Large Field-like Spin-Orbit Torque and Enhanced Magnetization Switching Efficiency Utilizing Amorphous Mo.

Spin-orbit torque magnetic random access memory (SOT-MRAM) has great promise in high write speed and low power consumption. Mo can play a vital role in constructing a CoFeB/MgO-based MRAM cell because of its ability to enhance the perpendicular magnetic anisotropy (PMA), thermal tolerance, and tunneling magnetoresistance. However, Mo is often considered as a less favorable candidate among SOT materials because of its weak spin-orbit coupling. In this study, we experimentally investigate the SOT efficiencies in Mo/CoFeB/MgO heterostructures over a wide range of Mo thicknesses and temperature. Decent damping-like SOT efficiency |ξDL| = 0.015 ± 0.001 and field-like SOT efficiency |ξFL| = 0.050 ± 0.001 are found in amorphous Mo. The ξFL/ξDL ratio is greater than 3. Furthermore, efficient current-induced magnetization switching is demonstrated with the critical current density comparable with heavy metal Ir and W. Our work reveals new understanding and possibilities for Mo as both an SOT source component and PMA buffer layer in the implementation of SOT-MRAMs.

Generation of tailored terahertz waves from monolithic integrated metamaterials onto spintronic terahertz emitters.

Recently emerging spintronic terahertz (THz) emitters, featuring many appreciable merits such as low-cost, high efficiency, ultrabroadband, and ease of integration, offer multifaceted capabilities not only in understanding the fundamental ultrafast magnetism physics but also for exploring multifarious practical applications. Integration of various flexible and tunable functions at the source such as polarization manipulation, amplitude tailoring, phase modulation, and radiation beam steering with the spintronic THz emitters and their derivatives can yield more compact and elegant devices. Here, we demonstrate a monolithic metamaterial integrated onto a W/CoFeB/Pt THz nanoemitter for a purpose-designed functionality of the electromagnetically induced transparency analog. Through elaborate engineering the asymmetry degree and geometric parameters of the metamaterial structure, we successfully verified the feasibility of monolithic modulations for the radiated THz waves. The integrated device was eventually compared with a set of stand-alone metamaterial positioning scenarios, and the negligible frequency difference between two of the positioning schemes further manifests almost an ideal realization of the proposed monolithic integrated metamaterial device with a spintronic THz emitter. We believe that such adaptable and scalable devices may make valuable contributions to the designable spintronic THz devices with pre-shaping THz waves and enable chip-scale spintronic THz optics, sensing, and imaging.

Unidirectional Magneto-Resistance in Modulation-Doped Magnetic Topological Insulators.

Nonlinear unidirectional spin Hall magnetoresistance (USMR) has been reported in heavy metal/ferromagnet bilayers, which could be employed as an effective method in detecting the magnetization orientation in spintronic devices with two-terminal geometry. Recently, another unidirectional magnetoresistance (UMR) was reported in magnetic topological insulator (TI)-based heterostructures at cryogenic temperature, whose amplitude is orders of magnitude larger than the USMR measured in heavy metal-based magnetic heterostructures at room temperature. Here, we report the UMR effect in the modulation-doped magnetic TI structures. This UMR arises due to the interplay between the magnetic dopant's magnetization and the current-induced surface spin polarization, when they are parallel or antiparallel to each other in the TI material. By varying the dopant's position in the structure, we reveal that the UMR is mainly originating from the interaction between the magnetization and the surface spin-polarized carriers (not bulk carriers). Furthermore, from the magnetic field-, the angular rotation-, and the temperature-dependence, we highlight the correlation between the UMR effect and the magnetism in the structures. The large UMR versus current ratio in TI-based magnetic bilayers promises the easy readout in TI-based spintronic devices with two-terminal geometry.

Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures.

Magnetic topological insulators such as Cr-doped (Bi,Sb)2Te3 provide a platform for the realization of versatile time-reversal symmetry-breaking physics. By constructing heterostructures exhibiting Néel order in an antiferromagnetic CrSb and ferromagnetic order in Cr-doped (Bi,Sb)2Te3, we realize emergent interfacial magnetic phenomena which can be tailored through artificial structural engineering. Through deliberate geometrical design of heterostructures and superlattices, we demonstrate the use of antiferromagnetic exchange coupling in manipulating the magnetic properties of magnetic topological insulators. Proximity effects are shown to induce an interfacial spin texture modulation and establish an effective long-range exchange coupling mediated by antiferromagnetism, which significantly enhances the magnetic ordering temperature in the superlattice. This work provides a new framework on integrating topological insulators with antiferromagnetic materials and unveils new avenues towards dissipationless topological antiferromagnetic spintronics.

