Spin-based Devices Research Articles

Modeling for Spin-FET and Design of Spin-FET-Based Logic Gates

Spintronics-based devices and circuits attract massive research interest from both academia and industry. A number of the devices and logic circuits have been proposed such as spin-based magnetic tunnel junction and all spin logic gate. A fundamental spin-based device, spin field-effect transistor (spin-FET) is one of the most interesting spin-based devices to address the power issue of semiconductor transistors which is still a research focus. In this paper, we first present an electrical model for the spin-FET based on both theoretical and experimental results. The theories of spin injection and detection are considered by a current driver of the spin-FET. Gate voltage modulation following Datta–Das theory is combined with the experimental results from several works of literature. Afterward, through the dc analysis of two spin-FETs with different channel materials, we demonstrate that the channel using InAs is a better choice to make a feasible spin-FET. The channel length is also optimized by the comparison of simulation results. Finally, a local geometry spin-FET model suitable for logic design is implemented with Verilog-A language and integrated on Cadence platform. Using our model, a low-power inverter is designed based on the concept of complementary spin-FET, and a logic circuit is proposed to implement AND and NOR logic functions. Simulation results validate the behaviors of the logic circuits and availability of our model.

Polarized emission in II–VI and perovskite colloidal quantum dots

The polarized emission of colloidal quantum dots from II–VI and perovskite semiconductors were investigated thoroughly, revealing information about the optical transitions in these materials and their potential use in various opto-electronic or spintronic applications. The studies included recording of the micro-photoluminescence of individual nanostructures at cryogenic temperatures, with or without the influence of an external magnetic field. The experimental conditions enabled detection of circular and/or linear polarized emission to elucidate the exciton manifolds, angular momentum of the emitting states, Landé g-factors, single exciton and bi-exciton binding energies, the excitons’ effective Bohr radii, and the unique influence of the Rashba effect. The study advances the understanding of other phenomena such as electron–hole dissociation, long diffusion lengths, and spin coherence, facilitating appropriate design of optical and spin-based devices.

Capping Layer (CL) Induced Antidamping in CL/Py/β-W System (CL: Al, β-Ta, Cu, β-W).

For achieving ultrafast switching speed and minimizing dissipation losses, the spin-based data storage device requires a control on effective damping (αeff) of nanomagnetic bits. Incorporation of interfacial antidamping spin orbit torque (SOT) in spintronic devices therefore has high prospects for enhancing their performance efficiency. Clear evidence of such an interfacial antidamping is found in Al capped Py(15 nm)/β-W(tW)/Si (Py = Ni81Fe19 and tW = thickness of β-W), which is in contrast to the increase of αeff (i.e., damping) usually associated with spin pumping as seen in Py(15 nm)/β-W(tW)/Si system. Because of spin pumping, the interfacial spin mixing conductance (g↑↓) at Py/β-W interface and spin diffusion length (λSD) of β-W are found to be 1.63(±0.02) × 1018 m-2 (1.44(±0.02) × 1018 m-2) and 1.42(±0.19) nm (1.00(±0.10) nm) for Py(15 nm)/β-W(tW)/Si (β-W(tW)/Py(15 nm)/Si) bilayer systems. Other different nonmagnetic capping layers (CL), namely, β-W(2 nm), Cu(2 nm), and β-Ta(2,3,4 nm) were also grown over the same Py(15 nm)/β-W(tW). However, antidamping is seen only in β-Ta(2,3 nm)/Py(15 nm)/β-W(tW)/Si. This decrease in αeff is attributed to the interfacial Rashba like SOT generated by nonequilibrium spin accumulation subsequent to the spin pumping. Contrary to this, when interlayer positions of Py(15 nm) and β-W(tW) is interchanged irrespective of the fixed top nonmagnetic layer, an increase of αeff is observed, which is ascribed to spin pumping from Py to β-W layer.

