Spin-dependent Tunneling Research Articles

Determination of layer-resolved composition, magnetization, and electronic structure of an Fe/MgO tunnel junction by standing-wave core and valence photoemission

Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attention due to its potential applications in various spintronic devices. However, the interfacial magnetic and electronic structure of a prototypical realization of this in Fe/MgO/Fe and the effective band gap of the MgO layer are still under debate. In order to resolve these issues, we have employed standing-wave excited core and valence photoemission, as well as core-level magnetic circular dichroism (MCD) in photoemission, to study the Fe/MgO interface with subnanometer depth resolution. For our synthetic procedure, we show that the Fe/MgO interface is linearly intermixed in composition over a length of \ensuremath{\sim}8 \AA{} (\ensuremath{\sim}4 monolayers) and that there is a magnetic dead layer \ensuremath{\sim}2--3 \AA{} thick. The unambiguous extraction of depth-resolved density of states (DOS) reveals that the interfacial layer composition is mostly metallic and nonmagnetic FeO${}_{\mathrm{x}}$, with x \ensuremath{\cong} 1, which accounts for a smaller magnetoresistance compared to theoretical predictions. The formation of the magnetic dead layer (FeO) at the interface should also reduce the tunneling spin polarization. The analysis of our data also shows a clear valence band edge of ultrathin MgO layer at \ensuremath{\sim}3.5 eV below the Fermi level (${E}_{\mathrm{F}}$) that is very close to that of single crystal bulk MgO. An analysis that does not consider the interdiffused region separately exhibits the valence band edge for MgO layer \ensuremath{\sim}1.3 eV below ${E}_{\mathrm{F}}$, which is significantly closer to the MgO barrier height estimated from magnetotransport measurements and further suggests that the Fe/MgO interdiffusion effectively reduces the MgO band gap.

Enhanced low-field magnetoresistance in LSMO/SFO composite system

The granular composites of (1−x)La0.7Sr0.3MnO3/xSrFe12O19 [(1−x)LSMO/xSFO] were prepared. The magnetic, electrical, and magnetoresistive properties of the composites were investigated systematically. Two magnetic transitions originating from LSMO and SFO are observed for x=0.05, 0.10, and 0.20. The addition of hard-magnetic SFO ferrite leads to the increased substantially resistivity of the composites and the shift of insulating-metallic transition temperature TIM correlated with LSMO. Enhanced low-field magnetoresistance (LFMR) in the composites can be mainly attributed to the enhanced spin disorder and spin-dependent tunneling at LSMO grain boundaries induced by the interaction between LSMO and SFO ferrite. The transport mechanisms in detail are analyzed in LSMO/SFO composite system.

Spin-polarized transport through ZnMnSe/ZnSe/ZnBeSe heterostructures

Using the transfer matrix method and Airy function, the spin-dependent tunneling through the ZnMnSe/ZnSe/ZnBeSe structure was investigated theoretically. The electron tunneling determined by the applied bias, external magnetic field, and spin orientations exhibited some interesting and complex features. It was found that the magnetic field could suppress the spin-up current, but enhance the spin-down current. Furthermore the spin-flip of current could be realized by changing the applied bias slightly. Therefore, it can be believed that our structure could behave as a good spin-filter.

Magnetoresistance and Exchange Bias Effect of (Co2MnSi)1-X-(NiO)X Nanocomposites

The (Co2MnSi)1-x-(NiO)x(x = 0.0, 0.1, 0.2, 0.3) nanocomposites were fabricated by mechanical alloying using Co2MnSi Heusler alloy and NiO nanoparticles. It is revealed that antiferromagnetic NiO nanocrystallines dramatically enhances the magnetoresistance of the nanocomposites more than 20 times larger than that of the NiO-free Co2MnSi alloys at 300 K. The Exchange bias effect of the nanocomposites suggests that the spin-dependent tunneling and scattering at the interfaces of ferromagnetic/antiferromagnetic are responsible for the enhancement of the magnetoresistance.

