Asymmetric Bias Dependence Research Articles

Tunnel magnetoresistance effect in a magnetic tunnel junction with a B2-Fe3Sn electrode

In magnetic tunnel junctions (MTJs), the tunnel resistance varies as a function of the relative magnetic configuration of the electrode, in an effect called tunnel magnetoresistance (TMR). The material of which the electrodes are composed is of great importance, because TMR is very sensitive to the electronic states of the electrodes. Additionally, structural defects at the interface also have a significant influence on TMR. In this study, we employ B2-Fe3Sn as the magnetic electrode of MTJs. The use of Fe3Sn could solve the problem of lattice mismatch between Fe and MgO. However, the presence of dissimilar atoms in the electrodes or interface oxidation could be a source of defects at the interface. We find that MTJs with Fe3Sn exhibit a TMR of 50% and an asymmetric bias dependence.

Solar‐blind AlGaN MSM photodetectors with 24% external quantum efficiency at 0 V

By utilising an asymmetric metallisation scheme to fabricate an AlGaN-based solar-blind metal–semiconductor–metal photodetector, a zero-bias external quantum efficiency (EQE) of 24% for illumination at 240 nm wavelength from the substrate side was obtained. Moreover, an asymmetric bias-dependence of dark current and EQE is observed. The EQE saturates at about 38% in reverse direction, whereas in forward direction the presence of an internal gain mechanism is indicated.

Bias dependence of tunneling magnetoresistance in magnetic tunnel junctions with asymmetric barriers

The transport properties of magnetic tunnel junctions (MTJs) are very sensitive to interface modifications. In this work we investigate both experimentally and theoretically the effect of asymmetric barrier modifications on the bias dependence of tunneling magnetoresistance (TMR) in single crystal Fe/MgO-based MTJs with (i) one crystalline and one rough interface, and (ii) with a monolayer of O deposited at the crystalline interface. In both cases we observe an asymmetric bias dependence of TMR and a reversal of its sign at large bias. We propose a general model to explain the bias dependence in these and similar systems reported earlier. The model predicts the existence of two distinct TMR regimes: (i) a tunneling regime when the interface is modified with layers of a different insulator, and (ii) a resonant regime when thin metallic layers are inserted at the interface. We demonstrate that in the tunneling regime, negative TMR is due to the high voltage which overcomes the exchange splitting in the electrodes, while the asymmetric bias dependence of TMR is due to the interface transmission probabilities. In the resonant regime, inversion of TMR could happen at zero voltage depending on the alignment of the resonance levels with the Fermi surfaces of the electrodes. Moreover, the model predicts a regime in which TMR has different signs at positive and negative bias, suggesting possibilities of combining memory with logic functions.

Voltage Asymmetry of Spin-Transfer Torques

Experimentally, it is seen that the free magnetic layer of a spin torque transfer (STT) device experiences a larger in-plane torque when a negative (rather than positive) voltage is applied to the fixed layer. This is surprising because magnets do not have any intrinsic asymmetry. In this paper, we 1) provide a simple physical explanation, based on the polarization of fixed layer in the energy range of transport; 2) extend it to explain the asymmetric bias dependence of out-of-plane torque as observed in some of the experiments; and 3) propose an asymmetric STT structure that can lead to a significant difference in the in-plane torques exerted on two contacts, even if they are identical. This effect 3 has not been observed to our knowledge and if demonstrated can find important applications.

Numerical evaluations on the asymmetric bias dependence of magnetoresistance in double spin filter tunnel junctions

In double spin filter tunnel junctions, the spin information is generated and analyzed purely from the spin filtering effect with nonmagnetic electrodes. In this article we numerically evaluate the bias dependence of magnetoresistance in such tunnel junctions (nonmagnetic metal/spin filter/nonmagnetic insulator/spin filter/nonmagnetic metal), particularly in cases when different spin filter materials are utilized. A large magnetoresistance with nonmonotonic and asymmetric bias dependence is expected within the framework of Wentzel–Kramers–Brillouin approximation. We illustrate the systematic influence of tunnel barrier height, tunnel barrier thickness, and exchange energy splitting on magnetoresistance, particularly focusing on the asymmetric behavior of the magnetoresistance bias dependence.

