Motion of domain walls under the action of spin-polarized current in a magnetic junction

The effect of spin-polarized current on a domain structure in a magnetic junction consisting of two ferromagnetic metallic layers separated by an ultrathin nonmagnetic layer is studied within a phenomenological theory. The magnetization of one ferromagnetic layer (layer 1) is assumed to be fixed, while that of the other ferromagnetic layer (layer 2) can be freely oriented both parallel and antiparallel to the magnetization of layer 1. Layer 2 can be split into domains. Charge transfer from layer 1 to layer 2 is not attended with spin scattering by the interface but results in spin injection. Due to s-d exchange interaction, injected spins tend to orient the magnetization in the domains parallel to layer 1. This causes the domain walls to move and “favorable” domains to grow. The average magnetization current injected into layer 2 and its contribution to the s-d exchange energy are found by solving the continuity equation for carriers with spins pointing up and down. From the minimum condition for the total magnetic energy of the junction, the parameters of the periodic domain structure in layer 2 are determined as functions of current through the junction and magnetic field. It is shown that the spin-polarized current can magnetize layer 2 up to saturation even in the absence of an external magnetic field. The associated current densities are on the order of 10⁵ A/cm². In the presence of the field, its effect can be compensated by such a high current. Current-induced magnetization reversal in the layer is also possible.     

Effect of single magnetic atom on spin-polarized transport of armchair graphene nanoribbons

Using a perturbative method, the influence of a single magnetic impurity on the spin-polarized current flowing through a metallic armchair graphene nanoribbon is investigated theoretically. It is shown that the nonlinear correction depends strongly on the relative spin orientations in the two spin-polarized reservoirs. Also, the effects of magnetic impurity position and width of nanoribbon on the nonlinear conductance are discussed.     

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.     

Spin injection into semiconductors using dilute magnetic semiconductors

So far, attempts at realizing spin-polarized current injection into a semiconductor using metallic ferromagnetic contacts have yielded unsatisfying results. In this paper, we present a simple model of diffusive transport, which shows that the principle reason for these negative results is a conductivity mismatch between the ferromagnetic contacts and the semiconductor. Moreover, we demonstrate that this problem can be addressed by using dilute magnetic semiconductor (DMS) contacts instead of metallic contacts. We present experimental results of optical measurements on a GaAs/AlGaAs diode fitted with a DMS spin injector contact. These measurements show a spin polarization of around 90% in the semiconductor. Furthermore, we discuss a novel magnetoresistance effect based on the suppression of one of the spin channels in the semiconductor which should allow the detection of a spin-polarized current by magnetoresistance measurements.     

Spin field emission and state switching in a magnetic tunnel junction

The phenomena of spin tunneling and spin torque transfer between magnetic layers of a tunnel spin-valve setup under weak and strong field emissions of spin-polarized electrons are considered. Bifurcational features of changes in the macrospin states under the impact of a tunnel current are discussed for varying directions of the spin-polarization vector.     

A simple model for maser action obtained through a magnetic tunnel junction placed inside a resonant cavity

Magnetic tunnel junctions are currently being used in magnetoresistive reading heads, magnetic field sensors and MRAMs, due to its giant magnetoresistance effect whose roots are linked to strong spin-dependent scattering mechanisms. The existence of spin-polarized currents in such devices posed us the question over the possibility to generate coherent microwave radiation in a spin inverted population medium, maintained through a spin-polarized current. In this paper we investigate the possibility of obtaining a maser effect considering a magnetic tunnel junction placed inside a resonant cavity. We put forward a simple model based on phenomenological rate equations, being the spin-polarized currents determined by the physics of the magnetic tunnel junction.     

Spin-polarization and x-ray magnetic circular dichroism in GaAs

The combination of angular spin momentum with electronics is a promising successor to charge-based electronics. The conduction bands in GaAs may become spin-polarized via optical spin pumping, doping with magnetic ions, or induction of a moment with an external magnetic field. We investigated the spin populations in GaAs with x-ray magnetic circular dichroism for each of these three cases. We find strong anti-symmetric lineshapes at the Ga L₃ edge indicating conduction band spin splitting, with differences in line width and amplitude depending on the source of spin polarization.     