Correlation between symmetry-selective transport and spin-dependent resonant tunneling in fully epitaxial Cr/ultrathin-Fe/MgO/Fe(001) magnetic tunnel junctions

Spin-dependent resonant tunneling was investigated in fully epitaxial Cr(001)/ultrathin Fe(001)/MgO(001)/Fe(001) pseudo double-barrier magnetic tunnel junctions (MTJs) by varying structural coherence of the MgO barrier through postdeposition annealing and Mg layer insertion. It was clearly demonstrated that not only the flatness of the Fe quantum-well layer, but also the structural coherence of the MgO barrier caused the appearance of sharp resonant conductance peaks which were not obtained for spin-dependent resonant tunneling in amorphous AlO-barrier MTJs. The symmetry-selective transport via MgO barrier plays a crucial role for the resonant tunneling as well as large tunneling magnetoresistance.

Resonant tunnel magnetoresistance in double-barrier planar magnetic tunnel junctions

We present a theoretical approach to calculate the spin-dependent current and tunnel magnetoresistance (TMR) in a double-barrier magnetic tunnel junction (DMTJ), in which the magnetization of the middle ferromagnetic metal layer can be aligned parallel or antiparallel in relation to the fixed magnetizations of the left and right ferromagnetic electrodes. The electron transport through the DMTJ is considered as a three-dimensional problem, taking into account all transmitting electron trajectories as well as the spin-dependent momentum conservation law. The dependence of the transmission coefficient and spin-polarized currents on the applied voltage is derived as an exact solution to the quantum-mechanical problem for the spin-polarized transport. In the range of the developed physical model, the resonant tunneling, nonresonant tunneling, and enhanced spin filtering can be explained; the simulation results are in good agreement with experimental data.

Resonant Tunnel Magnetoresistance in a Double Magnetic Tunnel Junction

We present quasi-classical approach to calculate a spin-dependent current and tunnel magnetoresistance (TMR) in double magnetic tunnel junctions (DMTJ) FML/I/FMW/I/FMR, where the magnetization of the middle ferromagnetic metal layer FMW can be aligned parallel or antiparallel with respect to the fixed magnetizations of the left FML and right FMR ferromagnetic electrodes. The transmission coefficients for components of the spin-dependent current, and TMR are calculated as a function of the applied voltage. As a result, we found a high resonant TMR. Thus, DMTJ can serve as highly effective magnetic nanosensor for biological applications, or as magnetic memory cells by switching the magnetization of the inner ferromagnetic layer FMW.

Spin-Polarized Tunneling between Optically Pumped GaAs(110) Surface and Spin-Polarized Tip

We performed spin-polarized scanning tunneling spectroscopy (SP-STS) on the optically pumped GaAs(110) surface with the Fe-coated W tip to investigate the spin-polarized tunneling effect between them. The optical effects that could be superimposed on the spin-dependent signal were carefully investigated with the nonmagnetic W tip. In the SP-STS experiment, the intensity and circular-polarization of the light were simultaneously modulated, and the modulation signals of the tunneling current were compared in order to distinguish the spin dependent component. We demonstrated that the spin-dependent tunneling effect could be detected when we carefully choose the energy range to avoid the parasitic optical effect.

Proposed spin-qubit controlled-not gate robust against noisy coupling

We propose an implementation of the two-qubit gate in a quantum dot spin qubit system which is immune to charge noise problems. Our proposed implementation, if it could be realized in a physical system, would have the advantage of being robust against uncertainties and fluctuations in the tunnel coupling and barrier gate voltage pulse area. The key idea is to introduce an auxiliary dot and use an analog to the stimulated Raman adiabatic passage pulse sequence in three-level atomic systems, often referred to in the context of electron transport in quantum dot systems as Coherent Tunneling by Adiabatic Passage. Spin-dependent tunneling opens the possibility of performing two-qubit gate operations by this method.

