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Ultrashort-laser-induced terahertz (THz) emission is a contact-less probe for charge current generation, which has been recently highlighted in electronics by exploring ultrafast charge dynamics. Here we show it is also applicable in magnetics by exploring spin dynamics. In particular, the generation and transport of an ultrashort spin current, preferably reaching a frequency up to THz, is the core of modern spintronics. First, we demonstrate a picosecond-long spin current generation in a Weyl semimetal Td-WTe2 thin film. The anisotropic dependence of the circular photogalvanic effect in the THz emission indicates that the spin current is generated along the crystalline low-symmetric a-axis, which can be further modulated by the helicity of the laser excitation. Moreover, the generated spins are polarized along the surface normal, which potentially provides an efficient means to manipulate magnetic devices with perpendicular magnetic anisotropy. Since the prior studies have been limited to the static measurements with in-plane spin orientation, our work provides an insight into dynamic feature of the anisotropic spin current and shows a potential for the THz-spintronic devices with Weyl semimetals. Second, we further show the transport of an ultrashort spin current using THz emission. In a magnetic multiplayer Bi2Se3/NiO/NiFe, we show that a spin current generated in the ferromagnetic NiFe layer is transferred to the Bi2Se3 layer through the antiferromagnetic insulator NiO layer. The magnetic and insulating features of NiO enables magnons, i.e., the quanta of spin waves, to carry spin currents without moving charges, which therefore can avoid substantial energy dissipation caused by Joule heating and subsequently provides an efficient means to manipulate the magnetization. Our works, which are relevant to the high-speed and energy-efficient control of spintronic devices, will invigorate magnon-based memory and logic devices.

MATERIALS FOR HUMANITY (MH 21)

Exploring generation and transport of ps-long spin currents with terahertz probes

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

technical paper

Antiferromagnetic (AFM) materials have attracted wide attention in spin-orbit torque (SOT)-based spintronic due to its abundant spin-dependent properties and
unique advantage of immunity against external field perturbations. To act as the charge-to-spin conversion source in energy-saving spintronic devices, it is of great importance for the AFM material to possess a large spin torque efficiency (ξDL). In this work, using the spin torque ferromagnetic resonance (ST-FMR) technique and a Mn2Au/NiFe(Py) bilayer system, we systemically study the ξDL of AFM Mn2Au films with different crystal structures. Compared with polycrystalline Mn2Au with effective ξDL < 0.051, we show a much larger ξDL of ~0.333 in single-crystal Mn2Au, which arises from the large spin Hall conductivity instead of electrical resistivity. Moreover, with a further contribution of interfacial effects, the effective ξDL of single-crystalline Mn2Au/Py system increases to 0.731, which is more than two times larger than the value of ~0.22 reported for the Mn2Au/CoFeB system. By utilizing the large ξDL of Mn2Au in a perpendicularly magnetized MnGa/Mn2Au system, energy-efficient deterministic magnetization switching with a current density at ~10^6 A cm−2 is achieved. Our results reveal a significant potential of Mn2Au as an efficient SOT source and shed light on its application in future AFM material-based SOT integration technology.

Giant spin torque efficiency in single-crystalline antiferromagnet Mn2Au films

We describe a simple experimental method to detect electron paramagnetic resonance (EPR) in polycrystalline 2,2-diphenyl-1-picrylhydrazyl (DPPH) sample, the standard g-marker for EPR spectroscopy, without using a cavity resonator or a prefabricated waveguide. It is shown that microwave (MW) current injected into a layer of silver paint coated on an insulating DPPH sample can excite the paramagnetic resonance in DPPH. As the applied dc magnetic field H is swept, the high-frequency resistance of the Ag-paint layer, measured at room temperature with a single port impedance analyzer in the MW frequency range f = 1 to 2.5 GHz, exhibits a sharp peak at a critical value of the dc field (H = Hr) while the reactance exhibits a dispersion-like behavior around the same field value for a given f. Hr increases linearly with f. We interpret the observed features in the impedance to EPR in DPPH driven by the Oersted magnetic field arising from the MW current in the Ag-paint layer. We also confirm the occurrence of EPR in DPPH independently using a co-planar waveguide based broadband technique in the frequency range 2 – 4 GHz. This technique has the potential to investigate other EPR active inorganic and organic compounds.

