Ferromagnetic Junctions Research Articles

Electric and thermal spin transfer torques across ferromagnetic/normal/ferromagnetic graphene junctions

We investigate the spin transfer torque (STT) driven by electric bias voltages across and temperature gradients through ferromagnetic/normal/ferromagnetic graphene junctions. Due to the unique band structure of the ferromagnetic graphene, there exists two transport regimes: the electron to electron (I) and hole to electron (II) transport. The electric STTs originated from the two regimes have opposite sign and can be reduced by the competition between the two transport processes. On the contrary, the thermal STTs originated from the transport regimes I and II have the same sign and are enhanced when the two regimes coexist. Remarkably, the thermal STT is comparable with the electric STT. Furthermore, the electric and thermal counterpart can be manipulated by the Fermi level. The controllable STT reported here makes the ferromagnetic graphene junction ideal for future spintronics applications.

Thermoelectric properties of graphene-like nanoribbon studied from the perspective of symmetry

We have studied the charge and spin thermopower systematically in a ferromagnetic junction of graphene-like zigzag nanoribbon modified by two on-site disorders in the tight-binding model. Symmetries of the transmission spectra and geometry configuration of the two disorders are important factors in determining the thermoelectric properties of the system. Conditions to achieve pure charge and pure spin thermopower are discussed from the perspective of symmetry. Symmetry breaking is required sometimes to obtain large figure of merit. The type and strength of the disorders can be used to further manipulate the spin polarization of thermal current. Disorders inside nanoribbon instead of on edge can then be used to finely tune the performance of the junction. The results may have great application value in designing thermoelectric devices.

Charge transport in silicene-based ferromagnetic p-wave superconductor junctions

Using Blonder-Tinkham-Klapwijk (BTK) formalism we studied the transport properties in silicene-based normal/ferromagnetic p-wave superconductor (N/FpS) junctions. Our study shows that the amplitude of the normalized conductance strongly depends on the pairing symmetry in superconductor such as px, py and px+ipy-wave symmetries, the applied electric field (EZ) and the exchange field (h) in FpS. In N/FpxS-silicene junctions the zero-bias conductance peak is obtained due to the Andreev bound states. When the electric field and magnetic field are parallel, called P configuration, there is one conductance dip (CD) for the voltage eV=lEz−λSO with λSO the spin obit coupling term and 2l the distance between two sublattices in the buckled silicene. In the anti-parallel (AP) configuration there is one CD at eV=lEz−λSO for 0<lEz−λSO<h and for lEz−λSO≥h there are two CDs at eV=h and eV=lEz−λSO, respectively. A sudden transition of conductance from zero to maximum (henceforth referred to as“switching of conductance”) at eV=lEz−λSO occurs in both N/FpyS-silicene junction and N/Fpx+ipyS-silicene junction. In N/FpyS-silicene junction the Andreev resonant peak appears at eV=h for lEz−λSO−h=0.1Δ00 in the AP case, where Δ00 is the zero temperature gap of the FpS. In N/Fpx+ipyS-silicene junctions the maximum value of the conductance appears at eV=Δ00−h in P junction while in AP junction at eV=Δ00+h.

Even-odd chain effect and perpendicular electric field induced spin valve in a zigzag silicene ferromagnetic junction

We study the spin transport in a zigzag silicene nanoribbon (ZSNR) ferromagnetic junction with different even(odd)-chain widths and a perpendicular electric field on its drain electrode. By using the tight-binding lattice model and Keldysh Green’s function method, we calculate the spin-dependent transverse conductance and find that the spin switching effect always exists in the system regardless of the zigzag chain number is an even or an odd. The spin valve phenomena come from the combination of the even-odd chain effect and the perpendicular electric field induced energy gap. When a transmission electron flows from the left lead to the middle scattering region, the transport exhibits a peculiar even-odd chain effect, and then the electron may be prohibited to pass through the middle-right tunneling junction when a perpendicular electron field is applied on the right terminal. The spin transport in the proposed device can be controlled by almost all the system parameters such as Fermi energy, electric field, spin orbit coupling, ferromagnetic magnetization and width of the ZSNR. Furthermore, the proposed device might act as an electrical tunable spin valve to produce and detect a 100% polarized spin current.