Relaxation of low-temperature magnetoresistance and magnetization of polycrystalline (La0.5Eu0.5)0.7Pb0.3MnO3

Hysteresis and relaxation of magnetoresistance and magnetization of substituted (La0.5Eu0.5)0.7Pb0.3MnO3 lanthanum manganite in a low-temperature region (<40 K) are investigated. It is shown that at these temperature features of the magnetoresistive effect are determined mainly by spin-dependent tunnelling of carriers via insulating grain boundaries. As was demonstrated previously, the grain boundaries may be antiferromagnetically ordered. Therefore, relaxation of magnetization and resistance is determined by the processes of relative orientation of the magnetic moments of ferromagnetic domains neighbouring the antiferromagnetic boundary of ferromagnetic domains under the action of temperature fluctuations. It is shown that relaxation follows the logarithmic law within the time interval t ∼ 102–3×103 s. A comparison between time evolutions of the magnetic moment and resistance shows that magnetoresistance and magnetization are related as δR = δMn, where n = 2.5. The obtained value n is close to the characteristic value n = 2 for tunnel magnetoresistance of granular ferromagnetic metal/insulator systems.

Non-equilibrium Green's function approach to the quasiparticle transport and magnetoresistance in hybrid magnetic tunnel junctions

A generalized approach to study quasiparticle transport across hybrid magnetic tunnel junctions (MTJs) is formulated using the non-equilibrium Green's function technique. This formalism allows for arbitrary thicknesses of the electrodes and the central scattering region comprising of materials with multiple electronic bands, and incorporates the many body interactions present in the electrode regions. While the method can be used to study the transport characteristics of various types of MTJs, we have used it to study the tunneling characteristics and magnetoresistance (MR) of MTJs in which s– f interaction is present at the electrode layers. It is also used to study the transport characteristics of MTJs with hybrid electrodes and double barrier. The magnetic correlation present in the electrodes is found to strongly influence the TMR. Eventhough the magnetic correlation in general suppress the TMR, the TMR is found to be enhanced strongly for certain band occupations of the electrodes. We observe a fall of TMR with increase in the number of layers in the insulating region. Band occupation of the metallic layer present at the middle of the insulating layers in the double barrier MTJ is found to be important in deciding its tunnel characteristics. Origin of the different types of behavior of TMR is analyzed in terms of the spin-dependent tunnel currents.

Spin-dependent tunneling through double-barrier quantum wells with random corrugation interfacial roughness

We use the transfer matrix method to study spin-dependent tunneling transmission through the NM/EuS/NM/EuS/NM junction which affected by random corrugation interfacial roughness. The temperature dependence of spin-splitting energy is calculated for EuS magnetic barrier within the mean-field approximation. We perform several calculations to study the effect of random nonplanar interfacial on transmission probability for several widths of quantum well at different temperatures. Our results show that the scattering process due to random roughness open the new conduction channels (the dip and peak resonance energies). The results may be useful in designing spintronic devices.

Single-Shot Detection of Electrons Generated by Individual Photons in a Tunable Lateral Quantum Dot

We demonstrate single-shot detection of single electrons generated by single photons using an electrically tunable quantum dot and a quantum point contact charge detector. By tuning the quantum dot in a Coulomb blockade before the photoexcitation, we observe the trapping and subsequent resetting of single photogenerated electrons. The photogenerated electrons can be stored in the dot for a tunable time range from shorter to longer than the spin-flip time T1. We combine this trap-reset technique with spin-dependent tunneling under magnetic fields to observe the spin-dependent photon detection within the T1.

Prospects of spin transport electronics

This review provides basic information on spintronics. Briefly described the effects on which the development of spintronic nanoscale devices are based: giant magneto-resistance, spin-dependent tunnelling effect, transport of spin-polarized current, the creation of spinpolarized current torque for a magnetic switch and the motion of the magnetization of magnetic domains. As a example of successive applications spin-dependent devices are given parameters of magnetic memories based on use of spintronics components. It is shown that such memory is competitive to nowadays standard memories (at 90 nm) and has the potential for future development (for example, reducing the critical size to 32 nm)

Tunneling magnetoresistance in epitaxial discontinuous Fe/MgO multilayers

Epitaxial discontinuous Fe/MgO multilayers have been grown by pulsed laser deposition on MgO(001) single-crystal substrates. The multilayers with 0.6 nm nominal Fe layers thickness are superparamagnetic and demonstrate tunneling magnetoresistance (TMR) in the current-in-plane configuration. Increasing deposition temperature causes an improvement in crystal quality and is accompanied by higher TMR ratios. The maximum value (9.2% TMR at room temperature and 18 kOe magnetic field) trebles that of polycrystalline samples deposited simultaneously on glass substrates. A model formula for TMR ratio that includes both spin-dependent tunneling and spin-filtering effect is proposed to explain these results.

