Double Magnetic Barriers Research Articles

Tunneling conductance regulated by the electrostatic barrier in monolayer black phosphorene

We investigate the tunneling conductance of monolayer black phosphorene through double magnetic barriers modulated by the electrostatic barrier. This magnetic barrier is generated by two ferromagnetic strips deposited on monolayer phosphorene and can form parallel (P) and antiparallel (AP) structures. Applying a voltage to a ferromagnetic strip will create an electrostatic barrier. The transmission of P and AP magnetic structures is calculated using the transfer matrix. Tunneling conductance is given by Landauer–Büttiker theory. Our results show that under double electrostatic barriers modulation, the AP conductance of the system is zero except for sharp peaks, while the P conductance has a certain value, which implies a large tunneling magnetoresistance. However, in the case of single electrostatic barrier modulation, the conductance becomes flat. For the Fermi level in the valence band, the movement of the Fermi level leads to the severe mismatching of transmitted wave vectors in the barrier region, resulting a positive and negative oscillation magnetoresistance. This may be useful for designing memory using a two-dimensional electron gas system.

Conductance of graphene based normal-superconductor junction with double magnetic barriers

We study conductance of a graphene based normal metal-superconductor junction with two magnetic barriers. The magnetic barriers are induced via two applied magnetic fields with the same magnitudes and opposite directions accompanied by an applied electrostatic potential. We solve Dirac-Bogoliubov-De-Gennes (DBdG) equation to calculate conductance of the junction. We find that applying the magnetic field leads to suppression of the Andreev reflection and conductance for all energies. On the other hand, we observe a crossover from oscillatory to tunneling behavior of the conductance as a function of the applied potential by increasing the magnetic field.

Electronic transport in armchair graphene nanoribbon under double magnetic barrier modulation

We present a theoretical investigation of the transport properties and the magnetoresistance effect in armchair graphene nanoribbons (AGNRs) under modulation by two magnetic barriers. The energy levels are found to be degenerate for a metallic AGNR but are not degenerate for a semiconducting AGNR. However, the conductance characteristics show quantized plateaus in both the metallic and semiconducting cases. When the magnetization directions of the barriers change from parallel to antiparallel, the conductance plateau in the metallic AGNR shows a degenerate feature due to matching between the transport modes in different regions. As the barrier height increases, the conductance shows more oscillatory behavior with sharp peaks and troughs. Specifically, the initial position of nonzero conductance for the metallic AGNR system moves towards a higher energy regime, which indicates that an energy gap has been opened. In addition, the magnetoresistance ratio also shows plateau structures in certain specific energy regions. These results may be useful in the design of electron devices based on AGNR nanostructures.

Electron tunneling through double magnetic barriers in Weyl semimetals

We theoretically investigate the transport in a magnetic/normal/magetic hybrid structure on the surface of a Weyl semimetal. We find a directional-dependent tunneling which is sensitive to the magnetic field configuration and the electric gate voltage. The momentum filtering behavior becomes more significant for two-delta-function-shaped magnetic barriers. There are many Fabry-Pérot resonances in the transmission determined by the distance between the two magnetic barriers. The combined effects of the magnetic field and the electrostatic potential can enhance the difference in the transmission between the parallel and antiparallel magnetization configurations, and consequently lead to a giant magnetoresistance.

Gate-tunable valley-filter based on suspended graphene with double magnetic barrier structures

We theoretically investigate the effects of strain-induced pseudomagnetic fields on the transmission probability and the ballistic conductance for Dirac fermion transport in suspended graphene. We show that resonant tunneling through double magnetic barriers can be tuned by strain in the suspended region. The valley-resolved transmission peaks are apparently distinguishable owing to the sharpness of the resonant tunneling. With the specific strain, the resonant tunneling is completely suppressed for Dirac fermions occupying the one valley, but the resonant tunneling exists for the other valley. The valley-filtering effect is expected to be measurable by strain engineering. The proposed system can be used to fabricate a graphene valley filter with the large valley polarization almost 100%.

Tunneling magnetoresistance tuned by a vertical electric field in an AA-stacked graphene bilayer with double magnetic barriers

We investigate the effect of a vertical electric field on the electron tunneling and magnetoresistance in an AA-stacked graphene bilayer modulated by the double magnetic barriers with parallel or antiparallel configuration. The results show that the electronic transmission properties in the system are sensitive to the magnetic-barrier configuration and the bias voltage between the graphene layers. In particular, it is found that for the antiparallel configuration, within the low energy region, the blocking effect is more obvious compared with the case for the parallel configuration, and even there may exist a transmission spectrum gap which can be arbitrarily tuned by the field-induced interlayer bias voltage. We also demonstrate that the significant discrepancy between the conductance for both parallel and antiparallel configurations would result in a giant tunneling magnetoresistance ratio, and further the maximal magnetoresistance ratio can be strongly modified by the interlayer bias voltage. This leads to the possible realization of high-quality magnetic sensors controlled by a vertical electric field in the AA-stacked graphene bilayer.

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.

