Magnetic Junctions Research Articles

High tunneling magnetoresistance induced by symmetry and quantum interference in magnetic molecular junctions

A higher tunneling magnetoresistance ratio is obtained in the symmetric magnetic molecular junction due to the quantum interference, and it shows stronger ability to resist the influence of electron-vibration interaction and temperature.

THz-излучение в магнитном переходе Fe/Mo

An investigation was made of THz radiation in the Fe/Mo magnetic junction arising from the pumping by a high-density current of the upper spin subband in the induced ferromagnetism layer in Mo. The tensor character of the exchange interaction constant at the Fe / Mo interface, associated with the presence of the Dzyaloshinsky-Moriya interaction, promotes radiative relaxation.

Frequency of Spin-Injection Radiation in the Magnetic Junction as a Function of the Spin Mobility of Electrons

Taking into account the difference in the mobilities of electrons with different spin projections, the frequencies of spin-injection radiation in the current-carrying magnetic contact formed by two ferromagnets are calculated. It is shown that these frequencies are in the terahertz range at a certain current density exceeding the threshold value, and the current density required to generate terahertz radiation can be, even at small values of the electron mobility difference, two orders of magnitude less than that in the absence of the electron mobility difference. The effect of the spin polarization ratio of ferromagnets that form a magnetic junction on the radiation frequency is analyzed.

Spin Relaxation in s-Wave Superconductors in the Presence of Resonant Spin-Flip Scatterers.

Employing analytical methods and quantum transport simulations we investigate the relaxation of quasiparticle spins in graphene proximitized by an s-wave superconductor in the presence of resonant magnetic and spin-orbit active impurities. Off resonance, the relaxation increases with decreasing temperature when electrons scatter off magnetic impurities-the Hebel-Slichter effect-and decreases when impurities have spin-orbit coupling. This distinct temperature dependence (not present in the normal state) uniquely discriminates between the two scattering mechanisms. However, we show that the Hebel-Slichter picture breaks down at resonances. The emergence of Yu-Shiba-Rusinov bound states within the superconducting gap redistributes the spectral weight away from magnetic resonances. The result is opposite to the Hebel-Slichter expectation: the spin relaxation decreases with decreasing temperature. Our findings hold for generic s-wave superconductors with resonant magnetic impurities, but also, as we show, for resonant magnetic Josephson junctions.

Hydrogenated silicene based magnetic junction with improved tunneling magnetoresistance and spin-filtering efficiency

In this paper, hydrogenation is used for the generation of band gap in silicene and the hydrogenated silicene is then studied for its spintronic application. Upon hydrogenation, silicene transforms into a wide band gap material with a band gap of 3.32 eV. Parameters like magneto-resistance and spin-filtering efficiency of magnetic tunnel junction (MTJ) with CrO2 as semi-metallic electrodes and hydrogenated silicene as scattering region are found to increase compared to pristine silicene as scattering region. The simulation results show that the magneto-resistance of hydrogenated silicene remains above 85% (higher than the pristine counterpart) for the entire bias range. In addition, the spin-filtering efficiency in hydrogenated silicene reaches a value as high as 96% whereas in case of pristine silicene it remains below 90% for the entire bias range.

The Potential of the Josephson Energy

The concept of the Josephson energy emerges directly from the original predictions of Brian Josephson. Although it can be used to explain the behaviour of Josephson devices, the Josephson energy has no present applications. In this paper, we show that in the context of magnetic Josephson junctions, the Josephson energy offers a potential mechanism for controlling magnetic memory devices.

Theory of universal differential conductance of magnetic Weyl type-II junctions

We study the transport properties of junctions of normal and superconducting Weyl semimetal with tilted dispersion, in the presence of magnetization induced by magnetic strips. The sub gap tunnelling conductance shows robust signatures in the presence of different orientation and strength of magnetization of the magnetic strips. We obtain the analytical results for the normal-magnetic-superconducting junction in the thin barrier limit and demonstrate that these results have no analogues to their conventional counterparts and junctions with Dirac electrons in two-dimensions. We discuss possible experimental setups to test our theoretical predictions.