Electrically Detected Paramagnetic Resonance in Ag-paint Coated Polycrystalline DPPH

Domain wall (DW) devices are gaining additional attention for their suitability in emerging neuromorphic computing (NC) 1. Underlying building blocks of NC are neurons and synapses, where neurons process the information and synapses control the information passing from pre-neurons to post-neurons. Such synaptic functions can be achieved by realizing multiple conductance (resistance) states. In DW devices, this virtue can be achieved by controllably stopping the DW at multiple specific positions. However, DW motion is stochastic, and therefore, there is a need to artificially form centers by amending magnetic/geometric properties that can controllably pin the DWs 2.
In the present study, we have proposed and studied the concept of nanoscale interfacial Dzyaloshinskii-Moriya interaction (iDMI) region to pin the DW. When an electrical current is passed through the ferromagnetic (FM) nanowire, the DW moves from the left end of the wire and gets pinned at the pinning site for a large range of iDMI constant, the width of the pinning site, and current density (J) values. The DWs can be depinned from the pinning site by applying a higher J. We found that local iDMI forms a barrier in energy locally and hence pins the DW. Besides, the magnetostatic interaction between DW and the pinning site plays an important role in pinning the DWs. In addition to DW pinning, we observed that the DW oscillates at GHz frequencies at the pinning site 3. Later, we introduced five pinning sites along the FM wire and observed that the DW can be pinned (and depinned) successively at every pinning site. Therefore, multiple resistance states and hence synaptic operations can be achieved. The results of this study will be discussed in full detail during the conference. 


References 
1. A. Sengupta and K. Roy, Applied Physics Express, 11 (2018) 030101. 
2. D. Kumar et al., IEEE Transactions on Magnetics, 55 (2018) 1-8. 
3. D. Kumar et al., Journal of Magnetism and Magnetic Materials, (2020) 166807.

Domain wall pinning through nanoscale interfacial Dzyaloshinskii-Moriya interaction

Spin-dependent thermoelectric research has drawn overwhelming attention during the last few years because of its potentiality in energy harvesting and communication applications. Waste heat from locomotives and industries can be converted into electrical and electronic signals to power their own electronics. The Anomalous Nernst effect (ANE) one of the thermomagnetic phenomena proposed to be useful for heat to electrical energy conversion even from a curved surface unlike a conventional thermoelectric module based on semiconductors. Here, we have systematically studied the ANE in polycrystalline metallic perovskite Pr0.5Sr0.5CoO3 as a function of temperature and external magnetic fields. The ANE shows a rapid increase at the onset of ferromagnetic transition (TC ~ 229 K) and after showing a broad maximum at lower temperatures, and it decreases linearly below 100 K. Magnetic field dependence of the ANE, quantified by the transverse Seebeck coefficient (Sxy), exhibits hysteresis below TC as like the magnetization. The maximum saturated value of Sxy due to ANE (~ 0.46 μVK¬-1) found near 175 K (below TC) is ~ 400 times higher than the manganite analog Pr0.5Sr0.5MnO3, and larger than other polycrystalline perovskite oxides reported to date. Our analysis suggests that ANE in this sample is most likely due to asymmetric skew scattering of charge carriers. This work may open a new pathway for the perovskite magnetic oxides for energy-saving applications.

