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Compared to a space group, a magnetic space group is much more complex, as both magnetic structure and magnetic moment direction can change the original symmetries of materials. The interplay between space symmetry and magnetism can generate versatile novel quantum states. However, detecting these topological phases experimentally, achieved through manipulating the magnetic configuration, has been restricted. It is mainly because the intrinsic link between the theory and the available experimental technique remains elusive. Here, we show that high harmonic generation (HHG) can identify these topological quantum states. We use rhombohedral ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ film as an example, and analyze the symmetry-dependent harmonic order and signal by combining first-principles calculations and time-dependent Liouville equation numerical computations. Our results provide a fundamental basis for using HHG to study the topological quantum phases mediated by various magnetic configurations that can be easily realized by applying an external magnetic field.

Structural characterization is essential to material engineering, but few tools can detect structural properties in the time domain. High harmonic generation (HHG) emerges as a new frontier that touches the heart of condensed-matter physics from the symmetry to quantum geometrical nature of electrons, but its capability in structural characterization has not been materialized. Here, we establish a crucial connection between the symmetry of a material and the helicity of light. We employ monolayer ${\mathrm{MoS}}_{2}$ as an example. We show that a linearly polarized laser pulse used in experiments is not ideal for structural characterization because it only generates in-plane anisotropy. It is the circularly polarized laser field that is capable of producing four distinctive HHG signals from the four phases of ${\mathrm{MoS}}_{2}$. This finally links the laser helicity to the crystal structure. The results are generic and are not affected by the Berry curvature, the interband or intraband contribution. Our study unleashes the power of HHG as a structural characterization tool for technologically important materials.

The exact measurement of spin-orbit torque (SOT) in magnetic heterostructures is the basis for a solid understanding of its physical mechanism. Although different SOT terms can be properly separated from harmonic Hall voltage analysis, owing to different symmetries and magnetic-field dependences, this method may be invalid for a ferromagnetic layer with a small Hall coefficient. Here, we develop a method to determine the SOT from harmonic longitudinal voltage measurements. Starting from a standard total-energy expression of a magnetic system, the equations to compute the corresponding SOT effective fields are derived and then applied to an in-plane magnetized $\mathrm{Pt}/\mathrm{Ni}\text{\ensuremath{-}}\mathrm{Fe}/\mathrm{Mg}\mathrm{O}$ system. The effective SOT fields and spin Hall angle obtained for the system are in accordance with previous results measured by other methods. Our results illustrate the applicability of harmonic longitudinal voltage measurements for studying current-induced torques in magnetic heterostructures.

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmOx and Pt/Co/AlOx structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (Jc) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained Jc for Pt/Co/SmOx and Pt/Co/AlOx structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and Jc also immensely relies on the depinning field.

As a new class of condensed matter, topological insulators challenge the traditional wisdom of condensed matter physics. Doping topological insulators with magnetic elements can realize the quantum anomalous Hall state, a two-dimensional bulk insulator with nonzero Chern number. High harmonic generation (HHG) has emerged as a very promising tool to probe electronic properties. However, its application to magnetically doped topological insulators has not been realized. Here, we predict that high harmonics from six quintuple layers of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$(6QL-${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$) carry crucial information about its structure. HHG sensitively depends on crystal symmetry and laser polarization. While in the parent compound 6QL-${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ all the harmonic orders are odd, once it is doped with a magnetic Cr atom, selective even and odd harmonics appear, depending on whether the laser field is in-plane or out-of-plane. With spin-orbit coupling, both even and odd harmonics appear. We find that these seemingly complicated harmonics have a simple origin: microscopic interplay between symmetry operations and laser polarization. Our finding demonstrates the unexplored power of HHG in topological insulators and will have a broad impact on future research.

We report on the magnetoresistance in different crystallographic directions of epitaxial ferromagnetic Fe30Co70 thin films with magnetization rotated in the film plane. A negative single crystal anisotropic magnetoresistance (SCAMR) is found when the current is along the easy magnetization axis [110], and the SCAMR can be tuned to the conventional positive one when the current flows along the hard magnetization axis [100]. This finding is explained comprehensively by a magnetocrystalline anisotropy (MCA) symmetry-adapted model expanded along the easy magnetization direction, with which the SCAMR can be represented as a MCA-independent conventional term cos 2 φ M and a series of MCA-dependent terms cos 2 n φ A ( n ≥ 1). The results show that the MCA-dependent twofold term contributes to the negative SCAMR, which cannot be used as a fingerprint of the half-metallicity. Our finding provides an approach to understand and design the magnetoresistance with ferromagnets by MCA.