Controllable photoenhanced spin- and valley-polarized transport in ferromagnetic MoS2 junction

We propose fully valley- and spin-polarized currents in a normal/ferromagnetic/normal MoS2 junction by applying a circularly polarized light in the off-resonance regime in the ferromagnetic (FM) region. It is found that the circularly polarized light and exchange field remove the valley and spin degeneracies of the conduction band edges, respectively. In this situation, by choosing appropriate values for gate voltage, the Fermi level intersects with one valley form one spin channel leading to the fully valley- and spin-polarized currents through the proposed junction. Interestingly, the sign of both valley and spin polarizations can be easily switched from −100% to 100% by varying the intensity of the circularly polarized light and gate voltage. The full valley- and spin-polarized currents provide desirable routines to construct spin/valley filter for MoS2 based devices.

Evolution of 0.7 conductance anomaly in electric field driven ferromagnetic CuO junction based resistive random access memory devices

This study demonstrates the origin of 0.7 conductance anomaly and non-volatile reversible control of switching in atomically controlled resistive random access memory devices. In quantum devices, 0.7 structure and zero bias anomaly (ZBA) are considered as the natural presence of spin state. Here, we show that similar behavior can be detected in an atomically thin electrically controllable break junction based on Cu/Ti/HfO2/TiN resistive switches in which the origin of the 0.7 structure is statistically controlled by the device design, filament geometry, and optimized electrical measurement scheme. Evidence of temperature dependent evolution of 0.7 plateau and low temperature ZBA is presented here. Lorentzians of the experimental result indicate the formation of a ZBA and non-ZBA like state at a low resistance state and a high resistance state of the resistive switches. Repeated spin-like switching is achieved by controlling the chemical composition of the filament in the sub-quantum region by the electric field driven ion redistribution process. During switching, formation of a ferromagnetic CuO atom in the break junction surrounded by Cu electron sources, i.e., nanoionics, manifests the suitable environment for spin-like behavior.

Thickness and composition-dependent spin-orbit torque behaviors in perpendicularly magnetized Ta/W (t)/CoFeB and Ta1-xWx/CoFeB junction structures

The spin-orbit torque (SOT) generated in normal metal (NM)/ferromagnet (FM) junctions is of technological interest as it can reduce the critical current density required for magnetization switching in memory devices. Ta and W, 5d NMs, exhibit large spin-orbit couplings and are compatible with the semiconductor processing. In this study, we investigate variations in SOT properties of Ta/W(t)/CoFeB/MgO/Ta (bilayer NM structures, where t is the W thickness) and Ta1-xWx/CoFeB/MgO/Ta (alloyed NM structures, where x is the W concentration in at%) junctions prepared on Si substrates with thermal oxides. In the former structure, the effective spin-orbit torque (SOT) field gradually increases with the W thickness reaching its maximum at 2.0 nm, followed by a decrease. In the latter structure, the effective SOT field behaves differently in the Ta- and W-rich regions. In-plane-current-induced switching measurements show similar trends, where the minimum and maximum switching current densities for the bilayer and alloyed NM structures are 3.4–10.3 and 0.81 to 6.3 × 107 A/cm2, respectively. Microstructural analyses by X-ray diffraction and transmission electron microscopy suggest that the crystal phase of W changes from the amorphous to the α phase in the bilayer NM structure with the increase in the W thickness and the alloyed NM structure with the change in the concentration.

