Giant magnetoresistance effect in hybrid ferromagnetic-Schottky-metal and semiconductor nanosystem

We report on a theoretical investigation of the giant magnetoresistance (GMR) effect in hybrid ferromagnetic-Schottky-metal and semiconductor nanosystem. Experimentally, this GMR device can be realized by the deposition of two ferromagnetic (FM) stripes and one Schottky normal metal (NM) in parallel way on the top of a semiconductor GaAs heterostructure. The GMR effect emanates from the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations of the device, and its magnetoresistance ratio (MR) can reach the order of ¹⁰⁶%. Furthermore, it is also shown that the MR of the device depends strongly on the relative location of the Schottky NM stripe between two FM stripes.     

Giant magnetoresistance effect in hybrid ferromagnetic/semiconductor nanosystems

We theoretically investigate the giant magnetoresistance (GMR) effect in general magnetically modulated semiconductor nanosystems, which can be realized experimentally by depositing two parallel ferromagnetic strips on the top of a heterostructure. Here the exact magnetic profiles and arbitrary magnetization direction of ferromagnetic strips are emphasized. It is shown that a considerable GMR effect can be achieved in such nanosystems due to the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations. It is also shown that the magnetoresistance ratio is strongly influenced by the magnetization direction of ferromagnetic strips in nanosystems, thus possibly leading to tunable GMR devices.     

Large Magnetoresistance Based on Double Spin Filter Tunnel Barriers

We propose and theoretically analyse a double magnetic tunnel device that takes advantages of the spin filter effect. Two magnetic tunnel barriers are formed by different spin filters which have different barrier heights. The magnetoresistance of the device is low （high） when the magnetic moments of the two spin filters are parallel （antiparallel）. We present a theoretical calculation of the magnetoresistance based on electric tunnel effect. In addition, the effect of the difference barrier heights and exchange splitting energies between the two spin filters are also analysed in detail. The numerical results show that the spin filter in this configuration gives a magnetoresistance larger than that with standard magnetic tunnel junctions.     

Giant magnetoresistance effect in nanostructures consisting of magnetic–electric barriers

The GMR effect in magnetic–electric barrier nanostructure, which can be realized experimentally by depositing two parallel metallic ferromagnetic strips with an applied voltage on the top of heterostructure, is investigated theoretically. It is shown that a considerable GMR effect can be achieved in such nanosystems due to the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations. It is also shown that the magnetoresistance ratio is strongly dependent upon the applied voltage to metallic ferromagnetic strips in nanosystems, thus may leading to voltage-tunable GMR devices.     

Interplay of spin accumulation and Coulomb blockade in double ferromagnetic junctions

Single-electron tunneling in a double junction in which a nonmagnetic metallic island is separated from two ferromagnetic electrodes by tunnel barriers, is analysed theoretically in the sequential tunneling regime and for an arbitrary intrinsic spin relaxation time on the island. It is shown that nonequilibrium spin polarization of the island results in tunnel magnetoresistance due to rotation of the electrode magnetizations from antiparallel to parallel alignment. It is also shown that discrete charging of the island gives rise to a fine structure in the voltage dependence of the island spin polarization and tunnel magnetoresistance     

Spin-flip and domain wall magnetoresistance in quantum magnetic nanocontacts

The theory of nanosize point contacts made of ferromagnetic metals is developed. A general quantum scattering theory is applied to calculate magnetoresistance of a nanocontact with a domain wall located in the constriction. The exact solution of the electron motion in a potential of the linear domain wall is used as a zero-order approximation. Spin-flip and spin-conserving quantized conductances of the nanocontact are calculated by the perturbation theory by the difference between the model and the Cabrera-Falicov potentials of the domain wall. It is explicitly shown that spin-flip conductance imposes natural limitation on magnetoresistance of the point contact, which otherwise diverges in the regime of complete spin-rectified conductance through the contact.     

The magnetoresistance effect in a nanostructure with the periodic magnetic barriers

The magnetoresistance (MR) effect is theoretically investigated in a periodic magnetically modulated nanostructure, which can be realized experimentally by depositing periodic parallel ferromagnetic strips on the top of a heterostructure. We find that there exists a significant conductance difference for electrons through the parallel (P) and antiparallel (AP) magnetization configurations, which results in a considerable magnetoresistance effect. We also find that the magnetoresistance effect depends not only on the temperature but also on the number of the periodic magnetic barriers.     

