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

Quantum oscillation of magnetoresistance in tunneling junctions with a nonmagnetic spacer

We make a theoretical study of the quantum oscillations of the tunneling magnetoresistance (TMR) as a function of the spacer layer thickness. Such oscillations were recently observed in tunneling junctions with a nonmagnetic metallic spacer at the barrier-electrode interface. It is shown that momentum selection due to the insulating barrier and conduction via quantum well states in the spacer, mediated by diffusive scattering caused by disorder, are essential features required to explain the observed period of oscillation in the TMR ratio and its asymptotic value for thick nonmagnetic spacer.Comment: 4 pages, 5 figures, two column, REVTex4 styl

Dynamics of the magnetic and structural a -> e phase transition in Iron

We have studied the high-pressure iron bcc to hcp phase transition by simultaneous X-ray Magnetic Circular Dichroism (XMCD) and X-ray Absorption Spectroscopy (XAS) with an X-ray dispersive spectrometer. The combination of the two techniques allows us to obtain simultaneously information on both the structure and the magnetic state of Iron under pressure. The magnetic and structural transitions simultaneously observed are sharp. Both are of first order in agreement with theoretical prediction. The pressure domain of the transition observed (2.4 $\pm$ 0.2 GPa) is narrower than that usually cited in the literature (8 GPa). Our data indicate that the magnetic transition slightly precedes the structural one, suggesting that the origin of the instability of the bcc phase in iron with increasing pressure is to be attributed to the effect of pressure on magnetism as predicted by spin-polarized full potential total energy calculations

Plants rustication in a solar greenhouse

La producción por propagación agámica o asexual mediante estacas semileñosas se realiza tradicionalmente en dos etapas, una destinada al enraizamiento y la otra al fortalecimiento radicular y crecimiento de la planta en condiciones próximas al aire libre. Este método presenta un bajo porcentaje de sobrevivencia, menor que el 45 %, debido especialmente al cambio brusco de hábitat que sufren las estacas. La solución que se propone es la incorporación un invernadero de plástico acondicionado térmicamente con energía solar como etapa intermedia del sistema de producción, para favorecer la adaptación del plantón, disminuir el estrés térmico y su estadía en la casa de vegetación (enraizamiento). El sistema se basa principalmente en colectar la radiación solar incidente y extraer el excedente de calor del aire del invernadero durante el día, para acumularlo y usarlo en el momento que sea necesario para calentar el aire y el suelo del mismo. En el presente trabajo se analizan los resultados experimentales y de la simulación numérica del sistema en pleno funcionamiento, utilizando una analogía térmica - eléctrica y el programa para resolución de circuitos eléctricos SCEPTRE. La desviación promedio diaria de la temperatura del aire interior al mismo fue de 1,4 °C con un error relativo promedio de 8,3 %. Se han logrado niveles de energía suficiente para asegurar un nivel térmico adecuado a las necesidades de las plantas para los días con condiciones climáticas típicas de la zona. Desde el punto de vista agronómico los resultados fueron muy satisfactorios lográndose un 100 % de sobrevivencia, una alta tasa de crecimiento y un material vegetal de excelente calidad.Production by agamic or asexual propagation using cuttings is traditionally carried out in two stages, one for rooting, and the other for root strengthening and plant growing in the open air. This method has a low percentage of survival. To reduce thermal stress and the period of staying in the vegetation house the present work proposed an intermediate step to facilitate the planton adaptation. The system is mainly based on collecting solar incident radiation and extracting air heating surplus in the greenhouse during the day and use it when necessary to heat the air and soil in the greenhouse. Experimental results and numeric simulation are analyzed using thermo – electric analogy and the program for electric circuit resolution SCEPTRE. Daily mean deviation of air temperature within the greenhouse was 14° C with a relative error average of 8.3%.. From the agronomic point of view, results were very satisfactory.Fil: Iriarte, Adolfo Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; ArgentinaFil: Saravia Mathon, Luis Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; ArgentinaFil: Matias, C.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; ArgentinaFil: Tomalino, L.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; Argentin

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

Conductance Oscillations in Transition Metal Superlattices

We present a numerical study of conductance oscillations of transition metal multilayers as a function of layer thickness. Using a material-specific tight-binding model, we show that for disorder-free layers with random thicknesses but clean interfaces, long-period oscillations in the conductance can occur, which are reminiscent of those found in structures exhibiting GMR. Using a heuristic effective mass model, we argue that these oscillations arise from beating between the Fermi wavevector and a class of wavevectors characteristic of the superlattice structure.Comment: 4 pages, 4 figure

The role of symmetry on interface states in magnetic tunnel junctions

When an electron tunnels from a metal into the barrier in a magnetic tunnel junction it has to cross the interface. Deep in the metal the eigenstates for the electron can be labelled by the point symmetry group of the bulk but around the interface this symmetry is reduced and one has to use linear combinations of the bulk states to form the eigenstates labelled by the irreducible representations of the point symmetry group of the interface. In this way there can be states localized at the interface which control tunneling. The conclusions as to which are the dominant tunneling states are different from that conventionally found.Comment: 14 pages, 5 figures, accepted in PRB, v2: reference 3 complete

Interplay of Chemical, Electronic, and Structural Effects in the Triple-Conducting BaFeO3-Ba(Zr,Y)O3 Solid Solution

Triple-conducting oxides with mobile protons, oxygen vacancies, and holes are key functional materials for protonic ceramic fuel/electrolysis cells. We comprehensively investigate the Ba(Zr,Y,Fe)O3-delta perovskite solid solution series ranging from electrolyte to electrode-type materials depending on iron content. From thermogravimetry and impedance spectroscopy, the proton and oxygen vacancy concentrations as well as electronic and ionic conductivities are determined. X-ray spectroscopy (Fe K-edge XANES, O K-edge Raman scattering, Fe, Zr, Y K-edge EXAFS) elucidates the finer features of the electronic structure and local distortions. A low Fe content of <= 10% strongly decreases the degree of hydration, while comparably high Fe concentrations of >= 70% are required to obtain an electronic conductivity sufficient for an electrode material. The transport of ionic and electronic carriers is interrelated in a complex way and is closely linked to details of the electronic structure (strength of Fe-O hybridization) and geometrical distortions (Fe-O-Fe and Fe-O-(Zr,Y) buckling). As a result, an optimum combination of proton concentration and electronic conductivity is not obtained in the middle of the solid solution series but rather found for Fe-rich materials with 20-30% doping with oversized, redox-inactive cations. A similar behavior is also expected for related solid solutions between a large-band gap electrolyte and small-band gap redox-active perovskites

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.