Spin-polarized Electric Current Research Articles

Spinovi khvyli u feromahnitniy nanotrubtsi. Urakhuvannya dysypatsiyi ta spin-polyaryzovanoho strumu

Dipole-exchange spin waves in a ferromagnetic nanotube with a circular cross-section have been studied in the presence of a spin-polarized electric current. The exchange and dipole-dipole magnetic interactions, anisotropy, dissipation effects, and the influence of a spin-polarized current are taken into consideration. An equation for the magnetic potential of spin excitations in the system concerned is derived, and the dispersion relation for spin waves is obtained. Depending on its direction, the spin-polarized current is demonstrated to either strengthen or weaken the effective dissipation. A condition, under which the presence of the spin-polarized current can lead to a generation of a spin wave, is determined.

Bifurcations in the Dynamical System for Three-Layered Magnetic Valve

Magnetization dynamics in a three-layered nanopillar Co/Cu/Co structure with one fixed and one free layer driven by external magnetic field and spin-polarized electric current was investigated using methods of the theory of bifurcations. Mathematical model is based on the Landau-Lifshits-Gilbert equation with the current term in the Sloncžewski–Berger form. Orientation of applied magnetic field was considered to be parallel to the anisotropy axis. Physical model included the magnetocrystalline anisotropy field and the demagnetizing field. Because of small size of the structure, the space dependence of magnetization, as usually, was not taken into account. The resulting system of equations has the form of the nonlinear dynamical system with the polynomial right sides. Mathematical simulation of magnetization dynamics for several typical values of field and current was performed. The numerical experiments revealed the features of switching process in more detail and permitted to find new regimes of magnetization dynamics, such as incomplete and accidental switching.

Alternating spin-polarized current induces parametric resonance in spin valves

Ferromagnetic systems under the influence of spin-polarized currents exhibit rich spatiotemporal dynamics at nanoscales. We study spin-transfer nano-oscillators driven by the combination of alternating and direct spin-polarized electric currents. We show here that the alternating current induces parametric instabilities on spin valves, that is, the magnetization responses at half the forcing frequency. A spatial self-organization emerges as a result of the oscillatory current, which includes dissipative solitons and Faraday-type waves. The parametric regime is described analytically by means of the Landau-Lifshitz-Gilbert-Slonczewski equation, in good agreement with micromagnetic simulations including the full dipolar field.

Layer-selective half-metallicity in bilayer graphene nanoribbons.

Half-metallicity recently predicted in the zigzag-edge graphene nanoribbons (ZGNRs) and the hydrogenated carbon nanotubes (CNTs) enables fully spin-polarized electric currents, providing a basis for carbon-based spintronics. In both carbon systems, the half-metallicity arises from the edge-localized electron states under an electric field, lowering the critical electric field Dc for the half-metallicity being an issue in recent works on ZGNRs. A properly chosen direction of the electric field alone has been predicted to significantly reduce Dc in the hydrogenated CNTs, which in this work turned out to be the case in narrow bilayer ZGNRs (biZGNRs). Here, our simple model based on the electrostatic potential difference between the edges predicts that for wide biZGNRs of width greater than ~2.0 nm (10 zigzag carbon chains), only one layer of the biZGNRs becomes half-metallic leaving the other layer insulating as confirmed by our density functional theory (DFT) calculations. The electric field-induced switching of the spin-polarized current path is believed to open a new route to graphene-based spintronics applications.

Symmetry-derived half-metallicity in atomic and molecular junctions.

Achieving highly spin-polarized electric currents in atomic-scale junctions is of great importance in the field of nanoelectronics and spintronics. Based on robust symmetry considerations, we propose a mechanism to block completely one of spin conduction channels for a broad class of atomic and molecular junctions bridging two ferromagnetic electrodes. This particular behavior is due to the wave function orthogonality between spin up s-like states in ferromagnetic electrode and available π channels in the junction. As a consequence, the system would ideally yield 100% spin-polarized current, with a junction acting thus as a "half-metallic" conductor. Using ab initio electron transport calculations, we demonstrate this principle on two examples: (i) a short carbon chain and (ii) a π-conjugated molecule (polythiophene) connected either to model semi-infinite Ni wires or to realistic Ni(111) electrodes. It is also predicted that such atomic-scale junctions should lead to very high (ideally, infinite) magneto-resistance ratios since the electric current gets fully blocked if two electrodes have antiparallel magnetic alignment.

