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The counterpropagating edge states of a two-dimensional topological insulator (TI) carry electrons of opposite spins. We investigate the transport properties of edge states in a two-dimensional TI which is contacted to ferromagnetic leads. The application of a side-gate voltage induces a constriction or quantum point contact (QPC) which couples the two edge channels. The transport properties of the system are calculated via the Keldysh nonequilibrium Green's function method. We found that inter-edge spin-flip coupling can significantly enhance (suppress) the charge current when the magnetization of the leads are anti-parallel (parallel) to one another. On the other hand, spin-conserving inter-edge coupling generally reduces the current by backscattering regardless of the magnetization configuration. The charge current and the conductance as a function of the bias voltage, also exhibit similar trends with respect to spin-flip coupling strength, for both parallel and anti-parallel configurations. Hence, gate voltage modulation of edge states via a QPC can provide a means of modulating the spin or charge current flow in TI-based spintronics devices. [ABSTRACT FROM AUTHOR]

It has been shown that three-terminal quantum dot (TTQD) devices and their variants can generate a rather high spin current in the presence of Rashba spin-orbit coupling (RSOC) at low temperature. The effect of contacting ferromagnetic (FM) electrodes to the TTQD devices is studied theoretically via the Keldysh non-equilibrium Green's function method. The spin torque and spin current in the system are subsequently calculated. It is found that the spin torque in the middle FM lead can be reversed symmetrically by applying a bias voltage with opposite sign. This suggests that current-induced magnetization switching can be applied for data writing. Moreover, the charge current shows significant modulation when the orientation of the FM moment is varied, a feature which can be utilized for reading data. Based on this, we propose an integrated Magnetic Random Access Memory (MRAM) device made up of Rashba TTQD unit cells. In each cell, the electron transport can be modulated by means of a gate voltage without the need for a transistor - unlike the case of conventional MRAMs. What is more, owing to the combination of RSOC and exchange interaction with the local FM moments, both the writing and reading process can be effected with a single FM layer. [ABSTRACT FROM AUTHOR]

Heusler alloy Co2FeAl was employed as ferromagnetic layers in Co2FeAl (3 nm)/Ru (x nm)/Co2FeAl (5 nm) synthetic antiferromagnet structures. The experimental results show that the structure with a Ru thickness of 0.45 nm is strongly antiferromagnetic coupled, which is maintained after annealing at 150 °C for 1 h. The structure has a very low saturation magnetization Ms of 425 emu/cm3, a low switching field Hsw of 4.3 Oe, and a high saturation field Hs of 5257 Oe at room temperature, which are favorable for application in ultrahigh density magnetic read heads or other magnetic memory devices. Crystal structure study testifies that the as-deposited Co2FeAl film is in the B2 phase. Therefore, Heusler alloys can be used to fabricate synthetic antiferromagnetic and it is possible to make “all-Heusler” spin valves or magnetic tunneling junctions with better magnetic switching properties and high magnetoresistance. [ABSTRACT FROM AUTHOR]

We investigate the spin transfer torque arising from a perpendicular-to-plane current in a ferromagnetic (FM)-nonmagnetic-FM trilayer. Our analysis is based on the spin drift-diffusion model, modified for the case of noncollinear magnetization, i.e., with arbitrary angle θ between the magnetization orientation of the two FM layers. By solving the electrochemical potential and spin accumulation across the trilayer, we obtain the θ dependence of the magnetoresistance and spin transfer torque in the free FM layer. The optimal magnetization orientation θmax and the corresponding maximum torque τmax are investigated as a function of the FM layer thicknesses. Based on the analysis, we propose that (i) the free (fixed) FM layer thickness be set at approximately the transverse (longitudinal) spin relaxation lengths, and (ii) the relative FM orientation be biased at some intermediate angles instead of the conventional collinear configuration, in order to maximize the current-induced magnetization switching effect. [ABSTRACT FROM AUTHOR]

