Spin-polarized Current Injection Research Articles

Spin polarized current in chiral organic radical monolayers.

The chirality-induced spin selectivity (CISS) effect is the capability of chiral molecules to act as spin filters, i.e. to selectively sort flowing electrons based on their spin states. The application of this captivating phenomenon holds great promise in the realm of molecular spintronics, where the primary focus lies in advancing technologies based on chiral molecules to regulate the injection and coherence of spin-polarized currents. In this context, we conducted a study to explore the spin filtering capabilities of a monolayer of the thia-bridged triarylamine hetero[4]helicene radical cation chemisorbed on a metallic surface. Magnetic-conductive atomic force microscopy revealed efficient electron spin filtering at exceptionally low potentials. Furthermore, we constructed a spintronic device by incorporating a monolayer of these molecules in between two electrodes, obtaining an asymmetric magnetoresistance trend with signal inversion in accordance with the handedness of the enantiomer involved, indicative of the presence of the CISS effect. Our findings underscore the significance of thia[4]azahelicene organic radicals as promising candidates for the development of quantum information operations based on the CISS effect as a tool to control the molecular spin states.

Efficient Spin-Selective Electron Transport at Low Voltages of Thia-Bridged Triarylamine Hetero[4]helicenes Chemisorbed Monolayer.

The Chirality Induced Spin Selectivity (CISS) effect describes the capability of chiral molecules to act as spin filters discriminating flowing electrons according to their spin state. Within molecular spintronics, efforts are focused on developing chiral-molecule-based technologies to control the injection and coherence of spin-polarized currents. Herein, for this purpose, we study spin selectivity properties of a monolayer of a thioalkyl derivative of a thia-bridged triarylamine hetero[4]helicene chemisorbed on a gold surface. A stacked device assembled by embedding a monolayer of these molecules between ferromagnetic and diamagnetic electrodes exhibits asymmetric magnetoresistance with inversion of the signal according to the handedness of molecules, in line with the presence of the CISS effect. In addition, magnetically conductive atomic force microscopy reveals efficient electron spin filtering even at unusually low potentials. Our results demonstrate that thia[4]heterohelicenes represent key candidates for the development of chiral spintronic devices.

Magnetic skyrmion manipulation in CrTe2/WTe2 2D van der Waals heterostructure

Magnetic skyrmions in two-dimensional van der Waals materials provide an ideal platform to push skyrmion technology to the ultimate atomically thin limit. In this work, we theoretically demonstrate the Dzyaloshinskii–Moriya interaction and the formation of a Néel-type skyrmion lattice at the CrTe2/WTe2 bilayer van der Waals heterostructure. Our calculations suggest a field-controlled Néel-type skyrmion lattice—a ferromagnet transition cycle. In addition, a spin-torque induced by spin-polarized current injection was simulated in order to study the motion of a skyrmion on a racetrack, where an increase in the skyrmion Hall angle is observed at high temperatures. Consequently, this study suggests that generation and annihilation of skyrmions can be achieved with temperature or field control and also manipulate the velocity and the direction of the Néel-type skyrmions through ultra-low current densities and temperature, thus shedding light on the general picture of magnetic skyrmion control and design of two-dimensional van der Waals heterostructures.

Fabrication of chemically and structurally abrupt Eu1−xLaxO/SrO/Si interfaces and their analysis by STEM-EELS

Precise control of molecular-beam epitaxy deposition conditions has enabled the fabrication of chemically and structurally abrupt ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{O}/\mathrm{SrO}/\mathrm{Si}$ interfaces by interposing as little as half a monolayer of SrO between the La-doped EuO and silicon. The interfaces were analyzed by scanning transmission electron microscopy-electron energy loss spectroscopy; no reaction products were detected. These abrupt interfaces might enable direct ohmic injection of spin-polarized currents from the doped half-metallic semiconductor EuO into silicon.

Controlled Localized Domain Wall Writing in Antiferromagnetic Nano Stripes With In-Plane Transverse Currents

In order for textures in intrinsic antiferromagnets to be applied in proposed spintronics devices, localized and controllable writing is required. Here, the feasibility of a simple scheme for spin-Hall torque driven localized writing of chiral antiferromagnetic (AFM) domain wall (DW) pairs at a chosen location in AFM nano stripes is investigated. Short voltage pulses causing in-plane current-injection across transverse contacts in the heavy metal trigger a localized horizontal boundary-nucleated reverse AFM domain that propagates throughout the width to the opposite boundary, defining a locally written DW pair without affecting information in other parts of the device. The nucleation mechanism is examined, indicating exchange torque governing. A diagram of the number of DW-pairs written as a function of applied writing pulse amplitude and pulse duration is shown for a generic antiferromagnet, illustrating the degree of control required of the excitation protocol. Simulations of write and drive pulses are also presented, showing that the slight current bending around the writing contacts does not impose any significant pinning nor nucleation of unwanted textures for a typical driving amplitude. The advantages of this simple scheme are that it does not require any component for vertical spin-polarized current injection nor lasers for writing as in previous proposals.

