Solitary magnon excitations in a one-dimensional antiferromagnet with Dzyaloshinsky-Moriya interactions

We investigate solitary excitations in a model of a one-dimensional antiferromagnet including a single-ion anisotropy and a Dzyaloshinsky-Moriya antisymmetric exchange interaction term. We employ the Holstein-Primakoff transformation, the coherent state ansatz and the time variational principle. We obtain two partial differential equations of motion by using the method of multiple scales and applying perturbation theory. By so doing, we show that the motion of the coherent amplitude must satisfy the nonlinear Schrodinger equation. We give the single-soliton solution.

DTADH and quantum critical phenomena caused by anisotropy and external magnetic field for spin-1/2 Heisenberg diamond chains

Using the transfer matrix renormalization group (TMRG) method, we study the connection between the first derivative of the thermal average of driving-term Hamiltonian (DTADH) and the trace of quantum critical behaviors at finite temperatures. Connecting with the exact diagonalization method, we give the phase diagrams and analyze the properties of each phase for both the ferromagnetic and anti-ferromagnetic frustrated J(3) anisotropy diamond chain models. The finite-temperature scaling behaviors near the critical regions are also investigated. Further, we show the critical behaviors driven by external magnetic field, analyze the formation of the 1/3 magnetic plateau and the influence of different interactions on those critical points for both the ferrimagnetic and anti-ferromagnetic distorted diamond chains.

Spin and charge currents through a quantum dot connected to ferromagnetic leads

We investigate the spin polarized current through a quantum dot connected to ferromagnetic leads in the presence of a finite spin-dependent chemical potential. The effects of the spin polarization of the leads p and the external magnetic field B are studied. It is found that both the magnitude and the symmetry of the current are dependent on the spin polarization of the leads. When the two ferromagnetic leads are in parallel configuration, the spin polarization p has an insignificant effect on the spin current, and an accompanying charge current appears with the increase of p. When the leads are in antiparallel configuration, however, the effect of p is distinct. The charge current is always zero regardless of the variation of p in the absence of B. The peaks appearing in the pure spin current are greatly suppressed and become asymmetric as p is increased. The applied magnetic field B results in an accompanying charge current in both the parallel and antiparallel configurations of the leads. The characteristics of the currents are explained in terms of the density of states of the quantum dot.

In-plane stray field induced spin-filtering in a two-dimensional electron gas under the modulation of surface ferromagnetic dual-gate

Based upon a hybrid ferromagnet/semiconductor structure consisting of two-dimensional electron gas and a pair of surface ferromagnetic stripes on top, we have theoretically investigated the effect of in-plane stray field omitted frequently in previous studies on the spin-dependent ballistic transport properties in hybrid structure. It is demonstrated here that, in combination with an external-controllable electrostatic modulation, the concerned structure shows a similar function as a lateral spin-polarized resonant tunneling device, where the strong spin-filtering effect occurs and nearly single-mode polarization is anticipated for the proper modulation. More importantly, the spin polarity of transmission electron can be easily transferred from one extreme to the other by switching the magnetization of stripes, showing the promising application as an efficient spin aligner in the developing semiconductor spintronics.

Ballistic Electron Transport In Hybrid Ferromagnet/Two-Dimensional Electron Gas Sandwich Nanostructure: Spin Polarization And Magnetoresistance Effect

We have theoretically investigated ballistic electron transport through a combination of magnetic-electric barrier based on a vertical ferromagnet/two-dimensional electron gas/ferromagnet sandwich structure, which can be experimentally realized by depositing asymmetric metallic magnetic stripes both on top and bottom of modulation-doped semiconductor heterostructures. Our numerical results have confirmed the existence of finite spin polarization even though only antisymmetric stray field B-z is considered. By switching the relative magnetization of ferromagnetic layers, the device in discussion shows evident magnetoconductance. In particular, both spin polarization and magnetoconductance can be efficiently enhanced by proper electrostatic barrier up to the optimal value relying on the specific magnetic-electric modulation. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3041477]

Electric tunable of electron spin polarization in hybrid magnetic-electric barrier structures

Electron spin-dependent transport properties have been theoretically investigated in two-dimensional electron gas (2DEG) modulated by the magnetic field generated by a pair of anti-parallel magnetization ferromagnetic metal stripes and the electrostatic potential provided by a normal metal Schottky stripe. It is shown that the energy positions of the spin-polarization extremes and the width of relative spin conductance excess plateau could be significantly manipulated by the electrostatic potential strength and width, as well as its position relative to the FM stripes. These interesting features are believed useful for designing the electric voltage controlled spin filters. (C) 2008 Elsevier B.V. All rights reserved.