Multi-heterostructured spin-valve junction of vertical FLG/MoSe2/FLG

Two-dimensional (2D) layered materials and their heterostructures have opened a new avenue for next-generation spintronic applications, benefited by their unique electronic properties and high crystallinity with an atomically flat surface. Here, we report magnetoresistance of vertical magnetic spin-valve devices with multi-layer (ML) MoSe2 and its heterostructures with few-layer graphene (FLG). We employed a micro-fabrication procedure to form ultraclean ferromagnetic–non-magnetic–ferromagnetic interfaces to elucidate the intrinsic spin-transferring mechanism through both an individual material and combinations of 2D layered materials. However, it is revealed that the polarity of tunneling magnetoresistance (TMR) is independent of non-magnetic spacers whether the spin valve is composed of a single material or a hybrid structure, but it strongly depends on the interfaces between ferromagnetics (FMs) and 2D materials. We observed positive spin polarizations in ML-MoSe2 and FLG/ML-MoSe2/FLG tunnel junctions, whereas spin-valve devices comprised of FLG/ML-MoSe2 showed a reversed spin polarization and demonstrated a negative TMR. Importantly, in Co/FLG/ML-MoSe2/FLG/NiFe devices, the polarization of spin carriers in the FM/FLG interface remained conserved during tunneling through MoSe2 flakes in spin-transferring events, which is understandable by Julliere’s model. In addition, large TMR values are investigated at low temperatures, whereas at high temperatures, the TMR ratios are deteriorated. Furthermore, the large values of driving ac-current also quenched the amplitude of TMR signals. Therefore, our observations suggest that the microscopic spin-transferring mechanism between ferromagnetic metals and 2D materials played a momentous role in spin-transferring phenomena in vertical magnetic spin-valve junctions.

Infinite attractors in a chaotic circuit with exponential memristor and Josephson junction resonator

Memristors play a vital role in the design of high density and low power memory devices. Considering the complex behaviour of memristor devices because of the feature that its memductance changes with the input current, these devices finds its applications in design of nonlinear circuits. Similarly, Josephson junction resonators also exhibit complex behaviours because of its inductive properties. To model the effect of magnetic flux effects on the Josephson junction devices, we introduce the memristor to produce the feedback effect between magnetic flux and junction potential. Such a circuit produce interesting complex behaviours and hence in this paper we propose a nonlinear chaotic oscillator in a memristor-Josephson junction combination. Due to the variation laws of the flux-controlled memductance, it will not always be the same when the excitation voltage changes and hence have their drawbacks in real word applications. Considering this, we use an exponential memristor which overcomes this problem and hence can be used in physical applications. By varying the type of the bias current, we could observe the property of megastability in the memristor-Josephson junction oscillator (when excited by an AC current), which was not been observed in real physical circuits. To show the applicational benefits of the proposed oscillator, we have discussed about the synchronisation of the master-slave memristor-Josephson junction oscillators and used it in secure communication systems.

A spin-wave magnetometer with a positive feedback

We demonstrate experimentally the operation of a spin-wave magnetometer integrated into a circuit with a positive feedback. The circuit consists of the passive magnetic and active electric parts. The magnetic part includes a sensing element, which is a magnetic cross junction made of Y3Fe2(FeO4)3. The electric part includes a non-linear amplifier and a phase shifter. The electric and magnetic parts are connected via micrometer size antennae. Spin waves are excited by two of these antennae while the output inductive voltage produced by the interfering spin waves is detected by the third antenna. Spin waves propagating in the orthogonal arms of the cross can accumulate significantly different phase shifts, depending on the direction and the strength of the external magnetic field. The output inductive voltage reaches its maximum in the case of constructive spin wave interference. The positive feedback provides further signal amplification. It appears possible to enhance the response function, compared to the passive circuits without a feedback, by a factor of ×100 without an increase in the noise level. The experimental data show a prominent response to the external magnetic field variation, exceeding 5×103V/T. The intrinsic noise spectral density of the device can be as low as 10−16 V2/Hz. The estimated sensitivity of the prototype device is 2×10-12T/√Hz at room temperature. We argue that spin-wave magnetometers can potentially be as sensitive as SQUIDs while operating at room temperature.

