involving two domain walls and focus on how the motion of the second domain wall is modified by the spin density waves caused by the presence of the first domain wall.

cond-mat.mes-hall↗
arXiv2011

Finite-size effects on the magnetoelectric response of field-driven ferroelectric/ferromagnetic chains

We study theoretically the coupled multiferroic dynamics of one-dimensional ferroelectric/ferromagnet chains driven by harmonic magnetic and electric fields as a function of the chain length. A linear magnetoelectric coupling is dominated by the spin-polarized screening charge at the interface. We performed Monte-Carlo simulations and calculations based on the coupled Landau-Lifshitz-Gilbert and Landau-Khalatnikov equations showing that the net magnetization and the total polarization of thin heterostructures, i.e. with up to ten ferroelectric and ferromagnetic sites counted from the interface, can be completely reversed by external electric and magnetic fields, respectively. However, for larger system solely a certain magnetoelectrical control can be achieved.

cond-mat.mes-hall↗
arXiv2011

Fast optical control of spin dynamics in a quantum wire

The spin dynamics in a quantum wire with a Rashba spin orbit interaction (SOI) is shown to be controllable via sub-picosecond electromagnetic pulses shaped appropriately. If the light polarization vector is along the wire's direction, the carriers experience a momentum boost while the phase coherence in different spin channels is maintained, a fact exploitable to control the speed of a photo-driven spin field effect transistor. A photon pulse with a polarization vector perpendicular to the wire results in a spin precession which is comparable to that due to the Rashba SOI and is tunable by the pulse field parameters, an effect utilizable in optically controlled spintronics devices.

cond-mat.mes-hall↗
arXiv2011

Electric field-effect on thermodynamics of multiferroic spin chain

Thermodynamic properties of the helical spins with magnetoelectric coupling is simulated by Monte Carlo algorithm. It is shown that the spin-spin correlations are affected by the thermal fluctuations and the electric field. Results for the temperature-dependence of ferroelectric polarization and the susceptibility are also presented in connection with the multiferroic phase transition.

cond-mat.str-el↗
arXiv2010

Thermoelectric effect of multiferroic oxide interfaces

We investigate the thermoelectric properties of electrons at the interface of oxide heterostructure and in the presence of a multiferroic oxide with spiral spin order. We find there is no (spin) Hall current generated by the temperature gradient. A Seebeck effect is however present. Due to the magnetoelectric coupling, the charge and thermal conductivities are electrically controllable via the spin spiral helicity. Moreover, the thermopower exhibits a sign change when tuning electro-statically the carrier density.

cond-mat.mes-hall↗
arXiv2010

Magnetotransport and spin dynamics in an electron gas formed at oxide interfaces

We investigate the spin-dependent transport properties of a two-dimensional electron gas formed at oxides' interface in the presence of a magnetic field. We consider several scenarios for the oxides' properties, including oxides with co-linear or spiral magnetic and ferroelectric order. For spiral multiferroic oxides, the magnetoelectric coupling and the topology of the localized magnetic moments introduce additional, electric field controlled spin-orbit coupling that affects the magneto-oscillation of the current. An interplay of this spin-orbit coupling, the exchange field, and of the applied magnetic field results in a quantum, gate-controlled spin and charge Hall conductance.

cond-mat.mes-hall↗
arXiv2010

Magneto-transport in impurity-doped few-layer graphene spin valve

Using Keldysh nonequilibrium Green's function method we study the spin-dependent transport through impurity-doped few layer graphene sandwiched between two magnetic leads with an arbitrary mutual orientations of the magnetizations. We find for parallel electrodes magnetizations that the differential conductance possesses two resonant peaks as the applied bias increases. These peaks are traced back to a buildup of a magnetic moment on the impurity due to the electrodes spin polarization. For a large mutual angle of the electrodes magnetization directions, the two resonant peaks approach each others and merge into a single peak for antiparallel orientation of the electrodes magnetizations. We point out that the tunneling magnetoresistance (TMR) may change sign for relatively small changes in the values of the polarization parameters. Furthermore, we inspect the behaviour of the differential conductance and TMR upon varying the temperature.

cond-mat.mtrl-sci↗
arXiv2010

Ultrafast control of inelastic tunneling in a double semiconductor quantum

In a semiconductor-based double quantum well (QW) coupled to a degree of freedom with an internal dynamics, we demonstrate that the electronic motion is controllable within femtoseconds by applying appropriately shaped electromagnetic pulses. In particular, we consider a pulse-driven AlxGa1-xAs based symmetric double QW coupled to uniformly distributed or localized vibrational modes and present analytical results for the lowest two levels. These predictions are assessed and generalized by full-fledged numerical simulations showing that localization and time-stabilization of the driven electron dynamics is indeed possible under the conditions identified here, even with a simultaneous excitations of vibrational modes.

cond-mat.mes-hall↗
arXiv2010

Role of non-collinear magnetization: from ferromagnetic nano wires to quantum rings