Two-dimensional van der Waals spinterfaces and magnetic-interfaces

Two-dimensional (2D) materials have brought fresh prospects for spintronics, as evidenced by the rapid scientific progress made in this frontier over the past decade. In particular, for charge perpendicular to plane vertical magnetic tunnel junctions, the 2D crystals present exclusive features such as atomic-level thickness control, near-perfect crystallography without dangling bonds, and novel electronic structure-guided interfaces with tunable hybridization and proximity effects, which lead to an entirely new group of spinterfaces. Such crystals also present new ways of integration of atomically thin barriers in magnetic tunnel junctions and an unprecedented means for developing composite barriers with atomic precision. All these new aspects have sparked interest for theoretical and experimental efforts, revealing intriguing spin-dependent transport and spin inversion effects. Here, we discuss some of the distinctive effects observed in ferromagnetic junctions with prominent 2D crystals such as graphene, hexagonal boron nitride, and transition metal dichalcogenides and how spinterface phenomena at such junctions affect the observed magnetoresistance in devices. Finally, we discuss how the recently emerged 2D ferromagnets bring upon an entirely novel category of van der Waals interfaces for efficient spin transmission and dynamic control through exotic heterostructures.

Symmetric and antisymmetric electric fields induced spin valves in a zigzag silicene nanoribbon ferromagnetic junction

We report a possible two dimensional spin switch based on an even-chain zigzag silicene nanoribbon (ZSR) ferromagnetic junction with two perpendicular staggered electric fields. By calculating the transverse conductance of the device, we find that the metal-insulator phase transition could always occur in the system regardless of two staggered electric fields are symmetric or antisymmetric along the y -axis of the ribbon, which can act as an electrical tunable spin valve to filter and detect one species of spin state. When two symmetric staggered electric fields are applied, both spin up and spin down electrons are prohibited to transport in the same energy region due to the energy gap opened by the homogeneous staggered potentials, which is independent of whether two electric fields are fully or partially distributed. When two staggered electric fields are anti-symmetrically put on the ZSR junction, it has the same energy band symmetry(pseudo-parity) as the pristine ZSR without applying the electric field, the energy band selection rule appears and can be used to generate, separate and detect 100% polarized spin currents combined with the staggered potential. Our findings may provide a possible way to design an electrical controllable spin switch in two dimensional graphene-like materials.

Non-Hermitian Weyl physics in topological insulator ferromagnet junctions

We introduce and investigate material junctions as a generic and tuneable electronic platform for the realization of exotic non-Hermitian (NH) topological states of matter, where the NH character is induced by the surface self-energy of a thermal reservoir. As a conceptually rich and immediately experimentally realizable example, we consider a three-dimensional topological insulator (TI) coupled to a ferromagnetic lead. Remarkably, the symmetry protected TI is promoted in a dissipative fashion to a non-symmetry protected NH Weyl phase with no direct Hermitian counterpart and which exhibits robustness against any perturbation. The transition between a gapped phase and the NH Weyl phase may be readily tuned experimentally with the magnetization direction of the ferromagnetic lead. Given the robustness of this exotic nodal phase, our general analysis also applies to, e.g., a two-dimensional electron gas close to criticality in proximity to a ferromagnetic lead. There, the predicted bulk Fermi arcs are directly amenable to surface spectroscopy methods such as angle-resolved photoemission spectroscopy.

Spin-dependent quantum beating of the conductance oscillations in asymmetric velocity modulated ferromagnetic graphene junction

Abstract The mismatch effect of a double Fermi-velocity barrier structures NG/FG 1 /NG/FG 2 /NG junction on the electrically driven SdH-like oscillations of spin-dependent conductance is investigated, where NG is normal graphene and FG 1,2 is the ferromagnetic-gate with Fermi-velocity modulation ξ 1,2 where ξ 1,2 is the Fermi velocity ratio between NG region and FG 1,2 region. The results show that the effect of asymmetry of the double Fermi-velocity barrier structures, the difference value between ξ 1 and ξ 2 cause of the oscillations the spin-dependent quantum beating pattern. Which the beating patterns of the spin transport appeared as a result of the superposition of the two oscillatory components in the double Fermi velocity barriers. In addition, we found that the shifting of the spin-dependent quantum beats of the G ↑ and G ↓ is due to the presence of the exchange field, H, that is, the G ↑ and G ↓ are shifted in the opposite direction. By the difference value of the phase shift can be estimated to be 2H . In case of U + η σ H > > E F which electrons are collimated, the oscillations behaviors show the quantum beating with holding constant amplitude and the period of the beating amplitude depend on the Fermi velocity ratio mismatch in FG regions. These interesting characteristic of spin-dependent quantum transport may identify the effect of Femi-velocity asymmetry on double proximity-induced ferromagnetic graphene structure for making spintronic devices.

