Spin-valve Effect Research Articles

Magnetoresistance of a Spin Metal–Oxide–Semiconductor Field-Effect Transistor with Ferromagnetic MnAs Source and Drain Contacts

Transport characteristics were investigated in a spin metal–oxide–semiconductor field-effect transistor (spin MOSFET) with ferromagnetic MnAs source and drain (S/D) contacts. A bottom-gate type spin MOSFET was fabricated by photolithography using an epitaxial MnAs film grown on a silicon-on-insulator (SOI) substrate. In-plane magnetoresistance showed a square like hysteretic behavior, when measurements were performed with constant source–drain and source–gate biases. From the comparison with the magnetization-related resistance change resulting from the MnAs contacts, a highly possible origin of the feature obtained for the spin MOSFET is the spin-valve effect originating from the spin-dependent transport in the Si channel.

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.

Spin-dependent transport in a Rashba ring connected to noncollinear ferromagnetic leads

We analyze spin-dependent transport through a quantum ring coupled to two ferromagnetic leads, whose magnetic moments lie in a common plane and form an arbitrary angle with respect to each other. The Rashba spin-orbit (RSO) interaction existed in the ring arms is taken into consideration. We calculate the linear conductance in terms of the Green’s functions method based on the equation of motion technique. It is found that due to the quantum interference effect arising from the RSO-induced spin precession phase factor, the conductance is greatly suppressed when the Fermi energy is aligned to the on-site energies of the ring, where the spin polarization and the tunnel magnetoresistance (TMR) have their maximums. The conductance, spin polarization, and the TMR are monotonously tuned by the relative angle of the leads’ magnetization directions, which shows the typical spin-valve effect. We pay special attention on the situation when one magnetic lead is polarized along z axis while the other one is pointing at x direction. The peak value of the TMR is suppressed now and can become either positive or negative when the on-site energies of the two ring arms are different from each other. This device is realizable with current technology and may practical applications in spintronics.

Conductance of functionalized nanotubes, graphene and nanowires: from ab initio to mesoscopic physics

AbstractWe review recent theoretical results aiming at understanding the impact of doping and functionalization on the electronic transport properties of nanotubes, nanowires and graphene ribbons. On the basis of ab initio calculations, the conductance of micrometer long tubes or ribbons randomly doped or grafted can be studied, allowing to extract quantities at mesoscopic length scales such as the elastic mean free path and localization length. While the random modification of a 1D conducting channel leads generaly to a significant loss of conductance, strategies can be found to either exploit or limitate such a detrimental effect. Spin‐filtering in transition metal doped nanotubes, the opening of a mobility gap in graphene ribbons, and the choice of molecules to limitate backscattering in covalently functionalized tubes are examples that will be discussed.magnified image Symbolic representation of a nanotube filled with Cobalt atoms or clusters with subsequent optimal spinvalve effect (see text).

Spin valve effect and negative tunnel magnetoresistance in a quantum dot transistor

We study the spin polarized transport through a quantum dot transistor. It is shown that the interplay of large Coulomb interaction and optically induced spin accumulation gives rise to the spin valve effect over a range of bias. We also find negative tunnel magnetoresistance for system with ferromagnetic electrodes.

Surface Spin Flip Probability of Mesoscopic Ag Wires

Spin relaxation in mesoscopic Ag wires in the diffusive transport regime is studied via nonlocal spin valve and Hanle effect measurements performed on Permalloy/Ag lateral spin valves. The ratio between momentum and spin relaxation times is not constant at low temperatures. This can be explained with the Elliott-Yafet spin relaxation mechanism by considering the momentum surface relaxation time as being temperature dependent. We present a model to separately determine spin flip probabilities for phonon, impurity and surface scattering and find that the spin flip probability is highest for surface scattering.

Inverse spin valve effect in multilayer graphene device

We report the gate-voltage dependence of the spin transport in multilayer graphene (MLG) studied experimentally by the local measurement. The sample consists of a Ni/MLG/Ni junction, where the thickness of the MLG is 9 nm and the spacing of two Ni electrodes is 300 nm. At zero gate voltage, we observed the normal spin valve effect, in which the resistance for the antiparallel alignment of magnetization in ferromagnetic electrodes is larger than that for the parallel alignment. By applying a large gate voltage, on the other hand, the spin valve effect is reversed: the resistance for the antiparallel alignment becomes smaller than that for the parallel alignment. The result is qualitatively interpreted as a quantum interference effect, indicating that the mean free path and the spin relaxation length of the MLG are longer than the electrode spacing (300 nm).

