Ferromagnetic Graphene Junction Research Articles

Analysis of spin current in ferromagnetic graphene junctions

Magnetization transport in a ferromagnetic/normal/ferromagnetic graphene junction has been investigated. Inspired by a seminal paper [Phys. Rev. B 66, 014,407, 2002], the longitudinal and transverse components of spin current i.e. components parallel and perpendicular to the magnetization direction in the ferromagnetic layer have been studied. It has been shown that only when the reflected and transmitted waves from the interface between two normal/ferromagnetic layers have the same angle as that of the incident wave, there will be no torque associated with the transport of longitudinal spin current. In general, for an arbitrary direction of magnetization, all spin current components are coexisted and therefore the parallel and the perpendicular components of spin transfer torque are existed. Therefore it is not possible to draw a general conclusion that the torque associated with the transport of longitudinal spin current is always zero.

Spin- and valley-polarized Goos–Hänchen-like shift in ferromagnetic mass graphene junction with circularly polarized light

We investigate the band structure and Goos–Hänchen-like shift in ferromagnetic mass graphene junction modulated by the circularly polarized light. It is found that both spin and valley-related energy gaps can be opened by employing the circularly polarized light and the exchange field in mass graphene. The valley-polarized Goos–Hänchen-like shift can be identified in the presence of circularly polarized light, and the spin-polarized Goos–Hänchen-like shift can be realized with introduction of exchange field in mass graphene. Furthermore, the spin and valley polarization-related Goos–Hänchen-like shift can be achieved by combination of circularly polarized light and exchange field in mass graphene. It is hopeful that our work will be more conducive for future applications in graphene polarization transport devices.

Tunable lateral spin polarization and spin-dependent collimation in velocity-modulated ferromagnetic-gate graphene structures

The influence of spatial variation of Fermi velocity on the spin-polarized transport properties of massless Dirac fermions in proximity-induced ferromagnetic graphene junction was investigated. We found that the velocity ratio ξ causing the width of the spin conductance dip near the Dirac point is broadened as the Fermi velocity ratio increases. In contrast, the effect of the Fermi velocity mismatch does not affect the shifting of the Dirac point. It leads to the indirect measurement of the Fermi velocity ratio that can be tuned by the spin-dependent conductance. In addition to the presence of both the exchange field and the Fermi velocity modulation, we found high spin filtering occurs when the Fermi velocity ratio ξ ≥ 1. Due to the transmission probability of electron with spin down, T↓ is blocked by the effect of Fermi velocity modulation similar to waves travelling through a different media. Moreover, we also found that the resonant transmission of the electron with spin up can be perfectly transmitted through the Fermi velocity barrier at the limit injected angle θ → π/2 for H/EF ≈ 1. The spin transport properties can be controlled by suitably modifying the strength of the ferromagnetic insulator and varying the Fermi velocity modulation, which leads to the highest spin polarization. By manipulating the behavior of spin-dependent collimation and spin beam splitting in this structure, we proposed a spin transport device for making lateral spin polarization. These interesting features will help make the development of spintronic devices in the graphene-based nanostructure.

Electric and thermal spin transfer torques across ferromagnetic/normal/ferromagnetic graphene junctions

We investigate the spin transfer torque (STT) driven by electric bias voltages across and temperature gradients through ferromagnetic/normal/ferromagnetic graphene junctions. Due to the unique band structure of the ferromagnetic graphene, there exists two transport regimes: the electron to electron (I) and hole to electron (II) transport. The electric STTs originated from the two regimes have opposite sign and can be reduced by the competition between the two transport processes. On the contrary, the thermal STTs originated from the transport regimes I and II have the same sign and are enhanced when the two regimes coexist. Remarkably, the thermal STT is comparable with the electric STT. Furthermore, the electric and thermal counterpart can be manipulated by the Fermi level. The controllable STT reported here makes the ferromagnetic graphene junction ideal for future spintronics applications.