Anomalous Nernst Effect in Bulk Pr0.5Sr0.5CoO3

The mutual synchronization of spin-torque oscillators (STOs) is critical for communication, energy harvesting and neuromorphic applications. Short range magnetic coupling-based synchronization has spatial restrictions (few μm), whereas the long-range electrical synchronization using vortex STOs has limited frequency responses in hundreds MHz (<500 MHz), restricting them for on-chip GHz-range applications. Here, we demonstrate electrical synchronization of four non-vortex uniformly-magnetized STOs using a single common current source in both parallel and series configurations at 2.4 GHz band, resolving the frequency-area quandary for designing STO based on-chip communication systems. Under injection locking, synchronized STOs demonstrate an excellent time-domain stability and substantially improved phase noise performance. By integrating the electrically connected eight STOs, we demonstrate the battery-free energy-harvesting system by utilizing the wireless radio-frequency energy to power electronic devices such as LEDs using the setup shown in Fig. 1a. We also demonstrate the capability of this energy harvesting system in holding dc power using a time-varying signal [Fig. 1b], which is useful in harvesting energy from discrete commercial sources such as a WiFi router. Our results highlight the significance of electrical topology (series vs. parallel) while designing an on-chip STOs system.

Electrically connected spin-torque oscillators array for 2.4 GHz WiFi band transmission and energy harvesting

Human sweat is one of the most available biofluids and the effect of sweat on the wearable electrodes has always been considered a deteriorating effect for electrode’s conductivity because of the enhanced corrosion behavior of bulk metals in the presence of saline solution. Herein, we demonstrate the first example of stretchable Ag electrodes in which the conductivity can be enhanced by human sweating behavior. The stretchable electrodes were fabricated by directly printing the ink composing of conductive filler silver flakes and rationally designed elastic binder (HPUA, thermoplastic and hydrophilic poly(urethane-acrylate) on hydrophilic textiles. Sweat can reduce the resistance of electrodes in both relaxed and stretched states. Our method uses sweat that is unharmful to human skin as the sintering agent that reduces the resistance of printed stretchable electrodes during wearing, providing a new route in the design of printed stretchable and wearable devices. In summary, we designed printable, photocurable, and textile-based Ag-HPUA electrodes whose resistance can be reduced by human sweat in both original and stretched states. The electrodes compose of Ag flakes and HPUA binder with hydrophilic, photocurable, and high stretchable properties. When the Ag-HPUA encounters the sweat, the lactic acid, and Cl- work synergistically to enhance the conductivity rapidly and permanently by removing the lubricant and sintering the exposed Ag flakes. The affecting factors and mechanism of the reaction between the Ag-HPUA electrode and sweat were well explored.

Printable Elastomeric Electrodes with Sweat-Enhanced Conductivity for Wearables

A recent proposal for a brain-inspired architecture, known as Neuromorphic Computing (NC), has received critical appraisal from many researchers charged with procuring low processing power that could one day rival the human brain. Its basic elements can be broadly classed into neurons and synapses, the latter of which is responsible for memory storage featuring multiply resistive states. Since then, spin-based neuromorphic elements have been a subject of study, amongst which the domain wall (DW) device expends the least energy. Nevertheless, the stochastic behaviour of DW motion complicates the identity of its position and renders its motion uncontrollable. This paper proposes the use of synthetic anti-ferromagnetic pinning sites embedded within a ferromagnetic device to control the position of the DW. We simulate both pinning and de-pinning of DWs at synthetic anti-ferromagnetic regions coupled by an inter-layer exchange interaction of up to -0.3 mJ/m2 and relate an increased magnitude of the inter-layer exchange coupling with a de-pinning current that grows in proportion in the affairs of longer pining sites (> 50 nm) with no dependence established on DMI or further length. On the contrary, we find both increasing and decreasing relationships between DMI and de-pinning current which are distinguished by two stable DW configurations at pinning sites of lower lengths. We also extend our study to accommodate five pinning sites to demonstrate the various resistive states from which its neuromorphic synaptic properties may be derived.