Temperature-dependent transport and spin accumulation in a quantum wire with Rashba spin-orbit interaction

We study the theory of temperature-dependent electron transport, spin polarization, and spin accumulation in a Rashba spin-orbit interaction (RSOI) quantum wire connected nonadiabatically to two normal conductor electrode leads. The influence of both the wire-lead connection and the RSOI on the electron transport is treated analytically by means of a scattering matrix technique and by using an effective free-electron approximation. Through analytical analysis and numerical examples, we demonstrate a simple way to design a sensitive spin-transfer switch that operates without applying any external magnetic fields or attaching ferromagnetic contacts. We also demonstrate that the antisymmetry of the spin accumulation can be destroyed slightly by the coupling between the leads and the wire. Moreover, temperature can weaken the polarization and smear out the oscillations in the spin accumulation.

Spin-dependent current noises in transport through coupled quantum dots

Current fluctuations can provide additional insight into quantum transport in mesoscopic systems. The present work is carried out for the fluctuation properties of transport through a pair of coupled quantum dots which are connected with ferromagnetic electrodes. Based on an efficient particle-number-resolved master equation approach, we are concerned with not only fluctuations of the total charge and spin currents, but also of each individual spin-dependent component. As a result of competition among the spin polarization, Coulomb interaction, and dot-dot tunnel coupling, rich behaviors are found for the self- and mutual-correlation functions of the spin-dependent currents.

Correlations between electronic energy bands spin splitting and magnetic profile symmetry of magnetic superlattice

We have theoretically investigated the energy band structures of two typical magnetic superlattices formed by perpendicular or parallel magnetization ferromagnetic stripes periodically deposited on a two-dimensional electron gas (2DEG), where the magnetic profile in the perpendicular magnetization is of inversion anti-symmetry, but of inversion symmetry in parallel magnetization, respectively. We have shown that the energy bands of perpendicular magnetization display the spin-splitting and transverse wave-vector symmetry, while the energy bands of the parallel magnetization exhibit spin degeneration and transverse wave-vector asymmetry. These distinguishing spin-dependent and transverse wave-vector asymmetry features are essential for future spintronics devices applications. (c) 2008 Elsevier B.V. All rights reserved.

Carrier spin relaxation in (Ga, Mn) As at room temperature

In this paper, the excitation energy density dependence of carrier spin relaxation is studied at room temperature for the as-grown and annealed (Ga, Mn) As samples using femtosecond time-resolved pump-probe Kerr spectroscopy. It is found that spin relaxation lifetime of electrons lengthens with increasing excitation energy density for both samples, and the annealed ( Ga, Mn) As has shorter carrier recombination and electron spin relaxation lifetimes as well as larger Kerr rotation angle than the as-grown ( Ga. Mn) As under the same excitation condition. which shows that DP mechanism is dominant in the spin relaxation process for ( Ga, Mn)As at room temperature. The enhanced ultrafast Kerr effect in the annealed (Ga,Mn)As shows the potential application of the annealed ( Ga, Mn) As in ultrafast all-optical spin switches, and also provides a further evidence for the p-d exchange mechanism of the ferromagnetic origin of (Ga, Mn) As.

Analytical model of spin filtering effect and its experimental verification in a forward-biased iron-metal-Al2O3-n-type GaAs tunneling structure under optical spin orientation

An analytical model for the spin filtering transport in a ferromagnetic-metal - Al2O3 - n-type semiconductor tunneling structure has been developed, and demonstrated that the ratio of the helicity-modulated photo-response to the chopped one is proportional to the sum of the relative asymmetry in conductance of two opposite spin-polarized tunneling channels and the MCD effect of the ferromagnetic metal film. The performed measurement in an iron-metal/Al2O3/n-type GaAs tunneling structure under the optical spin orientation has verified that all the aspects of the experimental results are very well in accordance with our model in the regime of the spin filtering. After the MCD effect of the iron film is calibrated by an independent measurement, the physical quantity of Delta G(t)/G(t) (Delta G(t) = G(t)(up arrow) - G(t)(down arrow) is the difference of the conductance between two opposite spin tunneling channels, G(t) =( G(t)(up arrow) + G(t)(down arrow))/2 the averaged tunneling conductance), which concerns us most, can be determined quantitatively with a high sensitivity in the framework of our analytical model. Copyright (c) EPLA, 2008.