Spin-dependent quantum beating of the conductance oscillations in asymmetric velocity modulated ferromagnetic graphene junction

Abstract The mismatch effect of a double Fermi-velocity barrier structures NG/FG 1 /NG/FG 2 /NG junction on the electrically driven SdH-like oscillations of spin-dependent conductance is investigated, where NG is normal graphene and FG 1,2 is the ferromagnetic-gate with Fermi-velocity modulation ξ 1,2 where ξ 1,2 is the Fermi velocity ratio between NG region and FG 1,2 region. The results show that the effect of asymmetry of the double Fermi-velocity barrier structures, the difference value between ξ 1 and ξ 2 cause of the oscillations the spin-dependent quantum beating pattern. Which the beating patterns of the spin transport appeared as a result of the superposition of the two oscillatory components in the double Fermi velocity barriers. In addition, we found that the shifting of the spin-dependent quantum beats of the G ↑ and G ↓ is due to the presence of the exchange field, H, that is, the G ↑ and G ↓ are shifted in the opposite direction. By the difference value of the phase shift can be estimated to be 2H . In case of U + η σ H > > E F which electrons are collimated, the oscillations behaviors show the quantum beating with holding constant amplitude and the period of the beating amplitude depend on the Fermi velocity ratio mismatch in FG regions. These interesting characteristic of spin-dependent quantum transport may identify the effect of Femi-velocity asymmetry on double proximity-induced ferromagnetic graphene structure for making spintronic devices.

Anomalous zero bias conductance peak in a ferromagnetic graphene junction with d-wave anisotropic superconducting pair symmetry

We theoretically present an anomalous zero bias conductance peak (ZBCP) in graphene junctions with proximity-induced ferromagnetism and d-wave anisotropic superconducting pair symmetry (ASPS) herein. It is revealed that in the thin insulator limit, the ZBCP can be periodically recovered by adjusting the sandwiched insulating barrier strength χ regardless of exchange field h and the phase of the periodical behaviours for h larger than its Fermi energy EF (h > EF) is exactly opposite to that for h < EF. Most interestingly, in the context of h > EF, the periodic oscillation of the nonzero bias conductance located in the ZBCP versus χ, is accompanied by an explicit splitting peak. Moreover, under the situation of the insulator with finite width, the conductance exhibits a stronger damping oscillation with bias voltage eV for any h, which is also accompanied by a splitting ZBCP at h > EF. These singular features originate from ferromagnetic-modulated midgap states characteristic by the relativistic nodal fermions, which confirms the spin polarization and ASPS of the graphene, and thus will be of great interest in the designing and fabrication of graphene superconducting spintronic devices.

Valley controlled spin-transfer torque in ferromagnetic graphene junctions

The presence of the valley degree of freedom in graphene leads to the valleytronics, in which information is encoded by the valley quantum number of the electron. We propose a valley controlled spin-transfer torque (STT) in graphene-based normal/normal/ferromagnetic junctions with the normal lead irradiated by the off-resonant circularly polarized light. The interplay of the spin–orbit interaction and the staggered potential in the central normal part results in the coupling between the valley and spin degrees of freedom, so a valley dependent spin polarized current can be demonstrated, which can exert a valley controlled STT on the ferromagnetic lead. The amplitude of the STT can be manipulated by the intensity of the light, the Fermi energy and the magnetization direction of the ferromagnetic lead. This valley controlled STT may find potential application in future valleytronics and spintronics.

Spin-transfer torque generated in graphene based topological insulator heterostructures

We studied the spin-transfer torque (STT) in graphene based normal-metal/topological-insulator/ferromagnet heterostructures (N/TI/F), which is induced by the helical spin-polarized current in the quantum spin Hall insulator. We found that the STT is comparable in magnitude to the STT in ferromagnetic-normal- ferromagnetic graphene junction, while not requiring additional ferromagnetic layer with fixed magnetization, which makes it advantageous for the manipulation of magnetic devices in spintronics. More interestingly, the STT is very robust in our proposed nanostructure, as it is immune to changes in the geometry due to an asymmetrically notch or the presence of random nanopores in the quantum spin Hall insulator. Our theoretical prediction suggests that graphene based quantum spin Hall insulator could be used for very efficient magnetization manipulation for magnetic materials.

