Spin-dependent Electron Tunneling Research Articles

Influence of dielectric matrix stoichiometry on electrical and magnetoresistive properties of Fe–Zr–O nanocomposites

Influence of the ZrOn matrix stoichiometry on electrical and magnetoresistive properties of Fe–Zr–O nanocomposites have been studied in wide range of Fe concentration (16–64 at.%). Two groups of the composite samples with different ZrOn stoichiometry were investigated: first group with the oxygen lack, the second one with the oxygen excess. It was established that the magnetoresistivity is not observed in the first group composites while the ensuring of the zirconium dioxide stoichiometry (the second group composites) leads to appearance of the magnetoresistive effect and raise (up to two orders) the composites resistivity. Magnetoresistivity in the first group composites appears only after heat treatment at 350 C. The obtained results are explained based on the difference in the density of localized states in the oxide matrix of the composites with lack and excess of oxygen and ratio between two types of conductivity (hopping conductivity and spin-dependent electron tunneling).

Beating the Thermal Limit of Qubit Initialization with a Bayesian Maxwell’s Demon

Fault-tolerant quantum computing requires initializing the quantum register in a well-defined fiducial state. In solid-state systems, this is typically achieved through thermalization to a cold reservoir, such that the initialization fidelity is fundamentally limited by temperature. Here, we present a method of preparing a fiducial quantum state with a confidence beyond the thermal limit. It is based on real time monitoring of the qubit through a negative-result measurement -- the equivalent of a `Maxwell's demon' that triggers the experiment only upon the appearance of a qubit in the lowest-energy state. We experimentally apply it to initialize an electron spin qubit in silicon, achieving a ground-state initialization fidelity of 98.9(4)%, corresponding to a 20$\times$ reduction in initialization error compared to the unmonitored system. A fidelity approaching 99.9% could be achieved with realistic improvements in the bandwidth of the amplifier chain or by slowing down the rate of electron tunneling from the reservoir. We use a nuclear spin ancilla, measured in quantum nondemolition mode, to prove the value of the electron initialization fidelity far beyond the intrinsic fidelity of the electron readout. However, the method itself does not require an ancilla for its execution, saving the need for additional resources. The quantitative analysis of the initialization fidelity reveals that a simple picture of spin-dependent electron tunneling does not correctly describe the data. Our digital `Maxwell's demon' can be applied to a wide range of quantum systems, with minimal demands on control and detection hardware.

Magnetic Modulation of Terahertz Waves via Spin-Polarized Electron Tunneling Based on Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

Conductivity of pressed powders of chromium dioxide with spin-dependent electron tunneling: The effect of thickness and composition of dielectric layers

The resistive, magnetoresistive, and magnetic properties for nine compacted CrO2 powder samples synthesized by hydrothermal method from chromic anhydride were studied. The proposed new synthesis method allows adjusting the thickness of the dielectric shells on the surface of CrO2 nanoparticles. The powders consisted of either rounded nanopartides (with an average diameter of ≈ 120 nm) or needle-like crystals (≈22.9 nm in diameter and 302 nm long). In all cases, nanoparticles were covered by dielectric shells of varied thickness and composition (for example, chromium oxide Cr2O3 or chromium oxyhydroxide β-CrOOH). The effect of material properties and thickness of the intergranular dielectric layers, as well as the shape of CrO2 nanoparticles, on the magnitude of the tunnel resistance and magnetoresistance (MR) of compacted powder samples was investigated. For all the samples studied, the nonmetallic temperature behavior of the resistance and the giant negative tunneling MR were detected at low temperatures. The maximum values of MR at T ≈ 5 K and relatively small magnetic field (H = 0.5 T) were approximately 37%. With increasing temperature, the MR rapidly decreased (down to ≈1% at H = 1 T, T ≈ 200 K).