Recently, electrical switching of interface states in nonferromagnet/ferromagnet (FM)/antiferromagnet (AFM) heterostructure using spin–orbit torque (SOT) is promising due to its high efficiency, zero magnetic field, and multilevel memory state. However, the reversal mechanism of the AFM interface state is still unclear. In this work, we explained the bipolar current switching of the AFM interface state at zero magnetic field by spin–orbit torque (SOT) in a perpendicularly magnetized Pt/Co/IrMn multilayer. By considering symmetry, we reveal that the mechanism behind the AFM interface bipolar current switching is consistent with FM layers perpendicularly switching induced by SOT. The distinct AFM bipolar current switching by SOT is contributed to the symmetry broken by adjacent FM interface coupling. Under such broken symmetry, the antiparallel interface configuration (AP) between FM and AFM could be switched to parallel configuration (P) for both positive and negative currents; however, P is only allowed to be switched to the AFM multiple domain configuration (M), instead of AP. Our result will be helpful for the formulation of a comprehensive understanding of AFM switching induced by SOT and for the development of the interface AFM spintronic devices.

Chemical waste-free, in situ synthesis of customized earth-abundant-metal based catalysts for high-efficiency water splitting still remains a formidable challenge. Here, Ni-Co hydroxide nanosheets (NCHS) are sprouted from Ni-Co foam (NCF) using an eco-friendly, pure-water-based method, and then are phosphorized and nitrided in N2 + P plasmas. The resulting N-doped Ni-Co phosphide catalyst (N-NiCoPx/NCF) exhibits excellent electrocatalytic performance, heralded by the low overpotentials of N-NiCoPx/NCF for generating a current density of 10 and 400 mA cm−2 (j10 and j400) in 1 M KOH of only 23 and 191 mV for the hydrogen evolution reaction (HER), and 298 and 410 mV for oxygen evolution reaction (OER), as well as by the low decay of high current density (j100) in the HER (6.02%) and OER (3.07%) processes for 100 h. Ab inito numerical simulations confirm that Co atom bonded with N represents the active center for overall water splitting.

Nano-size aluminum nitride (AlN) powders have been successfully synthesized with a high efficiency method through annealing from milling assisted by discharge plasma (p-milling) alumina (Al2O3) precursors. The characterization of the p-milling Al2O3 powders and the synthesized AlN are investigated. Compared to conventional ball milling (c-milling), it can be found that the precursors by p-milling have a finer grain size with a higher specific surface area, which lead to a faster reaction efficiency and higher conversion to AlN at lower temperatures. The activation energy of p-milling Al2O3 is found to be 371.5 kJ/mol, a value that is much less than the reported value of the unmilled and the conventional milled Al2O3. Meanwhile, the synthesized AlN powders have unique features, such as an irregular lamp-like morphology with uniform particle distribution and fine average particle size. The results are attributed to the unique synergistic effect of p-milling, which is the effect of deformation, fracture, and cold welding of Al2O3 powders resulting from ball milling, that will be enhanced due to the introduction of discharge plasma.

The microscopic mechanism for the current-induced spin–orbit torque (SOT) in magnetic heterostructures is still under debate. The accurate measurement of SOT effective fields and their thickness dependence is the basis for understanding this issue. In this work, we measured the SOT effective fields for Pt/NiFe bilayers by utilizing the harmonic longitudinal voltage (HLV) method with a Wheatstone bridge structure. Benefiting from the elimination of the linear resistance in the bridge structure and the large magnetoresistance difference resulting from the large length–width ratio of the bridge element, we achieved very high measurement accuracy for both field-like and damping-like effective fields in the Pt/NiFe bilayers. On this basis, we demonstrated the possibility of the SOT measurement with a relatively low current density (∼109 A/m2) by utilizing the HLV method with a Wheatstone bridge structure and found that the method we proposed is also applicable to the Pt/CoFeB system with a low anisotropic magnetoresistance ratio.

The production of ammonia (NH3) at ambient conditions has always faced enormous challenges in chemical industry. In this study, the potential of the electrocatalysts of transition-metal-embedded (TM = Sc, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd and Ag) graphdiyne monolayer (TM@GDY) has been systematically investigated as nitrogen (N2) Reduction Reaction (NRR) by Density Functional Theory (DFT). The results reveal that Mo-embedded graphdiyne monolayer (Mo@GDY) has high stability, high electrical conductivity and excellent catalytic activity with ultra-low onset potential (−0.33 V) by distal mechanism. The ultra-low onset potential is attributed to much stronger interactions between hydrogen and the next nearest nitrogen atoms (H-N2) for the distal mechanism compared with the enzymatic mechanism (−1.03 V) during the first hydrogenation. Moreover, Mo@GDY can also effectively suppress the Hydrogen Evolution Reaction (HER) during the entire NRR process. This study may provide a new approach for electrochemical N2 reduction to form NH3 at ambient conditions.