Pure valley and spin polarization current in ferromagnetic graphene junction

We investigate the band structure and spin- and valley-dependent transport in ferromagnetic graphene double junctions using the transfer matrix method. The ferromagnetic double junctions include a lateral layout of normal/magnetic vector potential/normal/effective exchange field/normal graphene regions. We find that the strain combined with magnetic vector potentials breaks the valley degeneracy, and the strain combined with the effective exchange field breaks the spin degeneracy, so that there exists a spin- and valley-dependent gap in the ferromagnetic graphene double junctions, which allows only one spin species with special valley characteristics to be transported. Thus, this type of junction can achieve simultaneously pure spin- and pure valley-polarized currents and function as a perfect valley filter and a perfect spin filter. The exact nature of the valley filtering and spin filtering can be tuned by the strain, magnetic barrier, and effective exchange field strength.

Spin and charge thermopower effects in the ferromagnetic graphene junction

Using wave function matching approach and employing the Landauer-Buttiker formula, a ferromagnetic graphene junction with temperature gradient across the system is studied. We calculate the thermally induced charge and spin current as well as the thermoelectric voltage (Seebeck effect) in the linear and nonlinear regimes. Our calculation revealed that due to the electron-hole symmetry, the charge Seebeck coefficient is, for an undoped magnetic graphene, an odd function of chemical potential while the spin Seebeck coefficient is an even function regardless of the temperature gradient and junction length. We have also found with an accurate tuning external parameter, namely, the exchange filed and gate voltage, the temperature gradient across the junction drives a pure spin current without accompanying the charge current. Another important characteristic of thermoelectric transport, thermally induced current in the nonlinear regime, is examined. It would be our main finding that with increasing thermal gradient applied to the junction the spin and charge thermovoltages decrease and even become zero for non zero temperature bias.

Anisotropic spin-dependent thermopower and current in ferromagnetic graphene junctions

We study the spin-dependent thermoelectric transport through two-dimensional normal/ferromagnetic/normal/ferromagnetic/normal graphene (NG/FG/NG/FG/NG) junctions. It is found that both charge and spin thermopowers depend on the FGʼs magnetization direction and exhibit an anisotropic behavior. Interestingly, the spin thermopower can be as large as the charge thermopower and even can exceed the latter in magnitude. Moreover, the pure spin thermopower and spin current emerge in this device. The results obtained here suggest a feasible way of enhancing thermospin effects and generating the pure spin current in two-dimensional graphene. • Spin-dependent thermopower in ferromagnetic graphene junctions is studied. • Spin and charge thermopowers are anisotropic. • Spin thermopowers can be as large as the charge thermopower. • A pure spin thermopower (spin current) is obtained.

Spin-polarized tunneling in a ferromagnetic graphene junction: Interplay between the exchange interaction and the orbital effect of the magnetic field

The influence of magnetic vector potential barrier (MVPB) on the spin-polarized transport of massless Dirac particles in ferromagnetic graphene is studied theoretically. The phenomenon of Klein tunneling of relativistic particles across a rectangular potential barrier prevents any of the massless fermions from being confined but they can be electrically confined by quantum dots with integrable dynamics (Bardarson et al., 2009) [36]. Utilization of only the in-plane exchange splitting in the ferromagnetic graphene cannot produce 100% spin polarization. This tunneling can be confined using the magnetic vector potential barrier, which leads to high degree of spin polarization. By combining the orbital effect and the Zeeman interaction in graphene junction, it is found that the junction mimics behavior of half-metallic tunneling junction, in which it acts as a metal to particles of one spin orientation but as an insulator or a semiconductor to those of the opposite orientation. The idea of the half-metallic tunneling junction can provide a source of ∼100% spin-polarized current, which is potentially very useful. Adjustment of the position of the Fermi level in ferromagnetic layer by placing a gate voltage on top of the ferromagnetic layer shows that reverse of the orientation of the completely spin-polarized current passing through the junction is controlled by adjusting the gate voltage. These interesting characteristics should lead to a practical gate voltage controlled spin filtering and spin-polarized switching devices as a perfect spin-polarized electron source for graphene-based spintronics.