Magnetotransport in a zigzag monolayer MoS2 nanoribbon with ferromagnetic electrodes

In this research, using the three-band tight-binding model combined with a non-equilibrium Green's function technique, spin-dependent electron transport was investigated in the quantum structure of zigzag monolayer molybdenum disulfide with ferromagnetic electrodes. It was shown that in parallel configurations, the conductance exhibited a quantized oscillating phenomenon, while in the antiparallel configurations with increasing magnetization, the conductance showed a zero platform in a large-energy region. We observed a giant magnetoresistance effect. Moreover, the length of the central part of the structure had a certain influence on the magnetoresistance ratio. It was found that as the length of the middle region increased, the magnetoresistance ratio decreased gradually. The results not only extended our understanding of novel electronic structures of monolayer MoS₂ but also provided the possibility for the technological applications of spintronics device.     

Spin-dependent transport through quantum dot with inelastic interactions

Using the Keldysh nonequilibrium Green function method, we theoretically investigate the electron transport properties of a quantum dot coupled to two ferromagnetic electrodes, with inelastic electron-phonon interaction and spin flip scattering present in the quantum dot. It is found that the electron-phonon interaction reduces the current, induces new satellite polaronic peaks in the differential conductance spectrum, and at the same time leads to oscillatory tunneling magnetoresistance effect. Spin flip scattering suppresses the zero-bias conductance peak and splits it into two, with different behaviors for parallel and anti-parallel magnetic configuration of the two electrodes. Consequently, a negative tunneling magnetoresistance effect may occur in the resonant tunneling region, with increasing spin flip scattering rate.     

Spin-valley-dependent transport and giant tunneling magnetoresistance in silicene with periodic electromagnetic modulations

The transport property of electrons tunneling through arrays of magnetic and electric barriers is studied in silicene.In the tunneling transmission spectrum, the spin-valley-dependent filtered states can be achieved in an incident energy range which can be controlled by the electric gate voltage. For the parallel magnetization configuration, the transmission is asymmetric with respect to the incident angle θ, and electrons with a very large negative incident angle can always transmit in propagating modes for one of the spin-valley filtered states under a certain electromagnetic condition. But for the antiparallel configuration, the transmission is symmetric about θ and there is no such transmission channel. The difference of the transmission between the two configurations leads to a giant tunneling magnetoresistance(TMR) effect.The TMR can reach to 100% in a certain Fermi energy interval around the electrostatic potential. This energy interval can be adjusted significantly by the magnetic field and/or electric gate voltage. The results obtained may be useful for future valleytronic and spintronic applications, as well as magnetoresistance device based on silicene.     

A valley-filtering switch based on strained graphene

We investigate valley-dependent transport through a graphene sheet modulated by both the substrate strain and the fringe field of two parallel ferromagnetic metal (FM) stripes. When the magnetizations of the two FM stripes are switched from the parallel to the antiparallel alignment, the total conductance, valley polarization and valley conductance excess change greatly over a wide range of Fermi energy, which results from the dependence of the valley-related transmission suppression on the polarity configuration of inhomogeneous magnetic fields. Thus the proposed structure exhibits the significant features of a valley-filtering switch and a magnetoresistance device.     

Radiofrequency magnetoresistance of Fe/Cr superlattices

The rf magnetoresistance of Fe/Cr superlattices is studied for two orientations of the current: parallel and across the superlattice layers. A mutually single-valued correspondence is established between the relative magnetoresistance measured at dc current and the change in the transmission coefficient of electromagnetic waves in the magnetic field. When rf currents flow across the layers, the relative change in the signal amplitude is proportional to twice the change in the electrical resistance of the superlattice and is of opposite sign. It is shown that the rf losses are determined by the surface resistance which is proportional to the superlattice thickness and inversely proportional to its conductivity. An equation is derived for the rf electric field distribution in the superlattice. It is established that when the thickness of the superlattice is small compared with the skin layer depth, field and current components which penetrate through the entire superlattice exist.     

Spin-dependent shot noise of inelastic transport through molecular quantum dots

Here we present a theoretical analysis of the effect of inelastic electron scattering on spin-dependent transport characteristics (conductance, current–voltage dependence, magnetoresistance, shot noise spectrum, Fano factor) for magnetic nanojunction. Such device is composed of molecular quantum dot (with discrete energy levels) connected to ferromagnetic electrodes (treated within the wide-band approximation), where molecular vibrations are modeled as dispersionless phonons. Non-perturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT–MT), which transforms the many-body electron–phonon interaction problem into a single-electron multi-channel scattering problem. The consequence of the localized electron–phonon coupling is polaron formation. It is shown that polaron shift and additional peaks in the transmission function completely change the shape of considered transport characteristics.     

