Parallel Magnetic Configuration Research Articles

Tunneling magnetoresistance and spin-valley polarization in magnetic silicene superlattices

We study the tunneling magnetoresistance and the spin-valley transport in silicene-based magnetic superlattices. The superlattice profile is obtained by the periodic modulation of the exchange field induced by ferromagnetic electrodes. The silicene band gap in the superlattice structure is also modulated through an external perpendicular electric field. The concept of parallel and antiparallel magnetization configurations of single magnetic junctions is extended to the periodic case by switching the magnetization orientation of the adjacent magnetic barriers in parallel and antiparallel fashion. The transfer matrix method and the Landauer-B\"uttiker formalism are used to obtain the transmission and transport properties, respectively. We find an oscillating conductance once the periodic modulation is incorporated. By tuning the external perpendicular electric field a conductance gap is obtained for the antiparallel configuration, which results in an enhancement of the tunneling magnetoresistance with respect to single magnetic junctions. In the case of the spin-valley polarization it is not possible to obtain two well-defined polarization states by simply switching the magnetization orientation, as in single magnetic junctions, due to the equivalence of the spin-valley conductance components. However, by inducing structural asymmetry in the width of barriers-wells, two well-defined polarization states can be reached. Moreover, an additional enhancement of the tunneling magnetoresistance is induced by the structural asymmetry. Our findings indicate that magnetic periodic modulation can be an option to improve the tunneling magnetoresistance and the spin-valley polarization of silicene-based structures.

Large magnetoresistance in a Co/MoS2/graphene/MoS2/Co magnetic tunnel junction

We demonstrate a large magnetoresistance (MR) in a $\mathrm{Co}/\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{Co}$ magnetic tunnel junction by means of ab initio transport calculations. A Co electrode turns out to be an excellent spin injector for a $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}$ barrier. The transmission spectrum, current-voltage characteristics, spin injection efficiency, and magnetoresistance are calculated for the modeled device at various bias voltages in the parallel and antiparallel magnetic configurations. A remarkable change in the transmission spectrum and a subsequent change in total current through the junction have been observed, when the relative magnetic orientations of the electrodes are altered. The huge change in current due to the change in the relative magnetic orientation of the Co electrodes produces a high magnetoresistance up to $1270%$. The obtained values of the device parameters clearly indicate that a $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}$ heterostructure would be an excellent compound for highly efficient spin-valve device applications.

Large enhancement of magnetic damping in the presence of domain walls in spin valves

Magnetic damping of the free layer of CoFeB in the spin valve IrMn/CoFe/Cu/CoFeB with large exchange bias has been characterized by frequency-swept ferromagnetic resonance under a series of fixed magnetic fields. The damping constant shows little difference between the parallel and antiparallel magnetization configurations, consistent with the theoretical prediction. Remarkably, in the intermediate states of the pinned CoFe layer under reversal, the effective damping constant of the CoFeB layer is significantly enhanced from 0.0119 up to 0.0292. This enhancement, exceeding the effect of the pumped spin current appreciably, is mainly due to the inhomogeneous broadening and/or two-magnon scattering caused by the stray field emerging from the domain walls (DW) of the pinned CoFe layer when its magnetization is partially reversed. Meanwhile, a resonance frequency shift is also observed in the presence of DW. Our result confirms the strong influence of the pinned layer DW on the magnetic damping in spin valves, which should be properly excluded while dealing with the nonlocal spin-transport-induced damping in heterostructures.

Spin pumping between noncollinear ferromagnetic insulators through thin superconductors

Dynamical magnets can pump spin currents into superconductors. To understand such a phenomenon, we develop a method utilizing the generalized Usadel equation to describe time-dependent situations in superconductors in contact with dynamical ferromagnets. Our proof-of-concept theory is valid when there is sufficient dephasing at finite temperatures, and when the ferromagnetic insulators are weakly polarized. We derive the effective equation of motion for the Keldysh Green's function focusing on a thin film superconductor sandwiched between two noncollinear ferromagnetic insulators of which one is dynamical. In turn, we compute the spin currents in the system as a function of the temperature and the magnetizations' relative orientations. When the induced Zeeman splitting is weak, we find that the spin accumulation in the superconducting state is smaller than in the normal states due to the lack of quasiparticle states inside the gap. This feature gives a lower backflow spin current from the superconductor as compared to a normal metal. Furthermore, in superconductors, we find that the ratio between the backflow spin current in the parallel and anti-parallel magnetization configuration depends strongly on temperature, in contrast to the constant ratio in normal metals.

