Spin‐Orbit Torque Switching of a Nearly Compensated Ferrimagnet by Topological Surface States

Utilizing spin‐orbit torque (SOT) to switch a magnetic moment provides a promising route for low‐power‐dissipation spintronic devices. Here, the SOT switching of a nearly compensated ferrimagnet Gd_x(FeCo)_1?x by the topological insulator [Bi₂Se₃ and (BiSb)₂Te₃] is investigated at room temperature. The switching current density of (BiSb)₂Te₃ (1.20 × 10⁵ A cm^?2) is more than one order of magnitude smaller than that in conventional heavy‐metal‐based structures, which indicates the ultrahigh efficiency of charge‐spin conversion (>1) in topological surface states. By tuning the net magnetic moment of Gd_x(FeCo)_1?x via changing the composition, the SOT efficiency has a significant enhancement (6.5 times) near the magnetic compensation point, and at the same time the switching speed can be as fast as several picoseconds. Combining the topological surface states and the nearly compensated ferrimagnets provides a promising route for practical energy‐efficient and high‐speed spintronic devices.     

Enhancing Light Emission in Interface Engineered Spin‐OLEDs through Spin‐Polarized Injection at High Voltages

The quest for a spin‐polarized organic light‐emitting diode (spin‐OLED) is a common goal in the emerging fields of molecular electronics and spintronics. In this device, two ferromagnetic (FM) electrodes are used to enhance the electroluminescence intensity of the OLED through a magnetic control of the spin polarization of the injected carriers. The major difficulty is that the driving voltage of an OLED device exceeds a few volts, while spin injection in organic materials is only efficient at low voltages. The fabrication of a spin‐OLED that uses a conjugated polymer as bipolar spin collector layer and ferromagnetic electrodes is reported here. Through a careful engineering of the organic/inorganic interfaces, it is succeeded in obtaining a light‐emitting device showing spin‐valve effects at high voltages (up to 14 V). This allows the detection of a magneto‐electroluminescence (MEL) enhancement on the order of a 2.4% at 9 V for the antiparallel (AP) configuration of the magnetic electrodes. This observation provides evidence for the long‐standing fundamental issue of injecting spins from magnetic electrodes into the frontier levels of a molecular semiconductor. The finding opens the way for the design of multifunctional devices coupling the light and the spin degrees of freedom.     

Engineering On‐Surface Spin Crossover: Spin‐State Switching in a Self‐Assembled Film of Vacuum‐Sublimable Functional Molecule

The realization of spin‐crossover (SCO)‐based applications requires study of the spin‐state switching characteristics of SCO complex molecules within nanostructured environments, especially on surfaces. Except for a very few cases, the SCO of a surface‐bound thin molecular film is either quenched or heavily altered due to: (i) molecule–surface interactions and (ii) differing intermolecular interactions in films relative to the bulk. By fabricating SCO complexes on a weakly interacting surface, the interfacial quenching problem is tackled. However, engineering intermolecular interactions in thin SCO active films is rather difficult. Here, a molecular self‐assembly strategy is proposed to fabricate thin spin‐switchable surface‐bound films with programmable intermolecular interactions. Molecular engineering of the parent complex system [Fe(H₂B(pz)₂)₂(bpy)] (pz = pyrazole, bpy = 2,2′‐bipyridine) with a dodecyl (C₁₂) alkyl chain yields a classical amphiphile‐like functional and vacuum‐sublimable charge‐neutral Fe^II complex, [Fe(H₂B(pz)₂)₂(C₁₂‐bpy)] (C₁₂‐bpy = dodecyl[2,2′‐bipyridine]‐5‐carboxylate). Both the bulk powder and 10 nm thin films sublimed onto either quartz glass or SiO_x surfaces of the complex show comparable spin‐state switching characteristics mediated by similar lamellar bilayer like self‐assembly/molecular interactions. This unprecedented observation augurs well for the development of SCO‐based applications, especially in molecular spintronics.     

Finite Size Effects on the Switching Dynamics of Spin‐Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse

Using ultrafast optical absorption spectroscopy, the room‐temperature spin‐state switching dynamics induced by a femtosecond laser pulse in high‐quality thin films of the molecular spin‐crossover (SCO) complex [Fe(HB(tz)₃)₂] (tz = 1,2,4‐triazol‐1‐yl) are studied. These measurements reveal that the early, sub‐picosecond, low‐spin to high‐spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out‐of‐equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high‐frequency all‐optical switches working at room temperature.     

绝缘介质中载流子注入与传导的机理

本文分析了与绝缘介质老化、失效过程密切相关的载流子注入与传导机理,包括具有电极限制特性的肖特基效应、具有体限制特性的普尔-弗兰凯尔效应、隧穿效应、离子跳跃传导以及空间电荷限制电流。研究结果表明:由普尔-弗兰凯尔效应引起的势垒降低的高度是由肖特基效应降低的势垒高度的两倍。如果In(I/E2)-1/E特性关系始终是线性的,说明载流子是由隧穿效应注入介质中的。在高电场下,离子跳跃传导的J-E特性曲线是一条斜率为eλ/2kT的直线,并可由此斜率计算得到离子跳跃的距离。由空间电荷限制电流的J-V特性可以确定临界电压,即电流从陷阱限制值迅速跳高至无陷阱的空间电荷限制电流值的电压。     

Field‐Free Programmable Spin Logics via Chirality‐Reversible Spin–Orbit Torque Switching

Spin–orbit torque (SOT)‐induced magnetization switching exhibits chirality (clockwise or counterclockwise), which offers the prospect of programmable spin‐logic devices integrating nonvolatile spintronic memory cells with logic functions. Chirality is usually fixed by an applied or effective magnetic field in reported studies. Herein, utilizing an in‐plane magnetic layer that is also switchable by SOT, the chirality of a perpendicular magnetic layer that is exchange‐coupled with the in‐plane layer can be reversed in a purely electrical way. In a single Hall bar device designed from this multilayer structure, three logic gates including AND, NAND, and NOT are reconfigured, which opens a gateway toward practical programmable spin‐logic devices.     