Spin supercurrent in two-dimensional superconductors with Rashba spin-orbit interaction

Spin current is a central theme in spintronics, and its generation is a keen issue. The spin-polarized current injection from the ferromagnet, spin battery, and spin Hall effect have been used to generate spin current, but Ohmic currents in the normal state are involved in all of these methods. On the other hand, the spin and spin current manipulation by the supercurrent in superconductors is a promising route for dissipationless spintronics. Here we show theoretically that, in two-dimensional superconductors with Rashba spin-orbit interaction, the generation of dissipationless bulk spin current by charge supercurrent becomes highly efficient, exceeding that in normal states in the dilute limit, i.e. when the chemical potential is close to the band edge, although the spin density becomes small there. This result manifests the possibility of creating new spintronic devices with long-range coherence.

Electrically controlled spin-switch and evolution of Hanle spin precession in graphene

Next generation of spintronic devices aims to utilize the spin-polarized current injection and transport to control the magnetization dynamics in the spin logic and memory technology. However, the detailed evolution process of the frequently observed bias current-induced sign change phenomenon of the spin polarization has not been examined in details and the underlying microscopic mechanism is not well understood. Here, we report the observation of a systematic evolution of the sign change process of Hanle spin precession signal in the graphene nonlocal spintronic devices at room temperature. By tuning the interface tunnel resistances of the ferromagnetic contacts to graphene, different transformation processes of Hanle spin precession signal are probed in a controlled manner by tuning the injection bias current/voltage. Detailed analysis and first-principles calculations indicate a possible magnetic proximity and the energy dependent electronic structure of the ferromagnet-graphene interface can be responsible for the sign change process of the spin signal and open a new perspective to realize a spin-switch at very low bias-current or voltage.

Spin-polarized quantum transport in Fe4N based current-perpendicular-to-plane spin valve

Fe4N has been confirmed to possess high spin polarization of 81.3% and low Gilbert damping constant of 0.021 ± 0.02 in the recent experiment. To explore the potential applications of Fe4N in spintronics devices, the current-perpendicular-to-plane spin valve employing Fe4N as electrode and Ag as spacer is simulated to study the spin polarized quantum transport by utilizing the first principles calculations combined with nonequilibrium Green’s function. The project density of states (PDOS), transmission coefficient, spin-polarized current, magnetoresistance (MR) ratio and spin injection efficiency (SIE) as a function of bias voltage are studied. Our calculations reveal that spin down electron is the majority spin polarized electron and the absolute value of MR ratio of Fe4N/Ag/Fe4N at equilibrium reaches up to 174%, and it decreases with the bias increases. Besides, our results indicate that Fe4N/Ag/Fe4N device has stable SIE value of about 40% and stable MR ratios of about 150% when bias increases from 0 V to 0.1 V, and the device has a better performance within this voltage range.

Resonant Hall effect under generation of a self-sustaining mode of spin current in nonmagnetic bipolar conductors with identical characters between holes and electrons

We have proposed an enhancement mechanism of the Hall effect, the signal of which is amplified due to the generation of a sustaining mode of spin current. Our analytic derivations of the Hall resistivity revealed the conditions indispensable for the observation of the effect: (i) the presence of the transverse component of an effective electric field due to spin splitting in chemical potential in addition to the longitudinal component; (ii) the simultaneous presence of holes and electrons each having approximately the same characteristics; (iii) spin-polarized current injection from magnetized electrodes; (iv) the boundary condition for the transverse current (Jc,y = 0). The model proposed in this study was experimentally verified by using van der Pauw-type Hall devices consisting of the nonmagnetic bipolar conductor YHx (x ≃ 2) and TbFeCo electrodes. Replacing Au electrodes with TbFeCo electrodes alters the Hall resistivity from the ordinary Hall effect to the anomalous Hall-like effect with an enhancement factor of approximately 50 at 4 T. We interpreted the enhancement phenomenon in terms of the present model.

