Spin-based Devices Research Articles

Activation and Enhancement of Room-Temperature Ferromagnetic Exchange in (Cu,N)-codoped In2O3 Dilute Magnetic Nanowires

Spin manipulation in dilute magnetic oxide nanowires is an important topic for achieving nanometer-sized spin-based magnetic devices. In this study, single-crystalline Cu-doped and (Cu,N)-codoped I...

Spin-Based Reconfigurable Logic for Power- and Area-Efficient Applications

Editor’s note: This article proposes a reconfigurable logic family comprised of spin-based devices that may be dynamically configured/reconfigured owing the versatility of the giant spin-Hall effect (GSHE) device. A GSHE device can be used to realize numerous Boolean logic functions, and the proposed spin-based polymorphic logic has the potential to be extremely area efficient. —Michael Niemier, University of Notre Dame

Enhancement of electrical conductivity and magnetic properties of bimetallic Schiff base complex on grafting to MWCNTs

In present work, we report a newly synthesized nano-inorganic hybrid magnetic material, [SBCu(II)Gd(III)]@MWCNT-COOH which is obtained by anchoring of a copper(II)–gadolinium(III) heteronuclear bimetallic complex, [triaqua(2-hydroxy-1,3-bis(3-methoxysalicylaldiminato)Cu(II)Gd(III)dinitrato)]nitrate monohydrate, [SBCu(II)Gd(III)] to carboxylated multiwalled carbon nanotubes (MWCNT-COOH) through a chemical method. MWCNT-COOH have been loaded with a number of [SBCu(II)Gd(III)] molecules on their surfaces, which generated multinuclear metal centers. [SBCu(II)Gd(III)]@MWCNT-COOH was characterized by FT-IR, UV–Vis, TGA, powder XRD, Raman spectroscopy, TEM, HRSEM, EDAX and elemental mapping. The DC electrical conductivity of materials was evaluated using two probe method suggesting that [SBCu(II)Gd(III)]@MWCNT-COOH is more conducting than [SBCu(II)Gd(III)]. The increased conductivity of [SBCu(II)Gd(III)]@MWCNT-COOH is confirmed by CV and EIS study. The capacitive and dielectric properties of the materials were investigated by Mott–Schottky electrochemical analysis. The positive value of slope signified n-type semiconducting nature of materials. The lower slope for [SBCu(II)Gd(III)] signified the strong dielectric behavior of the material. The positive shift in flat-band potential of [SBCu(II)Gd(III)]@MWCNT-COOH compared to [SBCu(II)Gd(III)] indicated its more conducting nature. Further small optical band gap obtained for [SBCu(II)Gd(III)]@MWCNT-COOH than [SBCu(II)Gd(III)] by UV–Vis studies suggested better conductivity of nano-inorganic hybrid material. Field dependent magnetization revealed enhanced saturation magnetization, remanent magnetization and coercivity of [SBCu(II)Gd(III)]@MWCNT-COOH due to generation of multinuclear metal centers on carboxylated MWCNTs. The temperature dependent magnetization measurement exhibited overlapping of field cooled and zero field cooled curves in entire temperature range (2–300 K), suggests intrinsic ferromagnetism in [SBCu(II)Gd(III)]@MWCNT-COOH. Hence, trend in variation of magnetization with magnetic field and temperature opens its utility in spin-based electronic devices.

Voltage-Controlled Topological Spin Switch for Ultralow-Energy Computing: Performance Modeling and Benchmarking

This research on a fully voltage-driven spin-based computing device, the ``vTOPSS'', could promote significant gains in energy efficiency, beyond the state of the art. Thus far the challenge in spin-based devices has been related to the large electric current required, and the associated Joule heating. A vTOPSS overcomes such limitations by driving a magnetic insulator via spin accumulation, generated by applying an electric field across a proximal topological insulator. Current flows perpendicular to the surface of the topological insulator and perpendicular to the applied electric field, making it nondissipative. For binary switching operations, a vTOPSS could consume mere attojoules.