In contrast to traditional concepts that eliminate magnetic impurities to achieve larger thermoelectric efficiencies, we report an enhanced thermoelectric efficiency for Cu through doping with the magnetic impurity Co. With doping concentrations from 15% to 30%, the amplitude of the Seebeck coefficient increases from 1.90 μV/K up to 16.3 μV/K, which greatly enhances the thermoelectric efficiency (i.e., power factor). Measuring the magnetoresistance and magnetothermoelectric powers at different temperatures indicates that the enhancement of thermoelectric efficiency is a result of spin-dependent scattering from Co nanoparticles, which are less sensitive to the superparamagnetic transitions. Our finding illustrates a path for the use of nanomagnets to develop potential thermoelectric materials.

Magnetic skyrmions have significant potential for applications in storage and logic devices, but the ability to control skyrmion motion is key to their success. To realize controlled skyrmion motion, vertical spin current-driven methods employing, e.g., the spin Hall or inverse spin galvanic effect, are efficient; thus, magnetic heterostructures featuring large spin-orbit torques are appealing. In this paper, we report on the observation of room-temperature magnetic skyrmions in Pt/Co/W multilayers. The interfacial Dzyaloshinskii-Moriya interaction was estimated to be $0.19\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{m}}^{2}$ based on the asymmetric domain-wall motion occurring upon the application of in-plane magnetic fields. The evolution of the magnetic structures from labyrinth domains to skyrmions with diameters of around 145 nm under magnetic fields was observed by performing Lorentz transmission electron microscopy. The skyrmion nucleation fields could be tuned by varying the repetition number. Large spin Hall angle systems such as Pt/Co/W multilayers are appealing for achieving current-driven skyrmion motion in future racetrack and logic applications.

The decomposition of acetaldehyde was investigated in a plasma-catalytic reactor. The gold-based catalysts were obtained by growing Au nanoparticles on silica beads or plates previously coated by a titanium oxo-alkoxy monolayer. The characterization of the synthesized catalysts was carried out by SEM, AFM and UV–Visible spectroscopy. Below an exposure time to UV of 20 min, a homogeneous surface coverage by the relatively small gold nanoparticle (R

We study the perpendicular magnetic anisotropy (PMA) and spin–orbit torques (SOTs) in Pt/Co/Cr stacks as a function of Cr thickness. An enhanced PMA is observed and ascribed to the strong interfacial anisotropy between Co and Cr. Moreover, SOTs induced magnetization switching is achieved under relatively small critical current density in the order of 106 A cm−2 due to the enhanced effective spin Hall angle up to 0.15–0.27. Significantly, the linear dependence between and the square of longitudinal resistivity suggests that the spin Hall effect may arise from the intrinsic mechanism in our systems.

We report a pronounced change of capacitance in silicon based p-n junction by manipulating the space charge region of the p-n junction under a magnetic field. By measuring the charge and discharge curves of p-n junction with a pulsed voltage, we find that with increasing the magnetic field the p-n junction resistance increases sharply, while the value of capacitance is obviously suppressed. Our results reveal that the magnetocapacitance and the magnetoresistance in p-n junction both arise from the redistribution of the space charge region under the magnetic field and give a new way for the development of future magnetic sensors with non-magnetic p-n junctions.

Creating spin current under temperature gradient in magnetic devices has resulted in a new emerging field of spin caloritronics, but extraction of material parameters i.e. Seebeck coefficient and interpretation of thermal transport characteristics are still great challenges due to the thermal contact effect, especially in a wide temperature range. Because the heat-driven voltages do not depend on the change of magnetic states in spin valve, this could be used to obtain the accurate temperature difference applied on sample and exclude the thermal contact effect. Based on this calibration, the electrical and thermal spin transport behaviors in the in-plane FeCo/Cu/FeNi spin valve were systematically studied with various temperature. We observed that the Seebeck coefficients of spin valve was negative and spin-dependent Seebeck coefficient was proportional to the asymmetry parameter in a wide temperature range from 100 to 300 K, indicating that the spin-dependent thermal transports are closely related with electrical transports in the in-plane spin valve.

