Spin-polarized Electric Current Research Articles

Controlled Quasi-Latitudinal Solutions for Ultra-Fast Spin-Torque Magnetization Switching

The aim of this paper is to present a novel class of time-dependent controls to realize ultra-fast magnetization switching in nanomagnets driven by spin-torques produced by spin-polarized electric currents. Magnetization dynamics in such complex systems is governed by the Landau–Lifshitz–Slonczewski equation which describes the precessional motion of (dimensionless) magnetization vector on the unit-sphere. The relevant case of nanoparticles with uniaxial anisotropy having in-plane easy and intermediate axes as well as out-of-plane hard axis is considered. By exploiting the characteristic smallness of damping and spin-torque intensity, the complexity of the magnetic system’s dynamic is dealt with by employing tools borrowed from Hamiltonian Perturbation Theory. More precisely, the aforementioned controls are constructed via suitable perturbative tools in a way to realize approximate latitudinal solutions (i.e. motions on a sphere in which the out-of-plane magnetization component stays constant) with the effect to fast “switch” the system from one stationary state to another. The possibility to keep a (“small”) bounded value of the out-of-plane coordinate throughout this process of “transfer” turns out to be advantageous in the applications as it sensibly reduces the post-switching relaxation oscillations that may cause the failure of switching in real samples. Further relevant quantitative results on the behavior of the solutions during the pre- and post-switching stages (termed “expulsion” and “attraction”, respectively) are given as a by-product. A selection of validating numerical experiments is presented alongside the corresponding theoretical results.

Dirac Half-Semimetallicity and Antiferromagnetism in Graphene Nanoribbon/Hexagonal Boron Nitride Heterojunctions.

Half-metals have been envisioned as active components in spintronic devices by virtue of their completely spin-polarized electrical currents. Actual materials hosting half-metallic phases, however, remain scarce. Here, we predict that recently fabricated heterojunctions of zigzag nanoribbons embedded in two-dimensional hexagonal boron nitride are half-semimetallic, featuring fully spin-polarized Dirac points at the Fermi level. The half-semimetallicity originates from the transfer of charges from hexagonal boron nitride to the embedded graphene nanoribbon. These charges give rise to opposite energy shifts of the states residing at the two edges, while preserving their intrinsic antiferromagnetic exchange coupling. Upon doping, an antiferromagnetic-to-ferrimagnetic phase transition occurs in these heterojunctions, with the sign of the excess charge controlling the spatial localization of the net magnetic moments. Our findings demonstrate that such heterojunctions realize tunable one-dimensional conducting channels of spin-polarized Dirac fermions seamlessly integrated into a two-dimensional insulator, thus holding promise for the development of carbon-based spintronics.

Domain wall damped harmonic oscillations induced by curvature gradients in elliptical magnetic nanowires

Understanding the domain wall (DW) dynamics in magnetic nanowires (NW) is crucial for spintronic-based applications demanding the use of DWs as information carriers. This work focuses on the dynamics of a DW displacing along a bent NW with an elliptical shape under the action of spin-polarized electric currents and external magnetic fields. Our results evidence that a curvature gradient induces an exchange-driven effective tangential field responsible for pinning a DW near the maximum curvature point in a NW. The DW equilibrium position depends on the competition between the torques produced by the external stimuli and the curvature-induced effective fields. When the external stimuli are below a certain threshold, the DW follows a damped harmonic oscillation around the equilibrium position. Above this threshold, DW displaces along the NW under an oscillatory translational motion.