Spin-polarized current injection induced magnetic reconstruction at oxide interface

Electrical manipulation of magnetism presents a promising way towards using the spin degree of freedom in very fast, low-power electronic devices. Though there has been tremendous progress in electrical control of magnetic properties using ferromagnetic (FM) nanostructures, an opportunity of manipulating antiferromagnetic (AFM) states should offer another route for creating a broad range of new enabling technologies. Here we selectively probe the interface magnetization of SrTiO3/La0.5Ca0.5MnO3/La0.7Sr0.3MnO3 heterojunctions and discover a new spin-polarized current injection induced interface magnetoelectric (ME) effect. The accumulation of majority spins at the interface causes a sudden, reversible transition of the spin alignment of interfacial Mn ions from AFM to FM exchange-coupled, while the injection of minority electron spins alters the interface magnetization from C-type to A-type AFM state. In contrast, the bulk magnetization remains unchanged. We attribute the current-induced interface ME effect to modulations of the strong double-exchange interaction between conducting electron spins and local magnetic moments. The effect is robust and may serve as a viable route for electronic and spintronic applications.

Транспортні властивості надпровідних мікромістків свинцю під дією спін-поляризованої струмової інжекції

Транспортні властивості надпровідних мікромістків свинцю під дією спін-поляризованої струмової інжекції

Fabrication of hybrid thin film structures from HTS and CMR materials

We present the preparation of bilayers from high-temperature superconductors (HTS) and half-metallic ferromagnetic (FM) manganite with a colossal magnetoresistance (CMR). We used YBa2Cu3O7-x (YBCO) and Tl2Ba2CaCu2O8 (TBCCO) thin films as a HTS material and La0.67Sr0.33MnO3 (LSMO) film as a CMR material. In the case of YBCO/LSMO, we prepared FM/HTS heterostructure for studying the spin-polarized current injection effect on the electrical properties of the YBCO strip in dc or low-frequency regimes and on the microwave characteristics of the strip. For the first time, we report the preparation of a TBCCO/LSMO bilayer. In some applications, the TBCCO offers better parameters (higher working temperature, lower surface resistance, lower 1/f noise) than YBCO.

Electrical injection and detection of spin-polarized currents in topological insulator Bi2Te2Se.

Topological insulators (TIs) are an unusual phase of quantum matter with nontrivial spin-momentum-locked topological surface states (TSS). The electrical detection of spin-momentum-locking of TSS has been lacking till very recently. Many of the results are from samples with significant bulk conduction, such as Bi2Se3, where it can be challenging to separate the surface and bulk contribution to the spin signal. Here, we report spin potentiometric measurements in flakes exfoliated from bulk insulating Bi2Te2Se crystals, using two outside nonmagnetic contacts for driving a DC spin helical current and a middle ferromagnetic (FM)-Al2O3 contact for detecting spin polarization. The voltage measured by the FM electrode exhibits a hysteretic step-like change when sweeping an in-plane magnetic field between opposite directions along the easy axis of the FM contact. Importantly, the direction of the voltage change can be reversed by reversing the direction of current, and the amplitude of the change as measured by the difference in the detector voltage between opposite FM magnetization increases linearly with increasing current, consistent with the current-induced spin polarization of spin-momentum-locked TSS. Our work directly demonstrates the electrical injection and detection of spin polarization in TI and may enable utilization of TSS for applications in nanoelectronics and spintronics.

Magnetization switching of a metallic nanomagnet via current-induced surface spin-polarization of an underlying topological insulator

We consider a thermally stable, metallic nanoscale ferromagnet (FM) subject to spin-polarized current injection and exchange coupling from the spin-helically locked surface states of a topological insulator (TI) to evaluate possible non-volatile memory applications. We consider parallel transport in the TI and the metallic FM, and focus on the efficiency of magnetization switching as a function of transport between the TI and the FM. Transport is modeled as diffusive in the TI beneath the FM, consistent with the mobility in the TI at room temperature, and in the FM, which essentially serves as a constant potential region albeit spin-dependent except in the low conductivity, diffusive limit. Thus, it can be captured by drift-diffusion simulation, which allows for ready interpretation of the results. We calculate switching time and energy consumed per write operation using self-consistent transport, spin-transfer-torque (STT), and magnetization dynamics calculations. Calculated switching energies and times compare favorably to conventional spin-torque memory schemes for substantial interlayer conductivity. Nevertheless, we find that shunting of current from the TI to a metallic nanomagnet can substantially limit efficiency. Exacerbating the problem, STT from the TI effectively increases the TI resistivity. We show that for optimum performance, the sheet resistivity of the FM layer should be comparable to or larger than that of the TI surface layer. Thus, the effective conductivity of the FM layer becomes a critical design consideration for TI-based non-volatile memory.