A heterostructure formed of topological and nontopological half-Heusler compounds provides a means to tune band inversion. In this work, we study the stabilities of [111] and [100] LuPtBi/LuAuSn half-Heusler superlattices. Since the [111] superlattice is more stable than the [100] one, we explore the band inversion in the [111] superlattice. By increasing the number of LuPtBi layers, the energy gap can be tuned from 0.63 to −1.50 eV, where the negative value indicates band inversion. The underlying mechanism involves band alignment, internal electric field, and quantum confinement. Our study offers an efficient means to obtain the desired band inversion in half-Heusler heterostructures.

We study the temperature (T) dependence of the anisotropy field (H-k), switching field (H-sw) saturation magnetization (M-s), spin-orbit torques (SOTs), and critical switching current density (J(c)) as well as Joule heating effect in the current-induced magnetization switching using Pt/Co/C stacks with perpendicular magnetic anisotropy. The results show that H-k remains roughly constant and gradually decreases as T increases. The increased dampinglike-SOT-induced effective field per unit current density (Delta H-DL/J(e)) with T increasing reveals the enhanced SOT efficiency at higher T. However, the calculated Delta H-DL, at J(c) is still a relatively small magnitude and cannot overcome the large H-k. In contrast, a significantly reduced H-sw was observed with the current or T increasing. It indicates that the magnetization switching is dominated by a depinning model. Only when the out-of-plane component of the Delta H-DL overcomes H-sw, the magnetization realizes a full reversal. Especially, both Delta H-DL and H-sw codetermine Meanwhile, T or current-generated Joule heating plays an assisted role in the switching by increasing SOT efficiency and decreasing H-sw. Moreover, our results also show that the enhanced Delta H-DL/J(e) at higher T is determined by the reduced M-s and the spin Ilan angle keeps invariable at different T. These findings could provide a new perspective towards improving the switching efficiency in SOT-based spintronic devices.

We present a simple model based on the classic Shockley model to explain the magnetotransport in nonmagnetic p-n junctions. Under a magnetic field, the evaluation of the carrier to compensate Lorentz force establishes the necessary space-charge region distribution. The calculated current-voltage (I-V) characteristics under various magnetic fields demonstrate that the conventional nonmagnetic p-n junction can exhibit an extremely large magnetoresistance effect, which is even larger than that in magnetic materials. Because the large magnetoresistance effect that we discussed is based on the conventional p-n junction device, our model provides new insight into the development of semiconductor magnetoelectronics. (C) 2018 The Japan Society of Applied Physics

We studied the current-induced spin-orbit torques in a perpendicularly magnetized Pt (1 nm)/Co (0.8 nm)/Pt (5 nm) heterojunction by harmonic Hall voltage measurements. Owing to similar Pt/Co/Pt interfaces, the spin-orbit torques originated from the Rashba effect are reduced, but the contribution from the spin Hall effect is still retained because of asymmetrical Pt thicknesses. When the temperature increases from 50 to 300 K, two orthogonal components of the effective field, induced by spin-orbit torques, reveal opposite temperature dependencies: the field-like term (transverse effective field) decreases from 2.3 to 2.1 (10(-6) Oe (A cm(-2))(-1)), whereas the damping-like term (longitudinal effective field) increases from 3.7 to 4.8 (10(-6) Oe (A cm(-2))(-1)). It is noticed that the damping-like term, usually smaller than the field-like term in the similar Pt/Co interfaces, is twice as large as the field-like term. As a result, the damping-like spin-orbit torque reaches an efficiency of 0.15 at 300 K. Such a temperature-dependent damping-like term in a Pt/Co/Pt heterojunction can efficiently reduce the switching current density which is 2.30 x 10(6) A cm(-2) at 300 K, providing an opportunity to further improve and understand spin-orbit torques induced by spin Hall effect.

Spin-orbit torques (SOTs) in Ta/CoFeB/MgO structures are studied by harmonic voltage method. The results indicate that both Slonczewski-like (HSL) and field-like (HFL) effective field are enhanced by annealing in the film stacks with various Ta thicknesses. Investigation of the crystallographic phase of the Ta layers and resistance of Hall bar devices suggest that annealing may induce a phase transformation in the Ta layers from the α to the β phase, which results in the enhanced HSL of the annealed samples. Current-induced magnetization switching experiments revealed a corresponding decrease of the switching current in the annealed samples because of their enhanced SOTs.