Universal 13 -suppression of magnonic shot noise in diffusive insulating magnets

Extending a Boltzmann-Langevin theory to magnons, we show a universality of current-noise suppression in diffusive systems against the difference of quantum-statistical properties of bosons and fermions. To this end, starting from a quantum kinetic equation for magnons subjected to thermal gradient in dilute impurities, we derive a bosonic counterpart of the semiclassical Boltzmann-Langevin equation for electrons and evaluate a magnonic current-noise in the diffusive insulating magnet. We theoretically discover that compared with a Poissonian shot noise of magnons in an insulating ferromagnetic junction, the magnonic shot noise is suppressed in the diffusive insulating bulk magnet and noise-to-current ratio (Fano factor) at low temperatures exhibits a universal behavior, i.e., the same suppression $1/3$ as the one for electron transport in diffusive conductors, despite the difference of quantum-statistical properties. Finally, we show that our predictions are within experimental reach with current device and measurement technologies.

Anomalous zero bias conductance peak in a ferromagnetic graphene junction with d-wave anisotropic superconducting pair symmetry

We theoretically present an anomalous zero bias conductance peak (ZBCP) in graphene junctions with proximity-induced ferromagnetism and d-wave anisotropic superconducting pair symmetry (ASPS) herein. It is revealed that in the thin insulator limit, the ZBCP can be periodically recovered by adjusting the sandwiched insulating barrier strength χ regardless of exchange field h and the phase of the periodical behaviours for h larger than its Fermi energy EF (h &amp;gt; EF) is exactly opposite to that for h &amp;lt; EF. Most interestingly, in the context of h &amp;gt; EF, the periodic oscillation of the nonzero bias conductance located in the ZBCP versus χ, is accompanied by an explicit splitting peak. Moreover, under the situation of the insulator with finite width, the conductance exhibits a stronger damping oscillation with bias voltage eV for any h, which is also accompanied by a splitting ZBCP at h &amp;gt; EF. These singular features originate from ferromagnetic-modulated midgap states characteristic by the relativistic nodal fermions, which confirms the spin polarization and ASPS of the graphene, and thus will be of great interest in the designing and fabrication of graphene superconducting spintronic devices.

Theoretical perspectives on transport of charge across ferromagnet/superconductor junction: New approach to verify quality of junction

A new approach to verify the quality of the magnetic tunnel junction between a ferromagnet and a superconductor single junction (FM/SC) is presented. In this work, we theoretically studied the tunneling conductance spectra of a FM/SC using the scattering method under a free electron approximation model in a two dimensional system. The main area of interest was the physical properties of a junction that was defined by the Dirac delta potential. It was found that the kinks occurring at the energy scale of the ferromagnetic material were suppressed when increasing the strength of the interfacial scattering, but they dominated at the metallic limit of the junction, where the potential strength at the junction was weak. Experimentally, this feature can be used to identify the quality of a ferromagnetic and superconductor junction by considering the shape of those kinks. Furthermore, when the effect of interfacial scattering Z and the conductivity mismatch on the conductance spectra were considered and compared, we found that the conductance spectra of the junction was sensitive to a change in the strength of Z.

Coherent spin transport properties of ferromagnetic graphene superlattice unit cell

The Aim of this paper is to study the properties of spin transport for ferromagnetic graphene superlattice unit cell junction taking into account the induced ac-field at different frequencies. Transfer matrix method is used for both spin alignments to calculate the conductance in terms of which spin polarization and giant magneto-resistance are also expressed. The results show that an oscillatory behavior is strongly exhibited by the three calculated quantities. These oscillations could be explained by the mutual effect between the magnetic field of the ferromagnetic insulator EuO, which causes spin filtering, and the induced photon energy of applied ac-field. The present research might have a scientific potential in the design and understanding of graphene superlattice based spin filters by optimizing the different parameters studied in this paper.