Spin valve effect in VN/GaN/VN van der Waals heterostructures

Nitride-based van der Waals magnetic heterostructures can simultaneously utilize both charge and spin degrees of electrons, which are important materials for low-dimensional spintronic devices. In this work, we study the stabilities and the electronic and magnetic properties of GaN/VN and VN/GaN/VN van der Waals heterostructures via first-principles calculations. The GaN/VN van der Waals heterostructure is a half-metal with 100% spin polarization, which can be applied in a spin filter and spin injection. Although monolayer GaN is a nonmagnetic semiconductor, it can introduce half-metallic characters via the magnetic proximity effect in both GaN/VN and VN/GaN/VN van der Waals heterostructures. The easy magnetization axis of hexagonal VN transits from the out-of-plane to the in-plane direction after combining with hexagonal GaN due to the different orbital occupations. Moreover, the electronic properties of the VN/GaN/VN van der Waals heterostructure depend largely on the magnetic configurations of the VN layers. By applying parallel or antiparallel magnetic configurations, the VN/GaN/VN van der Waals heterostructure presents half-metallic and semiconducting characters. The transformation indicates that the VN/GaN/VN van der Waals heterostructure is a promising candidate for room-temperature van der Waals spin valve devices.

Synthesis, Characterization, and Magnetoresistive Properties of the Epitaxial Pd0.96Fe0.04/VN/Pd0.92Fe0.08 Superconducting Spin-Valve Heterostructure.

A thin-film superconductor(S)/ferromagnet(F) F1/S/F2-type Pd0.96Fe0.04(20 nm)/VN(30 nm)/Pd0.92Fe0.08(12 nm) heteroepitaxial structure was synthesized on (001)-oriented single-crystal MgO substrate utilizing a combination of the reactive magnetron sputtering and the molecular-beam epitaxy techniques in ultrahigh vacuum conditions. The reference VN film, Pd0.96Fe0.04/VN, and VN/Pd0.92Fe0.08 bilayers were grown in one run with the target sample. In-situ low-energy electron diffraction and ex-situ X-ray diffraction investigations approved that all the Pd1−xFex and VN layers in the series grew epitaxial in a cube-on-cube mode. Electric resistance measurements demonstrated sharp transitions to the superconducting state with the critical temperature reducing gradually from 7.7 to 5.4 K in the sequence of the VN film, Pd0.96Fe0.04/VN, VN/Pd0.92Fe0.08, and Pd0.96Fe0.04/VN/Pd0.92Fe0.08 heterostructures due to the superconductor/ferromagnet proximity effect. Transition width increased in the same sequence from 21 to 40 mK. Magnetoresistance studies of the trilayer Pd0.96Fe0.04/VN/Pd0.92Fe0.08 sample revealed a superconducting spin-valve effect upon switching between the parallel and antiparallel magnetic configurations, and anomalies associated with the magnetic moment reversals of the ferromagnetic Pd0.92Fe0.08 and Pd0.96Fe0.04 alloy layers. The moderate critical temperature suppression and manifestations of superconducting spin-valve properties make this kind of material promising for superconducting spintronics applications.

Unusual transport property of two-dimensional semi-Dirac system modulated by magnetic barriers

We study the ballistic transport of a two-dimensional (2D) electron gas with anisotropic semi-Dirac (SD) band structure modulated by magnetic and electrical barriers. The magnetic barriers are induced by an inhomogeneous magnetic field, which can form a parallel (P) or antiparallel (AP) magnetic configuration. Using the transfer matrix approach, we calculate the transport-direction-dependent transmission and the conductance in P and AP configurations, respectively. For transport along the direction with linear dispersion, the magnetoresistance (TMR) oscillates with the Fermi energy and even varies from positive to negative due to the Klein tunneling. Whereas for transport along the direction with parabolic dispersion, the TMR is always positive, and there is a huge TMR within a range of energies due to the severe mismatch of the wavevector in the case of AP configuration. For the transport along an arbitrary direction, interestingly, the oscillation patterns of TMR are similar to those along the linear direction, but the amplitude of TMR is observably enhanced by two orders in magnitude. This unusual feature of TMR is originated from the anisotropic band structure of the system which suppresses the Klein tunneling. Our results are useful in designing magnetic memories based on 2D SD systems. • TMR in 2D SD system whose low energy states have both Schrodinger and Dirac electrons is first studied. • TMR oscillates with energy for transport along linear dispersion, and even varies from positive to negative due to the Klein tunneling, whereas along parabolic TMR is always positive, and there is a huge TMR due to the severe mismatch of the wavevector in the AP configuration. • Along an arbitrary direction the oscillation patterns of TMR are similar to those along the linear direction, but the amplitude of TMR is observably enhanced by two orders in magnitude.