Magnetization switching via charge current induced spin-orbit torques (SOTs) in heavy metal/ferromagnetic metal/heavy metal heterostructures has become an important issue due to its potential applications in high stability and low energy dissipation spintronic devices. In this work, based on a Pt/Co/Ta structure with perpendicular magnetic anisotropy (PMA), we report the effect of inserting a non-metal C interlayer between Co and Ta on the current-induced magnetization switching. A series of measurements based on the extraordinary Hall effect were carried out to investigate the difference of the anisotropy field, switching field, and damping-like and field-like SOT-induced effective fields as well as the current-induced spin Hall effect (SHE) torque after C decoration. The results show that PMA can be reduced by C decoration and the ratio of the effective SHE torque per unit current density and anisotropy field plays an essential role in the switching efficiency. In addition, the obtained switching curren...

In this work, we reported a large magnetoresistance effect in silicon by ion implantation of nitrogen atoms. At room temperature, the magnetoresistance of silicon reaches 125 % under magnetic field 1.7 T and voltage bias -80 V. By applying an alternating magnetic field with a frequency (f) of 0.008 Hz, we find that the magnetoresistance of silicon is divided into f and 2f two signal components, which represent the linear and quadratic magnetoresistance effects, respectively. The analysis based on tuning the magnetic field and the voltage bias reveals that electric-field-induced space-charge effect plays an important role to enhance both the linear and quadratic magnetoresistance effects. Observation as well as a comprehensive explanation of large MR in silicon, especially based on semiconductor CMOS implantation technology, will be an important progress towards magnetoelectronic applications.

To control the semiconductor device under low magnetic field is still a great challenge for semiconductor magnetoelectronics. In this work, we report the observation of the magneto-photogalvanic effect in periodic GaAs dot arrays. With an increase in magnetic field from 0 to 1500 Oe, the photovoltage increases linearly for a wide temperature range from 80 to 430 K. Compared with GaAs without the dot arrays, periodic GaAs dot arrays have a hundredfold increase of the magnetic-field-modulated photovoltage at room temperature. By changing the magnetic field orientation, the angular dependence of photovoltage reveals that the magneto-photogalvanic effect stems from the Hall electric field caused by optical current, and the enhancement of magneto-photogalvanic effect is attributed to the p-n junction coupling between GaAs dots. When the coupling between the GaAs dots is broken at the high temperatures, i.e., T = 430 K, we demonstrate that the enhancement effect disappears as expected. Our results not only illu...

Plasma hydrophilization is a general method to increase the surface free energy of materials. However, only a few works about plasma modification focus on the hydrophilization of tube inner and outer walls. In this paper, we realize simultaneous and long-lasting plasma hydrophilization on the inner and outer walls of polytetrafluoroethylene (PTFE) tubes by atmospheric pressure plasmas (APPs). Specifically, an Ar atmospheric pressure plasma jet (APPJ) is used to modify the PTFE tube's outer wall and meanwhile to induce transferred He APP inside the PTFE tube to modify its inner wall surface. The optical emission spectrum (OES) shows that the plasmas contain many chemically active species, which are known as enablers for various applications. Water contact angle (WCA) measurements, x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) are used to characterize the plasma hydrophilization. Results demonstrate that the wettability of the tube walls are well improved due to the replacement of the surface fluorine by oxygen and the change of surface roughness. The obtained hydrophilicity decreases slowly during more than 180 d aging, indicating a long-lasting hydrophilization. The results presented here clearly demonstrate the great potential of transferring APPs for surface modification of the tube's inner and outer walls simultaneously.

Large magnetoresistance discovered in nonmagnetic semiconductors offers an alternative route to renew magnetoelectronics without ferromagnets. However, it is still a great challenge to retain such large magnetoresistance under low magnetic fields. In this work, analogous to current amplification in the transistor, a magnetoresistance amplification effect is proposed in silicon transistor device, where the device current is significantly controlled by magnetic-field-manipulated coupling of two p-n junctions in transistor. As a direct consequence, large magnetoresistance of 50 000% with high sensitivity of 50% Oe(-1) is yielded at magnetic field of only 0.1 T. The results not only provide here a new proposal compatible with current semiconductor technology to achieve large magntoresistance at low magnetic field, but also realize magnetic-field-manipulated transistor, which is a step for magnetoelectronics.

In femtosecond magnetism, a femtosecond laser pulse affects the spin moment only indirectly through the orbital angular momentum and the spin–orbit coupling. A long-standing puzzle is what happens if the orbital angular momentum itself is quenched. Here, we employ a four-level system to resolve this puzzle. The results show that the quenching of the orbital angular moment in the ground state has no direct relation to the spin moment change. By contrast, the orbital moment can be restored partially after the pulsed optical excitation and can affect the demagnetization. Importantly, this study confirms that the orbital moment indeed responds to the laser field faster than spin if the pulse duration is short, consistent with the recent time-resolved X-ray magnetic circular dichroism experiment. Therefore, our finding shines new light on femtosecond magnetism.