Room-temperature highly anisotropic ferromagnetism and uniaxial spin amplification in (In,Mn)As quantum dots

The hole-mediated ferromagnetism in (In,Mn)As quantum dots is investigated using the k center dot p method and the mean field model. It is found that the (In,Mn)As quantum dot can be ferromagnetic at room temperature when there is one hole in the dot. For the spherical quantum dots, the Curie temperature decreases as the diameter increases, and increases as the effective composition of magnetic ions increases. It is interesting to find that the (In,Mn)As oblate quantum dot has highly anisotropic Zeeman splitting and ferromagnetism due to the spin-orbit coupling effect, which can be used as an uniaxial spin amplifier. (c) 2008 American Institute of Physics.

Thermodynamic properties of Cu-3(CO3)(2)(OH)(2) and the spin-1/2 distorted diamond Heisenberg chain

The thermodynamic properties of the spin-1/2 diamond quantum Heisenberg chain model have been investigated by means of the transfer matrix renormalization group (TMRG) method. Considering different crystal structures, by changing the interactions among different spins and the external magnetic fields, we first investigate the magnetic susceptibility, magnetization, and specific heat of the distorted diamond chain as a model of ferrimagnetic spin systems. The susceptibility and the specific heat show different features for different ferromagnetic (F) and antiferromagnetic (AF) interactions and different magnetic fields. A 1/3 magnetization plateau is observed at low temperature in a magnetization curve. Then, we discuss the theoretical mechanism of the double-peak structure of the magnetic susceptibility and the three-peak structure of the specific heat of the compound Cu-3(CO3)(2)(OH)(2), on which an elegant measurement was performed by Kikuchi [Phys. Rev. Lett. 94, 227201 (2005)]. Our computed results are consistent with the main characteristics of the experimental data. Meanwhile, we find that the double-peak structure of susceptibility can be found in several different kinds of spin interactions in the diamond chain. Moreover, a three-peak behavior is observed in the TMRG results of magnetic susceptibility. In addition, we perform calculations relevant for some experiments and explain the characteristics of these materials. (c) 2007 American Institute of Physics.

Spin relaxation and dephasing mechanism in (Ga,Mn)As studied by time-resolved Kerr rotation

Spin dynamics in (Ga,Mn)As films grown on GaAs(001) was investigated by Time-resolved magneto-optical Kerr effect. The Kerr signal decay time of (Ga,Mn)As without external magnetic field applied was found to be several hundreds picoseconds, which suggested that photogenerated polarized holes and magnetic ions are coupled as a ferromagnetic system. Nonmonotonic temperature dependence of relaxation and dephasing (R&D) time and Larmor frequency manifests that Bir-Aronov-Pikus mechanism dominates the spin R&D time at low temperature, while D'yakonov-Perel mechanism dominates the spin R&D time at high temperature, and the crossover between the two regimes is Curie temperature.

Structure, magnetization, and low-temperature spin dynamic behavior of zincblende Mn-rich Mn(Ga)As nanoclusters embedded in GaAs

We have fabricated a set of samples of zincblende Mn-rich Mn(Ga)As clusters embedded in GaAs matrices by annealing (Ga,Mn)As films with different nominal Mn content at 650 degrees C. For the samples with Mn content no more than 4.5%, the Curie temperature reaches nearly 360 K. However, when Mn content is higher than 5.4%, the samples exhibit a spin-glass-like behavior. We suggest that these different magnetic properties are caused by the competing result of dipolar and Ruderman-Kittel-Kasuya-Yosida interaction among clusters. The low-temperature spin dynamic behavior, especially the relaxation effect, shows the extreme creeping effect which is reflected by the time constant tau of similar to 10(11) s at 10 K. We explain this phenomenon by the hierarchical model based on the mean-field approach. We also explain the memory effect by the relationship between the correlation function and the susceptibility.