Manipulation of Coupling and Magnon Transport in Magnetic Metal-Insulator Hybrid Structures

Ferromagnetic metals and insulators are widely used for generation, control and detection of magnon spin signals. Most magnonic structures are based primarily on either magnetic insulators or ferromagnetic metals, while heterostructures integrating both of them are less explored. Here, by introducing a Pt/yttrium iron garnet (YIG)/permalloy (Py) hybrid structure grown on Si substrate, we studied the magnetic coupling and magnon transmission across the interface of the two magnetic layers. We found that within this structure, Py and YIG exhibit an antiferromagnetic coupling field as strong as 150 mT, as evidenced by both the vibrating-sample magnetometry and polarized neutron reflectometry measurements. By controlling individual layer thicknesses and external fields, we realize parallel and antiparallel magnetization configurations, which are further utilized to control the magnon current transmission. We show that a magnon spin valve with an ON/OFF ratio of ~130% can be realized out of this multilayer structure at room temperature through both spin pumping and spin Seebeck effect experiments. Thanks to the efficient control of magnon current and the compatibility with Si technology, the Pt/YIG/Py hybrid structure could potentially find applications in magnon-based logic and memory devices.

Tuning spin filtering and spin rectifying behaviors of zigzag silicon carbon nanoribbons by edge dual-hydrogenation

Abstract Using non-equilibrium Green's function combined with density functional theory, we investigate spin-resolved transport properties of zigzag silicon carbon nanoribbons (zSiCNRs) with the different edge hydrogenations. The dual-hydrogenation on edge C or Si atoms all can break the magnetic degeneracy of zSiCNRs with mono-hydrogenation, and change it from the initial metallicity to halfmetallic behavior under the ferromagnetic state. Under the parallel magnetic configuration, the dual-hydrogenation on edge C or Si atoms of zSiCNR devices can induce and reverse a prefect spin filtering behavior with nearly 100% spin filtering efficiency. Under the anti-parallel magnetic configuration, the dual-hydrogenation on edge C or Si atoms can enhance the spin rectifying behavior of zSiCNR devices obviously with the substantial increases in corresponding spin rectifying ratios of α-spin or β-spin currents. The above findings are very useful for its functional application in silicon carbon spin-dependent nanodevices.

Bias-Dependent Magnetoresistance Device Based on a Magnetically Confined GaAs/AlxGa1– xAs Heterostructure

We apply a bias to a magnetoresistance (MR) device, which is constructed on surface of GaAs/AlxGa1– xAs heterostructure by patterning two asymmetric ferromagnetic stripes. Using improved transfer matrix method and Landauer–Büttiker theory, bias-dependent transmission, conductance and magnetoresistance ratio are calculated numerically. An obvious MR effect appears, because of a significant difference of transmission or conductance between parallel (P) and antiparallel (AP) magnetization configurations. MR ratio can be tuned by adjusting magnitude or direction of applied bias. These interesting features not only propose an alternative way to control MR effect, but also put forward an electrically-tunable MR device for magnetic information storage.

Electrically-tunable magnetoresistance effect in magnetically modulated semiconductor heterostructure

We report on a theoretical study of magnetoresistance (MR) effect in a magnetically modulated semiconductor heterostructure (MMSH) under an applied bias, which can be constructed on surface of [Formula: see text] heterostructure by depositing two asymmetric ferromagnetic (FM) stripes. Bias-dependent transmission and conductance are calculated numerically, on the basis of both improved transfer matrix method (ITMM) and Landauer–Büttiker conductance theory. An obvious MR effect appears because of a significant difference of transmission between parallel and antiparallel (AP) magnetization configurations. Moreover, MR ratio can be tuned by the bias. These interesting features not only provide an alternative way to manipulate MR effect, but also may lead to an electrically-controllable MR device.