We performed a theoretical study and analysis of the effect of modifying the layer thicknesses of a current-perpendicular-to-plane (CPP) spin valve multilayer on its magnetoresistance (MR) ratio. An increase in the ferromagnetic (FM) layer thickness results in (i) an increase in the spin-dependent component of its total resistance, thereby resulting in higher MR, but also leads to (ii) greater spin relaxation in that layer and (iii) an anomalous MR effect in the high resistance regime, both of which suppress the MR ratio. The interplay of these effects results in a complex MR dependence on FM thickness, instead of the simple monotonic MR increase predicted by the two-current model. It also explains the existence of an optimum FM thickness for maximum MR ratio, as evidenced by experimental data. Finally, we consider the MR dependence on the strength and spin selectivity of interfacial resistances, which can either arise naturally or be engineered in the spin valve structure. The study of the combined effects of the FM layer thickness and resistivity, the MR suppression in the high resistive limit, and the competitive spin-dependent scattering in the bulk and at the interfaces is essential for optimizing the structure and material of a practical CPP spin valve to achieve the maximum MR ratio. [ABSTRACT FROM AUTHOR]

We propose a ballistic, coherent transmission system that utilizes the magnetic and electric barriers as Boolean input variables to realize functions similar in principle to the conventional logic gates. For practical implementation of these functions, we propose to use a device construct based on the high-electron-mobility transistor (HEMT) with ferromagnetic (FM) and nonmagnetic (NM) metal gates deposited on top of the HEMT heterostructure. This device system can be manipulated to realize multiple logic functions such as OR, AND, and their inverse by applying different magnetic and electric field configurations on the FM and the NM gates. The charge transport simulation is based on the single particle effective mass Hamiltonian and ballistic charge transport. The calculation results demonstrate clear binary outputs corresponding to various logic functions, with “high” (“low”) state having transmission probability of T>90% (T<10%). [ABSTRACT FROM AUTHOR]

We present a model of tunneling magnetotransport across a ferromagnetic single electron transistor (FM-SET). The model applies the “orthodox” theory and the Master equation method to describe the stochastic nature of separate tunneling events. The coherent transport of an individual charge through a tunnel barrier is modeled by a free-electron Hamiltonian through a trapezoidal barrier, and a two-band model within the FM electrodes. Single electron charging effect is incorporated into both stochastic and coherent parts of the model. The calculated tunneling resistance R[sub t] is dependent on the thickness and voltage-dependent profile of the tunnel barriers. A bias modulation of the tunneling magnetoresistance (TMR) is obtained with the same period as the Coulomb staircase pattern. This modulation is present even for “homogeneous” FM-SETs, where each junction is of the same material composition. By contrast, previous studies based on the Julliere model predict total suppression of TMR modulation for such FM-SETs. The TMR modulation is found to be highly dependent on barrier thickness and the material composition of the junctions, implying further adjustable parameters for optimal TMR ratio. Finally, the bias modulation of TMR persists at high temperatures, even though the Coulomb staircase steps have been virtually smeared out. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigate the tunnel magnetoresistance (TMR) of the double barrier magnetic tunnel junction, where a quantum dot (QD) with discrete electron and hole energy levels is sandwiched between ferromagnetic leads. The effects of the symmetry of the coupling between the leads and the dot on both the TMR and spin accumulation (SA) are studied for voltage ranges corresponding to the QD’s single and double occupancies. When the QD is singly occupied, both the TMR and SA assume at their minimum values for symmetrical junctions with identical coupling strengths. For the doubly occupied QD, the opposite occurs with the highest TMR and SA being observed for symmetrical junctions with identical coupling strengths. The TMR is found to be strongly correlated with the spin accumulation in the QD. [ABSTRACT FROM AUTHOR]

Tunneling transport across a double-junction system, consisting of a small magnetic metallic island, coupled to ferromagnetic contacts by tunnel barriers, is studied by incorporating the effects of source-drain V[SUBa] and gate V[SUBg] voltages, and the island charging energy into the model Hamiltonian. The transmission coefficients and current across the double barrier are evaluated using quantum mechanical transfer matrix method. The tunneling J - V[SUBa] characteristic exhibits a staircase pattern, while the tunneling current oscillates with the gate voltage. The device also exhibits a bias-dependent tunneling magnetoresistance with a peak value exceeding 35%. We attribute these behaviors to the combined effect of spin-polarized tunneling and discrete charging of the island. [ABSTRACT FROM AUTHOR]

