Spin-polarized Tunneling Current Research Articles

Precession coupled spin current in spin torque driven magnetic tunnel junctions

A spin-torque switchable magnetic tunnel junction contains two ferromagnetic electrodes across a barrier that supports spin-polarized tunnel current. The spin-torque induced magnetic switching of its more agile, or “free” layer provides the “write” mechanism. Often the dynamics of the non-switching “reference” layer is also important. Here, we illustrate such dynamics involving both the free and the reference layers by using an exchange-coupled two-macrospin-moment numerical model, described by a set of Landau–Lifshitz–Gilbert (LLG) equations, together with a stochastic Langevin-field for finite temperature. Damping-like spin-transfer torque is included for both moments. In steady-state, the coupled precession is shown to reduce effective spin-current delivered to the free layer due to a precessional resonant spin-current back flow. This back-flow of spin current preferentially affects the parallel state dynamics. It is not directly related to the reference layer’s thermal stability, nor its spin-torque switching threshold, as determined by the total anisotropy energy and magnetic volume. Rather, the spin-current reduction relates primarily to the matching of precession frequency between the free- and the reference-layer. Therefore, a desirable materials choice is to avoid anisotropy fields giving the free and the reference layer similar dynamic frequencies, so as to prevent such resonance-related spin-current loss.

Switching Magnetism and Superconductivity with Spin-Polarized Current in Iron-Based Superconductor.

We explore a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single-crystal Sr_{2}VO_{3}FeAs shows that a spin-polarized tunneling current can switch the Fe-layer magnetism into a nontrivial C_{4} (2×2) order, which cannot be achieved by thermal excitation with an unpolarized current. Our tunneling spectroscopy study shows that the induced C_{4} (2×2) order has characteristics of plaquette antiferromagnetic order in the Fe layer and strongly suppresses superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases the C_{4} state. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors.

Role of coherence in transport through engineered atomic spin devices

We give a further step in the quantum mechanical description of engineered atomic spin structures by deriving a master equation of the Redfield type that governs the dynamics of the atomic spin density matrix. By generalizing this approach to charge-specific density matrices, we are able to describe magnetic transport quantities, such as the average inelastic current and the shot noise, accessible by tunneling spectroscopy. Our method suitably describes moderate lead-atom coupling regimes where quantum coherence effects cannot be disregarded. We contrast our approach with the existing descriptions in terms of rate equations and show examples where coherence effects are crucial to understand the physics of spin-polarized tunnel current through spin structures.

Control of the non-stationary spin-polarized tunneling currents by applied bias changing

We reveal that for the single Anderson impurity localized between non-magnetic leads of the tunneling contact $"$magnetic$"$ state can be distinguished from the $"$paramagnetic$"$ one only by the analysis of the non-stationary system characteristics or the behavior of the second order correlation functions for the localized electrons occupation numbers. We investigate the response of the system to the sudden shift of the applied bias and to the switching $"$on$"$ the coupling to the second lead of the tunneling contact. We demonstrate that in addition to the changes of the relaxation regimes and typical relaxation time scales, non-stationary spin-polarized currents flowing in the both leads are present in the system. Spin polarization and direction of the non-stationary currents in each lead can be simultaneously inverted by the sudden changing of the applied bias voltage.

Nonequilibrium Effect in Ferromagnet-Insulator-Superconductor Tunneling Junction Currents

Nonequilibrium effect due to the imbalance in the number of the ? and ? spin electrons has been studied for the tunneling currents in the ferromagnet-insulator-superconductor (FIS) tunneling junctions within a phenomenological manner. It has been stated how the nonequilibrium effect should be observed in the spin-polarized quasiparticle tunneling currents, and pointed out that the detectable nonequilibrium effect could be found in the FIS tunneling junction at 77 K using HgBa2Ca2Cu3O8+? (Hg-1223) high-Tc superconductor rather than Bi2Sr2CaCu2O8+? (Bi-2212) one.

On the valve nature of a monolayer of aligned molecular magnets in tunneling spin-polarized electrons: Towards organic molecular spintronics

We form a monolayer of magnetic organic molecules and immobilize their moments pointing either upwards or downwards with respect to the substrate through an electrostatic-binding process. Such a monolayer is probed with a scanning tunneling microscope tip, which is also magnetized with the magnetization vector pointing towards (or away from) apex of the tip. From spin-polarized tunneling current, we show that the current was higher when magnetization vectors of the tip and molecules were parallel as compared to that when they were anti-parallel. We show that for tunneling of spin-polarized electrons, aligned organic molecular magnets can act as a valve.

Detecting Larmor Precession of a Single Spin with a Spin-Polarized Tunneling Current

Detection and manipulation of a single spin is essential for spin-based quantum information and computation. We propose an experimental method to detect the Larmor precession of a single spin with a spin-polarized tunneling current. The calculation demonstrates that the modulation of the tunneling current is significantly increased as compared with the non-spin-polarized current. The signal-to-noise ratio in the current power spectrum will be dramatically improved by this method. The longitudinal relaxation time can be measured by temporal control of spin orientation with pulses of magnetic field or radio frequency electromagnetic wave, while the transverse relaxation time can be derived by analyzing the resonant peak profile with a spectrum analyzer. Such ability in characterizing spin relaxation is of particular interest in providing insights into the coherence/decoherence of a single spin.