Anomalous magnetic transport in ferromagnetic graphene junctions

We investigate magnetotransport in a ferromagnetic-normal-ferromagnetic graphene junction where a gate electrode is attached to the normal segment. It is shown that the charge conductance can be maximal at an antiparallel configuration of the magnetizations. Moreover, we demonstrate that both the magnitude and the sign of the spin-transfer torque can be controlled by means of the gate voltage in the normal segment. In this way, the present system constitutes a spin-transfer torque transistor. These anomalous phenomena are attributed to the combined effect of the exchange field and the Dirac dispersion of graphene. Our prediction opens up the possibility of moving domain walls parallel or antiparallel to the current in a controllable fashion by means of a local gate voltage.

Strain-tunable spin transport in ferromagnetic graphene junctions

We investigate spin-dependent electronic transport through normal/strained/normal/ferromagnetic/normal graphene junctions. The substrate strain leads to opposite shifts of the K and K′ valleys and thus modulates the orbital motion of Dirac electrons. The energy dependence of this effect together with the exchange splitting can be utilized to design a strain-tunable spin filter.

Spin switching effects in a ferromagnetic graphene junction having a second gate

The effects of the bias voltage placed on a second gate which is positioned to the left of the ferromagnetic graphene (FG) layer on the switching of the charge and spin transports in a ferromagnetic graphene junction are studied. We show that the change in the bias voltage from positive to negative (while maintaining the positive voltage on the first gate above the FG layer) can induce a switching from high to low conductance states in the junction. The voltage on the first gate is used to shift the Fermi level in the FG layer. Enhancement of the on/off conductance ratio on the switching is shown to be inversely proportional to the thickness of the second gate barrier. We show that by biasing the nanostructures with the voltage U 2, the spin polarization can be increased and that as a consequence, the thickness of the FG barrier layer needed for spin-polarized transport can be reduced.

Equilibrium spin current in ferromagnetic graphene junction

We report a theoretical study of the equilibrium spin current (ESC) flowing in a ferromagnet/ferromagnet (FM/FM) graphene junction with noncollinear magnetizations, in which the FM correlation in graphene is induced by depositing an FM insulator on it. Using the scattering matrix method, we derived an analytical expression of the ESC which is formally the same as that of a normal FM/FM junction. The ESC comes from the exchange coupling between the two FM magnetizations. Owing to the linear dispersion of graphene, the ESC disappears in the one-dimensional case, while it exists in the two-dimensional case. It exhibits damped oscillations with change in the Fermi energy, the layer length between the two FM regions, and the FM exchange splitting. It is also found that ESC in FM/FM graphene junctions has electron-hole inversion symmetry whereas it has electron-hole inversion antisymmetry in normal FM/FM junctions.

Negative tunnel magnetoresistance and spin transport in ferromagnetic graphenejunctions

We study the tunnel magnetoresistance (TMR) and spin transport in ferromagneticgraphene junctions composed of ferromagnetic graphene (FG) and normal graphene(NG) layers. It is found that the TMR in the FG/NG/FG junction oscillatesfrom positive to negative values with respect to the chemical potential adjustedby the gate voltage in the barrier region when the Fermi level is low enough.Particularly, the conventionally defined TMR in the FG/FG/FG junction oscillatesperiodically from a positive to negative value with increasing the barrier height at anyFermi level. The spin polarization of the current through the FG/FG/FG junctionalso has an oscillating behavior with increasing barrier height, whose oscillatingamplitude can be modulated by the exchange splitting in the ferromagnetic graphene.

Controllable spin transport in ferromagnetic graphene junctions

We study spin transport in normal/ferromagnetic/normal graphene junctions where a gate electrode is attached to the ferromagnetic graphene. We find that, due to the exchange field of the ferromagnetic graphene, spin current through the junctions has an oscillatory behavior with respect to the chemical potential in the ferromagnetic graphene, which can be tuned by the gate voltage. In particular, we obtain a spin current reversal controllable by the gate voltage. Our prediction of high controllability of spin transport in ferromagnetic graphene junctions may contribute to the development of spintronics.