Reversible Electric-Field-Driven Magnetic Domain-Wall Motion

Control of magnetic domain-wall motion by electric fields has recently attracted scientific attention because of its potential for magnetic logic and memory devices. Here, we report on a new driving mechanism that allows for magnetic domain-wall motion in an applied electric field without the concurrent use of a magnetic field or spin-polarized electric current. The mechanism is based on elastic coupling between magnetic and ferroelectric domain walls in multiferroic heterostructures. Pure electric-field-driven magnetic domain-wall motion is demonstrated for epitaxial Fe films on BaTiO3 with in-plane and out-of-plane polarized domains. In this system, magnetic domain-wall motion is fully reversible and the velocity of the walls varies exponentially as a function of out-of-plane electric-field strength.Received 22 August 2014DOI:https://doi.org/10.1103/PhysRevX.5.011010This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical Society

Transformation of the Domain-Wall Structure in Permalloy Nanotape Under Electrical Current Pulses

Motion of the magnetic domain walls under the pulses of magnetic field or spin-polarized electric current is studied experimentally by direct visualization of the walls. The experiments are performed in the temperature range from 300 down to 6 K. Coercivity, mobility of the walls, and their variations with temperature are investigated. Wall velocity is observed to increase by orders of magnitude due to cooling from room to helium temperature in the case of current stimulated motion. The velocity becomes as high as the Walker velocity limit, about 1000 m/s. The walls are found to expand and tilt under the current pulses, which leads to their transformation into the domains with transverse magnetization.

Spin-polarized electric currents in diluted magnetic semiconductor heterostructures induced by terahertz and microwave radiation

We report on the study of spin-polarized electric currents in diluted magnetic semiconductor (DMS) quantum wells subjected to an in-plane external magnetic field and illuminated by microwave or terahertz radiation. The effect is studied in (Cd,Mn)Te/(Cd,Mg)Te quantum-wells (QWs) and (In,Ga)As/InAlAs:Mn QWs belonging to the well-known II-VI and III-V DMS material systems, as well as in heterovalent AlSb/InAs/(Zn,Mn)Te QWs, which represent a promising combination of II-VI and III-V semiconductors. Experimental data and developed theory demonstrate that the photocurrent originates from a spin-dependent scattering of free carriers by static defects or phonons in the Drude absorption of radiation and subsequent relaxation of carriers. We show that in DMS structures, the efficiency of the current generation is drastically enhanced compared to nonmagnetic semiconductors. The enhancement is caused by the exchange interaction of carrier spins with localized spins of magnetic ions resulting, on the one hand, in the giant Zeeman spin splitting, and, on the other hand, in the spin-dependent carrier scattering by localized Mn${}^{2+}$ ions polarized by an external magnetic field.

Externally driven transformations of vortex textures in flat submicrometer magnets

Two effects of oscillatory transformations of vortex textures in flat nanomagnets due to the application of an external field or a spin-polarized electric current are analytically described with relevance to soft-magnetic structures of submicrometer sizes (whose thickness is significantly bigger than the magnetostatic exchange length). These are changes of a domain wall (DW) structure in a long magnetic stripe (oscillations between a transverse DW, a vortex DW, and an antivortex DW) and periodic vortex-core reversals in a circular magnetic dot which are accompanied by oscillatory displacements of the vortex from the dot center. In nanostructures of smaller thicknesses (comparable to the exchange length), where nonlocal magnetostatic effects are very strong because of fast spatial variation of the magnetization, similar phenomena have been widely studied previously. Here, the dynamics is investigated within a local approach including magnetostatic field via boundary conditions on solutions to the Landau-Lifshitz-Gilbert equation only. Both the DWs in stripes and vortex states of the dot are treated as fragments of a cross-tie DW. Despite similarity of the cyclic transformations of the ordering to the dynamics of more strongly confined nanomagnets, details of motion (trajectories) of the vortices and antivortices (Bloch lines) of the textures under study are different, which is related to prohibition of rapid jumps of the polarization of Bloch lines. In addition to the magnetization rotation about the direction of magnetic field or current polarization, the evolution of textures is shown to relate to oscillatory changes of the direction of a cross-tie DW with respect to any arbitrary axis in the magnet plane accompanied by oscillations of the DW width.