We present a model to investigate the lateral transport and anisotropic magnetoresistance (AMR) of a permalloy film, with a two-dimensional array of unfilled holes (antidotes). The current density distribution j is first obtained numerically by solving the Poisson equation in the absence of B field, while the magnetization M of the film is calculated via micromagnetics. The M and j distributions are linearly interpolated through the sample, and then combined in a self-consistent scheme to obtain the galvanomagnetic effects (normal and planar Hall effects), which affect the current direction. The overall AMR is evaluated from the parallel contributions of the resulting current paths. Modifying the film geometry by reducing the antidot spacing results in an increase in the peak AMR ratio, which may be explained by the inhomogeneous current density and magnetization alignment induced by the antidots. [ABSTRACT FROM AUTHOR]

We perform a theoretical study of the electron transport through a normal metal—topological insulator—normal metal (NM-TI-NM) system, where a ferromagnetic insulator (FI) layer is deposited on top of the TI. The spin conductance of the system is analyzed as a function of parameters such as the strength of exchange coupling between the surface states of the TI and the magnetic moments of the FI layer, as well as the dimension of the TI channel. We find that the strength of the spin conductance can be optimized by tuning the parameters. [ABSTRACT FROM PUBLISHER]

We have investigated the evolution in the magnetization reversal mechanism of lithographically defined Ni80Fe20 nanowire arrays with varied film thicknesses. The nanowire array of width 185 nm and spacing 35 nm were fabricated using deep ultraviolet lithography at 248 nm exposing wavelength. We observed a cross-over from coherent rotation to curling reversal mode when the thickness to width ratio = 0.5. We have aided our understanding of the reversal process with theoretical modeling. A marked increase in the coercive field characterized the intermediate region between coherent rotation and magnetization curling. [ABSTRACT FROM AUTHOR]

In this work, we calculate the eigenstates and dispersion relations for the 2-D surface states of 3-D topological insulator (TI) planar nanostrips of finite width coupled to a proximate insulating ferromagnetic layer. The magnetization of the ferromagnetic layer in the transverse, longitudinal and out-of-plane directions have differing effects on the dispersion relations. We also calculated the transmission through a short central TI strip sandwiched between TI source and drain leads of semi-infinite length. We investigate the transport through the setup when the magnetization of the ferromagnetic layer on top of the central region differs from that of the leads. We found that when the lead magnetization is in the longitudinal direction, i.e., along the current direction, the transmission is maximal for both the parallel and anti-parallel alignment of the lead and central magnetizations. However, for the case where the lead magnetization is in the transverse direction, the transmission is maximal for parallel alignment and minimal for the antiparallel alignment. [ABSTRACT FROM AUTHOR]

We investigate the magneto-Coulomb effect in a single-electron tunneling transistor (SETT) with ferromagnetic leads. This effect enables us to achieve a magnetic field control, as opposed to the conventional electrical gate control of the SETT. By utilizing the sharp magnetic switching of the ferromagnetic (FM) source and drain electrodes, the magneto-Coulomb effect can attain a large magnetoconductance (MC) change in response to the applied magnetic B-field. We analyze the effect of the B-field on the effective charge Q on the island, and analytically obtain the step change ΔQ at the coercive field Bc of the FM leads. We also derive the optimal biasing conditions of Vg = (n ± 1/4)e/C9 and Vds ≈ Vth (threshold bias). Such bias conditions ensure the maximum sensitivity (dI/dQ) of the SETT modulation to Q, and hence to the applied B-field. The magnetoconductance response of the SETT under the optimal biasing condition is numerically calculated based on the assumption of single-domain switching of the FM leads. [ABSTRACT FROM AUTHOR]