Manipulating Current Spin Polarization in Magnetic Single-Molecule Junctions via Destructive Quantum Interference

By means of the first-principles method, the effects of destructive quantum interference (DQI) on spin-dependent transport properties in meta-phenylene ethylene-type oligomer (OPE)-based magnetic single-molecule junctions are investigated. We explore one possible strategy to manipulate the spin polarization (SP) of current based on controllable DQI features using chemical substituents. It is found that the position of DQI features can obviously be tuned by inserting one nitrogen atom into the meta-OPE core at different positions, which leads to selected suppression of the transmission peaks contributed by the spin-resolved hybrid interface state (HIS). A resulting enhancement or reduction of the SP of current is achieved by manipulating the position of DQI dips. The nature of shift of the DQI feature is analyzed in terms of Green’s function and frontier molecular orbitals. In addition, such a phenomenon is further verified in the molecular junctions with side group substituents. This work demonstrates the role of controllable DQI in spin-dependent transport properties, which provides a feasible way to manipulate the spin functionality of organic spintronic devices.

Magnetic field tunable superconducting transition in Nb/Co/Py/Nb exchange spring multilayers

Over the last decade, it has been shown that magnetic non-collinearity at an s-wave superconductor/ferromagnet interface is a key ingredient for spin-singlet to spin-triplet pair conversion. This has been verified in several synthetic non-collinear magnetic structures. A magnetically soft and hard ferromagnetic layer combination in a bilayer structure can function as a field tunable non-collinear magnetic structure, which may offer magnetic field tunability of singlet-to-triplet pair conversion. From magnetization measurements of Nb/Co/Py/Nb multilayers, we demonstrate a reversible enhancement of the superconducting critical temperature of 400 mK by measuring Tc with and without a non-collinear magnetic structure between Co and Py. The sensitivity of Tc in these structures offers the potential for realizing magnetic field tunable Josephson junctions in which pair conversion and Josephson critical currents are controllable using modest magnetic fields.

Rayleigh-Bénard Convection of Paramagnetic Liquid under a Magnetic Field from Permanent Magnets

The convection control is important in terms of the heat transfer enhancement and improvement of the applied devices and resultant products. In this study, the convection control by a magnetic field from block permanent magnets is numerically investigated on the Rayleigh-Bénard convection of paramagnetic fluid. To enhance the magnetic force from the available permanent magnets, pairs of alternating-pole magnets are employed and aligned near the bottom heated wall. The lattice Boltzmann method is employed for the computation of the heat and fluid flow with the consideration of buoyancy and magnetothermal force on the working fluid. It is found that, since the magnetic force at the junction of pair magnets becomes strong remarkably and in the same direction as the gravity, descending convection flow is locally enhanced and the pair of symmetrical roll cells near the magnet junction becomes longitudinal. The local heat transfer corresponds to the affected roll cell pattern; locally enhanced at the magnet junctions and low heat transfer area is shifted aside the magnet outer edge. The averaged Nusselt number on the hot wall also increases proportionally to the magnetic induction but it is saturated at high magnetic induction. This suggests the roll cell pattern is no more largely affected at extremely-high magnetic induction.

Effect of Environment on the Scattering of Electrons by a Junction of Different Topological Materials

AbstractThe studies of electron transport through a junction of topological materials in the literature so far ignore the coupling of a topological material to its surrounding environment. Here, the dynamics of an open system through a stochastic Hamiltonian are simulated to investigate the influence of the environment on the scattering of electrons by a junction of different topological materials, such as a Dirac–Weyl magnetic junction and a topological insulator. It is found that, although the detrimental effect of the environment is inevitable, the Landauer conductance can be enhanced via adjusting the system–environment coupling strength. This result supplies the possibilty of changing the transport feature of topological materials by modulating the surrounded environment. It is also demonstrated that a non‐Hermitian Hamiltonian can be used to replace the stochastic Hamiltonian for this study, when the system and the environment coupling are weak.

Enhanced spin-triplet pairing in magnetic junctions with s -wave superconductors

A common path to superconducting spintronics, Majorana fermions, and topologically-protected quantum computing relies on spin-triplet superconductivity. While naturally occurring spin-triplet pairing is elusive and even common spin-triplet candidates, such as Sr$_2$RuO$_4$, support alternative explanations, proximity effects in heterostructures can overcome these limitations. It is expected that robust spin-triplet superconductivity in magnetic junctions should rely on highly spin-polarized magnets or complex magnetic multilayers. Instead, we predict that the interplay of interfacial spin-orbit coupling and the barrier strength in simple magnetic junctions, with only a small spin polarization and s-wave superconductors, can lead to nearly complete spin-triplet superconducting proximity effects. This peculiar behavior arises from an effective perfect transparency: interfacial spin-orbit coupling counteracts the native potential barrier for states of a given spin and wave vector. We show that the enhanced spin-triplet regime is characterized by a huge increase in conductance magnetoanisotropy, orders of magnitude larger than in the normal state.

