Alan Kalitsov

Anomalous bias dependence of spin torque in magnetic tunnel junctions.

We predict an anomalous bias dependence of the spin transfer torque parallel to the interface, Tparallel, in magnetic tunnel junctions, which can be selectively tuned by the exchange splitting. It may exhibit a sign reversal without a corresponding sign reversal of the bias or even a quadratic bias dependence. We demonstrate that the underlying mechanism is the interplay of spin currents for the ferromagnetic (antiferromagnetic) configurations, which vary linearly (quadratically) with bias, respectively, due to the symmetric (asymmetric) nature of the barrier. The spin transfer torque perpendicular to interface exhibits a quadratic bias dependence.

Spin–orbit torque-assisted switching in magnetic insulator thin films with perpendicular magnetic anisotropy

As an in-plane charge current flows in a heavy metal film with spin–orbit coupling, it produces a torque on and thereby switches the magnetization in a neighbouring ferromagnetic metal film. Such spin–orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. Here we report the SOT-assisted switching in heavy metal/magnetic insulator systems. The experiments used a Pt/BaFe12O19 bilayer where the BaFe12O19 layer exhibits perpendicular magnetic anisotropy. As a charge current is passed through the Pt film, it produces a SOT that can control the up and down states of the remnant magnetization in the BaFe12O19 film when the film is magnetized by an in-plane magnetic field. It can reduce or increase the switching field of the BaFe12O19 film by as much as about 500 Oe when the film is switched with an out-of-plane field.

Fractional Diffusion in Silicon

Microscopic processes leading to ultrafast laser-induced melting of silicon are investigated by large-scale ab initio molecular dynamics simulations. Before becoming a liquid, the atoms are shown to be fractionally diffusive, which is a property that has so far been observed in crowded fluids consisting of large molecules. Here, it is found to occur in an elemental semiconductor.

THE INFLUENCE OF BOUNDARY SCATTERING ON TRANSPORT PHENOMENA IN FERROMAGNETIC METAL-DIELECTRIC NANOCOMPOSITES

It is the rather common for metal-dielectric composites to have metallic grains with size about 10–100A. This dimension is of the order of the bulk mean-free path of conduction electrons. Then the electron scattering at the boundaries between the adjacent grains (intergranular contacts) and at the interfaces between metallic grains and dielectric matrix would give a significant contribution to the total scattering and thus to the transport phenomena. We consider a simple model for intergranular contact and interface scattering and calculate the extraordinary Hall effect (EHE) in ferromagnetic nanocomposites taking into account the boundary scattering. The results of these calculations are in qualitative agreement with the recent experimental data on giant EHE in nanocomposites (NiFe)x(SiO2)1−x.

Extraordinary Hall effect in magnetic granular alloys

Abstract We present the results of a theoretical investigation of the extraordinary Hall effect (EHE) in magnetic granular alloys which exhibit giant magnetoresistance. We consider the impurity scattering of spin-polarized electrons within the magnetic granules, the non-magnetic matrix and at the interfaces between the granules and the matrix in the Zhang-Levy model taking into account the skew scattering due to the spin-orbit interaction. The calculation of the EHE coefficient Rs was carried out using the Boltzmann equation. We show that Rs for granular systems may be larger than that for an homogeneous ferromagnet. Moreover, these coefficients may have opposite sign. The theory predicts that the EHE resistivity is proportional to the concentration of the granules when the electron mean-free path in the matrix is much larger than within the granules. We do not find any correlation between Rs and p2, where p is the resistivity of the granular alloy. However, for some values of the parameters, the EHE resistivity is proportional to p3.9. This is in agreement with the experimental data for Co20Ag80.

