Feng Zhai

Theory of huge tunneling magnetoresistance in graphene

We investigate theoretically the spin-independent tunneling magnetoresistance effect in a graphene monolayer modulated by two parallel ferromagnets deposited on a dielectric layer. For the parallel magnetization configuration, Klein tunneling can be observed in the transmission spectrum but at specific oblique incident angles. For the antiparallel magnetization configuration, the transmission can be blocked by the magnetic-electric barrier provided by the ferromagnets. Such a transmission discrepancy results in a tremendous magnetoresistance ratio and can be tuned by the inclusion of an electric barrier.

Tunneling magnetoresistance on the surface of a topological insulator with periodic magnetic modulations

We investigate the band and transport features of Dirac electrons on the surface of a three-dimensional topological insulator under the modulation of a periodic exchange field provided by an array of ferromagnetic insulating (FI) stripes. The Dirac point is shifted (unchanged) by the superlattice when the magnetizations of adjacent FI stripes are parallel (antiparallel). For a finite magnetic superlattice, there exists a full transmission gap for both the parallel and antiparallel configurations. When the two kinds of transmission gaps have no overlap, a large magnetoresistance ratio with a tunable sign can be achieved.

Spin filtering in single magnetic barrier structures revisited

We reexamine spin-dependent transport properties of two-dimensional electrons modulated by the stray field of a ferromagnetic metal (FM) stripe on top. When the magnetization is along the transport direction of the electrons, the FM stripe generates not only a magnetic barrier but also an in-plane magnetic field component. Previous studies have omitted the in-plane magnetic field component and shown that such a device does not possess any spin-filtering effect. We show that with inclusion of this in-plane magnetic field component, a spin polarization of nearly 100% can be achieved in such a single FM-stripe modulated two-dimensional electron gas device.

Spin-filter diode based on ZnSe/Zn1−xMnxSe/Zn1−yMnySe/ZnSe heterostructures

We propose and demonstrate a spin-filter diode based on semimagnetic semiconductor ZnSe/Zn1−xMnxSe/Zn1−yMnySe/ZnSe heterostructures. It is found that the degree of spin polarization greatly depends on the electric field direction and its magnitude. There is significant difference of the spin polarization between under forward bias and under reverse bias. It is also found that the spin polarization will reverse under relatively small magnetic field, which is originated from resonant enhancement effect for spin-up electrons tunneling through effective steplike potential of the corresponding structure.

Generation of a fully valley-polarized current in bulk graphene

The generation of a fully valley-polarized current (FVPC) in bulk graphene is a fundamental goal in valleytronics. To this end, we investigate valley-dependent transport through a strained graphene modulated by a finite magnetic superlattice. It is found that this device allows a coexistence of insulating transmission gap of one valley and metallic resonant band of the other. Accordingly, a substantial bulk FVPC appears in a wide range of edge orientation and temperature, which can be effectively tuned by structural parameters. A valley-resolved Hall configuration is designed to measure the valley polarization degree of the filtered current.

Current and spin-filtering dual diodes based on diluted magnetic semiconductor heterostructures with a nonmagnetic barrier

We have investigated the ballistic spin-polarized transport through a diluted magnetic semiconductor heterostructure with the inclusion of a nonmagnetic barrier. It is found that at suitable magnetic fields, the output current of the system exhibits a nearly 100% spin polarization and large values under the forward bias, while under the reverse bias it is small and shows a weak polarization. Such current and spin-filtering dual diode functions are robust under the change of the nonmagnetic barrier width.

Spin current diode based on an electron waveguide with spin-orbit interaction

We propose a spin current diode which can work even in a small applied bias condition (the linear-response regime). The prototypal device consists of a hornlike electron waveguide with Rashba spin-orbit interaction, which is connected to two leads with different widths. It is demonstrated that when electrons are incident from the narrow lead, the generated spin conductance fluctuates around a constant value in a wide range of incident energy. When the transport direction is reversed, the spin conductance is suppressed strongly. Such a remarkable difference arises from spin-flipped transitions caused by the spin-orbit interaction. (c) 2008 American Institute of Physics.

Effects of conduction band offset on spin-polarized transport through a semimagnetic semiconductor heterostructure

We investigate the role played by the zero-field conduction band offset in spin-dependent transport through a ZnSe/Zn1−xMnxSe heterostructure with a single paramagnetic layer. It is shown that the zero-field band offset can strongly affect features of spin-polarized transport: the spin polarization is greatly weakened for the negative zero-field offset while in the positive case it is drastically enhanced. The reason is that the polarization is determined by the discrepancy between the effective potential for spin-up electrons and that for spin-down ones. Furthermore, the magnitude of the effective potential as well as its configuration are conduction-band-offset dependent and field tunable.

Rectification of spin-bias-induced charge currents

When a spin bias is applied to a two-terminal device, a charge current will be generated. We demonstrate the rectification of such a current by studying spin-dependent transport through a quantum point contact (QPC) modulated by a spin-orbit interaction. When the polarization orientation of the spin bias (which is the spin-quantization axis) is along the transverse direction of the QPC, the two spin-conserved transmissions show a distinct variation with the incident energy. As a result, the charge current can turn from zero to a remarkable value by switching the spin bias from one lead to the other lead.

Spin-polarized transport through a magnetic heterostructure: tunneling and spin filtering effect

We present tight-binding Greens function investigations on spin-polarized transport through a magnetic heterostructure system, which is made of two superlatticed magnetic layers separated by a nonmagnetic low-density-carrier spacer. The results confirm that in such a system, tunneling is one of the key mechanisms in electron transport. When tunneling effect is taken into account, the magnetoresistance ratios are reduced significantly. We also study the influence of the applied bias on the spin filtering effect. It is found that the current density is almost completely spin-polarized in a wide range of the applied bias, which indicates the heterostructure system can act as a good spin filter.