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The axion coupling in topological insulators (TI), which couples electric polarization (magnetization) with the magnetic (electric) field, is known to support a small-distance Casimir repulsion and a large-distance Casimir attraction with a zero-force stable equilibrium between TI plates. By enhancing the reflection properties of the TI interface through mirrors that introduce multiple reflections, we show that it is possible to maintain these trends while tuning the position of the zero-force point and its binding energy: the former by an order of magnitude and latter by over four orders. Moreover, surface charge on the TI allows for intermediate tuning of the zero-force point between coarse settings determined by the axion coupling. [ABSTRACT FROM AUTHOR]

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]

We compared the magnetoresistance (MR) ratios of current-confined-path (CCP) current-perpendicular-to-plane (CPP) giant MR spin valves (SVs) calculated using the two-current-model. The simplified two-current model, which places the parasitic resistance external to the two-current network, has been inaccurate in determining the overall resistance of a SV in parallel magnetization configuration. The characteristics of the error in MR depend on the parasitic resistance value, the spin asymmetry coefficient, and the spacer layer resistivity. In comparison to the complete two-current model, we found that although the simplified model could generally predict the trend of various MR behaviors, it also overestimates MR for different CCP densities. A more comprehensive calculation on the CCP SVs was then performed using the spin-drift-diffusion equations based on the Valet–Fert model. It was found that the MR behavior upon considering spin relaxation in the ferromagnetic layers has a closer match with results predicted by the simplified model due to the strong reduction in spin accumulation in the parasitic layers. Spin relaxation in the spacer layer, however, shows a competing effect that tends to reduce the MR ratios. Our results in this paper are useful for accurate evaluation of MR performance in a CPP SV sensor with current-confined paths. [ABSTRACT FROM AUTHOR]

We present a numerical simulation of current-induced magnetization switching (CIMS) in pseudospin-valve (PSV) structures exhibiting perpendicular anisotropy. The PSV structures consist of a reference layer of Co/Pt and Co/Ni multilayers and a free layer composed of Co/Ni multilayers. The spin torque induced by the passage of a spin-polarized current is modeled by means of a modified Landau–Lifshitz–Gilbert equation, which incorporates two additional spin-torque terms. Numerical micromagnetic simulations of the magnetization switching process predict a critical current density of the order of 107 A/cm2, a value consistent with available experimental results. In addition, we investigate the influence of the current sweep rate and the ratio of the in-plane and out-of-plane torque coefficients on the stability of the magnetization switching process. Our analysis provides a useful guide for optimal utilization of the CIMS effect in perpendicularly magnetized multilayers, for use in prospective spintronic applications. [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]

A hybrid Monte Carlo (MC) method is proposed to study the full magnetization dynamics of a magnetic nanoparticle, comprising damping, thermal fluctuations, and precessional motion involved in a magnetization reversal process. The precessional motion is an athermal process, and has been neglected in previous MC schemes used to model magnetization dynamics. We introduce it into our hybrid method by adding a precessional step of the appropriate size to the random walk of the heat-bath Metropolis MC method. This hybrid MC method is applied for the study of the role of precession in the magnetization switching process of a magnetic nanoparticle under the influence of an oblique field. We numerically predict two distinct behaviors in the switching process corresponding to the high and low damping limits. [ABSTRACT FROM AUTHOR]

The IV-VI diluted magnetic semiconductor Ge1-xMnxTe thin films on BaF2 (111) substrate have been prepared using solid-source molecular-beam epitaxy technique by varying the Mn concentrations from x=0.25 to 0.98. The chemical Mn concentration was determined by x-ray photoelectron spectroscopy measurement. The in situ reflection high-energy electron diffraction pattern indicates that the growth mechanism is in the island-growth mode. The x-ray diffraction shows that the Ge1-xMnxTe films crystallize in the NaCl phase with (111) orientation. A clear ferromagnetic ordering is observed in the detailed temperature-dependent magnetization measurement for 0.25C on Mn concentration x tends to follow a quadratic behavior. This phenomenon can be attributed to the increase of antiferromagnetic interaction since MnTe is an antiferromagnet. [ABSTRACT FROM AUTHOR]

Magnetotransport simulation is performed on granular nanomagnets (Co) in insulator, of average radius of 2.5 nm, over a temperature range (5200 K, the M-H curve approaches the Langevin function, but with a small discrepancy, due to the intrinsic anisotropy of Co. The magnetization results are then combined with a stochastic Monte Carlo transport model which combines the effects of stochastic spin-polarized tunneling, Coulomb blockade, and the magneto- and electrostatic influence of the contacts. The tunneling magnetoresistance shows a complex thermal dependence, with distinct behavior for different types of contact electrodes used. [ABSTRACT FROM AUTHOR]

Ferromagnetic films etched with a periodic array of holes (antidots) may be used as high-density storage media, where memory bits are stabilized by shape fields H[SUBs] near the edges of the antidots. A micromagnetic simulation is performed to study the conditions for well-defined bits at remanent state. The parameter under consideration is the bit signal-to-noise ratio (SNR), which is defined w.r.t. the ideal magnetization alignment, and is calculated for different values of intrinsic anisotropy K[SUBu] and transverse applied field H[SUBy]. Unlike previously thought, a transverse K[SUBu] hardly improves the SNR (< 10%) due to its sign independence, which leads to vortex formation around the antidots and, hence, increased noise in the interbit regions. By contrast, a relatively weak H[SUBy] field of 50 to 100 Oe can effectively separate neighboring bits by aligning the interbit regions, leading to a 250%-400% improvement in SNR. Further improvement in SNR is achieved by increasing the bit-aspect ratio. The SNR however, degrades sharply when the anisotropy H[SUBk] and transverse H[SUBy] fields approach the shape field value obtained via an analytical model. The model used is corroborated by the coercivity trend of different antidot size, obtained by micromagnetics. [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]

A hexagonal warping term has been proposed recently to explain the experimentally observed 2D equal energy contours of the surface states of the topological insulator Bi2Te3. Differing from the Dirac fermion Hamiltonian, the hexagonal warping term leads to the opening up of a band gap by an in-plane magnetization. We study the transmission between two Bi2Te3 segments subjected to different in-plane magnetizations and potentials. The opening up of a bandgap, and the accompanying displacement and distortion of the constant energy surfaces from their usual circular shapes by the in-plane magnetizations, modify the transverse momentum overlap between the two Bi2Te3 segments, and strongly modulate the transmission profile. The strong dependence of the TI surface state transport of Bi2Te3 on the magnetization orientation of an adjacent ferromagnetic layer may potentially be utilized in, e.g., a memory readout application. [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 performed numerical studies of the dynamics of thermally induced magnetization reversal of a coupled Heisenberg spin chain, based on the stochastic Landau-Lifshitz-Gilbert equation. Special attention is given to characterizing the transitions between different switching mechanisms (e.g., coherent rotation, soliton, and multidroplet reversals), as the chain length L is increased. Our calculations reveal that the switching time, switching time distribution, and the time evolution behavior of magnetization reversal display nontrivial dependence on the chain length L, and undergo distinct transitions as a function of L. In particular, we found that the switching time distribution is an important determinant of thermal stability, in addition to the mean switching time. [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]

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]