自旋场效应晶体管中隧道磁阻的势垒相关反转效应

考虑自旋场效应晶体管中Rashba自旋轨道相互作用和自旋输运量子相干性,研究了势垒强度对自旋场效应晶体管的自旋相关量子输运的影响. 研究发现,势垒强度较低时,隧道结电导随Rashba自旋轨道相互作用强度的变化呈现明显的振荡现象,势垒强度较高时,电导表现出明显的势垒相关“电导开关”现象. 当势垒强度逐渐增强时,平行结构电导呈现出单调下降趋势,而反平行结构电导产生波动,这种波动导致该隧道磁阻也随势垒强度的变化表现出振荡现象,且在合适的准一维电子气厚度情况下隧道磁阻值可以产生正负反转,这个效应将会在基于自旋的电子器件信息的存储上获得应用. 关键词： 自旋场效应管 开关效应 量子相干 隧道磁阻     

Modeling of magnetically controlled Si-based optoelectronic devices

We present a theoretical analysis and results of modeling of a new integrated device for spintronics application, which is based on a hybrid metal–semiconductor structure. The proposed device consists of a Si-based p–i–n photodetector sandwiched between two layers of a ferromagnetic metal (3d ferromagnet or half-metallic compound). Electron–hole pairs are created in the semiconductor part of the structure by light illumination. The photocurrent flowing in such a system is shown to depend on its magnetic configuration. This is due to a difference in the specular reflection (as well as in the diffuse scattering) of spin-up and spin-down electrons and holes from magnetically polarized layers—similar to giant magnetoresistance effect in magnetic multilayers. This, in turn, allows controlling the device performance by an externally applied magnetic field. We have estimated magnitude of the effect and also determined the role of relevant material parameters.     

Magnetoresistance of an asymmetric quantum-size structure in a parallel magnetic field: Field asymmetry independent of the current direction

A new phenomenon, viz., field-asymmetric transverse magnetoresistance of a doped asymmetric quantum-size structure discovered in a magnetic field parallel to the heteroboundary planes, is studied experimentally and theoretically. The magnetoresistance asymmetry relative to the field direction, which is independent of the direction of transport current, is observed when a lateral electric field is embedded in the structure with the help of alloyed metallic contacts. In the theoretical part of the paper, it is shown that the contribution to current, which is asymmetric in the magnetic field, can be consistently described in the framework of the theory of spontaneous current states and photovoltaic effect in systems without an inversion center; the reason behind the emergence of this current is associated with the asymmetry of the energy spectrum of charge carriers relative to the quasimomentum. It is shown that the change in the size and shape of Fermi contours in a magnetic field determines the magnitude of the strong negative magnetoresistance associated with the intersubband scattering under investigation and is found to be responsible for the emergence of a qualitatively new effect mentioned in the title of this paper.     

Anisotropic interface magnetoresistances in Pt(111)/Co n /Pt(111)

It is shown in terms of a fully relativistic spin-polarized ab initio-type approach that in Pt/Co/Pt trilayers two types of anisotropic magnetoresistance (AMR) have to be distinguished: an in-plane and an out-of-plane AMR. The obtained results, namely the magnetic field dependence as well as the thickness dependence of both AMR types are in very good agreement with a very recent experimental study, in which the in-plane as well as the out-of-plane AMR was reported for this system. The difference between the two types of AMR is visualized in terms of layer-resolved resistivities. In particular, it is confirmed that the anisotropic interface magnetoresistance (AIMR) introduced in the recent publication mainly originates in the vicinity of the Co/Pt interfaces.     

Magnetization reversal of single domain permalloy nanowires

Magnetization reversal process and magnetoresistance (MR) hysteresis of single domain permalloy nanowires are numerically investigated by using OOMMF. It is shown that the abrupt jumps in the magnetoresistance are due to the domain formation and domain wall propagation so that a magnetic domain suddenly switches from one state into another. A nonmonotonic angular dependence of the jump (switching) field is found. Coherent rotation mode is responsible for the smooth variation of MR curves. The nucleation pattern of newly born domains depends on the tilted angle of external field.     

The tunneling magnetoresistance in GaMnAs/GaAs/GaMnAs junctions

The tunneling magnetoresistance (TMR) in GaMnAs/GaAs/GaMnAs magnetic tunnel junctions is studied under an extended coherent tunneling approach where both the contributions of the light holes and the heavy holes and their mutual competitions are investigated. It is shown that the TMR ratio can increase with decreasing the barrier strength, which is different from the results in the conventional magnetic tunnel junctions but a good news for the applications. It is also shown that the presence of the pinholes in the thin barrier layer gives a possible explanation of the peak in the barrier thickness dependence of the TMR ratio.     

Anisotropic positive magnetoresistance of a nonplanar 2D electron gas in a parallel pagnetic field

Magnetotransport properties of a 2D electron gas in narrow GaAs quantum wells with AlAs/GaAs superlattice barriers were studied. It is shown that the anisotropic positive magnetoresistance observed in selectively doped semiconductor structures in a parallel magnetic field is caused by the spatial modulation of the 2D electron gas.