Synthesis and Characterization of Quinoidal Diketopyrrolopyrrole Derivatives with Exceptionally High Electron Affinities

Open-shell singlet biradicaloids are short-lived intermediates, but they exhibit fascinating properties for spin-based devices. Therefore, understanding the nature of their electronic structure and stability is critical for harnessing them in optoelectronic or spintronic devices. Toward this goal, we have synthesized a series of diketopyrrolopyrrole-based quinoidal molecules to investigate the contribution and relative importance of the biradical form on the ground-state electronic structure and distribution of spin density. Possibility of crossover from a closed-shell to an open-shell structure with increase in the C═C/C–C conjugation length was investigated. The ground-state properties were systematically investigated by nuclear magnetic resonance (NMR) spectroscopy, single-crystal X-ray diffraction, and electrochemical studies. Furthermore, n-doping has been carried out in solution at ambient conditions to understand the nature of doped species and demonstrate air stability. Doped species were probed b...

Development of spontaneous magnetism and half-metallicity in monolayer MoS2

Half-metallic behavior and ferromagnetism are predicted in strained MoS2 with different light elements adsorbed using density functional theory. We find that strain increases the density of states at the Fermi energy for Y doping (Y=H, Li, and F) at the S sites and strain-driven magnetism develops in agreement with the Stoner mean field model. Strain-driven magnetism requires less strain (∼3%) for H doping as compared with F and Li doping. No saturation of the spin-magnetic moment is observed in Li-doped MoS2 due to less charge transfer from the Mo d electrons and the added atoms do not significantly increase the Spin–orbit coupling. Half-metallic ferromagnetism is predicted in H and F-doped MoS2. Fixed magnetic moments calculations are also performed, and the DFT computed data is fitted with the Landau mean field theory to investigate the emergence of spontaneous magnetism in Y-doped MoS2. We predict spontaneous magnetism in systems with large (small) mag netic moments for H/F (Li) atoms. The large (small) magnetic moments are atttributed to the electronegativity difference between S and Y atoms. These results suggest that H and F adsorbed monolayer MoS2 is a good candidate for spin-based electronic devices.

Rashba Effect in a Single Colloidal CsPbBr3 Perovskite Nanocrystal Detected by Magneto-Optical Measurements.

This study depicts the influence of the Rashba effect on the band-edge exciton processes in all-inorganic CsPbBr3 perovskite single colloidal nanocrystal (NC). The study is based on magneto-optical measurements carried out at cryogenic temperatures under various magnetic field strengths in which discrete excitonic transitions were detected by linearly and circularly polarized measurements. Interestingly, the experiments show a nonlinear energy splitting between polarized transitions versus magnetic field strength, indicating a crossover between a Rashba effect (at the lowest fields) to a Zeeman effect at fields above 4 T. We postulate that the Rashba effect emanates from a lattice distortion induced by the Cs+ motion degree of freedom or due to a surface effect in nanoscale NCs. The unusual magneto-optical properties shown here underscore the importance of the Rashba effect in the implementation of such perovskite materials in various optical and spin-based devices.

Gate-dependent spin Hall induced nonlocal resistance and the symmetry of spin-orbit scattering in Au-clustered graphene

Engineering the electron dispersion of graphene to be spin-dependent is crucial for the realization of spin-based logic devices. Enhancing spin-orbit coupling in graphene can induce spin Hall effect, which can be adapted to generate or detect a spin current without a ferromagnet. Recently, both chemically and physically decorated graphenes have shown to exhibit large nonlocal resistance via the spin Hall and its inverse effects. However, these nonlocal transport results have raised critical debates due to the absence of field dependent Hanle curve in subsequent studies. Here, we introduce Au clusters on graphene to enhance spin-orbit coupling and employ a nonlocal geometry to study the spin Hall induced nonlocal resistance. Our results show that the nonlocal resistance highly depends on the applied gate voltage due to various current channels. However, the spin Hall induced nonlocal resistance becomes dominant at a particular carrier concentration, which is further confirmed through Hanle curves. The obtained spin Hall angle is as high as $\ensuremath{\sim}0.09$ at 2 K. Temperature dependence of spin relaxation time is governed by the symmetry of spin-orbit coupling, which also depends on the gate voltage: asymmetric near the charge neutral point and symmetric at high carrier concentration. These results inspire an effective method for generating spin currents in graphene and provide important insights for the spin Hall effect as well as the symmetry of spin scattering in physically decorated graphene.