Organic spin valves: effect of magnetic impurities on the spin transport properties of polymer spacers

We report the effect of magnetic impurities in the spacer layer of polymeric spin valves (PSV) with the sandwich configuration of La0.67Sr0.33MnO3(LSMO)/π-conjugated polymer regio-random poly(3-hexyl thiophene)/cobalt (Co), showing giant magnetoresistance (GMR) response. Different deposition rates of Co at the top electrode resulted in two types of devices: one with lower device resistance and linear current–voltage (I–V) characteristics and the other with very low inclusion of Co and exhibiting higher device resistance and nonlinear I–V characteristics. We observed an asymmetric dc bias dependence of magnetoresistance (MR) in devices with more Co inclusion, while for the other type of device, bias dependence was more symmetric. At higher bias, %MR of both types of device showed no significant difference (5–10%), but at low dc bias it ranged between 50 and 160% MR. This can be attributed to the higher tunneling probability of spin-polarized carriers from one ferromagnetic electrode to the other. Magnetic tunnel junction-like features are observed in the devices with greater Co inclusions. Anomalous MR peaks were also observed in these devices and their origin was explained in terms of presence of additional scattering centers around the included metal ions and increased spin relaxation due to high magnetic anisotropy in the system. Both types of PSVs showed a monotonic decrease in MR with temperature at high bias currents.

Negative spin polarization in (La,Sr)CoO3 probed by a magnetic tunnel junction with (La,Sr)MnO3

By using perovskite (La,Sr)MnO3 (LSMO)-based magnetic tunnel junctions with LaAlO3 barrier, we have experimentally revealed the spin state of (La,Sr)CoO3 (LSCO) that was employed as the counterelectrode of the junctions. Inverse tunnel magnetoresistance (TMR) up to −14% was observed at 10K in low-bias measurement. This indicates that t2g↓ electrons are the majority at the Fermi level in the LSCO band structure, taking into account the half-metallic character of LSMO. The temperature dependence of the interfacial spin polarization of LSCO as deduced from the TMR ratio indicates the reduction of the Curie temperature at the interface by 70K. The TMR ratio exhibits an asymmetric bias dependence in qualitative agreement with the calculated spin band structure of LSCO.

High inverted tunneling magnetoresistance in MgO-based magnetic tunnel junctions

Inverted tunneling magnetoresistance, where resistance decreases as the free layer in a magnetic tunnel junction flips its direction of magnetization after saturation, has been observed at zero bias in magnetic tunnel junctions with a thin CoFeB layer in the pinned synthetic antiferromagnetic CoFe∕Ru∕CoFeB stack. Magnetoresistance values as high as −55% at room temperature are measured in MgO-based tunnel junctions when the thickness of the pinned CoFeB layer is 1.5nm. The inverted magnetoresistance is associated with imbalance of the synthetic antiferromagnetic pinned layer. Asymmetric bias dependence with a magnetoresistance sign change is observed for a 0.5nm pinned CoFeB layer.

Spectroscopic Measurement of Spin-dependent Resonant Tunneling through a 3D Disorder: The Case ofMnAs/GaAs/MnAsJunctions

We propose an analytical model of spin-dependent resonant tunneling through a 3D assembly of localized states (spread out in energy and in space) in a barrier. An inhomogeneous distribution of localized states leads to resonant tunneling magnetoresistance inversion and asymmetric bias dependence as evidenced with a set of experiments with MnAs/GaAs(7-10 nm)/MnAs tunnel junctions. One of the key parameters of our theory is a dimensionless critical exponent beta scaling the typical extension of the localized states over the characteristic length scale of the spatial distribution function. Furthermore, we demonstrate, through experiments with localized states introduced preferentially in the middle of the barrier, the influence of an homogeneous distribution on the spin-dependent transport properties.

Influence of annealing on the bias voltage dependence of tunneling magnetoresistance in MgO double-barrier magnetic tunnel junctions with CoFeB electrodes

Double-barrier magnetic tunnel junctions with two MgO barriers and three CoFeB layers exhibiting tunneling magnetoresistance (TMR) values of more than 100% were fabricated. The bias voltage dependence of the TMR ratio is highly asymmetric after annealing at low temperatures, indicating dissimilar CoFeB∕MgO interfaces. The TMR effect decays very slowly for positive bias and is only reduced to half of its maximum value at V1∕2=1.88V when the junctions are processed at 200°C. The largest output voltage, 0.62V, is obtained after annealing at 300°C, a temperature that combines high TMR ratios with a considerable asymmetric bias dependence.