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 l sf = 4.5 µm and [Formula: see text] 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.

Magnetization switching through giant spin-orbit torque in a magnetically doped topological insulator heterostructure.

Recent demonstrations of magnetization switching induced by in-plane current in heavy metal/ferromagnetic heterostructures (HMFHs) have drawn great attention to spin torques arising from large spin-orbit coupling (SOC). Given the intrinsic strong SOC, topological insulators (TIs) are expected to be promising candidates for exploring spin-orbit torque (SOT)-related physics. Here we demonstrate experimentally the magnetization switching through giant SOT induced by an in-plane current in a chromium-doped TI bilayer heterostructure. The critical current density required for switching is below 8.9 × 10(4) A cm(-2) at 1.9 K. Moreover, the SOT is calibrated by measuring the effective spin-orbit field using second-harmonic methods. The effective field to current ratio and the spin-Hall angle tangent are almost three orders of magnitude larger than those reported for HMFHs. The giant SOT and efficient current-induced magnetization switching exhibited by the bilayer heterostructure may lead to innovative spintronics applications such as ultralow power dissipation memory and logic devices.

Electric-field control of ferromagnetism in Mn-doped ZnO nanowires.

In this Letter, the electric-field control of ferromagnetism was demonstrated in a back-gated Mn-doped ZnO (Mn-ZnO) nanowire (NW) field-effect transistor (FET). The ZnO NWs were synthesized by a thermal evaporation method, and the Mn doping of 1 atom % was subsequently carried out in a MBE system using a gas-phase surface diffusion process. Detailed structural analysis confirmed the single crystallinity of Mn-ZnO NWs and excluded the presence of any precipitates or secondary phases. For the transistor, the field-effect mobility and n-type carrier concentration were estimated to be 0.65 cm(2)/V·s and 6.82 × 10(18) cm(-3), respectively. The magnetic hysteresis curves measured under different temperatures (T = 10-350 K) clearly demonstrate the presence of ferromagnetism above room temperature. It suggests that the effect of quantum confinements in NWs improves Tc, and meanwhile minimizes crystalline defects. The magnetoresistace (MR) of a single Mn-ZnO NW was observed up to 50 K. Most importantly, the gate modulation of the MR ratio was up to 2.5 % at 1.9 K, which implies the electric-field control of ferromagnetism in a single Mn-ZnO NW.

Proximity induced high-temperature magnetic order in topological insulator--ferrimagnetic insulator heterostructure.

Introducing magnetic order in a topological insulator (TI) breaks time-reversal symmetry of the surface states and can thus yield a variety of interesting physics and promises for novel spintronic devices. To date, however, magnetic effects in TIs have been demonstrated only at temperatures far below those needed for practical applications. In this work, we study the magnetic properties of Bi2Se3 surface states (SS) in the proximity of a high Tc ferrimagnetic insulator (FMI), yttrium iron garnet (YIG or Y3Fe5O12). Proximity-induced butterfly and square-shaped magnetoresistance loops are observed by magneto-transport measurements with out-of-plane and in-plane fields, respectively, and can be correlated with the magnetization of the YIG substrate. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, an important step toward room-temperature application of TI-based spintronic devices.

Scale-invariant quantum anomalous Hall effect in magnetic topological insulators beyond the two-dimensional limit.

We investigate the quantum anomalous Hall effect (QAHE) and related chiral transport in the millimeter-size (Cr(0.12)Bi(0.26)Sb(0.62))2Te3 films. With high sample quality and robust magnetism at low temperatures, the quantized Hall conductance of e²/h is found to persist even when the film thickness is beyond the two-dimensional (2D) hybridization limit. Meanwhile, the Chern insulator-featured chiral edge conduction is manifested by the nonlocal transport measurements. In contrast to the 2D hybridized thin film, an additional weakly field-dependent longitudinal resistance is observed in the ten-quintuple-layer film, suggesting the influence of the film thickness on the dissipative edge channel in the QAHE regime. The extension of the QAHE into the three-dimensional thickness region addresses the universality of this quantum transport phenomenon and motivates the exploration of new QAHE phases with tunable Chern numbers. In addition, the observation of scale-invariant dissipationless chiral propagation on a macroscopic scale makes a major stride towards ideal low-power interconnect applications.

