Ferromagnetic Stripes Research Articles

Magnetically confined states and transport property on the surface of a topological insulator

We study the electronic structure and transport for a quasi-one-dimensional channel constructed via two ferromagnetic (FM) stripes on the surface of a three-dimensional (3D) topological insulator (TI) in parallel (P) or antiparallel (AP) magnetization configuration along the vertical z-direction. We demonstrate that the confined states which are localized inside the channel always exist due to the magnetic potential confinement. Interestingly, the channel is metallic because of the existence of a topologically protected gapless chiral edge mode in the case of AP configuration. The asymmetric spatial-distribution of both electron probability density and in-plane spin polarization for the confined states implies that in the case of P configuration there exists a chiral state near the channel edge owing to the Hamiltonian inversion symmetry broken in real space, while the distributions in AP case are always symmetry since the inversion symmetry is still kept. Furthermore, the transmission probability and the spatial-dependent distributions of charge and spin along a narrow–wide–narrow channel on the surface with P configuration confinement are also calculated, from which a fully in-plane spin-polarized electron output is achieved. Along with the mathematical analysis we provide an intuitive, topological understanding of these effects.

Structurally controllable spin spatial splitter in a hybrid ferromagnet and semiconductor nanostructure

We theoretically investigate modulation of a tunable δ-potential to the lateral displacement of electrons across a magnetically modulated semiconductor nanostructure. Experimentally, this nanostructure can be produced by depositing a nanosized ferromagnetic stripe with in-plane magnetization on top of a semiconductor heterostructure, while the δ-potential can be realized by means of the atomic layer doping technique. Theoretical analysis reveals that this δ-doping can break the intrinsic symmetry in nanostructure and a considerable spin polarization in the lateral displacement will appear. Numerical calculations demonstrate that both magnitude and sign of spin polarization can be manipulated by changing the height and/or position of the δ-doping, giving rise to a structurally tunable spin spatial splitter.

Spin filtering magnetic modulation and spin-polarization switching in hybrid ferromagnet/semiconductor structures

Electron spin-polarization modulation with a ferromagnetic strip of in-plane magnetization is analyzed in a hybrid ferromagnet/semiconductor filter device. The dependencies of electron spin-polarization on the strip’s magnetization strength, width and position have been systematically investigated. A novel magnetic control spin-polarization switch is proposed by inserting a ferromagnetic metal (FM) strip eccentric in relation to off the center of the spin filter, which produces the first energy level spin-polarization reversal. It is believed to be of significant importance for the realization of semiconductor spintronics multiplevalue logic devices.

Edge proximity-induced magnetoresistance and spin polarization in ferromagnetic gated bilayer graphene nanoribbon

Coherent spin-dependent transport through a junction containing normal/ferromagnetic/normal bilayer graphene nanoribbon with zigzag edges is investigated by using Landauer formalism. In a more realistic set-up, the exchange field is induced by two ferromagnetic insulator strips deposited on the ribbon edges while a perpendicular electric field is applied by the top gated electrodes. Our results show that, for antiparallel configuration, a band gap is opened giving rise to a semiconducting behavior, while for parallel configuration, the band structure has no band gap. As a result, a giant magnetoresistance is achievable by changing the alignment of induced magnetization. Application of a perpendicular electric field on the parallel configuration results in a spin field-effect transistor where a fully spin polarization occurs around the Dirac point. To compare our results with the one for monolayer graphene, we demonstrate that the reflection symmetry and so the parity conservation fail in bilayer graphene nanoribbons with the zigzag edges.

Adjustment of Demagnetizing Field in Permalloy Nanowires to Control Domain Wall Motion

The domain wall motion in permalloy nanowires has been investigated widely due to its potential for a new type of memory. In this paper. we use the LLG Simulator based on the Landau-Lifshitz-Gilbert (LLG) equation to investigate the field-driven domain-wall motion in a long, straight ferromagnetic strip. An injection field of 60 Oe is applied to inject a domain wall from an extended disk into the nanowire. We found a dependence of nanowire dimensions with the velocity of domain wall. By increasing the width of the nanowire, the velocity of the domain wall motion also increases, while the Walker breakdown field ( HWB) decreases. On the other hand, increasing the thicknesses of the nanowire, the domain wall velocity, HWB, and demagnetizing field all decrease. By applying a vertical field from 0 to 1000 Oe in order to enhance the demagnetizing field, it is found the HWB is increased from 16 to 20 Oe.

A method of local magnetic loss determination in punched ferromagnetic strips

The paper deals with results of measurement and numerical modelling of temperature increases at ferromagnetic strip heated by induced currents. The temperature rises are analysed both for transient and steady state. The ferromagnetic material used was subjected to mechanical punching. In the paper it is shown that magnetic losses at ferromagnetic material along the punched edge are several times larger than those in “green” material.