Chiral-induced switching of antiferromagnet spins in a confined nanowire

In the development of spin-based electronic devices, a particular challenge is the manipulation of the magnetic state with high speed and low power consumption. Although research has focused on the current-induced spin–orbit torque based on strong spin–orbit coupling, the charge-based and the torque-driven devices have fundamental limitations: Joule heating, phase mismatching, and overshooting. In this work, we investigate numerically and theoretically alternative switching scenario of antiferromagnetic insulator in one-dimensional confined nanowire sandwiched with two electrodes. As the electric field could break inversion symmetry and induce Dzyaloshinskii-Moriya interaction and pseudo-dipole anisotropy, the resulting spiral texture takes symmetric or antisymmetric configuration due to additional coupling with the crystalline anisotropy. Therefore, by competing two spiral states, we show that the magnetization reversal of antiferromagnets is realized, which is valid in ferromagnetic counterpart. Our finding provides promising opportunities to realize the rapid and energy-efficient electrical manipulation of magnetization for future spin-based electronic devices.

First-principles prediction of magnetically ordered half-metals above room temperature

Half-metallicity (HM) offers great potential for engineering spintronic applications, yet only few magnetic materials present metallicity in just one spin channel. In addition, most HM systems become magnetically disordered at temperatures well below ambient conditions, which further hinders the development of spin-based electronic devices. Here, we use first-principles methods based on density functional theory (DFT) to investigate the electronic, magnetic, structural, mixing, and vibrational properties of 90 XYZ half-Heusler (HH) alloys (X= Li, Na, K, Rb, Cs; Y= V, Nb, Ta; Z= Si, Ge, Sn, S, Se, Te). We disclose a total of 28 new HH compounds that are ferromagnetic, vibrationally stable, and HM, with semiconductor band gaps in the range of 1–4 eV and HM band gaps of 0.2–0.8 eV. By performing Monte Carlo simulations of a spin Heisenberg model fitted to DFT energies, we estimate the Curie temperature, TC, of each HM compound. We find that 17 HH HM remain magnetically ordered at and above room temperature, namely, 300≤TC≤450 K, with total magnetic moments of 2 and 4 μB. A further materials sieve based on zero-temperature mixing energies let us to conclude 5 overall promising HH HM that remain magnetically ordered at and above room temperature: NaVSi, RbVTe, CsVS, CsVSe, and RbNbTe. We also predict 2 semiconductor materials that are ferromagnetic at ambient conditions: LiVSi and LiVGe.

Enlarging spin-dependent transverse displacement of surface plasmon polaritons focus.

By introducing additional spin angular momentum (SAM) induced spiral phase, the spin-dependent transverse displacement of surface plasmon polaritons (SPPs) focus is effectively enlarged. The separation between the SPPs focuses generated by left circularly polarized (LCP) light and right circularly polarized (RCP) light reaches 1500 nm, which is six times larger than the previously reported values with semicircular plasmonic lens. The relationship between the displacement of the SPPs focus and the total spiral phase that consisted of the intrinsic and the additional spiral phase is theoretically established. Furthermore, the flexibility and versatility of the proposed mechanism is demonstrated by reversing or continuously controlling the SPPs focus. These findings hold great promise for spin-based plasmonic devices and the related applications, such as on-chip communication.

First-principles study of a vertical spin switch in atomic scale two-dimensional platform

Abstract High in-plane charge carrier mobility and long spin diffusion length makes graphene a unique material for spin-based devices. However, in a vertical graphene junction, the 2pz orbitals of carbon atoms in graphene can be tuned via suitable magnetic substrates; this would affect the spin injection into graphene. Here, a vertical spin switch has been designed by embedding a single layer of graphene as a tunnel layer between the Ni (1 1 1) substrate. Periodic density functional approach in conjunction with Julliere’s model is used to calculate the tunnel magnetoresistance (TMR). Further, single-layered hexagonal Boron Nitride (h-BN) is sandwiched between the graphene and Ni (1 1 1) substrate to understand the role of hybridization at the interface on TMR. Our calculation shows that in contrast to the graphene junction, a much higher TMR value is obtainable in the case of the graphene/h-BN multi-tunnel junction (MTJ). The TMR in graphene junction is found to decrease with the increase of an externally applied electric field, and drops to zero for a field greater than equal to 0.16 V/A. Similar phenomenon was observed in the case of h-BN/graphene MTJ, where TMR value remains unchanged for electric field up to 0.1 V/A beyond which it drops to zero. The change in hybridization and charge-carrier-population at the interface modifies the magnetic exchange interaction and magnetic anisotropy resulting in a spin flip at interface, leads to rapid drop in TMR after a threshold electric field. The high and tunable TMR value suggests h-BN assisted high performance graphene based vertical spin switch.