Spin-polarized tunneling current through a thin film of a topological insulator in a parallel magnetic field

We calculate the tunneling conductance \sigma between the surface states on the opposite sides of the ultra-thin film of a topological insulator in a parallel magnetic field B_y. The parallel magnetic produces a relative shift of the in-plane momenta of the two surfaces states. An overlap between the shifted Fermi circles and their spin structure define an unusual dependence of the tunneling conductance \sigma(B_y) on the magnetic field. Because the spin of the electronic surface states in topological insulators is locked with momentum, the spin-polarization of the tunneling current can be controlled by magnetic field B_y.

Antiferromagnetic coupling across silicon regulated by tunneling currents

We report on the enhancement of antiferromagnetic coupling in epitaxial Fe/Si/Fe structures by voltage-driven spin-polarized tunneling currents. Using the ballistic electron magnetic microscopy, we established that the hot-electron collector current reflects magnetization alignment and the magnetocurrent exceeds 200% at room temperature. The saturation magnetic field for the collector current corresponding to the parallel alignment of magnetizations rises up with the tunneling current, thus demonstrating stabilization of the antiparallel alignment and increasing antiferromagnetic coupling. We connect the enhancement of antiferromagnetic coupling with local dynamic spin torques mediated by spin-polarized tunneling electrons.

Indirect Control of Antiferromagnetic Domain Walls with Spin Current

The indirect controlled displacement of an antiferromagnetic domain wall by a spin current is studied by Landau-Lifshitz-Gilbert spin dynamics. The antiferromagnetic domain wall can be shifted both by a spin-polarized tunnel current of a scanning tunneling microscope or by a current driven ferromagnetic domain wall in an exchange coupled antiferromagnetic-ferromagnetic layer system. The indirect control of antiferromagnetic domain walls opens up a new and promising direction for future spin device applications based on antiferromagnetic materials.

Bias dependence of spin-polarized tunnel current through Fe/GaAs and Fe/GaAs/Fe junctions

Spin polarization of the tunnel current under finite bias voltage is studied theoretically for Fe/GaAs junctions with Schottky barriers and Fe/GaAs/Fe tunnel junctions, using a realistic tight-binding model and linear response theory. Calculating the spin polarization of the tunnel current across the interface as a function of bias voltage, we show that the spin-polarized interface resonant states within the Schottky barrier significantly influence spin-dependent tunnelling. The position of the resonant states depends on the Schottky barrier height, and the spin polarization of the tunnel current can be negative for a rather wide range of bias voltage, depending on the Schottky barrier height. The magnetoresistance of Fe/GaAs/Fe tunnel junctions is also negative under high bias voltage, because of the interfacial resonant states in the GaAs layer. The present results demonstrate the importance of controlling Schottky barrier formation to obtain high spin polarization in Fe/GaAs junctions.

The critical role of the barrier thickness in spin filter tunneling

Spin filter tunneling is considered in the low bias limit as functions of the temperature dependent barrier parameters. We demonstrate the generation of spin polarized tunneling currents in relation to the magnetic order parameter, and discuss how an interfacially suppressed order parameter leads to a temperature dependent tunneling current asymmetry. Analyzing the full parameter space reveals that the often overlooked barrier thickness plays a critical role in spin filter tunneling. With all else fixed, thicker barriers yield higher spin polarization, and allow a given polarization to be achieved at higher temperatures. This insight may open the door for new materials to serve as spin filter barriers.

Spin-dependent current in resonant tunneling diode with ferromagnetic GaMnN layers

The spin-polarized tunneling current through a double barrier resonant tunneling diode (RTD) with ferromagnetic GaMnN emitter/collector is investigated theoretically. Two distinct spin splitting peaks can be observed at current–voltage ( I–V) characteristics at low temperature. The spin polarization decreases with the temperature due to the thermal effect of electron density of states. When charge polarization effect is considered at the heterostructure, the spin polarization is enhanced significantly. A highly spin-polarized current can be obtained depending on the polarization charge density.

Spin-polarized tunneling through potential barriers at ferromagnetic metal/semiconductor Schottky contacts

A model for electron tunneling through a space-charge induced potential barrier at a semiconductor/ferromagnetic metal Schottky contact is developed and applied to the exploration of the bias dependence of the spin-polarized tunneling current. It is found that significant bias dependence of the spin polarization of the current can result from changes in the shape of the potential barrier due to the applied voltage. Specifically, we show that the dependence of the transmission coefficient on the shape of the barrier potential can lead to a nonmonotonic bias dependence of the spin current and may result in a reversal of the sign of the spin-current polarization at small voltages. Numerical results are presented for $\mathrm{Ga}\mathrm{As}∕\mathrm{Fe}$ Schottky contacts.