Effect of pressure and temperature on spin-dependent tunneling in InAs/GaAs heterostructure with Dresselhaus spin-orbit interaction

Abstract Using the transfer matrix, spin-dependent electron tunneling was studied in InAs/GaAs double barrier symmetrical heterostructure. The effect of Dresselhaus spin-orbit interaction in this system was analysed as a function of pressure and temperature. Both pressure and temperature influences the polarization efficiency, barrier transparency and dwell time of electrons. The increasing pressure increases polarization efficiency, tunneling life time and dwell time of electrons, where as the increasing temperature decreases the same parameters. The results might be helpful for the fabrication of spin-devices.

Gate-based single-shot readout of spins in silicon.

Electron spins in silicon quantum dots provide a promising route towards realizing the large number of coupled qubits required for a useful quantum processor1-7. For the implementation of quantum algorithms and error detection8-10, qubit measurements are ideally performed in a single shot, which is presently achieved using on-chip charge sensors, capacitively coupled to the quantum dots11. However, as the number of qubits is increased, this approach becomes impractical due to the footprint and complexity of the charge sensors, combined with the required proximity to the quantum dots12. Alternatively, the spin state can be measured directly by detecting the complex impedance of spin-dependent electron tunnelling between quantum dots13-15. This can be achieved using radiofrequency reflectometry on a single gate electrode defining the quantum dot itself15-19, significantly reducing the gate count and architectural complexity, but thus far it has not been possible to achieve single-shot spin readout using this technique. Here, we detect single electron tunnelling in a double quantum dot and demonstrate that gate-based sensing can be used to read out the electron spin state in a single shot, with an average readout fidelity of 73%. The result demonstrates a key step towards the readout of many spin qubits in parallel, using a compact gate design that will be needed for a large-scale semiconductor quantum processor.

Readout of singlet-triplet qubits at large magnetic field gradients

Visibility of singlet-triplet qubit readout is reduced to almost zero in large magnetic field gradients due to relaxation processes. Here we present a new readout technique that is robust against relaxation and allows for measurement when previously studied methods fail. This technique maps the qubit onto spin states that are immune to relaxation using a spin dependent electron tunneling process between the qubit and the lead. We probe this readout's performance as a function of magnetic field gradient and applied magnetic field, and optimize the pulse applied to the qubit through experiment and simulation.

Effect of the δ-potential on spin-dependent electron tunneling in double barrier semiconductor heterostructure

The effect of δ-potential was studied in GaAs/Ga0.6Al0·4As double barrier heterostructure with Dresselhaus spin-orbit interaction. The role of barrier height and position of the δ- potential in the well region was analysed on spin-dependent electron tunneling using transfer matrix method. The spin-separation between spin-resonances on energy scale depends on both height and position of the δ- potential, whereas the tunneling life time of electrons highly influenced by the position of the δ- potential and not on the height. These results might be helpful for the fabrication of spin-filters.

Influence of the surface morphology on the magnetoresistance of ultrathin films of ferromagnetic metals and their alloys

The microstructure, morphology and magnetoresistive properties of as-deposited and annealed at 700K discontinuous films of ferromagnetic metals and their alloys are investigated. It is established that the isotropic field dependences stemming from the spin-dependent electron tunneling between ferromagnetic islands are observed for as-deposited Co films with an effective thickness (d) of dCo = 5-25 nm as well as for Fe films (dFe = 10-30 nm). It is shown that the maximum values of the isotropic magnetoresistance take place in the case when the size of the islands is equal to 3-5nm and the width of the dielectric barrier between them is 1-3nm. The maximum TMR values for the films annealed at 700 K are obtained when the dielectric barrier is 2-5 nm wide.

Spin-dependent electron transport through a Mn-phthalocyanine molecule — A steady-state density functional theory (SS-DFT) study

We generalize the recently proposed steady-state density functional theory (SS-DFT) to spin-dependent cases and theoretically investigate the electronic and transport properties of a Mn-phthalocyanine molecule sandwiched between two graphene nanoribbon leads. The junction filters spin-up (minority spin) electrons while allowing spin-down (majority spin) electrons to pass with a filtering efficiency of about 99.5% at low biases. The spin-down electrons are found to tunnel through the junction via the HOMO orbital of the Mn-phthalocyanine molecule. Detailed analysis of the spin-dependent electron tunneling mechanism as well as the electronic/magnetic properties of the junction is presented.