Voltage-controlled tunneling anisotropic magnetoresistance of a ferromagnetic p++-(Ga,Mn)As∕n+-GaAs Zener-Esaki diode

The large tunneling anisotropic magnetoresistance of a ferromagnetic p++-(Ga,Mn)As∕n+-GaAs Zener-Esaki diode is shown in a perpendicular magnetic field over a large temperature and voltage range. Under an applied bias, the tunnel junction transparency is modified, allowing one to continuously tune anisotropic transport properties between the tunneling and the ohmic regimes. Furthermore, an asymmetric bias dependence of the tunneling anisotropic magnetoresistance is also observed: A reverse bias highlights the full (Ga,Mn)As valence-band states contribution, whereas a forward bias only probes part of the density of states and reveals opposite contributions from two valence subbands.

Resonant tunneling magnetoresistance in MnAs∕III-V∕MnAs junctions

This paper investigates the magnetoresistance of micron-sized MnAs/GaAs(AlAs)/MnAs magnetic tunnel junctions. We measure an asymmetric bias dependence of the magnetoresistance in which the negative contribution is attributed to resonant tunneling through a midgap defect band. Within this model we find a spin polarization of 60% for MnAs at the interface with GaAs. Moreover, we show that spin-dependent tunneling is a powerful technique for spectroscopic measurements of defects in a very thin layer.

Bias voltage effect on tunnel magnetoresistance in fully epitaxial MgO double-barrier magnetic tunnel junctions

Double-barrier magnetic tunnel junctions (DMTJs), consisting of a fully epitaxial Fe(001)∕MgO(001)∕Fe(001)∕MgO(001)∕Fe(001) structure, have been deposited onto MgO (001) single-crystal substrates using molecular-beam epitaxy, and have been characterized by measuring the bias voltage effects on both tunneling magnetoresistance (TMR) and conductance. The DMTJs are found to show large TMR ratios of up to 110% and extremely small bias voltage dependence(Vhalf=1.44V under a positive bias application) compared with conventional magnetic tunnel junctions (MTJs) with a single MgO barrier at room temperature. In addition, clear asymmetry is observed in the bias voltage dependence of the TMR ratios with respect to the signs of the bias, which corresponds to the asymmetric bias dependence of the conductance, especially for a parallel magnetization configuration. Such a high Vhalf with a large TMR ratio is relevant for a high-output MTJ cell for future spintronic devices.

Phase switching of a single isomeric molecule and associated characteristic rectification.

Variations in molecular electronic structures related to conformational change are exceedingly attractive because of their key role in the understanding and development of functional processes in molecular electronics and biology. We observed, for the first time, the novel phase switching of a photoactive isomeric molecule, N-(2-mercaptoethyl)-4-phenylazobenzamide (Azo molecule) at a single-molecule level, which exhibits a distinctive change in the conductive characteristic under scanning tunneling microscope (STM) measurement. In comparison with the results obtained by the measurement of photoactive isomerization of the isolated Azo molecule, which was performed also for the first time, the observed characteristics are attributed to the results of the trans and cis phase transformation of the Azo molecule, under the condition of an external electric field and current flow. A specific point is that the potential landscape of the system is controllable by the electric field and provides a conformational stability with asymmetric bias dependence resulting in rectification.

Inverse TMR Effect in Co0.9Fe0.1/SrTiO3/La0.7Sr0.3MnO3 Tunnel Junctions

ABSTRACTMagnetic tunnel junctions of Co0.9Fe0.1/SrTiO3 (STO)/ La0.7Sr0.3MnO3 (LSMO) with a spin-valve structure having an antiferromagnetic MnIr layer have been fabricated by sputtering. Junction magnetoresistance (MR) behavior and its dependence on the bias voltage are examined for junctions with epitaxial STO barrier formed under different sputtering conditions. Spin dependent transport measurements show that these junctions exhibit spin-valve type MR loops with an inverse (positive) MR of the ratio of 14-22 % at 4.2 K. The inverse MR observed is asymmetric with respect to the bias voltage direction. Stoichiometric STO barrier, as characterized by Rutherford backscattering (RBS) analysis, is found to result in very large asymmetric bias dependence, while the junctions with nonstoichiometric STO barrier exhibit the symmetric bias dependence. Our results suggest that the nonstoichiometry of STO barrier modifies the electronic structures of electrode/barrier interfaces, and thereby reducing the asymmetry of bias voltage dependence of junction MR.