Switching thresholds in MTJ using SPICE model – Effects of spin and Ampere torques

AbstractSpin torque due to spin polarized tunneling current can be used to switch the free layer in a magnetic tunnel junction (MTJ). This current also gives rise to an Ampere torque, which influences the switching threshold of the MTJ. We modified the Landau–Lifschitz–Gilbert equation (LLGE) to include an Ampere torque term and solved for the magnetization dynamics under the single domain approximation using a linear solver in SPICE. We also extend the model to a square array of MTJs to study the effect of nearest neighbour interactions in addition to effects like demagnetization and magnetostatic interactions with the pinned layer. The interlayer exchange field between the free and pinned layers of a MTJ and the spin torque are competing factors that decide the threshold current density for switching the MTJ. We used a two current model to study the effects of barrier height and barrier thickness on spin torque and exchange energy. We observe that both the spin torque and exchange energy decrease with an increase in barrier height (for ferromagnetic coupling) and barrier thickness. We find that the inclusion of Ampere torque causes a reduction in the switching current. Varying the thickness of MgO and Al2O3 barriers allows us to minimize the switching threshold voltage. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Subnanosecond switching of local spin-exchange coupled to ferromagnets

The dynamics of a single spin that is embedded in a tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. By using time-dependent equations of motion for the spin, which is under the influence of a spin-polarized tunneling current, it is shown that the magnetic field induced by bias voltage pulses allows for a subnanosecond switching of the local spin and the possibility of spin reversal is illustrated. Furthermore, it is shown that the time evolution of the Larmor frequency sharply peaks around the spin-flip event and it is argued that this feature can be used as an indicator for the spin flip.

Spin-polarized tunneling current through a ferromagnetic insulator between two metallic or superconducting leads

Using the Keldysh formalism (Zh. Eksp. Teor. Fiz. 47, 1515 (1964) [Sov. Phys. JETP 20, 1018 (1965)]), the tunneling current through a hybrid structure where a confined magnetic insulator (I) is sandwiched between two nonmagnetic leads is calculated. The leads can be either normal metals (M) or superconductors (S). Each region is modeled as a single band in tight-binding approximation in order to understand the formation of the tunneling current as clearly as possible. The tunneling process itself is simulated by a hybridization between the lead and insulator conduction bands. The insulator is assumed to have localized moments which can interact with the tunneling electrons. This is described by the Kondo lattice model and treated within an interpolating self-energy approach. For the superconductor, the mean-field BCS theory is used. The spin polarization of the current shows a strong dependence both on the applied voltage and on the properties of the materials. Even for this idealized three band model, there is a qualitative agreement with experiment.

Magnetoresistance in asymmetric ferromagnet/superconductor/ferromagnet double tunnel junctions

We investigate theoretically the spin-dependent transport in a ferromagnet/superconductor/ferromagnet double tunnel junction taking into account the junction asymmetry. The spin-polarized tunneling current induces spin imbalance in the superconductor giving rise to suppression of superconductivity. We find that this pair breaking effect exhibits sensitive dependence on the asymmetric natures of the tunnel junction. We show, in particular, that the junction asymmetry can result in substantial changes in the magnetoresistive behavior.

Spin-Polarized Scanning Tunneling Microscopy Study on Charge Ordering of Reconstructed Fe3O4(001) Film Surfaces

Fe3O4(001) film surfaces having a (√2 ×√2)R45° reconstruction are studied by scanning tunneling microscopy (STM) and spin-polarized STM (SP-STM). The reconstruction is induced by ordered arrays of oxygen vacancy at B-terminated surfaces as revealed by STM. Fe cation rows having a wavelike structure oriented along the [110] direction with a separation of 0.6 nm are observed. A single Fe cation with a 0.3 nm periodicity within the cation rows is resolved. Instead of a single Fe cation, SP-STM imaged an alternation of high and low corrugations representing Fe3+–Fe3+ dimers and Fe2+–Fe2+ dimers, respectively. The resolving of Fe dimers having different spin states by SP-STM is due to the contribution of spin-polarized tunneling current. The Fe3+–Fe3+ and Fe2+–Fe2+ dimers have a separation of about 0.6 nm in the [110] direction, while in the [110] direction these dimers alternate by about 0.65 and 0.55 nm. The separation is consistent with the wavelike row structure on the (√2×√2)R45° reconstructed Fe3O4(001) film surfaces.

Finite Tunneling Spin Polarization at the Compensation Point of Rare-Earth-Metal–Transition-Metal Alloys

We show, using superconducting tunneling spectroscopy and tunneling magnetoresistance measurements, that ferrimagnetic alloys of Co and Gd can exhibit both positive and negative spin polarization depending on temperature and composition. These observations can be understood by considering the relative contributions of independent spin-polarized tunneling currents from the rare-earth-metal and transition-metal subnetwork magnetizations, which are coupled antiferromagnetically. At the compensation point of the alloy, where the subnetwork magnetizations are equal and the alloy has nearly zero net magnetization, nevertheless large tunneling spin polarization is observed.