arXiv subjects

N. Ryzhanova

Publications and source records attributed to N. Ryzhanova.

12 recordsLinked to original sources

arXiv2015

Analytical description of ballistic spin currents and torques in magnetic tunnel junctions

In this work we demonstrate explicit analytical expressions for both charge and spin currents which constitute the 2x2 spinor in magnetic tunnel junctions with noncollinear magnetizations under applied voltage. The calculations have been performed within the free electron model in the framework of the Keldysh formalism and WKB approximation. We demonstrate that spin/charge currents and spin transfer torques are all explicitly expressed through only three irreducible quantities, without further approximations. The conditions and mechanisms of deviation from the conventional sine angular dependence of both spin currents and torques are shown and discussed. It is shown in the thick barrier approximation that all tunneling transport quantities can be expressed in an extremely simplified form via Slonczewski spin polarizations and our effective spin averaged interfacial transmission probabilities and effective out-of-plane polarizations at both interfaces. It is proven that the latter plays a key role in the emergence of perpendicular spin torque as well as in the angular dependence character of all spin and charge transport considered. It is demonstrated directly also that for any applied voltage, the parallel component of spin current at the FM/I interface is expressed via collinear longitudinal spin current components. Finally, spin transfer torque behavior is analyzed in a view of transverse characteristic length scales for spin transport.

cond-mat.mes-hall↗

arXiv2012

Artificial ferroelectricity due to anomalous Hall effect in magnetic tunnel junctions

We theoretically investigated Anomalous Hall Effect (AHE) and Spin Hall Effect (SHE) transversally to the insulating spacer O, in magnetic tunnel junctions of the form F/O/F where F are ferromagnetic layers and O represents a tunnel barrier. We considered the case of purely ballistic (quantum mechanical) transport, taking into account the assymetric scattering due to spin-orbit interaction in the tunnel barrier. AHE and SHE in the considered case have a surface nature due to proximity effect. Their amplitude is in first order of the scattering potential. This contrasts with ferromagnetic metals wherein these effect are in second (side-jump scattering) and third (skew scattering) order on these potentials. The value of AHE voltage in insulating spacer may be much larger than in metallic ferromagnetic electrodes. For the antiparallel orientation of the magnetizations in the two F-electrodes, a spontaneous Hall voltage exists even at zero applied voltage. Therefore an insulating spacer sandwiched between two ferromagnetic layers can be considered as exhibiting a spontaneous ferroelectricity.

cond-mat.mes-hall↗

arXiv2011

Spin Transfer Torques induced by Spin Hall Effect

Spin accumulation and spin transfer torques induced by Spin Hall Effect in bi-layer structures comprising ferromagnetic and paramagnetic materials are theoretically investigated. The charge and spin diffusion equations taking into account spin-flip and spin Hall effect are formulated and solved analytically and numerically for in structures with simplified and complex geometry, respectively. It is demonstrated that spin torques could be efficiently produced by means of Spin Hall effect which may be further enhanced by modifying structure geometry.

cond-mat.mes-hall↗

arXiv2011

Spin-current vortices in current-perpendicular-to-plane nanoconstricted spin-valves

The charge and spin diffusion equations taking into account spin-flip and spin-transfer torque were numerically solved using a finite element method in complex non-collinear geometry with strongly inhomogeneous current flow. As an illustration, spin-dependent transport through a non-magnetic nanoconstriction separating two magnetic layers was investigated. Unexpected results such as vortices of spin-currents in the vicinity of the nanoconstriction were obtained. The angular variations of magnetoresistance and spin-transfer torque are strongly influenced by the structure geometry.

cond-mat.mes-hall↗

arXiv2009

The two-bands model of the magnetic tunnel junctions in the Fe/Cr/MgO/Fe structure

In this paper we present theoretical studies of spin dependent transport in Fe/Cr/MgO/Fe tunnel junctions with non-collinear alignment of magnetizations of metallic layers comprising these MTJs. Calculations are performed with use of non-equilibrium Green function technique in the framework of Keldysh formalism. WKB approximation is used for wave and Green functions in the trapezoidal barrier region under applied voltage. Electronic structure of ferromagnetic electrodes is modeled with two bands model, i.e. with majority s-electrons and minority d-holes. Furthermore, we introduce s-d hybridization by calculating the corresponding perturbation corrections for the wave and Green functions.

cond-mat.mes-hall↗

arXiv2008

Quasi-Two-Dimensional Extraordinary Hall Effect

Quasi-two-dimensional transport is investigated in a system consisting of one ferromagnetic layer placed between two insulating layers. Using the mechanism of skew-scattering to describe the Extraordinary Hall Effect (EHE) and calculating the conductivity tensor, we compare the quasi- two-dimensional Hall resistance with the resistance of a massive sample. In this study a new mechanism of EHE (geometric mechanism of EHE) due to non-ideal interfaces and volume defects is also proposed.