Advances in Magnetic Domain Walls and Their Applications

This article explores the recent developments in spin-based domain wall (DW) memories. The physics behind the DW motion, device materials, current challenges, and applications have been discussed in detail. DWs can propagate through a magnetic nanowire by the application of external magnetic fields or by spin-polarized electric currents. Great progress has been made in these devices since the introduction of electric current-induced DW motion. However, driving DWs necessitates large spin-current densities that are incompatible with low-power devices. Therefore, significant efforts have been made to achieve highly efficient and controlled DW motion by material engineering and different mechanisms such as spin-orbit-torque (SOT), Dzyaloshinskii-Moriya interaction (DMI), and voltage-controlled magnetic anisotropy. The controlled manipulation of DWs in magnetic materials has inspired numerous strategies for high-density memory and energy-efficient logic implementation. Moreover, these devices are the potential candidates for neuromorphic computing applications that can be combined with logic-in-memory. The displacement of the DW can achieve multiple resistance states that have opened up possibilities of building artificial neurons and synapses. Despite the rapid progress, DW devices face several challenges such as low read margin, low speed, and sub-20nm scalability. Future research directions have to focus on material engineering and fabrication techniques to address these issues. Simultaneously, efforts from the circuit and system perspectives are extensively required in exploring the possible uses of these devices.

Curved domain walls in the ferromagnetic nanostructures with Rashba and nonlinear dissipative effects

This work reveals an analytical investigation of the curved domain wall motion in ferromagnetic nanostructures in the framework of the extended Landau-Lifshitz-Gilbert equation. To be precise, the study delineates the description of curved domain wall motion in the steady-state dynamic regime for metallic and semiconductor ferromagnets. The study is done under the simultaneous action of the Rashba field and nonlinear dissipative effects described via the viscous-dry friction mechanism. By means of reductive perturbation technique and realistic assumption on the considered parameters, we establish an analytical expression of the steady domain wall velocity that depends on mean curvature of domain wall surfaces, nonlinear dissipation coefficients, Rashba parameter, external magnetic field, and spin-polarized electric current. In particular, it is observed that the domain wall velocity, mobility, threshold, and Walker breakdown can be manipulated by the combined mechanism of the Rashba field and nonlinear dissipation coefficients. Finally, the obtained analytical results are illustrated numerically for the curved domain walls through constant-curvature surfaces under-considered scenarios. The results presented herein are in qualitatively good agreement with the recent observations.

Using Redox Titrations to Probe the Role of Trivalent Impurity Ions in the Ferromagnetism of Colloidal EuS Nanocrystals

EuS and related ferromagnetic semiconductors have long been model materials for spintronic device functionality because of their ability to generate highly spin-polarized electrical currents. Altho...

On the statics and dynamics of transverse domain walls in bilayer piezoelectric-magnetostrictive nanostructures

In this article, we investigate the static and dynamic properties of transverse Bloch domain wall in an isotropic, linearly elastic bilayer piezoelectric-magnetostrictive nanostructures under the influence of axial (driving), transverse magnetic fields and spin-polarized electric current. To be precise, we perform the analysis under the framework of the one-dimensional Extended Landau–Lifshitz–Gilbert equation in the presence of stresses generated by a piezoelectric actuator. First, we derive the magnetization profile in the two distant domains and then study the static magnetization profile in the sole presence of the applied transverse magnetic field. Next, we propose a new Walker’s type trial function and establish the analytical expressions of the dynamical quantities such as moving DW profile, velocity, displacement, and excitation angle by using a small angle approximation approach. Finally, we delineate the obtained analytical results with the aid of numerical illustrations.

Generation and control of photo-excited thermal currents in triple degenerate topological semimetal MoP with circularly polarized ultrafast light pulses

Ultrafast spectroscopy is a powerful method to generate and control topological phase transitions and spin-polarized electrical currents in topological quantum materials. These light-induced novel physical properties originate from the topologically nontrivial states of Dirac and Weyl fermions. The topological semimetal molybdenum phosphide (MoP) exhibits double and triple degenerate points in the momentum space. We present the preliminary results of spin-polarized electrical currents and optical response investigations of MoP. We design and construct an experimental setup to perform the photocurrent generation and control by circularly polarized light in topological insulator Bi<sub>2</sub>Se<sub>3</sub>. The results compare well with those reported, which confirms the validity and reliability of our experimental setup. Further, we conduct the photocurrent experiment on MoP by using 400 nm laser pulses for excitation and successfully detect the current signals at different sample positions. We attribute the observed currents to photo-induced thermal currents (not the photo current associated with the triple degenerate topological properties), which facilitates generating and controlling photocurrents in MoP in the future investigation. Our thermal current investigations are of essence for further exploring the photocurrents in various types of topological quantum materials.