Roles of spin-polarized current and spin accumulation in the current-induced magnetization switching

To clarify the contributions of spin-polarized current and spin accumulation to the current-induced magnetization switching, the effects of the top electrode size of the magnetic nanopillar are investigated both theoretically and experimentally. Theoretical calculation demonstrates that the spin-polarized current and the spin accumulation can be adjusted in opposite directions by modifying the size of the top electrode. Increase in the size of the top electrode suppresses the spin accumulation but enhances the spin-polarized current inside the nanopillar. On the other hand, it is shown experimentally that the nanopillar with a wide top electrode exhibits small critical switching current compared to the nanopillar with a narrow top electrode. The results suggest that the spin-polarized current contributes to the current-induced magnetization switching dominantly over the spin accumulation.     

Studies of novel magnetic systems by electron capture spectroscopy (ECS)

The electron spin polarization (ESP) at surfaces of bulk and artificially structured, thin film magnetic materials is studied using electron capture spectroscopy (ECS) and ion-induced emission of spin-polarized electron. For V and Tb surfaces, it is found that the temperature dependence of the surface magetic order is quite different from that of the bulk. For ultra-thin magnetic superlattices, V(100)/Ag(100) and Tb (0001)/W(110), we find novel magnetic surface anisotropies and critical behavior. At surfaces of 5 monolayers V(100) on Ag(100), the critical behavior of the surface-ESP is found to be identical to that of the two-dimensional Ising ferromagnet. At surfaces of PtMnSb and Fe₈₀B₂₀, we findnonzero ESP due to long-ranged or short-ranged ferromagnetic order. Thesign of the ESP at surfaces of the Heusler alloy PtMnSb is in agreement with recent band structure calculations. At surfaces of the spin glass Fe₈₀B₂₀, we find, in agreement with ECS data, that the ESP ofion-induced emitted spin-polarized electrons is extremely sensitive to the cleanness of thetopmost surface layer.     

Dependences of spin polarization on the control parameters in the spin-polarized injection through the magnetic p-n junction

Effective spin-polarized injection from magnetic semiconductor (MS) to nonmagnetic semiconductor (NMS) has been highlighted in recent years. In this paper we study theoretically the dependence of nonequilibrium spin polarization (NESP) in NMS during spin-polarized injection through the magnetic p-n junction. Based on the theory in semiconductor physics, a model is established and the boundary conditions are determined in the case of no external spin-polarized injection and low bias. The control parameters that may influence the NESP in NMS are indicated by calculating the distribution of spin polarization. They are the doping concentrations, the equilibrium spin polarization in MS and the bias. The effective spin-polarized injection can be realized more easily by optimizing the above parameters.     

Spin-polarized transport in inhomogeneous magnetic semiconductors: theory of magnetic/nonmagnetic p-n junctions

A theory of spin-polarized transport in inhomogeneous magnetic semiconductors is developed and applied to magnetic/nonmagnetic p-n junctions. Several phenomena with possible spintronic applications are predicted, including spin-voltaic effect, spin valve effect, exponential and giant magnetoresistance. It is demonstrated that only nonequilibrium spin can be injected across the space-charge region of a p-n junction, so that there is no spin injection (or extraction) at low bias.     

Spin torque, tunnel-current spin polarization, and magnetoresistance in MgO magnetic tunnel junctions

We employ the spin-torque response of magnetic tunnel junctions with ultrathin MgO tunnel barrier layers to investigate the relationship between spin transfer and tunnel magnetoresistance (TMR) under finite bias, and find that the spin torque per unit current exerted on the free layer decreases by < 10% over a bias range where the TMR decreases by > 40%. This is inconsistent with free-electron-like spin-polarized tunneling and reduced-surface-magnetism models of the TMR bias dependence, but is consistent with magnetic-state-dependent decay lengths in the tunnel barrier.     

Vertical quantum dots at high magnetic fields beyond the few-electron limit

We describe phenomena that can be studied in vertical quantum dot single electron transistors. Moving from the few-electron to the several- and many-electron regimes, features in the conductance peaks initially related to spin polarization evolve with magnetic field. This allows us to first probe the spin-flip region beyond the last single-particle crossing at low field, and then the formation and stability of the spin-polarized maximum density droplet at high field. According to a simple capacitance model, charge redistribution in the dot at higher magnetic fields is accompanied by abrupt changes in the area of the droplet.     

Analysis of effective spin-polarized transport through a ZnO based magnetic p–n junction at room temperature

ZnO based magnetic semiconductors (MSs) are prominent candidates for the spintronic devices because of their high Curie temperatures and low conductance mismatches. In this paper the spin-polarized transport in MS/nonmagnetic semiconductor (NMS) p–n junction is investigated. A model is established based on semiconductor drift–diffusion theory and continuity equation. Boundary conditions are obtained from the quasi-chemical potential (QCP) relations at the junction interface. For a ZnO based magnetic p–n junction, we calculate the distributions of carrier/spin density and spin polarization at room temperature. It is demonstrated that by choosing proper parameters, effective spin-polarized injection from ZnO based MS into ZnO can be achieved at room temperature without external spin-polarized injection (ESPI) or large bias.     