We consider interaction effects related to a nonuniform magnetization in ferromagnetic nanowires and their possible generalization to nanorings. First we show that the electric current in a ferromagnetic nanowire with more than one domain wall induces an exchange coupling between the walls mediated by the spin-dependent interference of scattered carriers. This interaction reveals a complex behavior as a function of mutual orientations and separation of the domain walls, thus affecting the domain wall dynamics. Then we consider how the theory should be modified in the case of a magnetized quantum ring. In this case the situation is more complicated because the vortex and onion magnetization states in the ring are not truly ferromagnetic.

cond-mat.mes-hall↗
arXiv2010

Photoinduced dynamics in quantum rings

We investigate the spin-dependent dynamical response of a semiconductor quantum ring with a spin orbit interaction (SOI) upon the application of a single and two linearly polarized, picosecond, asymmetric electromagnetic pulses in the presence of a static magnetic flux. We find that the pulse-generated electric dipole moment is spin dependent. It is also shown that the SOI induces an extra SU(2) effective flux in addition to the static external magnetic flux which is reflected in an additional periodicity of the spin-dependent dipole moment. Furthermore, the pulses may induce a net dynamical charge currents (CC) and dynamical spin currents (SC) when the clockwise and anti-clockwise symmetry of the carrier is broken upon the pulse application.

cond-mat.mes-hall↗
arXiv2010

Coupled Spin-Phonon Excitations in Helical Multiferroics

Both the Dzyaloshiskii-Moriya interaction and the exchange-striction are shown to affect dynamically the magnetoelectric excitations in the perovskite multiferroic RMnO3. The exchange-striction results in a biquadratic interaction between the spins and the transverse phonons, giving rise to quantum fluctuations of the ferroelectric polarization P. This leads to low-lying phonon modes that are perpendicular to P and to the helical spins at small wave vector but are parallel to P at a wave vector close to the magnetic modulation vector. For spin-1/2 helimagnet, the local polarization can be completely reversed by the spin fluctuation, and so does the direction of the on-site spin chirality, which allows for a finite differential scattering intensity of polarized neutrons from a cycloidal magnet.

cond-mat.str-el↗
arXiv2010

Kondo effect in graphene: single vs. two-channel character

Based on the tight-binding formalism, we investigate the Anderson and the Kondo model for an adaom magnetic impurity above graphene. Different impurity positions are analyzed. Employing a partial wave representation we study the nature of the coupling between the impurity and the conducting electrons. The components from the two Dirac points are mixed while interacting with the impurity. Two configurations are considered explicitly: the adatom is above one atom (ADA), the other case is the adatom above the center the honeycomb (ADC). For ADA the impurity is coupled with one flavor for both A and B sublattice and both Dirac points. For ADC the impurity couples with multi-flavor states for a spinor state of the impurity. We show, explicitly for a 3d magnetic atom, $d_{z^{2}}$, ($d_{xz}$,$d_{yz}$), and ($d_{x^{2}-y^{2}}$,$d_{xy}$) couple respectively with the $Γ_{1}$, $Γ_{5} (E_{1})$, and $Γ_{6} (E_{2})$ representations (reps) of $C_{6v}$ group in ADC case. The basses for these reps of graphene are also derived explicitly. For ADA we calculate the Kondo temperature.

cond-mat.str-el↗
arXiv2010

Single or multi-flavor Kondo effect in graphene

Based on the tight-binding formalism, we investigate the Anderson and the Kondo model for an adaom magnetic impurity above graphene. Different impurity positions are analyzed. Employing a partial wave representation we study the nature of the coupling between the impurity and the conducting electrons. The components from the two Dirac points are mixed while interacting with the impurity. Two configurations are considered explicitly: the adatom is above one atom (ADA), the other case is the adatom above the center the honeycomb (ADC). For ADA the impurity is coupled with one flavor for both A and B sublattice and both Dirac points. For ADC the impurity couples with multi-flavor states for a spinor state of the impurity. We show, explicitly for a 3d magnetic atom, $d_{z^{2}}$, ($d_{xz}$,$d_{yz}$), and ($d_{x^{2}-y^{2}}$,$d_{xy}$) couple respectively with the $Γ_{1}$, $Γ_{5} (E_{1})$, and $Γ_{6} (E_{2})$ representations (reps) of $C_{6v}$ group in ADC case. The basses for these reps of graphene are also derived explicitly. For ADA we calculate the Kondo temperature.

cond-mat.str-el↗
arXiv2010

Tunneling Anisotropic Magnetoresistance of Helimagnet Tunnel Junctions

We theoretically investigate the angular and spin dependent transport in normal-metal/helical-multiferroic/ferromagnetic heterojunctions. We find a tunneling anisotropic magnetoresistance (TAMR) effect due to the spiral magnetic order in the tunnel junction and to an effective spin-orbit coupling induced by the topology of the localized magnetic moments in the multiferroic spacer. The predicted TAMR effect is efficiently controllable by an external electric field due to the magnetoelectric coupling.

cond-mat.mes-hall↗