Asymmetrical domain wall propagation in bifurcated PMA wire structure due to the Dzyaloshinskii-Moriya interaction

Controlling domain wall (DW) motion in complex magnetic network structures is of paramount significance for the development of spin-based devices. Here, we report on the dynamics of a propagating DW in a bifurcated ferromagnetic wire with perpendicular magnetic anisotropy (PMA). The Dzyaloshinskii-Moriya interaction (DMI) in the wire structure induces a tilt angle to the injected DW, which leads to a quasi-selective propagation through the network branch. The DW tilting causes a field interval between DWs to arrive at Hall bars in the individual branches. Micromagnetic results further show that by tailoring the strength of the DMI, the control of DW dynamics in the PMA complex network structures can be achieved.

Doping-Concentration-Induced Ferromagnetism and Antiferromagnetism in In2S3:Dy3+ Quantum Dots

Diluted magnetic semiconductor (DMS) quantum dots have been researched extensively due to their potential applications in next-generation spin-based devices. Herein, the cubic In2S3:Dy3+ DMS quantum dots (3–5 nm) with different doping concentrations were synthesized via a gas–liquid phase chemical deposition method. The effect of Dy3+ content on the photoluminescence (PL) and ferromagnetism was investigated. The PL emission spectra exhibit a blue-shift compared with those reported previously due to the increased quantum size confinement and enhanced intensity attributed to the Dy3+ doping. The distinct and stronger room-temperature ferromagnetism is observed from vibrating sample magnetometer (VSM) measurement. The coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases and antiferromagnetic interation plays a dominant position after a certain doping concentration value can be further confirmed according to the zero field cooling/field cooling (ZFC/FC) curves. As revealed in the magnetic origin study from first-principles calculations, the ferromagnetism obtained arises not only from the Dy atoms but also from the In vacancies. In addition, we also proposed a spontaneous mechanism based on the bound magnetic polaron theory to explain the change of saturation magnetizations along with Dy3+ doping concentration. This work provides experimental and theoretical guidance for designing and synthesizing unique spintronic materials, which can promote development of spintronic applications.

Germanium Revived from the Spintronics Graveyard

Germanium produces a surprisingly large separation of electron spins in response to electric current—good news for spin-based devices, since germanium is highly compatible with silicon.

Revealing the spin optics in conic-shaped metasurfaces

Ellipse and hyperbola are two well-known curves in mathematics with numerous applications in various fields, but their properties and inherent differences in spin optics are less understood. Here, we investigate the peculiar optical spin properties of the two curves and establish a connection between their foci and the spin states of incident light by introducing a geometric-phase distribution along the conic curve. We show that the optical spin Hall effect is the intrinsic optical spin property of ellipse, where photons with different spin states can be exactly separated to each of its two foci, while a hyperbola exhibits optical spin-selective effect, where only photons with one particular spin state can be accumulated at its foci. These properties are then experimentally demonstrated in near field by arranging nanoslits in conic shapes. Based on the spin properties of the curves, we design spin-based plasmonic devices with various functionalities. Our results reveal the intrinsic optical spin properties behind conic curves and provide a route for designing spin-based plasmonic devices.

Low-Temperature Magnetic Force Microscopy on Single Molecule Magnet-Based Microarrays.