Anisotropic magnetoresistance in a Ni81Fe19/SiO2/Ca-Bi2Se3 hybrid structure

A topological insulator gives a great concern in the field of thin film devices because it delivers a conducting state and a high spin-momentum locking at the material surface. To investigate the interfacial coupling between ferromagnet and topological insulator, the anisotropic magnetoresistance (AMR) in a Ni81Fe19/SiO2/Ca-doped Bi2Se3 structure is observed. The AMR is determined by the alignment between the magnetization direction of a ferromagnetic electrode and the bias current direction. The bias current induces a strong spin-momentum locking along the transverse direction which changes the magnetic anisotropy and switching process of the ferromagnetic layer. Furthermore, the angle dependence of magnetoresistance clearly shows that the amplitude of AMR is enhanced due to the coupling of Ni81Fe19/SiO2/Ca-doped Bi2Se3 hybrid structure. These results provide an efficient technique for manipulating magnetization reversal of the ferromagnetic material in spin-based devices.

Effects of Elastic Dephasing on Scaling of Ultra-Small Magnetic Tunnel Junctions

The study of the effects of scaling on magnetic tunnel junction (MTJ) devices has become an important topic in the field of spin-based memory devices. Here, we investigate the effect of elastic dephasing on trilayer and pentalayer MTJ considered at small transverse cross-sectional areas using the non-equilibrium Green's function spin transport formalism. We consider the structures with and without dephasing effects and clearly point out as to how the tunnel magnetoresistance effect gets affected by dephasing. We attribute the trends noted by analyzing the transmission spectra and hence the currents across the devices. Although dephasing affects the tunnel magnetoresistance ratio (TMR) values for both devices, we note that the obtained TMR values are still in a reasonable range that may not hinder their usability for practical applications.

PARC: A Novel Design Methodology for Power Analysis Resilient Circuits Using Spintronics

A prevalent class of side-channel attacks relies on Differential Power Analysis (DPA) methods, which monitor power traces during cryptographic processing to discover secret keys. Reconfigurability via Polymorphic Gate Modules (PGMs) offers an approach to obscure DPA information by dynamically rearranging the operation of constituent sub-circuits, albeit at the cost of increased area and power consumption. Thus, we develop the Power Analysis-Resilient Circuit (PARC) design methodology to instantiate the use of spin-based devices as an extension to conventional Register Transfer Language (RTL) specifications. PARC replaces a specific portion of the circuit to maximize a new Effectiveness of Design (EoD) metric, which quantifies DPA impact versus its performance overhead. To validate functionality, PARC is applied to various benchmark circuits including ISCAS-89, MCNC, and ITC-99. EoD results indicate that PARC significantly increases the number of power traces that an adversary would need to use in order to extract power information but incurs a low cost to the functional circuit itself.

Ambient Degradation-Induced Spin Paramagnetism in Phosphorene.

The sizeable direct bandgap, high mobility, and long spin lifetimes at room temperature offer black phosphorus (BP) potential applications in spin-based semiconductor devices. Toward these applications, a critical step is creating a magnetic response in BP, which is arousing much interest. It is reported here that ambient degradation of BP, which is immediate and inevitable and greatly changes the semiconducting properties, creates magnetic moments, and any degree of degradation leads to notable paramagnetism. Its Landau factor g measured is ≈1.995, revealing that the magnetization mainly results from spin rather than orbital moments. Such magnetism most likely results from the unsaturated phosphorus in the vacancies which are stabilized by O adatoms. It can be tuned by changing any one of the ambient factors of ambient temperature, humidity, and light intensity, and can be stabilized by exposing BP in argon. The findings highlight the importance of evaluating the effect of ambient degradation-induced magnetism on BP's spin-based devices. The work seems an essential milestone toward the forthcoming research upsurge on BP's magnetism.