Direct observation of magneto-Peltier effect in current-in-plane giant magnetoresistive spin valve

We report on the direct observation of the magneto-Peltier effect in a current-in-plane giant magnetoresistive (CIP-GMR) spin valve. By means of the recently developed thermoelectric imaging technique based on lock-in thermography, we demonstrate that thermoelectric cooling and heating are generated by applying a local magnetic field to the CIP-GMR spin-valve film, confirming the different Peltier coefficients of the spin valve between the parallel and antiparallel magnetization configurations. The cooling and heating positions are found to be tuned simply by changing the magnitude of the local magnetic field. This versatile and reconfigurable thermoelectric conversion functionality may provide a thermal management method for CIP-GMR magnetic sensors.

Giant magnetoresistance ratio in a current-perpendicular-to-plane spin valve based on an inverse Heusler alloy Ti2NiAl.

A Ti2NiAl inverse Heusler alloy based current-perpendicular-to-plane (CPP) spin valve (SV) with various kinds of atomic terminated interfaces has been designed to explore the potential application of Heusler alloys in spintronics devices. By performing first principles calculations combined with the nonequilibrium Green’s function, it is revealed that spin magnetic moments of interfacial atoms suffer a decrease, and the electronic structure shows that the TiNiB-terminated structure possesses the largest interface spin polarization of ≈55%. Our study on spin-transport properties indicates that the total transmission coefficient at the Fermi level mainly comes from the contribution from the spin up electrons, which are regarded as the majority of the spin electrons. When the two electrodes of the CPP-SV device are in parallel magnetization configuration, the interface containing Ti and Ni atoms possesses a higher spin up transmission coefficient than the interface containing Ti and Al atoms. The device with the TiNiB-terminated interface possesses the largest magnetoresistance ratio of 3.28 × 105, and it has great application potential in spintronics devices.

Surface state transport in double-gated and magnetized topological insulators with hexagonal warping effects

We explore the scattering of Dirac electrons in a double-gated topological insulator in the presence of magnetic proximity effects and warped surface states. It is found that a magnetic field can shift the Dirac cone in momentum space and deform the constant-energy contour, or opens up a band gap at the Dirac point, depending on the magnetization orientation. The double gate voltage induces quantum wells and/or quantum barriers on the surface of topological insulators, generating surface resonant tunnelling states. It is found that the hexagonal warping effect can increase the electronic transport at high energies when the constant-energy contour exhibits a snowflake shape. The energy-dependent conductances in the parallel and antiparallel magnetic configurations exhibit out-of-phase oscillations due to the quantum interference of propagating waves in the region between the two magnetized segments. Although the conductance spectrum of the double-well structure is higher than that of the double-barrier structure, the magnetoresistance ratio versus the separation distance between the two magnetized barriers exhibits pronounced oscillations due to the resonant tunnelling states. We show that the surface state transport can be controlled by the exchange field and gate voltage without breaking time reversal symmetry, suggesting that the double gated and magnetized topological insulators can be utilized to achieve a large magnetoresistance ratio with a tunable sign.

Giant magnetoresistance and spin-valley polarization of Dirac fermions modulated by magnetic fields

In the presence of magnetic fields, we studied the giant magnetoresistance (MR) effect of Dirac fermions in graphene and silicene, which is related to the spin and valley degrees of freedom. It is found that the electron could resonantly tunnel through the parallel magnetization configuration, but it is blocked in the antiparallel situation. Such a transmission difference leads to a giant MR ratio, which can be controllable electrically in silicene. Furthermore, for the asymmetric structure in silicene, the transport depends strongly on the spin and valley degrees of freedom. Accordingly, this MR device achieves a remarkable spin-valley polarized current which can be effectively controlled by the magnetic fields and electric fields.

Large rectifying ratio and prefect spin-filtering effect in a zigzag SiC nanoribbon heterojuction with boron and nitrogen impurities

Based on a mono hydrogen-passivated zigzag SiC nanoribbon with width 4, we construct a heterojunction through different substitutional impurities. The left part of the heterojunction is boron-doped at the Si-edge of zigzag SiC nanoribbon, and the right part is nitrogen-doped at the C-edge. Our results show that boron and nitrogen doped zigzag SiC nanoribbons exhibit half-metallic features at ferromagnetic state, which provides the possibility to design spin nanodevices. By performing first-principle quantum transport calculations, spin-dependent transport properties in different magnetic configurations are studied. Current-voltage curves show perfect spin-filtering and negative differential resistance effects when the spin orientations of the two electrodes are opposite. In the parallel magnetization configuration of two electrodes, a strong rectifying behavior is observed and the maximum rectifying ratio reaches the order of 109. Through detailed analysis of two electrodes' band structures along with the transmission spectra, the interesting transport behaviors can be understood. Besides, the Bloch states of spin-polarized bands crossing the Fermi level can help us to understand the appearance of the negative differential resistance effect.