Distinguishing between tunneling and injection regimes of ferromagnet/organic semiconductor/ferromagnet junctions

Magnetoresistance effects in organic semiconductor spin valves have recently been reported, and have been variously interpreted as being due to tunneling magnetoresistance or giant magnetoresistance. We introduce a criterion for distinguishing between tunneling and injection conductivity necessary for properly analyzing organic spin-valve phenomena. We measure current-voltage $(I\text{\ensuremath{-}}V)$ characteristics in $\text{Co}/{\text{AlO}}_{x}/\text{rubrene}/\text{Fe}$ junctions with a rubrene layer thickness, $d$, ranging from 5 to 50 nm. For $d\ensuremath{\le}10\text{ }\text{nm}$, the $I\text{\ensuremath{-}}V$ traces are typical of tunnel junctions. At $dg15\text{ }\text{nm}$, the tunneling current becomes negligibly small. At larger biases, however, a second type of conductivity sets in. In this regime, the $I\text{\ensuremath{-}}V$ curves are strongly nonlinear and temperature dependent. By comparing these to $I\text{\ensuremath{-}}V$ curves measured in organic light-emitting diodes, we assign the latter mode to injection into the organic layer followed by hopping transport. We observe a spin-valve effect only in the tunneling regime.

Ca 3 ( Ru 1 − x Cr x ) 2 O 7 : A new paradigm for spin valves

A spin valve is a device structure whose electrical resistance can be manipulated by controlling the relative spin alignment of adjacent metallic, magnetic layers separated by nonmagnetic insulating layers. The spin valve effect is thought to be a delicate quantum phenomenon that depends upon the precision deposition and nanoscale patterning of artificial thin-film heterostructures whose quality and performance are difficult to control. We have observed a novel, strong spin valve effect in bulk single crystals of Ca3(Ru1−xCrx)2O7 having an anisotropic, bilayered crystal structure. This discovery opens new avenues for understanding the underlying physics of the spin valve effect, and for realizing their potential in practical devices.

Cu2Oas a nonmagnetic semiconductor for spin transport in crystalline oxide electronics

We probe spin transport in Cu_{2}O by measuring spin valve effect in La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/Co and La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/La_{0.7}Sr_{0.3}MnO_{3} epitaxial heterostructures. In La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/Co systems we find that a fraction of out-of-equilibrium spin polarized carrier actually travel across the Cu_{2}O layer up to distances of almost 100 nm at low temperature. The corresponding spin diffusion length dspin is estimated around 40 nm. Furthermore, we find that the insertion of a SrTiO_{3} tunneling barrier does not improve spin injection, likely due to the matching of resistances at the interfaces. Our result on dspin may be likely improved, both in terms of Cu_{2}O crystalline quality and sub-micrometric morphology and in terms of device geometry, indicating that Cu_{2}O is a potential material for efficient spin transport in devices based on crystalline oxides.

Electrical bistability and spin valve effect in a ferromagnet/organic semiconductor/ferromagnet heterojunction

We report a study of the electrical bistability and bias-controlled spin valve effect in an organic device using rubrene (C42H28) as an organic semiconductor channel. The halfmetallic La0.7Sr0.3MnO3 (LSMO) and Fe are used as the two ferromagnetic electrodes. The device displays reproducible switching between a low-impedance (ON) state and a highimpedance (OFF) state by applying different polarities of high biases. In the ON state, the device shows a spin valve effect with magnetoresistance values up to 3.75%. The observed spin valve effect disappears when the device recovers to the initial OFF state.

Superconducting triplet spin valve

We study the critical temperature T c of SFF trilayers (S is a singlet superconductor, F is a ferromagnetic metal), where the long-range triplet superconducting component is generated at noncollinear magnetizations of the F layers. We demonstrate that T c can be a nonmonotonic function of the angle α between the magnetizations of the two F layers. The minimum is achieved at an intermediate α, lying between the parallel (P, α = 0) and antiparallel (AP, α = π) cases. This implies a possibility of a “triplet” spin-valve effect: at temperatures above the minimum T Tr c but below T P c and T AP c , the system is superconducting only in the vicinity of the collinear orientations. At certain parameters, we predict a reentrant T c (α) behavior. At the same time, considering only the P and AP orientations, we find that both the “standard” (T P c < T AP c ) and “inverse” (T P c > T AP c ) switching effects are possible depending on parameters of the system.

Positive Giant Magnetoresistance in La0.67Sr0.33MnO3/Alq3/Co Organic Spin-Valves

We report a positive spin valve effect—low resistance for parallel electrodes configuration—in organic spin valves fabricated by vacuum thermal evaporation using half-metal perovskite manganites La0.67Sr0.33MnO3 as the bottom electrode, cobalt as the top electrode, and tris(8-hydroxyquinoline) aluminum (Alq3) as the organic semiconductor spacer. A positive giant magnetoresistance (GMR) of ∼12% has been observed at 100 K. The origination mechanism of negative and positive GMR in organic spin valves has been discussed in detail.

Spin-valve effect in zigzag graphene nanoribbons by defect engineering

We report on the possibility for a spin valve effect driven by edge defect engineering of zigzag graphene nanoribbons. Based on a mean-field spin unrestricted Hubbard model, electronic band structures and conductance profiles are derived, using a self-consistent scheme to include gate-induced charge density. The use of an external gate is found to trigger a semiconductor-metal transition in clean zigzag graphene nanoribbons, whereas it yields a closure of the spin-split bandgap in the presence of Klein edge defects. These features could be exploited to make novel charge and spin based switches and field effect devices.