Spin transport and spin pump in graphene-like materials: effects of tilted Dirac cone

We study the spin transport phenomena in two-dimensional graphene-like materials with arbitrary tilted Dirac cones. The tilt arises due to next-nearest hopping when the bottom of the conduction band and top of the valence band does not simultaneously coincide at Dirac point. We consider normal-ferromagnetic-normal (N-F-N) junction of the materials and using the generalized scattering approach calculate the spin current. Here, we show that tilting the Dirac cones can strongly change the transport properties by modifying the period of oscillation of the spin current. The spin conductance can be effectively tuned by the tilt with taking advantage of the modified interference condition. A pure spin current reversal also possible with a smooth variation of the tilting. We further study the spin current by the adiabatic precession of a doped ferromagnet on top of the material. It is shown that the spin-mixing conductance and hence the spin current can become zero by turning the tilt of the Dirac cone. Our findings provide an efficient way towards high controllability of spin transport by tuning the tilt of the ferromagnetic junction and can be very useful in the field of spintronics. The model also presents a simplified way to measure the tilt of Dirac cone of those materials.

Tuning of Magnetic Activity in Spin-Filter Josephson Junctions Towards Spin-Triplet Transport.

The study of superconductor-ferromagnet interfaces has generated great interest in the last decades, leading to the observation of spin-aligned triplet supercurrents and 0-π transitions in Josephson junctions where two superconductors are separated by an itinerant ferromagnet. Recently, spin-filter Josephson junctions with ferromagnetic barriers have shown unique transport properties, when compared to standard metallic ferromagnetic junctions, due to the intrinsically nondissipative nature of the tunneling process. Here we present the first extensive characterization of spin polarized Josephson junctions down to 0.3K, and the first evidence of an incomplete 0-π transition in highly spin polarized tunnel ferromagnetic junctions. Experimental data are consistent with a progressive enhancement of the magnetic activity with the increase of the barrier thickness, as neatly captured by the simplest theoretical approach including a nonuniform exchange field. For very long junctions, unconventional magnetic activity of the barrier points to the presence of spin-triplet correlations.

Spin Polarization Dynamics of Nonequilibrium Conduction Electrons in Magnetic Junctions

The dynamics of the motion of the magnetic moment averaged over an ensemble of nonequilibrium spin-injected electrons in a ferromagnetic junction is considered with allowance for the exchange inter ...

Spin dependence of crossed Andreev reflection and electron tunneling induced by Majorana fermions

We investigate spin dependence of the nonlocal transport induced by Majorana fermions in a one-dimensional ferromagnet–ferromagnetic-insulator–superconductor–ferromagnetic-insulator–ferromagnet junction on the edge of a two-dimensional topological insulator. The results show that coupled Majorana fermions lead to the nonlocal transport processes including electron tunneling and crossed Andreev reflection, which can be tuned by adjusting the spin polarizations of the Majorana fermions. By manipulating the bands in the two ferromagnets, the nonlocal transport can be selected as either pure electron tunneling or pure crossed Andreev reflection, the transmission probability of which could be 100%. Furthermore, the pure electron tunneling and the pure crossed Andreev reflection are well controlled by the spin directions of the electron states in the two ferromagnets.

Valley controlled spin-transfer torque in ferromagnetic graphene junctions

The presence of the valley degree of freedom in graphene leads to the valleytronics, in which information is encoded by the valley quantum number of the electron. We propose a valley controlled spin-transfer torque (STT) in graphene-based normal/normal/ferromagnetic junctions with the normal lead irradiated by the off-resonant circularly polarized light. The interplay of the spin–orbit interaction and the staggered potential in the central normal part results in the coupling between the valley and spin degrees of freedom, so a valley dependent spin polarized current can be demonstrated, which can exert a valley controlled STT on the ferromagnetic lead. The amplitude of the STT can be manipulated by the intensity of the light, the Fermi energy and the magnetization direction of the ferromagnetic lead. This valley controlled STT may find potential application in future valleytronics and spintronics.