Unconventional superfluid phases and the phase dynamics in spin-orbit-coupled Bose systems

We study the phase distribution and its dynamics in spin-orbit coupled two component ultracold Bosons for finite size system. Using an inhomogeneous meanfield analysis we demonstrate how phase distribution evolves as we tune the spin-orbit coupling $\gamma$ and $t$, the spin-independent hopping. For $t>>\gamma$ we find the homogeneous superfluid phase. As we increase $\gamma$, differences in the phases of the order parameter grows leading to twisted superfluid phase. For $t \sim \gamma$ competing orderings in the phase distribution is observed. At large $\gamma$ limit a Ferro-Magnetic stripe ordering appears along the diagonal. We explain that this is due to the frustration bought in by the spin-orbit interaction. Isolated vortex formation is also shown to appear. We also investigate the possible collective modes. In deep superfluid regime we derive the equation of motion for the phases following a semi-classical approximation. Imaginary frequencies indicating the damped modes are seen to appear and the dynamics of lowest normal modes are discussed.

Magnetization characteristic of ferromagnetic thin strip by measuring anisotropic magnetoresistance and ferromagnetic resonance

The magnetization characteristic in a Permalloy thin strip is investigated by electrically measuring the anisotropic magnetoresistance and ferromagnetic resonance in in-plane and out-of-plane configurations. Our results indicate that the magnetization vector can rotate in the film plane as well as out of the film plane by changing the intensity of external magnetic field of certain direction. The magnetization characteristic can be explained by considering demagnetization and magnetic anisotropy. Our method can be used to obtain the demagnetization factor, saturated magnetic moment and the magnetic anisotropy.

Electrical and proximity-magnetic effects induced quantum Goos–Hänchen shift on the surface of topological insulator

We use scattering matrix method to theoretically demonstrate that the quantum Goos–Hänchen shift of the surface on three-dimensional topological insulator coated by ferromagnetic strips is sensitive to the magnitude of ferromagnetic magnetization. The dependence of quantum Goos–Hänchen shift on magnetization and gate bias is investigated by performing station phase approach. It is found that quantum Goos–Hänchen shift is positive and large under the magnetic barrier but may be positive as well as negative values under the gate bias. Furthermore, the position of quantum Goos–Hänchen peak can also be modulated by the combination of gate bias and proximity magnetic effects. Our results indicate that topological insulators are another candidates to support quantum Goos–Hänchen shift.

Strong spin-dependent negative differential resistance in composite graphene superlattices

We find clear signatures of spin-dependent negative differential resistance in compound systems comprising a graphene nanoribbon and a set of ferromagnetic insulator strips deposited on top of it. The periodic array of ferromagnetic strips induces a proximity exchange splitting of the electronic states in graphene, resulting in the appearance of a superlattice with a spin-dependent energy spectrum. The electric current through the device can be highly polarized and both the current and its polarization manifest non-monotonic dependence on the bias voltage. The device operates therefore as an Esaki spin diode, which opens possibilities to design new spintronic circuits.

Manipulating spin beam splitter by electric field in hybrid ferromagnetic-Schottky-stripe and semiconductor nanostructure

Recently, a spin beam splitter, which operates via the Goos–Hanchen (GH) effect of electrons, was fabricated by depositing ferromagnetic and Schottky metal stripes on top of a semiconductor heterostructure. To explore effective manipulation of its spin polarization of GH shifts, we introduce a transverse electric field into the device. Theoretical analysis and numerical simulation show that both magnitude and sign of the spin polarization are related closely to this electric field. Thus, this spin beam splitter can be used as an electrically controllable spin-polarized source for spintronics applications.

Complexes of Domain Walls in Ferromagnetic Stripes

Interaction of domain walls (DWs) in ferromagnetic stripes is studied with relevance to the formation of stable complexes of many domains. Two DW system is described with the Landau-Lifshitz-Gilbert equation including regimes of narrow and wide stripes which correspond the presence of transverse and vortex DWs. The DWs of both kinds are characterized with their chiralities (the direction of the magnetization rotation in the stripe plane) and polarities (the magnetization orientation in the center of a vortex and/or halfvortices), hence, their interactions are analyzed with dependence on these properties. In particular, pairs of the DWs of opposite or like both chiralities and polarities are investigated as well as pairs of opposite (like) chiralities and of like (opposite) polarities. Conditions of the creation of stationary magnetic bubbles built of two interacting DWs are formulated with relevance to the situations of presence and absence of the external magnetic field.

THE ELECTRON TRANSPORT IN A NANOSTRUCTURE MODULATED BY THE MAGNETIC FIELD AND THE δ-DOPING

We theoretically investigate the effect of the δ-doping on the electron transport in a magnetic nanostructure, which can be experimentally realized by depositing one ferromagnetic stripe on the top of a semiconductor heterostructure. We find that the position and the strength of the δ-doping as well as the distance between the two magnetic fields play an important role on the electron transport properties such as the transmission probability, the conductance and the spin polarization. These interesting results may be very helpful for analyzing the spin-dependent transport mechanism of the electron and making the new types of the spintronic devices.