Superlattice of Fe(x)Ge(1-x) nanodots and nanolayers for spintronics application.

Fe(x)Ge(1-x) superlattices with two types of nanostructures, i.e. nanodots and nanolayers, were successfully fabricated using low-temperature molecular beam epitaxy. Transmission electron microscopy (TEM) characterization clearly shows that both the Fe(x)Ge(1-x) nanodots and nanolayers exhibit a lattice-coherent structure with the surrounding Ge matrix without any metallic precipitations or secondary phases. The magnetic measurement reveals the nature of superparamagnetism in Fe(x)Ge(1-x) nanodots, while showing the absence of superparamagnetism in Fe(x)Ge(1-x) nanolayers. Magnetotransport measurements show distinct magnetoresistance (MR) behavior, i.e. a negative to positive MR transition in Fe(x)Ge(1-x) nanodots and only positive MR in nanolayers, which could be due to a competition between the orbital MR and spin-dependent scatterings. Our results open a new growth strategy for engineering Fe(x)Ge(1-x) nanostructures to facilitate the development of Ge-based spintronics and magnetoelectronics devices.

Electrical spin injection and detection in Mn5Ge3/Ge/Mn5Ge3 nanowire transistors.

In this Letter, we report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline ferromagnetic Mn5Ge3 as source/drain contacts formed by thermal reactions. Degenerate indium dopants were successfully incorporated into as-grown Ge nanowires as p-type doping to alleviate the conductivity mismatch between Ge and Mn5Ge3. The magnetoresistance (MR) of the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias in the temperature range from T = 2 K up to T = 50 K, which clearly indicated the electrical spin injection from ferromagnetic Mn5Ge3 contacts into Ge nanowires. In addition to the bias effect, the MR amplitude was found to exponentially decay with the Ge nanowire channel length; this fact was explained by the dominated Elliot-Yafet spin-relaxation mechanism. The fitting of MR further revealed a spin diffusion length of lsf = 480 ± 13 nm and a spin lifetime exceeding 244 ps at T = 10 K in p-type Ge nanowires, and they showed a weak temperature dependence between 2 and 50 K. Ge nanowires showed a significant enhancement in the measured spin diffusion length and spin lifetime compared with those reported for bulk p-type Ge. Our study of the spin transport in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor points to a possible realization of spin-based transistors; it may also open up new opportunities to create novel Ge nanowire-based spintronic devices. Furthermore, the simple fabrication process promises a compatible integration into standard Si technology in the future.

Manipulating surface-related ferromagnetism in modulation-doped topological insulators.

A new class of devices based on topological insulators (TI) can be achieved by the direct engineering of the time-reversal-symmetry (TRS) protected surface states. In the meantime, a variety of interesting phenomena are also expected when additional ferromagnetism is introduced to the original topological order. In this Letter, we report the magnetic responses from the magnetically modulation-doped (Bi(z)Sb(1-z))2Te3/Cr(x)(Bi(y)Sb(1-y))2Te3 bilayer films. By electrically tuning the Fermi level across the Dirac point, we show that the top TI surface carriers can effectively mediate the magnetic impurities and generate robust ferromagnetic order. More importantly, such surface magneto-electric effects can be either enhanced or suppressed, depending on the magnetic interaction range inside the TI heterostructures. The manipulation of surface-related ferromagnetism realized in our modulation-doped TI device is important for the realization of TRS-breaking topological physics, and it may also lead to new applications of TI-based multifunctional heterostructures.

Efficient Orbitronic Terahertz Emission Based on CoPt Alloy.