Optical characterization of nonlocal spin transfer torque acting on a single nanomagnet

Time-resolved scanning Kerr microscopy (TRSKM) has been used to examine the effect of nonlocal spin transfer torque (NL-STT) within a two-terminal current perpendicular-to-plane nonlocal spin valve. A combination of Oersted field and NL-STT was used to excite and control the magnetization dynamics. By comparison with a macrospin model, the strength of the NL-STT was quantified and found to be comparable to that achieved by direct injection of spin-polarized current. The sensitivity of the magneto-optical probe to small out-of-plane deflections of the magnetization also allows the NL-STT to be quantified in quasi-dc measurements, greatly simplifying the investigation of STT phenomena.

Spin torque in the framework of random magnetization dynamics driven by a jump-noise process

It is demonstrated that the Slonczewski spin-torque term can be naturally derived from the equation for magnetization dynamics driven by a jump-noise process. The central point of the derivation is the modification of transition probability rate of the jump-noise process caused by spin-polarized current injection. This modification results in two distinct terms in the expected value of the jump-noise process: the traditional one corresponding to the Landau–Lifshitz damping and another one representing the Slonczewski spin-torque term.

Spin polarized carrier injection from full Heusler alloy Co2MnSi into superconducting NbN

The spin-polarized current injection (SPCI) experiments, where the spin-polarized current (IS) is injected into a 25 μm wide superconducting NbN bridge from a ferromagnetic Co2MnSi layer by tunneling through MgO barrier, show a large (≈67%) suppression of superconducting critical current (IC) due to IS injection at T/TC ≈ 0.4. This corresponds to a large dynamic gain of ≈36 at 3 K, which is 40 times higher than the gain at 6.5 K. Such a rapid rise of gain at lower temperatures strongly suggests a dominant role of SPCI in IC suppression as compared to the effect of Joule-heating.

New method of spin injection into organic semiconductors using spin filtering tunnel barriers

In an organic light emitting diode, injection of spin-polarized current can be expected to influence the light emission. For the emission to occur, the bias voltage must exceed the device turn-on voltage. However in magnetic junctions with conventional ferromagnetic electrodes, the magnetoresistance decreases as bias increases. This response limits the use of such ferromagnets for spin injection in organic light emitting diodes where high bias is needed. A way out of this limitation is shown here, by utilizing the spin-filter phenomenon to generate current spin polarization, and inject into an organic semiconductor. Europium sulfide was used as the spin-filter layer with rubrene as the barrier to form a quasi-magnetic junction. The junctions showed magnetoresistance which increased as bias voltage was increased, with a high value of 6% at 1.8 V. These results show the potential to use spin-filter systems for investigating electroluminescence in organic light emitting diodes with spin-polarized electrons. Ferrite compound spin-filters with Curie temperature much above room temperature are a potential class of materials to realize room temperature spintronics devices.

Influence of surface treatment and interface layers on electrical spin injection efficiency and transport in InAs

Spin-valve, weak localization/antilocalization, and scanned probe microscopy measurements are used to investigate the influence of sulfur-based surface treatments and electrically insulating barrier layers on spin injection into, and spin transport within, the two-dimensional electron layer at the surface of p-type InAs at 4.2 K. An electrically insulating barrier layer is found to be required to achieve nonzero spin injection efficiency, with a 3 nm Al2O3 electrically insulating barrier providing a spin injection efficiency of 5±2%. Conductive atomic force microscopy suggests that localized leakage through the InAs native oxide is sufficient to suppress spin-polarized current injection in the absence of a more highly insulating barrier layer. Spin scattering lengths are determined experimentally from both weak localization/antilocalization and spin-valve measurements. Spin and elastic scattering lengths of 230±20 and 85±5 nm, respectively, are measured, with a sulfur-based surface treatment increasing the spin scattering length to 250±20 nm and decreasing the elastic scattering length to 65±5 nm.

Permalloy-based carbon nanotube spin-valve

In this paper we demonstrate that permalloy (Py), a widely used Ni/Fe alloy, forms contacts to carbon nanotubes (CNTs) that meet the requirements for the injection and detection of spin-polarized currents in carbon-based spintronic devices. We establish the material quality and magnetization properties of Py strips in the shape of suitable electrical contacts and find a sharp magnetization switching tunable by geometry in the anisotropic magnetoresistance (AMR) of a single strip at cryogenic temperatures. In addition, we show that Py contacts couple strongly to CNTs, comparable to Pd contacts, thereby forming CNT quantum dots at low temperatures. These results form the basis for a Py-based CNT spin-valve exhibiting very sharp resistance switchings in the tunneling magnetoresistance, which directly correspond to the magnetization reversals in the individual contacts observed in AMR experiments.