Current-driven domain wall motion along high perpendicular anisotropy multilayers: The role of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction

The current-induced domain wall motion along a thin cobalt ferromagnetic strip sandwiched in a multilayer (Pt/Co/AlO) is theoretically studied with emphasis on the roles of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction. The results point out that these ingredients, originated from the spin-orbit coupling, are consistent with recent experimental observations in three different scenarios. With the aim of clarifying which is the most plausible the influence of in-plane longitudinal and transversal fields is evaluated.

Electric control of spin-dependent Goos–Hänchen shift in a magnetically modulated semiconductor nanostructure

Abstract We theoretically investigate how to manipulate spin-dependent Goos–Hanchen (GH) shifts by an applied bias in a realistic magnetic-barrier nanostructure, which is experimentally created by depositing a ferromagnetic stripe with perpendicular magnetization on the top of heterostructure. GH shifts of transmitted electron beams are calculated numerically with the help of the stationary phase method. It is shown that both magnitude and sign of spin polarization in GH shifts are closely relative to the applied bias, which can give rise to a bias-controllable spin beam splitter.

Topological floating phase in a spatially anisotropic frustrated Ising model

We present new results for the ordering process of a two-dimensional Ising model with anisotropic frustrating next-nearest-neighbor interactions. We concentrate on a specific wide temperature and parameter region to confirm the existence of two particular phases of the model. The first phase is an incommensurate algebraically-ordered (floating) phase emerging at the transition from the paramagnetic high-temperature phase. Then the model undergoes a transition to an antiferromagnetically ordered second phase with diagonal ferromagnetic stripes (ordering wave vector $\vec q = (\pi/2, \pi/2)$). We analyze the unconventional features appearing in several observables, e.g., energy, structure factors, and correlation functions by means of extensive Monte-Carlo simulations and examine carefully the influence of the lattice sizes. For the analytical study of the intermediate phase the Villain-Bak theory is adapted for the present model. Combining both the numerical and analytical work we present the quantitative phase diagram of the model, and, in particular, argue in favor of an intermediate topological floating phase.

The influence of the spin-orbit torques on the current-driven domain wall motion

The current-induced domain wall motion along a ferromagnetic strip with high perpendicular magnetocristalline anisotropy sandwiched in an multilayer stack is theoretically studied, by means of micromagnetic simulations and the one-dimensional model, with emphasis on the roles of the Rashba spin-orbit coupling and the spin Hall effect. The results point out that in the presence of a strong Rashba field the domain wall motion can be either in the direction of the current or opposing to it depending on the amplitude of the spin Hall effect. The predictions are in agreement with the experiments only in a reduced range of positive spin Hall angles under a strong Rashba torque.

Tunable electron wave filter and Goos–Hänchen shift in asymmetric graphene double magnetic barrier structures

Electronic transmission and Goos–Hänchen shift for Dirac fermions in asymmetric graphene double magnetic barrier structure are investigated theoretically. Numerical results for rectangular barriers demonstrate that a transmission gap appears owing to the magnetic confinement and the width of gap is dependent on the incident angle and changed by the width of barrier and the distance between the double barriers. At the edge of the transmission gap, the Goos–Hänchen shifts of electrons have strong peaks and are tuned by the structure parameters. Furthermore, for the real magnetic structure induced by ferromagnetic strips, the transmission gap and GH shift remain. The tunable transmission gap indicates that the graphene magnetic structure can be use as electron wave filter and the tunable GH shift shows that the structure can be applied to laterally manipulate Dirac fermions in graphene.

Controllable electron-spin polarization by δ-doping in a hybrid ferromagnet and semiconductor nanostructure

By using modern materials growth techniques such as molecular beam epitaxy, a tunable δ-potential can be intentionally doped into semiconductor nanostructures. In this work, we theoretically investigate modulation of electron-spin polarization by such a δ-doping in a hybrid ferromagnet and semiconductor nanostructure, where a nanosized ferromagnetic stripe with horizontal magnetization is deposited on top of a semiconductor heterostructure. It is shown that this nanosystem possesses a considerable spin polarization effect due to breaking the intrinsic symmetry by the δ-doping. It is also shown that both magnitude and sign of spin polarization vary dramatically with the height and/or position of the δ-doping. These interesting properties may provide an alternative scheme to achieve a controllable spin-polarized source, and this nanostructure may serve as a structurally tunable spin filter.

Anomalous electron collimation in HgTe quantum wells with inverted band structure

We investigate the electron collimation behavior in HgTe quantum wells (QWs) with a magnetic–electric barrier induced by a ferromagnetic metal stripe. We find that electrons can transmit perfectly through the magnetic–electric barrier at some specific incidence angles. These angles can be controlled by the tuning gate voltage, local magnetic field and Fermi energy of incident electrons in QWs with appropriate barrier length. This collimation feature can be used to construct momentum filters in HgTe QWs and has potential application in nanodevices.