Left-handed polarized spin waves in ferromagnets induced by spin-transfer torque

Polarization is a fundamental property of waves that refers to the orientation of the oscillations. It has been widely used to encode information in photonics and phononics. However, the polarization of spin waves is rarely used yet in magnonics. The reason for this is that only the right-handed polarized spin waves can be accommodated in ferromagnets. Here, we report that stable left-handed polarized spin waves can be introduced into ferromagnets if a spin-polarized electrical current is presented. The right-handed and left-handed polarized spin waves coexist when the current density is larger than a critical value while the system keeps stable. The results are confirmed by micromagnetic simulations. This work provides new playgrounds to study spin waves and points to new findings for future experimental studies.

Twisted skyrmions at domain boundaries and the method of image skyrmions

We predict a twisted skyrmion structure at the boundary of two antiparallel magnetic domains stabilized by a (synthetic or crystal) antiferromagnetic coupling on the border. Through this novel state, skyrmions with opposite polarities can be freely switched between each other by spin-polarized electric currents. We therefore propose the concept of double-track skyrmion racetrack memories and logic gates where the binary data are represented by skyrmions with different polarities. The dynamics of the skyrmion polarity reversal is theoretically studied. Using the method of image, we derive the analytical formula of the repulsive potential when a normal skyrmion approaches the domain boundary. A harmonic attractive potential is obtained for the twisted skyrmion across the border. Micromagnetic simulations compare well with analytical results. The method of image skyrmion proposed in this work can be used to deal with a large class of skyrmion-boundary interaction problems.

Magnetic droplet solitons generated by pure spin currents

Magnetic droplets are dynamical solitons that can be generated by locally suppressing the dynamical damping in magnetic films with perpendicular anisotropy. To date, droplets have been observed only in nanocontact spin-torque oscillators operated by spin-polarized electrical currents. Here, we experimentally demonstrate that magnetic droplets can be nucleated and sustained by pure spin currents in nanoconstriction-based spin Hall devices. Micromagnetic simulations support our interpretation of the data, and indicate that in addition to the stationary droplets, propagating solitons can be also generated in the studied system, which can be utilized for the information transmission in spintronic applications.

Fast vortex wall motion in wide permalloy strips from double switching of the vortex core

We study vortex domain wall dynamics in wide permalloy strips driven by applied magnetic fields and spin-polarized electric currents. As recently reported [V. Est\'evez and L. Laurson, Phys. Rev. B 93, 064403 (2016)], for sufficiently wide strips and above a threshold field, periodic dynamics of the vortex core are localized in the vicinity of one of the strip edges, and the velocity drop typically observed for narrow strips is replaced by a high-velocity plateau. Here, we analyze this behavior in more detail by means of micromagnetic simulations. We show that the high-velocity plateau originates from a repeated double switching of the magnetic vortex core, underlying the periodic vortex core dynamics in the vicinity of the strip edge, i.e., the ``attraction-repulsion'' effect. We also discuss the corresponding dynamics driven by spin-polarized currents, as well as the effect of including quenched random structural disorder to the system.

Direct Observation of Current-Induced Motion of a 3D Vortex Domain Wall in Cylindrical Nanowires

The current-induced dynamics of 3D magnetic vortex domain walls in cylindrical Co/Ni nanowires are revealed experimentally using Lorentz microscopy and theoretically using micromagnetic simulations. We demonstrate that a spin-polarized electric current can control the reversible motion of 3D vortex domain walls, which travel with a velocity of a few hundred meters per second. This finding is a key step in establishing fast, high-density memory devices based on vertical arrays of cylindrical magnetic nanowires.

Core-protective half-metallicity in trilayer graphene nanoribbons

Half-metals, playing an important role in spintronics, can be described as materials that enable fully spin-polarized electrical current. Taking place in graphene-based materials, half-metallicity has been shown in zigzag-edged graphene nanoribbons (ZGNRs) under an electric field. Localized electron states on the edge carbons are a key to enabling half-metallicity in ZGNRs. Thus, modification of the localized electron states is instrumental to the carbon-based spintronics. Our simple model shows that in a trilayer ZGNRs (triZGNRs) only the middle layer may become half-metallic leaving the outer layers insulating in an electric field, as confirmed by our density functional theory (DFT) calculations. Due to the different circumstances of the edge carbons, the electron energies at the edge carbons are different near the Fermi level, leading to a layer-selective half-metallicity. We believe that triZGNRs can be the tiniest electric cable (nanocable) form and can open a route to graphene-based spintronics applications.