Magnetization Manipulation of a Flexible Magnetic Sensor by Controlled Stress Application

Spin-based electronic devices on polymer substrates have been intensively investigated because of several advantages in terms of weight, thickness, and flexibility, compared to rigid substrates. So far, most studies have focused on maintaining the functionality of devices with minimum degradation against mechanical deformation, as induced by stretching and bending of flexible devices. Here, we applied repetitive bending stress on a flexible magnetic layer and a spin-valve structure composed of Ta/NiFe/CoFe/Cu/Ni/IrMn/Ta on a polyimide (PI) substrate. It is found that the anisotropy can be enhanced or weakened depending upon the magnetostrictive properties under stress. In the flat state after bending, due to residual compressive stress, the magnetic anisotropy of the positive magnetostrictive free layer is weakened while that of the pinned layer with negative magnetostriction is enhanced. Thus, the magnetic configuration of the spin-valve is appropriate for use as a sensor. Through the bending process, we design a prototype magnetic sensor cell array and successfully show a sensing capability by detecting magnetic microbeads. This attempt demonstrates that appropriate control of stress, induced by repetitive bending of flexible magnetic layers, can be effectively used to modify the magnetic configurations for the magnetic sensor.

Room temperature ferromagnetic resonance in hetero-epitaxial BTO − BFO/LSMO magnetoelectric composite

We synthesized epitaxial BTO-BFO heterostructure with decreased leakage and simultaneously improved the multiferroic properties. This study provides new direction for ferromagnetic resonance studies, in high quality BTO-BFO films grown on LSMO. We observed small Gilbert damping (α=0.004) and the absence of large inhomogeneous broadening, in a film with 80 nm thickness of BTO-BFO on LSMO (110). This fact offers opportunities for employing this material system for spin transfer in multifunctional materials where controlling magnetization by a flow of spin angular momentum, or spin current, is crucial toward developing nanoscale spin-based memory and devices. Magnetic insulators, such as BTO-BFO on LSMO, are potentially excellent candidates for pure spin current without the existence of charge current.

Novel Cascadable Magnetic Majority Gates for Implementing Comprehensive Logic Functions

In the quest for novel, scalable and energy-efficient computing technologies, spin-based logic devices are being extensively explored due to their potential for nonvolatility, small cell area, and low operational power. Spin torque majority gate (STMG) is one of the most promising options for beyond CMOS nonvolatile logic circuits for normally-off computing. However, significant problems arose with cascade-ability, signal nonreciprocity, and complicated circuit configurations based on STMG. In this paper, a novel magnetic majority gate (MMG) logic has been proposed, utilizing both spin transfer torque and spin–orbit torque effects. A logic family including and/nand and or/nor functions can be achieved with an easy configuration and under a stable operation. Communication between logic units is realized by spin current injection through a nonferromagnetic metal wire to ensure its cascade-ability and nonreciprocity to design multiple logic-depth circuits. With all of these advantages, the proposed cascadable MMGs can be utilized to design logic functions such as the BUFFER/ NOT, XOR/ XNOR, and complicated logic gates, which pave the pathway for designing robust and comprehensive logic circuits using full spintronic devices.

Modulation of spin transport in DNA-based nanodevices by temperature gradient: A spin caloritronics approach

Spin caloritronics, a novel and rich research field, combining spin, charge and heat transport in materials, provides alternative strategies for thermoelectric waste heat recovery, and the information technologies. Understanding of the spin transport properties of materials has important implications for spin-based devices. Flexibility, low cost, and adjustable conductance are the important factors to consider the spin transport properties of DNA chains. The spin current corresponding to spin-up and spin-down electrons in the presence of a temperature gradient along the DNA chain are calculated. ∇T=1K/Å is the most appropriate temperature gradient in which the maximum spin current flows. The spin current of different sequences determines the best sequence for spin transport in different situations. Applying the external magnetic field can amplify the spin current along the DNA chain, and one can choose the most appropriate field. Is−V characteristic diagrams corresponding to different sequences are distinctive and determine the spin-dependent negative differential resistance (SNDR) regions corresponding to the negative slope intervals. Eventually, the spin-dependent Seebeck effect (SDSE) in DNA and the SDSE coefficients as a function of temperature are studied. The peak of the SDSE diagrams is different for different species of sequences. Poly(AT), hc1, and poly(CG) show maximum peaks of SDSE coefficient at 300, 310, and 320 K, respectively. Therefore, one can design a DNA based spin nanodevice with maximal efficiency in different situations.