The magnetic properties of some single molecule magnets (SMM) on surfaces can be strongly modified by the molecular packing in nanometric films/aggregates or by interactions with the substrate, which affect the molecular orientation and geometry. Detailed investigations of the magnetism of thin SMM films and nanostructures are necessary for the development of spin-based molecular devices, however this task is challenged by the limited sensitivity of laboratory-based magnetometric techniques and often requires access to synchrotron light sources to perform surface sensitive X-ray magnetic circular dichroism (XMCD) investigations. Here we show that low-temperature magnetic force microscopy is an alternative powerful laboratory tool able to extract the field dependence of the magnetization and to identify areas of in-plane and perpendicular magnetic anisotropy in microarrays of the SMM terbium(III) bis-phthalocyaninato (TbPc2) neutral complex grown as nanosized films on SiO2 and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and this is in agreement with data extracted from nonlocal XMCD measurements performed on homogeneous TbPc2/PTCDA films.

Investigation of Ni-doped CeO2 nanoparticles–spintronics application

In recent years, spin-based electric devices, i.e., spintronics, have attracted interest in next generation electrical devices—charge flows faster and energy loss is lower. In the case of ferromagnetic material, charge may flow faster but energy loss is high due to its heating effect. The aim is to explore paramagnetic material in spintronics on the basis of spin Hall effect (SHE). This study presents the preparation and characterization of Ni-doped CeO2 by a combustion technique. The characterization was studied by powder X-ray diffraction analysis (XRDA), high-resolution transmission electron microscopy (HR-TEM), vibrating sample magnetometry (VSM), electrical impedance studies, and X-ray photoelectron spectroscopy (XPS) analysis. VSM studies reveal Ni-doped CeO2 is paramagnetic in nature, which will open the possibility of realizing spintronics devices.

Selective addressing and readout of optically detected electron spins

Optically detected magnetic resonance (ODMR) is known to allow the ultrasensitive detection of a small number of electron spins for species such as nitrogen vacancy (NV) centers in diamond, down to the single spin level. This method was recently combined with the pulsed magnetic resonance imaging (MRI) technique known as optically detected magnetic resonance imaging (ODMRI) to obtain images of NVs at resolutions ranging from the micron- to the nanoscale. The great advantage of ODMRI is that the quantum state of the spins can be acquired (i.e., readout) in parallel from all parts of the sample without any loss of spectroscopic information. Here we further enhance the utility of our ODMRI technique by improving its spatial resolution and adding a spatially selective spin addressing and manipulation capability. We demonstrate this capability by selectively manipulating a specific group of spins in our sample and subsequently imaging the entire sample to show this selectivity effect. Such capabilities are of fundamental importance in the field of spin-based quantum devices and sensors.

Spin pumping driven auto-oscillator for phase-encoded logic—device design and material requirements

In this work, we propose a spin nano-oscillator (SNO) device where information is encoded in the phase (time-shift) of the output oscillations. The spin current required to set up the oscillations in the device is generated through spin pumping from an input nanomagnet that is precessing at RF frequencies. We discuss the operation of the SNO device, in which either the in-plane (IP) or out-of-plane (OOP) magnetization oscillations are utilized toward implementing ultra-low-power circuits. Using physical models of the nanomagnet dynamics and the spin transport through non-magnetic channels, we quantify the reliability of the SNO device using a “scaling ratio”. Material requirements for the nanomagnet and the channel to ensure correct logic functionality are identified using the scaling ratio metric. SNO devices consume (2-5)× lower energy compared to CMOS devices and other spin-based devices with similar device sizes and material parameters. The analytical models presented in this work can be used to optimize the performance and scaling of SNO devices in comparison to CMOS devices at ultra-scaled technology nodes.

The circular current in a conducting mesoscopic ring with coupled quantum dot

In this paper, we investigate the circular current in the metallic ring coupled with two QDs connected with two ferromagnetic leads with the in-plane electric field ξ which is perpendicular to the wire applied to the ring by using the non-equilibrium Green’s function technique. We find that in this systems the circular current in the ring can be tuned by the voltages between left and right electrodes, the temperature, the spin polarization, and the energy of the QDs and the electric field e. With these parameters varied, the values and direction of the circular current can be controlled, so we can control the values and the directions of the magnetic field produced at the ring center. In addition, there also has the function of spin filter in the P and has the function of suppression of the circular current for large 𝒫 of the AP. So this structure has potential applications in designing spin-based quantum devices.