Switching of a spin valve with three magnetic layers

A spin valve with two pinned ferromagnetic layers sandwiching a free ferromagnetic layer with a thickness smaller than the spin diffusion length in the same layer and than the domain wall thickness is considered. The instability conditions are determined for various mutual orientations of the magnetization of the layers. The possibility of a considerable decrease in the instability threshold due to joint action of spin-polarized current and an external magnetic field is indicated. It is shown that in addition to collinear states, a nonequilibrium noncollinear state can exist, into which the system is switched after exceeding the instability threshold.     

Voltage-controllable spin-polarized transport through parallel-coupled double quantum dots with Rashba spin-orbit interaction

A spin device, consisting of parallel-coupled double quantum dots and three normal metal leads, is proposed to realize spin-polarized current without the help of magnetic field and magnetic material. Based on the Keldysh nonequilibrium Green function technique and equation of motion method, the spin-dependent current formula in each lead is derived. It is shown that not only a fully polarized current but also a tunable pure spin current can be obtained by modulating the structure parameters, strength of Rashba spin-orbit interaction and bias voltages properly. It further demonstrates the dependence of the spin-polarized current on the strength of the Rashba spin-orbit interaction.     

Simulation of spin-polarized scanning tunneling microscopy images of nanoscale non-collinear magnetic structures

We apply an efficient method to calculate spin-polarized scanning tunneling microscopy (SP-STM) images of nanostructures with complex non-collinear magnetic order. The model is based on the spin-polarized version of the Tersoff–Hamann model of STM and the independent orbital approximation for the surface electronic structure. For its application, only the knowledge of the arrangement of the magnetic moments of the surface atoms is required. In spite of its simplifications, calculated SP-STM images of periodic collinear and non-collinear magnetic spin structures are in many cases in excellent agreement with those obtained from computationally much more demanding ab initio calculations. Especially for surfaces of chemically equivalent atoms, the atomic scale SP-STM images are dominated by the magnetic structure and depend much less on the accurate electronic structure. This suggests the application of the method to more complex non-collinear magnetic structures such as domain walls in antiferromagnets, spin-spiral states, spin glasses, or disordered states. Based on the model, we predict SP-STM images of helical spin-spiral states in ultra-thin films. PACS 68.37; 75.70; 75.30     

基于磁动力学方程的电流感应磁化翻转效应的宏观模型

提出了一个基于磁动力学方程的宏观唯象理论模型，对纳米级赝自旋阀结构的电流感应磁化翻转效应给出了明晰的物理解释：流入自由层的净自旋流和自由层内的自旋弛豫过程的共同作用，导致自由层总磁矩随时间的改变，甚至产生磁化方向的翻转.模型将“铁磁/非铁磁”界面的自旋相关散射，以及铁磁层中的自旋积累和弛豫过程，统一于宏观的磁动力学方程中.通过求解该方程的解析解，给出了赝自旋阀在电流激励下的磁化翻转条件和临界电流密度的表达式.对该效应的定性解释和数值模拟结果都和实验报道良好符合.根据模型分析了影响临界电流密度的诸因素，并指出提高器件性能的途径. 关键词： 电流感应磁化翻转 磁动力学方程 自旋电子学     

Spin-polarized transmission through a mesoscopic ring with a magnetic impurity

Based on one-dimensional quantum waveguide theory we study the symmetry of the spin-polarized transmission through an Aharonov–Bohm ring with a magnetic impurity, in which the spin-exchange interaction between an incident electron and the magnetic impurity leads to spin–flip scattering. It shows that for some special Fermi energies, both spin-up and spin-down transmission coefficients are symmetric under the flux reversal in the spin–flip scattering process and the spin-polarized conductance also is symmetric. In above case, AB oscillations of spin-down transmission and reflection are perfectly identical. The effect of the exchange interaction strength and Fermi wave vector on transmission behavior of spin-state electrons is examined.     

Spin-dependent quantum transport through an Aharonov--Bohm structure spin splitter

Using the tight-binding model approximation, this paper investigates theoretically spin-dependent quantum transport through an Aharonov-Bohm （AB） interferometer. An external magnetic field is applied to produce the spinpolarization and spin current. The AB interferometer, acting as a spin splitter, separates the opposite spin polarization current. By adjusting the energy and the direction of the magnetic field, large spin-polarized current can be obtained.