A selfconsistent theory of current-induced switching of magnetization

A selfconsistent theory of the current-induced switching of magnetization using nonequilibrium Keldysh formalism is developed for a junction of two ferromagnets separated by a nonmagnetic spacer. It is shown that the spin-transfer torques responsible for current-induced switching of magnetization can be calculated from first principles in a steady state when the magnetization of the switching magnet is stationary. The spin-transfer torque is expressed in terms of one-electron surface Green functions for the junction cut into two independent parts by a cleavage plane immediately to the left and right of the switching magnet. The surface Green functions are calculated using a tight-binding Hamiltonian with parameters determined from a fit to an {\it ab initio} band structure.This treatment yields the spin transfer torques taking into account rigorously contributions from all the parts of the junction. To calculate the hysteresis loops of resistance versus current, and hence to determine the critical current for switching, the microscopically calculated spin-transfer torques are used as an input into the phenomenological Landau-Lifshitz equation with Gilbert damping. The present calculations for Co/Cu/Co(111) show that the critical current for switching is $\approx 10^7A/cm^2$, which is in good agreement with experiment.Comment: 23 pages, 16 figure

Reflection mechanism for generating spin transfer torque without charge current

A reflection mechanism for generating spin-transfer torque is proposed. It is due to interference of bias-driven nonequilibrium electrons incident on a switching junction, with the electrons reflected from an insulating barrier inserted in the junction after the switching magnet. It is shown, using the rigorous Keldysh formalism, that this out-of-plane torque T⊥ is proportional to an applied bias and is as large as the torque in a conventional junction generated by a strong charge current. However, the charge current and the in-plane torque T∥ are almost completely suppressed by the insulating barrier. This junction thus offers the highly applicable possibility of bias-induced switching of magnetization without charge current

On the Uniform Random Generation of Non Deterministic Automata Up to Isomorphism

In this paper we address the problem of the uniform random generation of non deterministic automata (NFA) up to isomorphism. First, we show how to use a Monte-Carlo approach to uniformly sample a NFA. Secondly, we show how to use the Metropolis-Hastings Algorithm to uniformly generate NFAs up to isomorphism. Using labeling techniques, we show that in practice it is possible to move into the modified Markov Chain efficiently, allowing the random generation of NFAs up to isomorphism with dozens of states. This general approach is also applied to several interesting subclasses of NFAs (up to isomorphism), such as NFAs having a unique initial states and a bounded output degree. Finally, we prove that for these interesting subclasses of NFAs, moving into the Metropolis Markov chain can be done in polynomial time. Promising experimental results constitute a practical contribution.Comment: Frank Drewes. CIAA 2015, Aug 2015, Umea, Sweden. Springer, 9223, pp.12, 2015, Implementation and Application of Automata - 20th International Conferenc

Interplay between local structure and electronic properties on CuO under pressure

The electronic and local structural properties of CuO under pressure have been investigated by means of X-ray absorption spectroscopy (XAS) at Cu K edge and ab-initio calculations, up to 17 GPa. The crystal structure of CuO consists of Cu motifs within CuO$_4$ square planar units and two elongated apical Cu-O bonds. The CuO$_4$ square planar units are stable in the studied pressure range, with Cu-O distances that are approximately constant up to 5 GPa, and then decrease slightly up to 17 GPa. In contrast, the elongated Cu-O apical distances decrease continuously with pressure in the studied range. An anomalous increase of the mean square relative displacement (EXAFS Debye Waller, \sigma$^2$) of the elongated Cu-O path is observed from 5 GPa up to 13 GPa, when a drastic reduction takes place in \sigma$^2$. This is interpreted in terms of local dynamic disorder along the apical Cu-O path. At higher pressures (P>13 GPa), the local structure of Cu$^{2+}$ changes from a 4-fold square planar to a 4+2 Jahn-Teller distorted octahedral ion. We interpret these results in terms of the tendency of the Cu$^{2+}$ ion to form favorable interactions with the apical O atoms. Also, the decrease in Cu-O apical distance caused by compression softens the normal mode associated with the out-of-plane Cu movement. CuO is predicted to have an anomalous rise in permittivity with pressure as well as modest piezoelectricity in the 5-13 GPa pressure range. In addition, the near edge features in our XAS experiment show a discontinuity and a change of tendency at 5 GPa. For P < 5 GPa the evolution of the edge shoulder is ascribed to purely electronic effects which also affect the charge transfer integral. This is linked to a charge migration from the Cu to O, but also to an increase of the energy band gap, which show a change of tendency occurring also at 5 GPa