Optimized Gaussian basis sets for Goedecker–Teter–Hutter pseudopotentials

We have optimized the exponents of Gaussian s and p basis functions for the elements H, B–F and Al–Cl using the pseudopotentials of (Goedecker, Teter and Hutter 1996 Phys. Rev. B 54 1703) by minimizing the total energy of dimers. We found that this procedure causes the Gaussians to be somewhat more localized than the usual procedure, where the exponents are optimized for atoms. We further found that three exponents, equal for s and p orbitals, are sufficient to reasonably describe the electronic structure of all elements that we have studied. For Li and Be results are presented for pseudopotentials of (Hartwigsen et al 1998 Phys. Rev. B 58 3641). We expect that our exponents will be useful for density functional theory studies where speed is important.

Spin-polarized current-induced torque in magnetic tunnel junctions

We present tight-binding calculations of the spin torque in noncollinear magnetic tunnel junctions based on the nonequilibrium Green functions approach. We have calculated the spin torque via the effective local magnetic moment approach and the divergence of the spin current. We show that both methods are equivalent, i.e., the absorption of the spin current at the interface is equivalent to the exchange interaction between the electron spins and the local magnetization. The transverse components of the spin torque parallel and perpendicular to the interface oscillate with different phase and decay in the ferromagnetic layer (FM) as a function of the distance from the interface. The period of oscillations is inversely proportional to the difference between the Fermi momentum of the majority and minority electrons. The phase difference between the two transverse components of the spin torque is due to the precession of the electron spins around the exchange field in the FM layer. In the absence of applied b...

Bias dependence of tunneling magnetoresistance in magnetic tunnel junctions with asymmetric barriers.

The transport properties of magnetic tunnel junctions (MTJs) are very sensitive to interface modifications. In this work we investigate both experimentally and theoretically the effect of asymmetric barrier modifications on the bias dependence of tunneling magnetoresistance (TMR) in single crystal Fe/MgO-based MTJs with (i) one crystalline and one rough interface, and (ii) with a monolayer of O deposited at the crystalline interface. In both cases we observe an asymmetric bias dependence of TMR and a reversal of its sign at large bias. We propose a general model to explain the bias dependence in these and similar systems reported earlier. The model predicts the existence of two distinct TMR regimes: (i) a tunneling regime when the interface is modified with layers of a different insulator, and (ii) a resonant regime when thin metallic layers are inserted at the interface. We demonstrate that in the tunneling regime, negative TMR is due to the high voltage which overcomes the exchange splitting in the electrodes, while the asymmetric bias dependence of TMR is due to the interface transmission probabilities. In the resonant regime, inversion of TMR could happen at zero voltage depending on the alignment of the resonance levels with the Fermi surfaces of the electrodes. Moreover, the model predicts a regime in which TMR has different signs at positive and negative bias, suggesting possibilities of combining memory with logic functions.

Bias-dependence of the tunneling electroresistance and magnetoresistance in multiferroic tunnel junctions

We predict that the tunneling electroresistance effect is present at finite bias even in multiferroic tunnel junctions (MFTJs) with inversion symmetry. The effect is highly sensitive to the relative magnetization orientation in the electrodes. In addition, we demonstrate control of the bias-dependence of the tunneling magnetoresistance (TMR) in MFTJs via switching of the ferroelectric polarization of the barrier. The polarization induces a monotonic bias behavior in TMR which can be reversed by polarization switching. The magnitude of both effects is proportional to the polarization. The underlying mechanism is the inversion symmetry breaking due to the polarization combined with the interplay of the bias-induced and polarization-induced spin-dependent interfacial screening. These results expand the possibilities for the next-generation multifunctional devices.

Signatures of nonthermal melting

Intense ultrashort laser pulses can melt crystals in less than a picosecond but, in spite of over thirty years of active research, for many materials it is not known to what extent thermal and nonthermal microscopic processes cause this ultrafast phenomenon. Here, we perform ab-initio molecular-dynamics simulations of silicon on a laser-excited potential-energy surface, exclusively revealing nonthermal signatures of laser-induced melting. From our simulated atomic trajectories, we compute the decay of five structure factors and the time-dependent structure function. We demonstrate how these quantities provide criteria to distinguish predominantly nonthermal from thermal melting.