Effect of boron in Fe/MgO interface on structural stability and state coupling

The structural stability and electronic state coupling in Fe/MgO heterojunction with a B interlayer at the interface are studied by using first-principles calculations. The B localized at the interface can effectively prevent interfacial oxidization and maintain a stable structure on the basis of a comparison with the Fe/MgO and Fe/FeO/MgO heterojunctions. The Fe/FeB/MgO heterojunction preserves the preferential transmission of the majority-spin Δ1 states, which dominate the spin-dependent electron tunneling in MgO-based magnetic tunnel junctions (MTJs). The Fe/FeB/MgO/FeB/Fe MTJ is expected to maintain high tunneling magnetoresistance ratio similar to that of the Fe/MgO/Fe MTJ. This work provides a basic method to explore interfacial influence on MTJs.

Magnetoresistance of drop-cast film of cobalt-substituted magnetite nanocrystals.

An oleic acid-coated Fe2.7Co0.3O4 nanocrystal (NC) self-assembled film was fabricated via drop casting of colloidal particles onto a three-terminal electrode/MgO substrate. The film exhibited a large coercivity (1620 Oe) and bifurcation of the zero-field-cooled and field-cooled magnetizations at 300 K. At 10 K, the film exhibited both a Coulomb blockade due to single electron charging as well as a magnetoresistance of ∼-80% due to spin-dependent electron tunneling. At 300 K, the film also showed a magnetoresistance of ∼-80% due to hopping of spin-polarized electrons. Enhanced magnetic coupling between adjacent NCs and the large coercivity resulted in a large spin-polarized current flow even at 300 K.

Impurity-band transport in organic spin valves.

The central phenomenon in the field of organic spintronics is the large magnetoresistance in thick organic spin valves. A prerequisite for understanding the magnetoresistance is a reliable description of the device resistance, or the I-V characteristics. Here I show that the observed I-V characteristics in the organic spin valves is incompatible with charge injection into the organic's lowest unoccupied molecular orbital or highest occupied molecular orbital but can be explained by electrons tunnelling into a broad impurity band located in the gap between these molecular orbitals. Voltage drop takes place mainly across depletion layers at the two electrode/organic interfaces, giving rise to electrode-limited charge transport. Spin-dependent electron tunnelling into the impurity band from the ferromagnetic electrodes results in spin accumulations inside the organic, which rapidly diffuses through the organic primarily via the exchange between impurity-band electrons. This picture explains the major magnetoresistance features and predicts enhanced capacitance in these devices.

Coulomb blockade of spin-dependent shuttling

We show that nanomechanical shuttling of single electrons may enable qualitatively new functionality if spin-polarized electrons are injected into a nanoelectromechanical single-electron tunneling (NEM-SET) device. This is due to the combined effects of spin-dependent electron tunneling and Coulomb blockade of tunneling, which are phenomena that occur in certain magnetic NEM-SET devices. Two effects are predicted to occur in such structures. The first is a reentrant shuttle instability, by which we mean the sequential appearance, disappearance and again the appearance of a shuttle instability as the driving voltage is increased (or the mechanical dissipation is diminished). The second effect is an enhanced spin polarization of the nanomechanically assisted current flow.