A theoretical investigation of ferromagnetic tunnel junctions with 4-valued conductances

In considering a novel function in ferromagnetic tunnel junctions consisting of ferromagnet(FM)/barrier/FM junctions, we theoretically investigate multiple valued (or multi-level) cell property, which is in principle realized by sensing conductances of four states recorded with magnetization configurations of two FMs; that is, (up,up), (up,down), (down,up), (down,down). To obtain such 4-valued conductances, we propose FM1/spin-polarized barrier/FM2 junctions, where the FM1 and FM2 are different ferromagnets, and the barrier has spin dependence. The proposed idea is applied to the case of the barrier having localized spins. Assuming that all the localized spins are pinned parallel to magnetization axes of the FM1 and FM2, 4-valued conductances are explicitly obtained for the case of many localized spins. Furthermore, objectives for an ideal spin-polarized barrier are discussed.Comment: 9 pages, 3 figures, accepted for publication in J. Phys.: Condens. Matte

Studies of concentration and temperature dependencies of precipitation kinetics in iron-copper alloys using kinetic monte carlo and stochastic statistical simulations

The earlier-developed ab initio model and the kinetic Monte Carlo method (KMCM) are used to simulate precipitation in a number of iron-copper alloys with different copper concentrations x and temperatures T. The same simulations are also made using the improved version of the earlier-suggested stochastic statistical method (SSM). The results obtained enable us to make a number of general conclusions about the dependencies of the decomposition kinetics in Fe-Cu alloys on x and T. We also show that the SSM describes the precipitation kinetics in a fair agreement with the KMCM, and employing the SSM in conjunction with the KMCM enables us to extend the KMC simulations to the longer evolution times. The results of simulations seem to agree with available experimental data for Fe-Cu alloys within statistical errors of simulations and the scatter of experimental results. Comparison of results of simulations to experiments for some multicomponent Fe-Cu-based alloys enables us to make certain conclusions about the influence of alloying elements in these alloys on the precipitation kinetics at different stages of evolution.Comment: 18 pages, 17 postscript figures, LaTe

Sub-gap conductance in ferromagnetic-superconducting mesoscopic structures

We study the sub-gap conductance of a ferromagnetic mesoscopic region attached to a ferromagnetic and a superconducting electrode by means of tunnel junctions. In the absence of the exchange field, the ratio $r= \gamma / \epsilon_T$ of the two tunnel junction resistances determines the behaviour of the sub-gap conductance which possesses a zero-bias peak for $r\gg 1$ and for $r\ll 1$ a peak at finite voltage. We show that the inclusion of the exchange field leads to a peak splitting for $r\ll 1$, while it shifts the zero-bias anomaly to finite voltages for $r\gg 1$.Comment: 5 pages revte

Exponential behavior of the interlayer exchange coupling across non-magnetic metallic superlattices

It is shown that the coupling between magnetic layers separated by non-magnetic metallic superlattices can decay exponentially as a function of the spacer thickness $N$, as opposed to the usual $N^{-2}$ decay. This effect is due to the lack of constructive contributions to the coupling from extended states across the spacer. The exponential behavior is obtained by properly choosing the distinct metals and the superlattice unit cell composition.Comment: To appear in Phys. Rev.

Theory of a quodon gas. With application to precipitation kinetics in solids under irradiation

Rate theory of the radiation-induced precipitation in solids is modified with account of non-equilibrium fluctuations driven by the gas of lattice solitons (a.k.a. quodons) produced by irradiation. According to quantitative estimations, a steady-state density of the quodon gas under sufficiently intense irradiation can be as high as the density of phonon gas. The quodon gas may be a powerful driver of the chemical reaction rates under irradiation, the strength of which exponentially increases with irradiation flux and may be comparable with strength of the phonon gas that exponentially increases with temperature. The modified rate theory is applied to modelling of copper precipitation in FeCu binary alloys under electron irradiation. In contrast to the classical rate theory, which disagrees strongly with experimental data on all precipitation parameters, the modified rate theory describes quite well both the evolution of precipitates and the matrix concentration of copper measured by different methodsComment: V. Dubinko, R. Shapovalov, Theory of a quodon gas. With application to precipitation kinetics in solids under irradiation. (Springer International Publishing, Switzerland, 2014