Magnetic field effect on silicon based solar cells

In this study, the impact of DC magnetic field on the power production, open-circuit voltage, photocurrent density and fill factor of a silicon photovoltaic (PV) cell/module is assessed. In this regard, the influence of DC magnetic field is first evaluated by presenting a novel theoretical electromagnetic force analysis formulating essential basis and concepts. Then, experimental measurements and data obtained from several tests conducted by arranging two different setups are given that verify theoretical results. In theory and practice, it is shown that depending on the direction of a DC magnetic field applied to a monocrystalline or polycrystalline silicon PV cell/module, different effects are observed. In detail, when the DC magnetic field points along the junction electric field of the silicon PV cell(s), i.e., the directions of the DC magnetic field and junction electric field are same or opposite, it has no effect on the parameters of the silicon PV cell/module. But, the power production, open-circuit voltage and photocurrent density of the silicon PV cell/module decrease when the DC magnetic field points in the other directions. Moreover, the reduction in the power production, open-circuit voltage and photocurrent density reaches its maximum when the DC magnetic field is applied in the direction perpendicular to the junction electric field. The above-mentioned points concluded in this paper are the contributions of this work to the state of the art of the topic.

Effect of a Potential Jump in a Magnetic Junction upon Spin Injection by an Electric Current on the Effectiveness of Electromagnetic Wave Radiation

Results are presented from studying the effect a jump in potential has on the modes of operation of different magnetic junctions, including the Ni/NiO/Fe tunnel magnetic junction. The difference between the effectiveness of radiation in them is explained by the effect of jumps in potential have on the formation of Fermi quasi-levels in spin-energy sub-bands.

Spin-flip excitations induced by dehydrogenation in a magnetic single-molecule junction.

Recent scanning tunneling microscopy experiments on electron transport through iron(ii) phthalocyanine (FePc) molecules adsorbed on Au(111) surfaces have revealed that the measured differential conductance signals can be modulated through a selective dehydrogenation process [R. Li et al., Chem. Commun. 54, 9135 (2018)]. To understand the physical origin of the variation of line shapes in the measured dI/dV spectra, we employ a first-principles-based quantum transport simulation to calculate the electronic structures and transport properties in the dehydrogenated FePc/Au(111) composite system. Theoretical results indicate that the removal of eight outermost hydrogen atoms in the FePc molecule distorts the planar molecular conformation by increasing the distance between the Fe center and substrate and thus breaks the ligand field exerted on the Fe 3d orbitals. Such variations lead to a weaker coupling with the substrate along with a different local electronic configuration of the Fe center compared with that of the intact case, which is responsible for the suppression of Kondo resonance and the appearance of spin-flip excitation in the system. The simulated dI/dV plots are consistent with the experimental observations, showing the typical step features at finite bias associated with spin-flip excitations of a spin-1 quantum magnet. These findings provide important insights into the electron correlation effects modulated by the structural and chemical environment of the molecular spin center.

Spin Polarization of Nonequilibrium Conduction Electrons in Magnetic Junctions

An original equation for nonequilibrium spin polarization in magnetic junctions based on dynamic equations for magnetic moment was obtained. Spatial nonuniformity of the distribution of carriers is taken into account in the equations, and the magnetic moment is averaged over the ensemble of nonequilibrium spin-injected electrons. The solution to the dynamic equations for the magnetic moment is used to estimate the probability of spin-flip electron transitions upon spin injection, and the solution to the equation for the nonequilibrium spin polarization in magnetic junctions is used to calculate frequencies of photon emission or absorption at energies corresponding to the energy of effective exchange splitting of spin subbands.

Exploration of Spin-Dependent Thermoelectricity in the Chiral Double-Stranded DNA Molecule Coupled to Ferromagnetic Leads

Spin selectivity in double-stranded DNA (dsDNA) molecules is unambiguously linked to their intrinsic chirality. Nevertheless, how the chiral-induced spin selectivity (CISS) effect in dsDNA influences spin-dependent thermoelectricity needs to be understood well, to comprehend the relationship between spin and heat transport. To this end, the authors explore the spin Seebeck effect of a chiral dsDNA-based magnetic junction, and elucidate the underlying physics relevant to the CISS effect by means of linear-response theory and nonequilibrium Green's functions. Their results provide valuable guidelines for engineering innovative spin-caloritronic devices based on chiral organic molecules.