Domain-wall enhancement of superconductivity in superconductor/ferromagnet hybrids: Case of weak ferromagnets

We use magnetotransport measurements to study the influence of the domain structure on the superconducting transition temperature ${T}_{c}$ in ferromagnetic $(F)$/superconducting $(S)$ bilayers and $F/S/F$ trilayers based on Nb and the weak ferromagnet ${\text{Cu}}_{43}{\text{Ni}}_{57}$. By comparing the anisotropic magnetoresistance above ${T}_{c}$ with the resistance behavior in the transition and with critical current measurements below ${T}_{c}$, we show that in bilayers enhanced superconductivity is found when the $F$ layer is in a domain state. We also find that, below ${T}_{c}$, the magnetic fields where domains occur are significantly higher than above or around ${T}_{c}$, suggesting that the magnetization rotation in the $F$ layers is influenced by the adjacent superconductor. In trilayers the effects are similar. A domain-state dominated mechanism even for reported spin-valve effects therefore cannot be ruled out.

Transport of Spin-Polarized Electrons through an NEM-SET Structure with Ferromagnetic Electrodes

We have investigated the spin-polarized electron transport in a magnetic nanoelectromechanical single-electron-transistor (NEM-SET), with an oscillating quantum dot (QD) coupled to two ferromagnetic electrodes. The interplay between the electronic and mechanical degrees of freedom is considered by using the quantum master equation method within Wigner phase-space. We present a concrete picture for the transition of the QD oscillations from the tunneling state to the shuttling one by analysis of the electron occupation, the effective potential and amplitude probability distribution. It is found that the development of dynamic shuttle instability is dependent on the relative orientation of two leads’ magnetizations, which arises a pronounced spin valve effect. For an asymmetric NEM-SET structure, besides the spin valve effect, we unexpectedly find that the shuttle instability is additionally dependent on the the bias-voltage polarities, exhibiting a sizable current rectification. The coexistence of two effects makes it possible to control the spin valve effect electrically or control the rectification magnetically. Subject Index: 304, 355, 375, 394

Observation of the “Inverse” spin valve effect in a Ni/V/Ni trilayer system

An experimental study of magnetic and superconducting properties of a trilayer Ni/V/Ni thin film system grown on single-crystalline MgO(001) substrate is reported. The field dependence of the superconducting transition temperature Tc for samples comprising Ni layers with similar values of the coercive field Hc reveals no anomalies. However, in samples with different thicknesses of the nickel layers the difference in Hc amounts up to ΔHc ∼ 1.8 kOe, thus enabling to manipulate the relative orientations of the layers’ magnetization by an external magnetic field. Surprisingly, for these samples the Tc for the parallel orientation of the magnetizations of the Ni layers is higher, in a certain magnetic field range, than for the antiparallel one, at odds with theoretical predictions. Possible reasons of this contradiction are discussed.

Electrical spin injection and detection in lateral all-semiconductor devices

Both electrical injection and detection of spin-polarized electrons are demonstrated in a single wafer all-semiconductor GaAs-based lateral spintronic device, employing ${p}^{+}\text{\ensuremath{-}}(\text{Ga},\text{Mn})\text{As}/{n}^{+}$-GaAs ferromagnetic Esaki diodes as spin aligning contacts. Spin-dependent phenomena, such as spin precession and spin-valve effect, are observed in nonlocal signal and the measurements reveal the unusual origin of the latter in the investigated devices. The conversion of spin-polarized holes into spin-polarized electrons via Esaki tunneling leaves its mark in a bias dependence of the spin-injection efficiency, which at maximum reaches the value of 50%.

Spin-valve effect of spin-accumulation resistance in a double ferromagnet/superconductor junction

We have measured the transport properties of ferromagnet-superconductor nanostructures, where two superconducting aluminum (Al) electrodes are connected through two ferromagnetic iron (Fe) ellipsoids in parallel. We find that, below the superconducting critical temperature of Al, the resistance depends on the relative alignment of the ferromagnets' magnetization. This spin-valve effect is analyzed in terms of spin accumulation in the superconducting electrode submitted to inverse proximity effect.

Enhanced spin injection and detection in spin valves with integrated tunnel barriers

We study the spin-dependent transport in lateral spin-valve devices with aluminum oxide tunnel barriers at the interfaces between NiFe electrodes and an interconnecting Al strip. Different total conductivities per cross-sectional area are achieved by varying the oxygen pressure, the oxidation time, and the thickness of the oxidized aluminum layer. The experimental data are consistent with our theoretical description including spin diffusion, spin relaxation, and tunnel barriers at the interfaces between electrodes and aluminum strip. With decreasing tunnel conductance the amplitude of the nonlocal spin-valve effect increases by two orders of magnitude up to saturation.