Magnetic Susceptibility Study of Sub‐Pico‐emu Sample Using a Micromagnetometer: An Investigation through Bistable Spin‐Crossover Materials

A promising and original method to study the spin‐transition in bistable spin‐crossover (SCO) materials using a magnetoresistive multiring sensor and its self‐generated magnetic field is reported. Qualitative and quantitative studies are carried out combining theoretical and experimental approaches. The results show that only a small part of matter dropped on the sensor surface is probed by the device. At a low bias‐current range, the number of detected nanoparticles depends on the amplitude of the current. However, in agreement with the theoretical model, the stray voltage from the particles is proportional to the current squared. By changing both the bias current and the concentration of particle droplet, the thermal hysteresis of an ultrasmall volume, 1 × 10^?4 mm³, of SCO particles is measured. The local probe of the experimental setup allows a highest resolution of 4 × 10^?14 emu to be reached, which is never achieved by experimental methods at room temperature.     

Structural Modifications in Bilayered Molecular Systems Lead to Predictable Changes in Their Electronic Properties

This study uses a novel surface engineering approach to demonstrate the influence of organic functional group substitutions on molecular electronic properties. Specifically, bilayered organic monomolecular systems immobilized on an inorganic electrode as the charge‐injecting components of organic electronic devices are compared. Recent literature reports demonstrate that structural modification in functional monolayers have unpredictable effects on their electronic properties. These studies indicate that the structure most certainly plays an important role, but its effect on the molecular resistance is diminished due to differences in other monolayer parameters. It is demonstrated that a separate control over the monolayer geometry and its chemical structure is required in order to observe predictable structure‐property relations. Here, bilayered molecular interfaces, comprising inert and functional layers whose properties can be independently controlled, are formed. It is shown that 1) the charge transfer through the bilayered system is sensitive to small structural molecular changes; 2) that it can be controlled and predicted by controlling the electron‐withdrawing or donating nature of the organic moiety; and 3) that the differences in the charge transfer dynamics of two bilayered systems can be visualized via patterned electroluminescence.     

Large‐Area Atomic Layers of the Charge‐Density‐Wave Conductor TiSe2

Layered transition metal (Ti, Ta, Nb, etc.) dichalcogenides are important prototypes for the study of the collective charge density wave (CDW). Reducing the system dimensionality is expected to lead to novel properties, as exemplified by the discovery of enhanced CDW order in ultrathin TiSe₂. However, the syntheses of monolayer and large‐area 2D CDW conductors can currently only be achieved by molecular beam epitaxy under ultrahigh vacuum. This study reports the growth of monolayer crystals and up to 5 × 10⁵ µm² large films of the typical 2D CDW conductor—TiSe₂—by ambient‐pressure chemical vapor deposition. Atomic resolution scanning transmission electron microscopy indicates the as‐grown samples are highly crystalline 1T‐phase TiSe₂. Variable‐temperature Raman spectroscopy shows a CDW phase transition temperature of 212.5 K in few layer TiSe₂, indicative of high crystal quality. This work not only allows the exploration of many‐body state of TiSe₂ in 2D limit but also offers the possibility of utilizing large‐area TiSe₂ in ultrathin electronic devices.     

Spin–Orbit Torque Switching in a Nearly Compensated Heusler Ferrimagnet

Ferrimagnetic materials combine the advantages of the low magnetic moment of an antiferromagnet and the ease of realizing magnetic reading of a ferromagnet. Recently, it was demonstrated that compensated ferrimagnetic half metals can be realized in Heusler alloys, where high spin polarization, zero magnetic moment, and low magnetic damping can be achieved at the same time. In this work, by studying the spin–orbit torque induced switching in the Heusler alloy Mn₂Ru_{1? x} Ga, it is found that efficient current‐induced magnetic switching can be realized in a nearly compensated sample with strong perpendicular anisotropy and large film thickness. This work demonstrates the possibility of employing compensated Heusler alloys for fast, energy‐efficient spintronic devices.     

Organic Spin‐Valves and Beyond: Spin Injection and Transport in Organic Semiconductors and the Effect of Interfacial Engineering

Since the first observation of the spin‐valve effect through organic semiconductors, efforts to realize novel spintronic technologies based on organic semiconductors have been rapidly growing. However, a complete understanding of spin‐polarized carrier injection and transport in organic semiconductors is still lacking and under debate. For example, there is still no clear understanding of major spin‐flip mechanisms in organic semiconductors and the role of hybrid metal–organic interfaces in spin injection. Recent findings suggest that organic single crystals can provide spin‐transport media with much less structural disorder relative to organic thin films, thus reducing momentum scattering. Additionally, modification of the band energetics, morphology, and even spin magnetic moment at the metal–organic interface by interface engineering can greatly impact the efficiency of spin‐polarized carrier injection. Here, progress on efficient spin‐polarized carrier injection into organic semiconductors from ferromagnetic metals by using various interface engineering techniques is presented, such as inserting a metallic interlayer, a molecular self‐assembled monolayer (SAM), and a ballistic carrier emitter. In addition, efforts to realize long spin transport in single‐crystalline organic semiconductors are discussed. The focus here is on understanding and maximizing spin‐polarized carrier injection and transport in organic semiconductors and insight is provided for the realization of emerging organic spintronics technologies.