Orbitronic devices operate by manipulating orbitally polarized currents. Recent studies have shown that these orbital currents can be excited by femtosecond laser pulses in a ferromagnet such as Ni and converted into ultrafast charge currents via orbital-to-charge conversion. However, the terahertz emission from orbitronic terahertz emitters based on Ni is still much weaker than that of the typical spintronic terahertz emitter. Here, this work reports a more efficient light-induced generation of orbital current from a CoPt alloy, and the terahertz emission from CoPt/Cu/MgO is comparable to that of benchmark spintronic terahertz emitters. By varying the composition of the CoPt alloy, the thickness of Cu, and the capping layer, this work confirms that THz emission primarily originates from the orbital accumulation generated within CoPt, propagating through Cu, followed by subsequent orbital-to-charge conversion due to the inverse orbital Rashba-Edelstein effect at the Cu/MgO interface. This study provides strong evidence for the efficient orbital current generation in CoPt alloy, paving the way for efficient orbital terahertz emitters.

Generation of Tunable Terahertz Waves from Tailored Versatile Spintronic Meta-Antenna Arrays.

Broadband spintronic terahertz (THz) radiation can be efficiently generated by spin-to-charge current conversion in a ferromagnetic/nonmagnetic heterostructure. There had been many studies on realizing the enhancement or the modulation of spintronic terahertz waves. However, reported devices so far focus on implementing certain specific modulation methods, either related to the spintronic stacks or related to the metamaterial structures. In this study, a set of femtosecond laser-driven versatile spintronic terahertz devices are proposed by integrating meta-antenna structures with W/CoFeB/Pt nanolayer stacks. These monolithic integrated devices exhibit spintronic terahertz wave emission, spectral modulation, and polarization manipulation simultaneously. The terahertz pulses are generated within the ferromagnetic heterostructure interfaces and transmitted along the metallic structures, leading to the modulation of the spintronic terahertz waves. Results have shown that the center-frequency shift is up to 140 GHz and the value of ellipticity can reach 0.6, demonstrating a set of integrated and efficient spintronic terahertz devices to modulate the emitted wave. In addition, compared with the slotline antenna, the maximum peak value of the bandpass band is enhanced up to 1.63 times by carefully designing the metamaterial structure. The spintronic meta-antenna array proposed here paves an integrated way for the manipulation of spintronic terahertz optoelectronic devices.

Room temperature energy-efficient spin-orbit torque switching in two-dimensional van der Waals Fe3GeTe2 induced by topological insulators.

Two-dimensional (2D) ferromagnetic materials with unique magnetic properties have great potential for next-generation spintronic devices with high flexibility, easy controllability, and high heretointegrability. However, realizing magnetic switching with low power consumption at room temperature is challenging. Here, we demonstrate the room-temperature spin-orbit torque (SOT) driven magnetization switching in an all-van der Waals (vdW) heterostructure using an optimized epitaxial growth approach. The topological insulator Bi2Te3 not only raises the Curie temperature of Fe3GeTe2 (FGT) through interfacial exchange coupling but also works as a spin current source allowing the FGT to switch at a low current density of ~2.2×106 A/cm2. The SOT efficiency is ~2.69, measured at room temperature. The temperature and thickness-dependent SOT efficiency prove that the larger SOT in our system mainly originates from the nontrivial topological origin of the heterostructure. Our experiments enable an all-vdW SOT structure and provides a solid foundation for the implementation of room-temperature all-vdW spintronic devices in the future.

Interfacial engineering of ferromagnetism in wafer-scale van der Waals Fe4GeTe2 far above room temperature.

Despite recent advances in exfoliated vdW ferromagnets, the widespread application of 2D magnetism requires a Curie temperature (Tc) above room temperature as well as a stable and controllable magnetic anisotropy. Here we demonstrate a large-scale iron-based vdW material Fe4GeTe2 with the Tc reaching ~530 K. We confirmed the high-temperature ferromagnetism by multiple characterizations. Theoretical calculations suggested that the interface-induced right shift of the localized states for unpaired Fe d electrons is the reason for the enhanced Tc, which was confirmed by ultraviolet photoelectron spectroscopy. Moreover, by precisely tailoring Fe concentration we achieved arbitrary control of magnetic anisotropy between out-of-plane and in-plane without inducing any phase disorders. Our finding sheds light on the high potential of Fe4GeTe2 in spintronics, which may open opportunities for room-temperature application of all-vdW spintronic devices.

Fabrication of crystal α-Si3N4/Si-SiOx core-shell/Au-SiOx peapod-like axial double heterostructures for optoelectronic applications.