Emergent phenomena induced by spin-orbit coupling at surfaces and interfaces.

Spin-orbit coupling (SOC) describes the relativistic interaction between the spin and momentum degrees of freedom of electrons, and is central to the rich phenomena observed in condensed matter systems. In recent years, new phases of matter have emerged from the interplay between SOC and low dimensionality, such as chiral spin textures and spin-polarized surface and interface states. These low-dimensional SOC-based realizations are typically robust and can be exploited at room temperature. Here we discuss SOC as a means of producing such fundamentally new physical phenomena in thin films and heterostructures. We put into context the technological promise of these material classes for developing spin-based device applications at room temperature.

Tailoring Spin Textures in Complex Oxide Micromagnets.

Engineered topological spin textures with submicron dimensions in magnetic materials have emerged in recent years as the building blocks for various spin-based memory devices. Examples of these magnetic configurations include magnetic skyrmions, vortices, and domain walls. Here, we show the ability to control and characterize the evolution of spin textures in complex oxide micromagnets as a function of temperature through the delicate balance of fundamental materials parameters, micromagnet geometries, and epitaxial strain. These results demonstrate that in order to fully describe the observed spin textures, it is necessary to account for the spatial variation of the magnetic parameters within the micromagnet. This study provides the framework to accurately characterize such structures, leading to efficient design of spin-based memory devices based on complex oxide thin films.

Defect-Rich Dopant-Free ZrO2 Nanostructures with Superior Dilute Ferromagnetic Semiconductor Properties.

Control of the spin degree of freedom of an electron has brought about a new era in spin-based applications, particularly spin-based electronics, with the potential to outperform the traditional charge-based semiconductor technology for data storage and information processing. However, the realization of functional spin-based devices for information processing remains elusive due to several fundamental challenges such as the low Curie temperature of group III-V and II-VI semiconductors (<200 K), and the low spin-injection efficiencies of existing III-V, II-VI, and transparent conductive oxide semiconductors in a multilayer device structure, which are caused by precipitation and migration of dopants from the host layer to the adjacent layers. Here, we use catalyst-assisted pulsed laser deposition to grow, for the first time, oxygen vacancy defect-rich, dopant-free ZrO2 nanostructures with high TC (700 K) and high magnetization (5.9 emu/g). The observed magnetization is significantly greater than both doped and defect-rich transparent conductive oxide nanomaterials reported to date. We also provide the first experimental evidence that it is the amounts and types of oxygen vacancy defects in, and not the phase of ZrO2 that control the ferromagnetic order in undoped ZrO2 nanostructures. To explain the origin of ferromagnetism in these ZrO2 nanostructures, we hypothesize a new defect-induced bound polaron model, which is generally applicable to other defect-rich, dopant-free transparent conductive oxide nanostructures. These results provide new insights into magnetic ordering in undoped dilute ferromagnetic semiconductor oxides and contribute to the design of exotic magnetic and novel multifunctional materials.

Electrical detection of spin transport in Si two-dimensional electron gas systems

Spin transport in a semiconductor-based two-dimensional electron gas (2DEG) system has been attractive in spintronics for more than ten years. The inherent advantages of high-mobility channel and enhanced spin–orbital interaction promise a long spin diffusion length and efficient spin manipulation, which are essential for the application of spintronics devices. However, the difficulty of making high-quality ferromagnetic (FM) contacts to the buried 2DEG channel in the heterostructure systems limits the potential developments in functional devices. In this paper, we experimentally demonstrate electrical detection of spin transport in a high-mobility 2DEG system using FM Mn-germanosilicide (Mn(Si0.7Ge0.3)x) end contacts, which is the first report of spin injection and detection in a 2DEG confined in a Si/SiGe modulation doped quantum well structure (MODQW). The extracted spin diffusion length and lifetime are lsf = 4.5 μm and at 1.9 K respectively. Our results provide a promising approach for spin injection into 2DEG system in the Si-based MODQW, which may lead to innovative spintronic applications such as spin-based transistor, logic, and memory devices.