Large, Negative Magnetoresistance in an Oleic Acid-Coated Fe3O4 Nanocrystal Self-Assembled Film

An oleic acid-coated Fe3O4 nanocrystal self-assembled film was fabricated via drop casting of colloidal particles on a SiO2/Si substrate. The film exhibited bifurcation of the zero-field-cooled and field-cooled magnetizations around 250 K. The nonlinear current-voltage (I-V) characteristics between the source and drain electrodes in both zero and non-zero magnetic fields (H) were observed above and below the bifurcation temperature. A large negative magnetoresistance (MR ≈ -60%) was achieved at 200 K and H = 1 T. Even at 295 K and 0.2 T, the negative MR (∼ -50%) persisted. A Fowler-Nordheim plot and power-law scaling of the I-V characteristics revealed that the current flows through two-dimensional (2D) percolated electron tunneling paths. The enlargement of MR can be attributed to spin-dependent electron tunneling between magnetically coupled Fe3O4 nanocrystals self-assembled in 2D ordered arrays.

Spin Accumulation in Cr Nanoparticles in Single Electron Tunneling Regime

We have studied spin-dependent single electron tunneling in Cr nanoparticles with a diameter of 4 nm using Fe/MgO/Cr-nanoparticles/MgO/Fe double tunnel junctions. The transport properties are governed by the Coulomb blockade, showing suppression of current at low bias voltages. Magnetoresistance is clearly observed at a bias voltage over 0.25 V, and the magnetoresistance ratio increases with increasing bias voltage. These results suggest that the magnetoresistance is due to the spin accumulation in Cr nanoparticles. With some assumption, the minimum value of spin relaxation time in Cr nanoparticles is roughly estimated to be on the order of tens of nanoseconds .

Effects of a magnetic field on growth of porous alumina films on aluminum

The effects induced by a magnetic field on the oxide film growth on aluminum in sulfuric, oxalic, phosphoric and sulfamic acid, and on current transients during re-anodizing of porous alumina films in the barrier-type electrolyte, were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on Si wafer substrates. We could show that the duration of the anodizing process increased by 33% during anodizing in sulfuric acid when a magnetic field was applied (0.7 T), compared to the process without a magnetic field. Interestingly, such a magnetic field effect was not found during anodizing in oxalic and sulfamic acid. The pore intervals were decreased by ca. 17% in oxalic acid. These findings were attributed to variations in electronic properties of the anodic oxide films formed in various electrolytes and interpreted on the basis of the influence of trapped electrons on the mobility of ions migrating during the film growth. The spin dependent tunneling of electrons into the surface layer of the oxide under the magnetic field could be responsible for the shifts of the current transients to lower potentials during re-anodizing of heat-treated oxalic and phosphoric acid alumina films.

Hysteresis of magnetoresistance in granular La0.7Ca0.3MnO3 at low temperatures

The magnetoresistance of granular La0.7Ca0.3MnO3 is studied experimentally over wide ranges of temperatures and magnetic fields. The emphasis is on anomalously large hysteresis of magnetoresistance at low temperatures (T = 4.2 K). The observed ρ(H) dependence can be qualitatively explained by spin-dependent tunneling of electrons through the dielectric boundaries of conducting granules characterized by a wide spread in the magnetic-moment magnitudes.

Spin-dependent tunneling of single electrons into an empty quantum dot

Using real-time charge sensing and gate pulsing techniques we measure the ratio of the rates for tunneling into the excited and ground spin states of a single-electron AlGaAs/GaAs quantum dot in a parallel magnetic field. We find that the ratio decreases with increasing magnetic field until tunneling into the excited spin state is completely suppressed. However, we find that by adjusting the voltages on the surface gates to change the orbital configuration of the dot we can restore tunneling into the excited spin state and that the ratio reaches a maximum when the dot is symmetric.

Current-induced tunnel magnetoresistance due to spin accumulation in Au nanoparticles

Spin-dependent single electron tunneling was investigated in a magnetic double tunnel junction including Au nanoparticles as a center electrode. Tunnel magnetoresistance (TMR) clearly emerged with increasing spin-polarized current injected into Au nanoparticles and reached a maximum value of about 12% at 4.2K. The observation indicates that spin accumulation occurs in Au nanoparticles and causes TMR. The spin relaxation time in Au nanoparticles, as estimated from the critical current for the appearance of TMR, is of the order of 10ns, which is much longer than that in the bulk state.