A metallic surface state with uniaxial spin polarization on Tl/Ge(111)-(1 × 1)

We show that a metallic surface state is formed on Tl/Ge(111)-(1 × 1). The surface state forms electron pockets around of the surface Brillouin zone. A first-principles calculation reveals that the electron pockets are composed of a single branch of a spin-split surface-state band. The spin quantization axis is along the surface normal and inverts according to the time-reversal symmetry. Since this spin-split branch is the unique metallic band on this surface, the surface conductivity should be governed by this spin-split branch, suggesting a possible spin-polarized electric current.

Ultrafast heating as a sufficient stimulus for magnetization reversal in a ferrimagnet

The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field.

Control over topological insulator photocurrents with light polarization

Three-dimensional topological insulators represent a new quantum phase of matter with spin-polarized surface states that are protected from backscattering. The static electronic properties of these surface states have been comprehensively imaged by both photoemission and tunnelling spectroscopies. Theorists have proposed that topological surface states can also exhibit novel electronic responses to light, such as topological quantum phase transitions and spin-polarized electrical currents. However, the effects of optically driving a topological insulator out of equilibrium have remained largely unexplored experimentally, and no photocurrents have been measured. Here, we show that illuminating the topological insulator Bi(2)Se(3) with circularly polarized light generates a photocurrent that originates from topological helical Dirac fermions, and that reversing the helicity of the light reverses the direction of the photocurrent. We also observe a photocurrent that is controlled by the linear polarization of light and argue that it may also have a topological surface state origin. This approach may allow the probing of dynamic properties of topological insulators and lead to novel opto-spintronic devices.

Externally driven transmission and collisions of domain walls in ferromagnetic wires

Analytical multidomain solutions to the dynamical (Landau-Lifshitz-Gilbert) equation of a one-dimensional ferromagnet including an external magnetic field and spin-polarized electric current are found using the Hirota bilinearization method. A standard approach to solve the Landau-Lifshitz equation (without the Gilbert term) is modified in order to treat the dissipative dynamics. I establish the relations between the spin interaction parameters (the constants of exchange, anisotropy, dissipation, external-field intensity, and electric-current intensity) and the domain-wall parameters (width and velocity) and compare them to the results of the Walker approximation and micromagnetic simulations. The domain-wall motion driven by a longitudinal external field is analyzed with especial relevance to the field-induced collision of two domain walls. I determine the result of such a collision (which is found to be an elastic one) on the domain-wall parameters below and above the Walker breakdown (in weak- and strong-field regimes). Single-domain-wall dynamics in the presence of an external transverse field is studied with relevance to the challenge of increasing the domain-wall velocity below the breakdown.

Lagrangian formulation of the linear autonomous magnetization dynamics in spin-torque auto-oscillators

A Lagrangian formalism is used to find steady-state solution of the Landau–Lifshitz–Gilbert–Slonczewski equation corresponding to the linear autonomous dynamics of a magnetic auto-oscillatory system subject to the action of a spin-polarized electric current. In such a system, two concurrent dissipative mechanisms, arising from the positive intrinsic dissipation and the negative current-induced one, take place simultaneously and make the excitation of a steady precessional motion of the magnetization vector conceivable. The proposed formulation leads to the definition of a complex generalized non-Hermitian Eigenvalue problem, both in the case of a macrospin model and in the more general case of an ensemble of magnetic particles interacting each other through magnetostatic and exchange interactions. This method allows to identify the spin-wave normal modes which become unstable in the presence of the two competing dissipative contributions and provides an accurate estimation of the value of the excitation threshold current.