Spin-dependent photoelectron tunneling from GaAs into magnetic cobalt

The spin dependence of the photoelectron tunnel current from free-standing GaAs films into out-of-plane magnetized cobalt films is demonstrated. The measured spin asymmetry ($A$), resulting from a change in light helicity, reaches $\ifmmode\pm\else\textpm\fi{}6%$ around zero applied tunnel bias and drops to $\ifmmode\pm\else\textpm\fi{}2%$ at a bias of $\ensuremath{-}1.6 \mathrm{V}$ applied to the GaAs. This decrease is a result of the drop in the photoelectron-spin polarization that results from a reduction in the GaAs surface-recombination velocity. The sign of $A$ changes with that of the cobalt magnetization. In contrast, $A$ is negligible on nonmagnetic gold films.

Etch-stop technique for patterning of tunnel junctions for a magnetic field sensor

Spin-dependent tunnelling devices, e.g. magnetic random access memories and highly sensitive tunnelling magnetoresistance (TMR) sensors, often consist of a large number of magnetic tunnel junctions (MTJs) of uniform quality over the whole device. The uniformity and yield of the fabrication of such a device are therefore very important. A major source of yield loss is the short-circuiting of junctions by redeposition of etch residues. This can be prevented by terminating of the etch in the typically 1 nm thick tunnelling barrier. Here, electron spectroscopy for chemical analysis for monitoring the etching semi-continuously is proposed. The fabrication scheme employs Ar ion milling for etching the MTJs, and photoelectron spectroscopy for analysing the composition of the etched surface in situ. Junctions etched either to or through the barrier were used for this. The quality of the etch stop was investigated using transmission electron microscopy (TEM), and it was confirmed that the etch could be stopped in the MgO barrier. The TEM imaging also showed clear signs of redeposition. Such redeposition was attributed to being partly caused by the reduction of the TMR ratio of the junctions etched through the barrier, which was only 15% as compared with 150% for junctions etched to the barrier. Also, the latter junctions exhibited 2.7 times less noise in the low-frequency regime, resulting in a 27 times improvement of the signal-to-noise ratio with the etch stop. The barrier also proved effective in protecting the bottom contact from oxidation during the capping and contacting of the junctions.

Large and inverted spin signals in nonlocal spin valves

A nonlocal spin valve (NLSV) is a nanoscale planar heterostructure, consisting of a spin injector, a spin detector, and a nonmagnetic channel. A pure spin current can be induced in the nonmagnetic channel by electrical spin injection. We report large but inverted spin signals in a set of NLSV structures. The interface between the ferromagnetic spin detector and the nonmagnetic channel is found to be a break junction formed by electrostatic discharge. A large (g80%) but negative tunneling spin polarization across the break junction is inferred. The large magnitude is due to a strong coupling between the charges and spins at the low-conductance break junction. The inverted sign results from the specific interfacial atomic structures and bonding states, which influence the spin-dependent tunneling matrices.

Spin current through double quantum dots with spin- and time-dependent interdot coupling

We analyze theoretically the characteristics of electron transport through double quantum dots between which time oscillating and spin-polarization-dependent tunneling occurs. General formulas for the time-averaged current (spin current and charge current) and its differential are obtained by Keldysh nonequilibrium, Green’s function, and the transport properties derived from those formulas, can be qualitatively explained by spin-dependent resonant tunneling images.

Spin-dependent electronic structure of self-organized Co nanomagnets

The magnetism of small (only a few nanometers in diameter) Co nanoparticles (NPs) grown on Au(111) was investigated by means of spin-dependent scanning tunneling microscopy and spectroscopy in a broad energy range. Direct evidence is provided for the existence of localized d-states of minority and majority character that govern the spin polarization of the NPs below the Fermi level. On the other hand, the discrete electronic states resulting from the spatially confined sp-like Co surface state electrons above the Fermi level are found to be of majority character. This confirms the theoretically predicted spin-polarized character of the delocalized surface state electrons of Co NPs on Au(111).