Influence of perpendicular magnetic field and polarized current on the dynamics of coupled magnetic vortices in a thin nanocolumnar trilayer conducting structure

A micromagnetic investigation of the dynamics of two dipole-coupled magnetic vortices in a magnetic tunnel nanocolumn under the action of an external magnetic field directed perpendicularly to the plane of the sample and of a spin-polarized electric current has been carried out. Three regimes of motion of the vortices have been shown to exist that differ in critical values of the current. The dependence of the magneticfield strength that separately switches the polarity of the cores of the vortices depending on the density of the spin-polarized current has been found. The possibility of controlling the frequency of the stationary motion of the vortices and of the fine adjustment of the amplitude of the controlling currents using an external magnetic field has been suggested.

Photoinduced topological phase transition and spin polarization in a two-dimensional topological insulator

A great deal of attention has been paid to the topological phases engineered by photonics over the past few years. Here, we propose a topological quantum phase transition to a quantum anomalous Hall (QAH) phase induced by off-resonant circularly polarized light in a two-dimensional system that is initially in a quantum spin Hall phase or a trivial insulator phase. This provides an alternative method to realize the QAH effect, other than magnetic doping. The circularly polarized light effectively creates a Zeeman exchange field and a renormalized Dirac mass, which are tunable by varying the intensity of the light and drive the quantum phase transition. Both the transverse and longitudinal Hall conductivities are studied, and the former is consistent with the topological phase transition when the Fermi level lies in the band gap. A highly controllable spin-polarized longitudinal electrical current can be generated when the Fermi level is in the conduction band, which may be useful for designing topological spintronics.

Current-Induced Pinwheel Oscillations in Perpendicular Magnetic Anisotropy Spin Valve Nanopillars

Nanopillar spin valve devices are typically comprised of two ferromagnetic layers: a reference layer and a free layer whose magnetic orientation can be changed by both an external magnetic field and through the introduction of spin-polarized electric current. Here we report the continuous repeated switching behavior of both the reference and free layers of a perpendicular spin valve made of Co/Pd and Co/Ni multilayers that arises for sufficiently large DC currents. This periodic switching of the two layers produces an oscillating signal in the MHz regime but is only observed for one sign of the applied current. The observed behavior agrees well with micromagnetic simulations.

All-electrical production of spin-polarized currents in carbon nanotubes: Rashba spin-orbit interaction

We study the effect of the Rashba spin-orbit interaction in the quantum transport of carbon nanotubes with arbitrary chiralities. For certain spin directions, we find a strong spin-polarized electrical current that depends on the diameter of the tube, the length of the Rashba region and on the tube chirality. Predictions for the spin-dependent conductances are presented for different families of achiral and chiral tubes. We have found that different symmetries acting on spatial and spin variables have to be considered in order to explain the relations between spin-resolved conductances in carbon nanotubes. These symmetries are more general than those employed in planar graphene systems. Our results indicate the possibility of having stable spin-polarized electrical currents in absence of external magnetic fields or magnetic impurities in carbon nanotubes.

Femtosecond control of electric currents in metallic ferromagnetic heterostructures.

The idea to use not only the charge but also the spin of electrons in the operation of electronic devices has led to the development of spintronics, causing a revolution in how information is stored and processed. A novel advancement would be to develop ultrafast spintronics using femtosecond laser pulses. Employing terahertz (10(12) Hz) emission spectroscopy and exploiting the spin-orbit interaction, we demonstrate the optical generation of electric photocurrents in metallic ferromagnetic heterostructures at the femtosecond timescale. The direction of the photocurrent is controlled by the helicity of the circularly polarized light. These results open up new opportunities for realizing spintronics in the unprecedented terahertz regime and provide new insights in all-optical control of magnetism.

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.