cond-mat.mtrl-sci↗

arXiv2004

Diode effect in magnetic tunnel junctions

The influence on the I-V characteristics and tunnel magnetoresistance (TMR), of impurities embedded into the insulating barrier I separating the two ferromagnetic electrodes F of a magnetic tunnel junction, was theoretically investigated. When the energy of the electron's bound state at the impurity site is close to the Fermi energy, it is shown that the current and TMR are strongly enhanced in the vicinity of the impurity. If the position of the impurity inside the barrier is asymmetric, e.g. closer to one of the interfaces F/I, the I-V characteristic exhibits a quasidiode behavior. The case of a single impurity and of a random distribution of impurities within a plane were both studied.

cond-mat.other↗

arXiv2004

Magnetic tunnel junctions with impurities

The influence of impurities, embedded into the isolating spacer (I) between two ferromagnetic electrodes (F), on the I-V curve and tunnel magnetoresistance (TMR), is theoretically investigated. It is shown, that the current and TMR are strongly enhanced in the vicinity of the impurity under the condition that the energy of the electron's bound state on the impurity is close to the Fermi energy. If the position of the impurity inside the barrier is asymmetric, e.g. closer to the one of the interfaces F/I the I-V curve exhibits quasidiode behavior.

cond-mat.other↗

arXiv2004

Influence of s-d scattering on the electron density of states in ferromagnet/superconductor bilayer

We study the dependence of the electronic density of states (DOS) on the distance from the boundary for a ferromagnet/superconductor bilayer. We calculate the electron density of states in such structure taking into account the two-band model of the ferromagnet (FM) with conducting s and localized d electrons and a simple s-wave superconductor (SC). It is demonstrated that due to the electron s-d scattering in the ferromagnetic layer in the third order of s-d scattering parameter the oscillation of the density of states has larger period and more drastic decrease in comparison with the oscillation period for the electron density of states in the zero order.

cond-mat.other↗

arXiv2004

Resonance magneto-resistance in double barrier structure with spin-valve

The conductance and tunnel magneto-resistance (TMR) of the double barrier magnetic tunnel junction with spin-valve sandwich (F/P/F) inserted between two insulating barrier, are theoretically investigated. It is shown, that resonant tunnelling, due to the quantum well states of the electron confined between two barriers, sharply depends on the mutual orientation of the magnetizations of ferromagnetic layers F. The calculated optimistic value of TMR exceeds 2000% .

cond-mat↗

arXiv2001

Extraordinary Hall effect in hybrid ferromagnetic/superconductor (F/S) bilayer

Extraordinary Hall effect (EHE) in bilayer F/S(N) was investigated theoretically. The conductivity tensor $σ_{αβ}$ is calculated in the Kubo formalism with Green functions found as the solutions of the Gorkov equations. We considered diffuse transport in the ferromagnetic layer, taking into account as a main mechanism of electron resistivity s-d scattering. In this model Gorkov equations for s-electrons in the ferromagnetic layer remain linear and are solved easily. It is shown that Hall field $E^H$ for both F/S and F/N contacts are step-functions of the coordinate perpendicular to the planes of the layers and have zero value in S(N) layer. The Andreev reflection increases the value of Hall constant $R_s$ for F/S case. The value of the Hall constant is $R_H^{F/S} = R_H^{bulk} (σ^{\uparrow} + σ^{\downarrow})^2 / 4 σ^{\uparrow}σ^{\downarrow}$, where $σ^{\uparrow}$ and $σ^{\downarrow}$ are conductivities of electrons with up and down spins, and $R_H^{bulk}$ is the Hall constant in the bulk ferromagnetic metal. In fact, $R_H^{F/S}$ coincides with EHE constant of the bilayer of two ferromagnetic metals with equal thickness and opposite directions of their magnetizations. So we can make a conclusion, that the ideal interface between ferromagnetic metal and superconductor may be considered like a mirror with inversion in spin space.

cond-mat.mtrl-sci↗

arXiv2000

Does Giant Magnetoresistance Survive in Presence of Superconducting Contact?

The giant magnetoresistance (GMR) of ferromagnetic bilayers with a superconducting contact (F1/F2/S) is calculated in ballistic and diffusive regimes. As in spin-valve, it is assumed that the magnetization in the two ferromagnetic layers F1 and F2 can be changed from parallel to antiparallel. It is shown that the GMR defined as the change of conductance between the two magnetic configurations is an oscillatory function of the thickness of F2 layer and tends to an asymptotic positive value at large thickness. This is due to the formation of quantum well states in F2 induced by Andreev reflection at the F2/S interface and reflection at F1/F2 interface in antiparallel configuration. In the diffusive regime, if only spin-dependent scattering rates in the magnetic layers are considered (no difference in Fermi wave-vectors between spin up and down electrons) then the GMR is supressed due to the mixing of spin up and down electron-hole channels by Andreev reflection.

cond-mat.mtrl-sci↗