Ferromagnetic Terminals Research Articles

Modulation of magnetocrystalline anisotropy in FePt/PbTiO3 heterostructures by ferroelectric polarization

Reducing the power consumption required for magnetization reversal is an urgent problem for spin storage device. Electric-field control of the magnetic anisotropy energy (MAE) using multiferroics materials is a promising method to solve this problem. Based on density functional theory, we investigated the effects of the ferroelectric polarization on MAE of FePt/PbTiO3 multiferroic heterostructures. The MAEs of FePt monolayer with different polarization intensity are calculated. Our results indicated that the interfaces coupling between ferroelectric terminals and ferromagnetic terminals have a very large impact on the MAE of FePt monolayer. Moreover, with the reversal of the polarization orientation of ferroelectric PbTiO3 films, the MAE of ferromagnetic FePt monolayer has a monotonous but non-linear change. We demonstrated that the reversal of the polarization orientation results in a redistribution of charge density at the interface, thus resulting in a monotonic change in MAE with polarization intensity. It is provided an effective way to modulate the MAE by controlling the polarization intensity of ferroelectric layers.

Reconfigurable spin logic device using electrochemical potentials

A reconfigurable spin logic device is realized using an asymmetric quantum well displaying strong Rashba spin splitting. This design is developed to remove the inefficient spin injection process and to utilize the Rashba-effect-induced electrochemical potential shifts. In this logic device, two ferromagnetic electrodes were deposited on two quantum well channels, respectively. The magnetization orientations of the ferromagnetic electrodes determine the function of the logic operation, and the polarity of the charge current is assigned to the logic input. The output voltage corresponds to the difference between potentials of the two ferromagnetic terminals, which read the electrochemical potentials of the individual quantum well channels. The detected signal induced by the Rashba channel is observed to be two orders of magnitude greater than the spin injection signal from the ferromagnetic source into the semiconductor channel. Four logic functions, i.e., the AND, OR, NAND, and NOR operations, are illustrated for a single device up to room temperature.

Photon and phonon-assisted shot noise in a molecular quantum-dot-ferromagnetic system in the Coulomb blockade regime

The dynamic shot noise in a molecular quantum dot connected to two non-collinear ferromagnetic terminals under the perturbation of ac fields has been investigated by the nonequilibrium Green’s function approach. The formulas of current, current correlation, and shot noise have been presented in the weak electron-phonon (el-ph) interaction regime to address the cooperated behaviors generated by the photon and phonon perturbation. The photon and phonon perturbations induce quite different current correlations, and the noise suppression occurs unambiguously by increasing the phonon energy in the valley regime. The suppression of shot noise comes from drawing the unbalanced current correlation towards the balanced one, where coherent current correlation takes major role. The suppression of photon-assisted shot noise can be completed mainly by rotating polarization angle of the terminals, and increasing the phonon energy ħω 0. The Fano factor is enhanced considerably by the el-ph interaction, and it is also enhanced by increasing the polarization angle from θ = 0 to θ = π.

Current Induced Heat Generation in Ferromagnet-Quantum Dot-Ferromagnet System.

We study the heat generation in ferromagnet-quantum dot-ferromagnet system by the non-equilibrium Green’s functions method. Heat generation under the influence of ferromagnet leads is very different compared with a system with normal metal leads. The significant effects in heat generation are caused by the polarization angle θ associated with the orientation of polarized magnetic moment of electron in the ferromagnetic terminals. From the study of heat generation versus source drain bias (Q-eV) curves, we find that the heat generation decreases as θ increases from 0 to 0.7π. The heat generation versus gate voltage (Q-eVg) curves also display interesting behavior with increasing polarization angle θ. Meanwhile, heat generation is influenced by the relative angle θ of magnetic moment in the ferromagnetic leads. These results will provide theories to this quantum dot system as a new material of spintronics.

Full counting statistics of generic spin entangler with quantum dot-ferromagnet detectors

Entanglement between spatially separated electrons in nanoscale transport is a fundamental property, yet to be demonstrated experimentally. Here we propose and analyse theoretically the transport statistics of a generic spin entangler coupled to a hybrid quantum dot-ferromagnet detector system. We show that the full distribution of charges arriving at the ferromagnetic terminals provides complete information on the spin state of the particles emitted by the entangler. This provides means for spin entanglement detection via electrical current correlations, with optimal measurement strategies depending on the a priori knowledge of ferromagnet polarization and spin-flip rates in the detector dots. The scheme is exemplified by applying it to Andreev and triple dot entanglers.