Limits for the vortex state spin torque oscillator in magnetic nanopillars: Micromagnetic simulations for a thin free layer

We report micromagnetic simulations of magnetization dynamics of a vortex state in the free layer of a circular nanopillar excited by the spin transfer torque effect of a perpendicular to the layer (dot) plane spin-polarized electrical current. The magnetization of the reference layer (polarizer) is assumed to be fixed. A new regime of the dynamic magnetization response to the current is reported: vortex expelling from the dot, subsequent in-plane magnetization oscillations in single domain state, and the vortex return with an opposite core polarization. We analyze conditions (limits of the vortex state as a nano-oscillator) to achieve steady magnetization oscillations corresponding to a gyrotropic motion of the vortex core in terms of the current intensity. These conditions are formulated via the critical currents and vary greatly with the magnetic damping parameter and the cell size used for micromagnetic simulations. The existing experiments on the current induced magnetization dynamics in nanopillars and nanocontacts are discussed.

Spin polarized electric currents in semiconductor heterostructures induced by microwave radiation

We report on microwave (mw) radiation induced electric currents in (Cd,Mn)Te/(Cd,Mg)Te and InAs/(In,Ga)As quantum wells subjected to an external in-plane magnetic field. The current generation is attributed to the spin-dependent energy relaxation of electrons heated by mw radiation. The relaxation produces equal and oppositely directed electron flows in the spin-up and spin-down subbands yielding a pure spin current. The Zeeman splitting of the subbands in the magnetic field leads to the conversion of the spin flow into a spin-polarized electric current.

Nonadiabatic Stochastic Resonance of a Nanomagnet Excited by Spin Torque

We report microwave-frequency magnetization dynamics coexcited by alternating spin torque and thermal fluctuations. In these dynamics, temperature strongly enhances the amplitude of magnetization precession and enables excitation of nonlinear dynamic states of magnetization by weak alternating spin torque. We explain these thermally-activated dynamics in terms of nonadiabatic stochastic resonance of magnetization driven by spin torque.

Spin-polarized electric currents in quantum transport through tubular two-dimensional electron gases

Scattering theory is employed to derive a Landauer-type formula for the spin and the charge currents, through a finite region where spin-orbit interactions are effective. It is shown that the transmission matrix yields the spatial direction and the magnitude of the spin polarization. This formula is used to study the currents through a tubular two-dimensional electron gas. In this cylindrical geometry, which may be realized in experiment, the transverse conduction channels are not mixed (provided that the spin-orbit coupling is uniform). It is then found that for modest boundary scattering, each step in the quantized conductance is split into two, and the new steps have a non-zero spin conductance, with the spin polarization perpendicular to the direction of the current.

Dependence of Magnetization Dynamics on Anisotropy in Thin Films Driven by Spin-Polarized Currents

An analytical study of magnetization dynamics in nanomagnets subjected to magnetic fields and spin-polarized electrical currents is discussed. It is shown how the presence of various dynamical regimes depends on magnetic properties of the investigated system, in particular, it is demonstrated that the magnetization response is different if we consider magnetic systems with either uniaxial in-plane or easy-plane anisotropy.

Spin-Wave Instabilities in Spin-Transfer-Driven Magnetization Dynamics

We study the stability of magnetization precessions induced in spin-transfer devices by the injection of spin-polarized electric currents. Instability conditions are derived by introducing a generalized, far-from-equilibrium interpretation of spin waves. It is shown that instabilities are generated by distinct groups of magnetostatically coupled spin waves. Stability diagrams are constructed as a function of external magnetic field and injected spin-polarized current. These diagrams show that the application of larger fields and currents has a stabilizing effect on magnetization precessions. Analytical results are compared with numerical simulations of spin-transfer-driven magnetization dynamics.