ELECTRON OPTICS WITH DIRAC FERMIONS: ELECTRON TRANSPORT IN MONOLAYER AND BILAYER GRAPHENE THROUGH MAGNETIC BARRIER AND THEIR SUPERLATTICES

In this review article we discuss the recent progress in studying ballistic transport for charge carriers in graphene through highly inhomogeneous magnetic field known as magnetic barrier in combination with gate voltage induced electrostatic potential. Starting with cases for a single or double magnetic barrier we also review the progress in understanding electron transport through the superlattices created out of such electromagnetic potential barriers and discuss the possibility of experimental realization of such systems. The emphasis is particularly on the analogy of such transport with propagation of light wave through medium with alternating dielectric constant. In that direction we discuss electron analogue of optical phenomena like Fabry–Perot resonances, negative refraction, Goos–Hänchen effect, beam collimation in such systems and explain how such analogy is going to be useful for device generation. The resulting modification of band structure of Dirac fermions, the emergence of additional Dirac points was also discussed accompanied by brief section on the interconvertibility of electric and magnetic field for relativistic Dirac fermions. We also discuss the effect of such electromagnetic potential barrier on bilayer graphene (BLG) in a similar framework.

Spin-filtering Effect in Graphene with Double Magnetic Barrier Structures

Spin-filtering Effect in Graphene with Double Magnetic Barrier Structures

The interfacial roughness effect on spin-dependent transport in nonplanar junctions with double magnetic barriers

In this paper, we use the transfer matrix method to study the effect of roughness with nonplanar interfaces on spin-dependent transport properties in a magnetic tunnel junction, which consists of two ferromagnetic semiconductor barriers separated by a nonmagnetic (NM) layer. This trilayer is sandwiched between two NM electrodes. The roughness is modeled as a periodic corrugation interface. Based on the effective mass approximation, the spin-dependent transmission probability, and also the dependence of tunnel magnetoresistance (TMR) and electron spin polarization (SP) on the center nonmagnetic layer quantum well width are studied, theoretically, at several different temperatures. The numerical results show that the SP and TMR have an oscillatory behavior as a function of NM layer thickness and the interface roughness/islands degrade the transmission probability. Our results may be useful for the development of nanoelectronic devices.

Electron tunneling through double magnetic barriers on the surface of a topological insulator

We study electron tunneling through a planar magnetic and electric barrier on the surface of a three-dimensional topological insulator. For the double barrier structures, we find (i) a directional-dependent tunneling which is sensitive to the magnetic field configuration and the electric gate voltage, (ii) a spin rotation controlled by the magnetic field and the gate voltage, (iii) many Fabry-P\'erot resonances in the transmission determined by the distance between the two barriers, and (iv) the electrostatic potential can enhance the difference in the transmission between the two magnetization configurations, and consequently lead to a giant magnetoresistance. Points (i), (iii), and (iv) are alike with that in graphene stemming from the same linear-dispersion relations.

Angular dependence of shot noise in diluted magnetic semiconductors resonant tunnelling diodes

In this work, spin-dependent current and shot noise transport in double magnetic barrier resonant tunnelling diodes is theoretically investigated. All components of the junctions, including external and central (spacer) electrodes are non-magnetic semiconductor, which have been separated by two II-VI diluted magnetic semiconductors (II-IV DMSs) layers. Our calculations are based on parabolic energy band approximation and transfer matrix method in the presence of external magnetic field and also applied voltage, within the phase coherent transport regime. In order to calculate the magnetisation of DMS, we have used the mean-field approximation in low magnetic field (B < 5T). We also examine the dependence of shot noise and tunnelling current densities on applied voltage and the relative angle between the magnetisations of two DMS layers in CdTe/CdMnTe heterostructures. All current and shot-noise components show a negative differential resistor (NDR), which originates from the resonant states tunnelling through quantum well. Also, the produced high spin polarisation makes the nanostructure a good candidate for new spin filter devices. Thus, these numerical results may shed light on the next applications of the quantum multilayer systems and make them convenient for future spintronics devices.

Magneto conductance for tunnelling through double magnetic barriers

The temperature-dependent magnetoresistance effect is investigated in a magnetically modulated two-dimensional (2D) electron gas (2DEG) which can be realized by depositing two parallel ferromagnets on top of a 2DEG electron gas. In the resonant tunnelling regime the transmission for the parallel and antiparallel magnetization configurations shows a quite distinct dependence on the longitudinal wave vector of the incident electrons. This leads to a very large magneto resistance ratio with a strong temperature dependence.

Wave-vector filtering and spin filtering in a magnetic double-barrier and double-well structure

We investigate wave-vector filtering and spin filtering for electrons tunneling through a magnetic double-barrier and double-well quantum structure in the presence of an external electric field, where the double magnetic wells are within or outside the double magnetic barriers. It is shown that electronic resonant tunneling shows strong wave-vector-dependent enhancement and suppression as the applied bias increases. It is also shown that although the current density decreases with the strength of the inhomogeneous magnetic field increasing, the degree of spin polarization is generally enhanced and shows a more wider and nearly 100% spin polarization plateau in small external bias range. The general rules for relations between wave-vector filtering and the magnetic configuration as well as between spin filtering and the magnetic configuration are summarized and explained.

Comment on “Tunable spin‐injection and magnetoconductance in a novel 2DEG‐ferromagnet structure” [phys. stat. sol. (b) 235, No. 1, 157–161 (2003)

AbstractWe point out that the predicted strong spin‐injection effect by Jiang and Jalil [phys. stat. sol. (b) 235, 157 (2003)] for a double magnetic barrier structure is based on a wrong calculation of the transmission probability. We corrected the result and found no significant spin‐injection. (© 2003 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)