Pinning Domain Walls with Synthetic Antiferromagnets

Artificial electronic synapses with two-terminal floating-gate (FG) configurations have undergone enormous development because of the advantage in terms of easy modulation and quick fabrication. Although synaptic features can be effectively modulated via the FG field effect, the tunability remain limited, especially for the commonly used n-type transition metal dichalcogenides (TMDCs). This paper focuses on a two-terminal MoS2 FG artificial synaptic device, the surface of which has been functionalized by an ultrathin deposition (0.1 nm) of MoO3 on the channel region. The surface functionalization together with the gating effect greatly enhance hysteresis behaviors of the device with expanded memory window from 6 to 14.8 V and significantly increased ON/OFF current ratio from 10 to 107. In addition, essential synaptic functions were successfully emulated in this synaptic two-terminal device, including transition from short-term to long-term potentiation, paired-pulse facilitation/depression, and spike timing-dependent plasticity. These findings promise surface charge transfer doping as an effective method to broaden the functionality of two-dimensional TMDCs-based electronic devices and provide a platform for the development of electric-modulated neuromorphic architectures.

Artificial synapses based on two-dimensional van der Waals materials for brain-inspired computing

2D transition-metal dichalcogenides (2D TMDs) exhibit a multitude of exotic properties. In particular, if 2D van der Waals magnets exist, they would be ideal atomically thin building blocks for 2D spintronics [1]. Theories have predicted intrinsic magnetism in 2D VX2, such as vanadium diselenide and vanadium ditelluride. Bonilla et al. reported strong room-temperature ferromagnetism in VSe2 monolayers on van der Waals substrates [2]. We show however, that 2D VSe2 is not intrinsically ferromagnetic, but displays evidence of spin frustration [3]. Nevertheless, a magnetic transition in 2D VSe2 can be induced at the contamination-free interface between Co and VSe2 via interface hybridization [4]. Promotion of ferromagnetism in 2D VSe2 is accompanied by antiferromagnetic coupling to Co and a reduction in the spin moment of Co. Consistent results are obtained for 2D VTe2 [5].

We have addressed the conflicting reports on the ferromagnetism of clean monolayer VSe2. We studied the controllable formation of 1D defect line patterns in VSe2 monolayers using STM and q-plus AFM techniques [6]. We found that the reconstructed VSe2 monolayer with Se-deficient line defects displays room-temperature ferromagnetism under X-ray magnetic circular dichroism and magnetic force microscopy, consistent with the DFT calculations. This work possibly resolves the controversy on whether monolayer VSe2 is intrinsically ferromagnetic, and highlights the importance of controlling surface defects in 2D crystals.

References

[1] W. Zhang, P. K. J. Wong, R. Zhu, A. T. S. Wee, InfoMat. (2019) 1, 1–17.
[2] M. Bonilla et al., Nat. Nano. (2018) 13, 289-293. 
[3] P. K. J. Wong et al., Adv. Mat. (2019) 31, 1901185.
[4] W. Zhang et al., ACS Nano (2019) 13, 8997-9004 
[5] P. K. J. Wong et al., ACS Nano (2019) 13, 11, 12894–12900
[6] R. Chua et al., Adv. Mater. (2020) 200069 


Is 2D Vanadium Diselenide Ferromagnetic?

Topological phonons in crystalline materials have been attracting great interest. However, most cases studied so far are direct generalizations of the topological states from electronic systems. Here, we reveal a novel class of topological phonons -- the symmetry-enforced nodal-chain phonons, which manifest features unique for phononic systems. We show that with D2d little co-group at a non-time-reversal-invariant-momentum point, the phononic nodal chain is guaranteed to exist owing to the vector basis symmetry of phonons, which is a unique character distinct from electronic and other systems. Combined with the spinless character, this makes the proposed nodal-chain phonons enforced by symmorphic crystal symmetries. We further screen all 230 space groups, and find five candidate groups. Interestingly, the nodal chains in these five groups exhibit two different patterns: for tetragonal systems, they are one-dimensional along the fourfold axis; for cubic systems, they form a three-dimensional network structure. Based on first-principles calculations, we identify K2O as a realistic material hosting almost ideal nodal-chain phonons. We show that the effect of LO-TO splitting, another unique feature for phonons, helps to expose the nodal-chain phonons in K2O in a large energy window. In addition, all the five candidate groups have spacetime inversion symmetry, so the nodal chains also feature a quantized π Berry phase. This leads to drumhead surface phonon modes that must exist on multiple surfaces of a sample.

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