The authors investigate the magnetic and transport behaviors of Ge1-xMnxTe thin films with high Mn composition (x=0.98) grown by solid-source molecular-beam epitaxy. The temperature-dependent magnetization (M-T) gives a Curie paramagnetic temperature θp∼120 K, in contrast to the Curie temperature of TC∼95 K obtained from the Arrott plot and temperature-dependent resistivity measurement. The resistivity and M-T behaviors can be attributed to weak localization effect of disordering. The authors discussed the ferromagnetism in Ge0.02Mn0.98Te on the basis of the Ruderman-Kittel-Kasuya-Yoshida interaction and clustering effect. [ABSTRACT FROM AUTHOR]

We report the synthesis of zinc-blende CrTe (zb-CrTe) as highly oriented crystalline grains in films of 100-nm thickness. The structural properties of the film were characterized using high-resolution transmission electron microscopy (HRTEM) and selective-area electron diffraction (SAED). Temperature-dependent magnetization measurements show that the Curie temperature of the film is ∼327 K. Angular M-H measurements indicate that the CrTe films have an in-plane easy axis of magnetization along the [01¯1] direction of GaAs. [ABSTRACT FROM AUTHOR]

In this paper, we propose an alternative method to improve magnetoresistance by inserting a thin ferromagnetic (FM) layer into the nonmagnetic (NM) spacer of a basic spin-valve (SV) trilayer (FM1-NM-FM1), thus creating a penta-layer SV structure (FM1-NM- FM2-NM-FM1). We investigated the effect of increasing the resistivity (ρF2) of the FM2 on overall magnetoresistance (MR). We performed both analytical and numerical studies on the MR of the current perpendicular-to-plane (CPP) structure using the phenomenological spin drift-diffusion models. For finite ρF2, the MR profile is dependent on the intrinsic conductance polarization (αF2) of FM2. We found that inserting FM2 enhances MR when αF2 exceeds a critical value of α2C. It is found that MR can be doubled by inserting a FM layer with high αF2, such as the half-metallic Cr2O. We have numerically calculated MRmax and the corresponding ρF20 values for different αF2 values. Finally, we studied the effect of spin relaxation on the MR of the CPP SV. [ABSTRACT FROM AUTHOR]

We present an iterative scheme which combines the non-equilibrium Green's function (NEGF) for evaluating the quantum spin transport in a ferromagnetic quantum dot device and the Landau-Lifshitz (LL) equation for modeling the magnetization dynamics of the dot. For a given initial magnetization, the spin polarization of current and the resulting spin torque in the dot are calculated using the NEGF formalism. The torque acts on the magnetic moment of the dot, and the resultant magnetization dynamics is obtained from the LL equation. The new value of the dot's magnetization is then used as an input for the next round of NEGF calculation, and the whole process is repeated iteratively. The spin torque is thus calculated self-consistently with the dynamics of the magnetic moment of the dot. We apply this self-consistent iterative scheme to study the magnetization dynamics in an exemplary quantum dot system with an induced spin bias in the leads under varying damping conditions. [ABSTRACT FROM AUTHOR]

We investigate the applicability of a triple quantum dot (TQD) device for memory operation, in which information is coded by the magnetization of a ferromagnetic (FM) electrode. In the presence of Rashba spin-orbit coupling, a high spin polarization of current can be generated in the TQD device to induce spin transfer switching of the FM electrode. We evaluate the spin current and spin torque via the Keldysh nonequilibrium Green's function method. The calculated spin torque can be reversed symmetrically by applying an opposite bias voltage, thus enabling current-induced magnetization switching to be applied for data writing. Additionally, the charge current shows a large modulation when the magnetization of the FM electrode is switched, a feature which can be utilized for reading data. The ability to write and read data demonstrates the applicability of the TQD device for spin transfer torque-based memory. [ABSTRACT FROM AUTHOR]

Reweighting of trajectories in stochastic dynamical processes leads to significant reduction in simulation effort as it allows stochastic dynamical result corresponding to one temperature to be ported to another temperature. In this paper, we propose an improved reweighting technique which incorporates a time scaling factor. This results in an improved distribution in the trajectory's weight space, and broadens the range of temperature over which the reweighting technique can be applied. The technique is employed in numerical simulation of an exemplary stochastic process, i.e., the thermally activated magnetization reversal process of an Ising system. [ABSTRACT FROM AUTHOR]