Dynamic spin-polarized shot noise in a quantum dot coupled to ferromagnetic terminals under the perturbation of ac fields

We have investigated the shot noise in the mesoscopic system composed of a quantum dot (QD) coupled to ferromagnetic terminals under the perturbation of ac fields. The shot noise has been derived using the nonequilibrium Green's function (NGF) technique to describe the spin polarization effect along with photon absorption and emission processes in the Coulomb blockade regime. We have examined the influence of spin polarization on the shot noise under the perturbation of ac fields in the nonadiabatic regime. The Coulomb blockade effect results in the modification of shot noise compared with the noninteracting case. The spin orientation contributes a spin valve effect for controlling electron tunnelling through this QD, and different resonant forms appear around the Coulomb blockade channel. The photon-assisted spin-splitting and spin-polarization effect contributes a photon-assisted spin valve to adjust the electron tunnelling current and shot noise. The spin-polarization effect varies the value of the Fano factor. However, it does not change the noise type from sub-Poissonian to super-Poissonian.

Proposal for quantum spin tomography in ferromagnet-normal conductors

We present a theory for a complete reconstruction of nonlocal spin correlations in ferromagnet-normal conductors. This quantum spin tomography is based on cross-correlation measurements of electric currents into ferromagnetic terminals with controllable magnetization directions. For normal injectors, nonlocal spin correlations are universal and strong. The correlations are suppressed by spin-flip scattering and, for ferromagnetic injectors, by increasing injector polarization.

Enhanced spin injection efficiency in a four-terminal quantum dots system

Within the scheme of quantum rate equations, we investigate the spin-resolved transport through a double quantum dot system with four ferromagnetic terminals. We find that the injection efficiency of spin-polarized electrons can be significantly improved. When the magnetization in one of four ferromagnetic terminals is antiparallel with the other three, the current polarization rate through one dot can be greatly enhanced, accompanied by the drastic decrease through the other one. The mechanism is the exchange interaction between electrons in the two dots, which can be a promising candidate for the improvement of the spin injection efficiency.

Spin transport theory in ferromagnet/semiconductor systems with noncollinear magnetization configurations

We present a comprehensive theory of spin transport in a nondegenerate semiconductor that is in contact with multiple ferromagnetic terminals. The spin dynamics in the semiconductor is studied during a perturbation of a general, noncollinear magnetization configuration and a method is shown to identify the various configurations from current signals. The conventional Landauer-B\"uttiker description for spin transport across Schottky contacts is generalized by the use of a nonlinearized $I\text{\ensuremath{-}}V$ relation, and it is extended by taking into account noncoherent transport mechanisms. The theory is used to analyze a three terminal lateral structure where a significant difference in the spin accumulation profile is found when comparing the results of this model with the conventional model.

Electric readout of magnetization dynamics in a ferromagnet-semiconductor system

We apply an analysis of time-dependent spin-polarized current in a semiconductor channel at room temperature to establish how the magnetization configuration and dynamics of three ferromagnetic terminals, two of them biased and third connected to a capacitor, affect the currents and voltages. In a steady state, the voltage on the capacitor is related to spin accumulation in the channel. When the magnetization of one of the terminals is rotated, a transient current is triggered. This effect can be used for electrical detection of magnetization reversal dynamics of an electrode or for dynamical readout of the alignment of two magnetic contacts.

Semiclassical theory of spin-polarized shot noise in mesoscopic diffusive conductors

We study fluctuations of spin-polarized currents in a three-terminal spin-valve system consisting of a diffusive normal metal wire connected by tunnel junctions to three ferromagnetic terminals. Based on a spin-dependent Boltzmann-Langevin equation, we develop a semiclassical theory of charge and spin currents and the correlations of the currents fluctuations. In the three terminal system, we show that current fluctuations are strongly affected by the spin-flip scattering in the normal metal and the spin polarizations of the terminals, which may point in different directions. We analyze the dependence of the shot noise and the cross-correlations on the spin-flip scattering rate in the full range of the spin polarizations and for different magnetic configurations. Our result demonstrate that noise measurements in multi-terminal devices allow to determine the spin-flip scattering rate by changing the polarizations of ferromagnetic terminals.