Effect of 4d and 5d Transition-Metal Insertions to Spin-Dependent Transports in Fe/MgO Superlattices

We systematically investigated the effect of the spin–orbit coupling (SOC) to the tunneling magnetoresistance (TMR) ratio of the Fe/MgO (001) superlattices with inserted 4d (Ru, Rh, Pd) and 5d (Os, Ir, Pt) transition-metal (TM) monolayers by the first-principles full-potential linearized augmented plane wave method. We find that the TMR ratios for both 4d and 5d TM insertions decrease due to the introduction of the SOC. We observe that in all systems, the TMR values are mainly determined by the electric conductivities in the parallel magnetic configuration, as they are two orders of magnitude larger than the conductivities in the antiparallel magnetic configuration. When the SOC is introduced, we find that the TMR values are reduced depending on the SOC strength, and the largest TMR reduction occurs for the Pt monolayer insertion. In-plane wave vector (k∥) dependence of the electric conductivity shows that the reduction of TMR is indeed related to the decrease of electric conductivity in the parallel magnetic configuration.

Signature of quantum Griffiths singularity state in a layered quasi-one-dimensional superconductor

Quantum Griffiths singularity was theoretically proposed to interpret the phenomenon of divergent dynamical exponent in quantum phase transitions. It has been discovered experimentally in three-dimensional (3D) magnetic metal systems and two-dimensional (2D) superconductors. But, whether this state exists in lower dimensional systems remains elusive. Here, we report the signature of quantum Griffiths singularity state in quasi-one-dimensional (1D) Ta2PdS5 nanowires. The superconducting critical field shows a strong anisotropic behavior and a violation of the Pauli limit in a parallel magnetic field configuration. Current-voltage measurements exhibit hysteresis loops and a series of multiple voltage steps in transition to the normal state, indicating a quasi-1D nature of the superconductivity. Surprisingly, the nanowire undergoes a superconductor-metal transition when the magnetic field increases. Upon approaching the zero-temperature quantum critical point, the system uncovers the signature of the quantum Griffiths singularity state arising from enhanced quenched disorders, where the dynamical critical exponent becomes diverging rather than being constant.

Tunable magnetoresistance in spin-orbit coupled graphene junctions

Using the Landauer-B\"utikker formalism, we study the graphene magneto-transport in the presence of Rashba spin-orbit interaction (RSOI). We show that the angle resolved transmission probability in the proposed structures can be tuned by the RSOI strength. The transmission spectrum show Klein tunneling in the parallel (P) magnetization configuration which can be blocked by the RSOI. This effect is also observable for the anti-parallel (AP) magnetization configuration in different incident angle. The numerical results shows that the spin-polarized conductance strongly depends on the strength of the RSOI and can be generated by tuning the magnetic exchange field and RSOI strength. This spin-polarized conductance is a sensitive oscillatory function of the thickness of the RSO region. Because of the spin-flip effect, the junction shows a spin-valve effect with large and negative magnetoresistance (MR) and spin-magnetoresistance (SMR) in the presence of RSOI. When the RSOI is on, the frequency and amplitude of shot-noise and Fano factor's oscillations are also increased. These results can provide a way to extending the application of graphene-based junctions in spintronics.

Spin-polarized and thermospin-polarized transport properties of phthalocyanine dimer based molecular junction with different transition metal atoms.

Based on the first-principles density functional theory combined with the non-equilibrium Green's function method, we have studied the spin-polarized and thermospin-polarized transport properties of phthalocyanine (Pc) dimer based molecular junction with different transition metal (TM = Mn, Fe, Co, Ni) atoms. Our results show that the spin-polarized and thermospin-polarized transport properties can be effectively tuned by changing the central TM atoms, and only the MnPc dimer system exhibits perfect spin/thermal-spin filtering and sizeable giant magnetoresistance (GMR)/thermal-GMR effects. Meantime, the MnPc dimer system reveals a low-bias negative differential resistance effect under the parallel magnetic configuration. These findings suggest that the MnPc dimer system has great potential in developing the high-performance multifunctional spintronic and spin caloritronic devices.