Novel crystal α-Si(3)N(4)/Si-SiO(x) core-shell/Au-SiO(x) peapod-like axial double heterostructural nanowires were obtained by directly annealing a Au covered SiO(2) thin film on a Si substrate. Our extensive electron microscopic investigation revealed that the α-Si(3)N(4) sections with a mathematical left angle bracket 101 mathematical right angle bracket growth direction were grown first, followed by growth of the Si-SiO(x) core-shell sections and finally growth of the Au-SiO(x) peapod-like sections. Through a series of systematically comparative experiments, a temperature-dependent multi-step vapor-liquid-solid growth mechanism is proposed. Room temperature photoluminescence measurement of individual nanowires reveals two emission peaks (410 and 515 nm), indicating their potential applications in light sources, laser or light emitting display devices.

Generation and Control of Terahertz Spin Currents in Topology-Induced 2D Ferromagnetic Fe3 GeTe2 |Bi2 Te3 Heterostructures.

Future information technologies for low-dissipation quantum computation, high-speed storage, and on-chip communication applications require the development of atomically thin, ultracompact, and ultrafast spintronic devices in which information is encoded, stored, and processed using electron spin. Exploring low-dimensional magnetic materials, designing novel heterostructures, and generating and controlling ultrafast electron spin in 2D magnetism at room temperature, preferably in the unprecedented terahertz (THz) regime, is in high demand. Using THz emission spectroscopy driven by femtosecond laser pulses, optical THz spin-current bursts at room temperature in the 2D van der Waals ferromagnetic Fe3 GeTe2 (FGT) integrated with Bi2 Te3 as a topological insulator are successfully realized. The symmetry of the THz radiation is effectively controlled by the optical pumping incidence and external magnetic field directions, indicating that the THz generation mechanism is the inverse Edelstein effect contributed spin-to-charge conversion. Thickness-, temperature-, and structure-dependent nontrivial THz transients reveal that topology-enhanced interlayer exchange coupling increases the FGT Curie temperature to room temperature, which provides an effective approach for engineering THz spin-current pulses. These results contribute to the goal of all-optical generation, manipulation, and detection of ultrafast THz spin currents in room-temperature 2D magnetism, accelerating the development of atomically thin high-speed spintronic devices.

Flexible Control of Broadband Polarization in a Spintronic Terahertz Emitter Integrated with Liquid Crystal and Metasurface.

Flexible polarization control of the terahertz wave in the wide bandwidth is crucial for numerous applications, such as terahertz communication, material characterization, imaging, and biosensing diagnosis. However, this promise is impeded by the operating bandwidth of circular polarization states, control modes, and the efficiency of the regulation. Here, we report a spintronic terahertz emitter integrated with phase complementary elements, consisting of a liquid crystal and metasurface, to achieve broadband polarization control with high flexibility. This strategy allows the broadband conversion between linear, elliptical, and circular polarization by changing the rotation angle to modulate the space-variant Pancharatnam-Berry phase. The device is characterized with a terahertz time-domain spectroscopy system, demonstrating that the ellipticity of the circular polarization state could keep greater than 0.9 in 0.60-0.99 THz. In the case of an external electro-magnetic field, further polarization modulation experiments are carried out to provide multiple conversion approaches for multi-azimuth. We first propose a method of full broadband polarization state control of the terahertz emitter based on Pancharatnam-Berry phase modulation and an external electro-magnetic field. We believe that such integrated devices with broadband working bandwidth and multiple control modes will make valuable contributions to the development and multi-scene applications of ultrafast terahertz technologies.

Magnetic skyrmions for unconventional computing.

Improvements in computing performance have significantly slowed down over the past few years owing to the intrinsic limitations of computing hardware. However, the demand for data computing has increased exponentially. To solve this problem, tremendous attention has been focused on the continuous scaling of Moore's law as well as the advanced non-von Neumann computing architecture. A rich variety of unconventional computing paradigms has been devised with the rapid development of nanoscale devices. Magnetic skyrmions, spin swirling quasiparticles, have been endowed with great expectations for unconventional computing due to their potential as the smallest information carriers by exploiting their physics and dynamics. In this paper, we provide an overview of the recent progress of skyrmion-based unconventional computing from a joint device-application perspective. This paper aims to build up a panoramic picture, analyze the remaining challenges, and most importantly to shed light on the outlook of skyrmion based unconventional computing for interdisciplinary researchers.