Chemical Modification toward Long Spin Lifetimes in Organic Conjugated Radicals.

Organic semiconductors for spin-based devices require long spin relaxation times. Understanding their spin relaxation mechanisms is critical to organic spintronic devices and applications for quantum information processing. However, reports on the spin relaxation mechanisms of organic conjugated molecules are rare and the research methods are also limited. Herein, we study the molecular design and spin relaxation mechanisms by systematically varying the structure of a conjugated radical. We found that solid-state relaxation times of organic materials are largely different from that in solution state. We demonstrate that substitution of a lower gyromagnetic ratio nucleus (e. g. D, Cl) on the para-position of the aryl rings in the triphenylmethyl (TM) radical can significantly improve their coherence times (Tm ). Flexible thin films based on such radicals exhibit ultra-long spin-lattice relaxation times (T1 ) up to 35.6(6) μs and Tm up to 1.08(4) μs under ambient conditions, which are among the longest values in films. More importantly, using the TM radical derivative (5CM), we observed room-temperature quantum coherence and Rabi cycles in thin film for the first time, suggesting that organic conjugated radicals have great potentials for spin-based information processing.

0.2 λ0 Thick Adaptive Retroreflector Made of Spin-Locked Metasurface.

The metasurface concept is employed to planarize retroflectors by stacking two metasurfaces with separation that is two orders larger than the wavelength. Here, a retroreflective metasurface using subwavelength-thick reconfigurable C-shaped resonators (RCRs) is reported, which reduces the overall thickness from the previous record of 590 λ0 down to only 0.2 λ0 . The geometry of RCRs could be in situ controlled to realize equal amplitude and phase modulation onto transverse magnetic (TM)-polarized and transverse electric (TE)-polarized incidences. With the phase gradient being engineered, an in-plane momentum could be imparted to the incident wave, guaranteeing the spin state of the retro-reflected wave identical to that of the incident light. Such spin-locked metasurface is natively adaptive toward different incident angles to realize retroreflection by mechanically altering the geometry of RCRs. As a proof of concept, an ultrathin retroreflective metasurface is validated at 15 GHz, under various illumination angles at 10°, 12°, 15°, and 20°. Such adaptive spin-locked metasurface could find promising applications in spin-based optical devices, communication systems, remote sensing, RCS enhancement, and so on.

Electrical Control of the Zeeman Spin Splitting in Two-Dimensional Hole Systems.

Semiconductor holes with strong spin-orbit coupling allow all-electrical spin control, with broad applications ranging from spintronics to quantum computation. Using a two-dimensional hole system in a gallium arsenide quantum well, we demonstrate a new mechanism of electrically controlling the Zeeman splitting, which is achieved through altering the hole wave vector k. We find a threefold enhancement of the in-plane g-factor g_{∥}(k). We introduce a new method for quantifying the Zeeman splitting from magnetoresistance measurements, since the conventional tilted field approach fails for two-dimensional systems with strong spin-orbit coupling. Finally, we show that the Rashba spin-orbit interaction suppresses the in-plane Zeeman interaction at low magnetic fields. The ability to control the Zeeman splitting with electric fields opens up new possibilities for future quantum spin-based devices, manipulating non-Abelian geometric phases, and realizing Majorana systems in p-type superconductor systems.

(Keynote) Contemporary and Future Logic Devices

Headwinds faced by scaling efforts of high performance logic technology have forced engineering teams to go beyond dumb shrinking of device dimensions and into clever employment of new materials and structures, over last few technology nodes. We departed technology staple, planar transistors, and entered into 3D structures, half a dozen years ago. Such paradigm shifts will continue to happen more often in the years to come. We will look at today's state-of-the-art devices - FinFETs - and explore capability and suitability of new architectures for years to come. Those include variants of gate-all-around (GAA) transistors, horizontal and vertical, negative-capacitance FET (NCFET), tunnel FET (TFET), and spin-based devices. We'll assess feasibility of material contenders for better channel transport - III-V, SiGe/Ge, carbon nanotubes (CNT) and 2D transition metal di-chalcogenides (TMD). Finally, we discuss 3D monolithic device stacking and standard cell level design-technology co-optimization role in scaling.