Spin current from sub-terahertz-generated antiferromagnetic magnons.

Spin dynamics in antiferromagnets has much shorter timescales than in ferromagnets, offering attractive properties for potential applications in ultrafast devices1-3. However, spin-current generation via antiferromagnetic resonance and simultaneous electrical detection by the inverse spin Hall effect in heavy metals have not yet been explicitly demonstrated4-6. Here we report sub-terahertz spin pumping in heterostructures of a uniaxial antiferromagnetic Cr2O3 crystal and a heavy metal (Pt or Ta in its ß phase). At 0.240 terahertz, the antiferromagnetic resonance in Cr2O3 occurs at about 2.7 tesla, which excites only right-handed magnons. In the spin-canting state, another resonance occurs at 10.5 tesla from the precession of induced magnetic moments. Both resonances generate pure spin currents in the heterostructures, which are detected by the heavy metal as peaks or dips in the open-circuit voltage. The pure-spin-current nature of the electrically detected signals is unambiguously confirmed by the reversal of the voltage polarity observed under two conditions: when switching the detector metal from Pt to Ta, reversing the sign of the spin Hall angle7-9, and when flipping the magnetic-field direction, reversing the magnon chirality4,5. The temperature dependence of the electrical signals at both resonances suggests that the spin current contains both coherent and incoherent magnon contributions, which is further confirmed by measurements of the spin Seebeck effect and is well described by a phenomenological theory. These findings reveal the unique characteristics of magnon excitations in antiferromagnets and their distinctive roles in spin-charge conversion in the high-frequency regime.

Mapping the intrinsic photocurrent streamlines through micromagnetic heterostructure devices.

Photocurrent in quantum materials is often collected at global contacts far away from the initial photoexcitation. This collection process is highly nonlocal. It involves an intricate spatial pattern of photocurrent flow (streamlines) away from its primary photoexcitation that depends sensitively on the configuration of current collecting contacts as well as the spatial nonuniformity and tensor structure of conductivity. Direct imaging to track photocurrent streamlines is challenging. Here, we demonstrate a microscopy method to image photocurrent streamlines through ultrathin heterostructure devices comprising platinum on yttrium iron garnet (YIG). We accomplish this by combining scanning photovoltage microscopy with a uniform rotating magnetic field. Here, local photocurrent is generated through a photo-Nernst type effect with its direction controlled by the external magnetic field. This enables the mapping of photocurrent streamlines in a variety of geometries that include conventional Hall bar-type devices, but also unconventional wing-shaped devices called electrofoils. In these, we find that photocurrent streamlines display contortion, compression, and expansion behavior depending on the shape and angle of attack of the electrofoil devices, much in the same way as tracers in a wind tunnel map the flow of air around an aerodynamic airfoil. This affords a powerful tool to visualize and characterize charge flow in optoelectronic devices.

Giant Magnon-Polaron Anomalies in Spin Seebeck Effect in Double Umbrella-Structured Tb_{3}Fe_{5}O_{12} Films.

We report a giant hysteretic spin Seebeck effect (SSE) anomaly with a sign reversal at magnetic fields much stronger than the coercive field in a (001)-oriented Tb_{3}Fe_{5}O_{12} film. The high-field SSE enhancement reaches 4200% at approximately 105 K over its weak-field value and presents a nonmonotonic dependence on temperature. The unexpected high-field hysteresis of SSE is found to be associated with a magnetic transition of double-umbrella spin texture in TbIG. Nearly parallel dispersion curves of magnons and acoustic phonons around this neoteric transition are supported by theoretical calculations, leading to a high density of field-tuned magnon polarons and consequently an extraordinarily large SSE. Our study provides insight into the evolution of magnon dispersions of double-umbrella TbIG and could potentially boost the efficiency of magnon-polarons SSE devices.

Atomic Origins of Enhanced Ferroelectricity in Nanocolumnar PbTiO3 /PbO Composite Thin Films.

Nanocomposite films hold great promise for multifunctional devices by integrating different functionalities within a single film. The microstructure of the precipitate/secondary phase is an essential element in designing composites' properties. The interphase strain between the matrix and secondary phase is responsible for strain-mediated functionalities, such as magnetoelectric coupling and ferroelectricity. However, a quantitative microstructure-dependent interphase strain characterization has been scarcely studied. Here, it is demonstrated that the PbTiO3 (PTO)/PbO composite system can be prepared in nano-spherical and nanocolumnar configurations by tuning the misfit strain, confirmed by a three-dimensional reconstructive microscopy technique. With the atomic resolution quantitative microscopy with a depth resolution of a few nanometers, it is discovered that the strained region in PTO is much larger and more uniform in nanocolumnar compared to nano-spherical composites, resulting in much enhanced ferroelectric properties. The interphase strain between PbO and PTO in the nanocolumnar structure leads to a giant c/a ratio of 1.20 (bulk value of 1.06), accompanied by a Ti polarization displacement of 0.48 Å and an effective ferroelectric polarization of 241.7 µC cm-2 , three times compared to the bulk value. The quantitative atomic-scale strain and polarization analysis on the interphase strain provides an important guideline for designing ferroelectric nanocomposites.

Role of Magnon-Magnon Scattering in Magnon Polaron Spin Seebeck Effect.

The spin Seebeck effect (SSE) signal of magnon polarons in bulk-Y_{3}Fe_{5}O_{12} (YIG)/Pt heterostructures is found to drastically change as a function of temperature. It appears as a dip in the total SSE signal at low temperatures, but as the temperature increases, the dip gradually decreases before turning to a peak. We attribute the observed dip-to-peak transition to the rapid rise of the four-magnon scattering rate. Our analysis provides important insights into the microscopic origin of the hybridized excitations and the overall temperature dependence of the SSE anomalies.

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI3.

The magnetic properties in two-dimensional van der Waals materials depend sensitively on structure. CrI3, as an example, has been recently demonstrated to exhibit distinct magnetic properties depending on the layer thickness and stacking order. Bulk CrI3 is ferromagnetic (FM) with a Curie temperature of 61 K and a rhombohedral layer stacking, whereas few-layer CrI3 has a layered antiferromagnetic (AFM) phase with a lower ordering temperature of 45 K and a monoclinic stacking. In this work, we use cryogenic magnetic force microscopy to investigate CrI3 flakes in the intermediate thickness range (25-200 nm) and find that the two types of magnetic orders, hence the stacking orders, can coexist in the same flake with a layer of ∼13 nm at each surface being in the layered AFM phase similar to few-layer CrI3 and the rest in the bulk FM phase. The switching of the bulk moment proceeds through a remnant state with nearly compensated magnetic moment along the c-axis, indicating formation of c-axis domains allowed by a weak interlayer coupling strength in the rhombohedral phase. Our results provide a comprehensive picture on the magnetism in CrI3 and point to the possibility of engineering magnetic heterostructures within the same material.

Observation of Magnon Polarons in a Uniaxial Antiferromagnetic Insulator.

Magnon polarons, a type of hybridized excitations between magnons and phonons, were first reported in yttrium iron garnet as anomalies in the spin Seebeck effect responses. Here, we report an observation of antiferromagnetic (AFM) magnon polarons in a uniaxial AFM insulator Cr_{2}O_{3}. Despite the relatively higher energy of magnon than that of the acoustic phonons, near the spin-flop transition of ∼6 T, the left-handed magnon spectrum shifts downward to hybridize with the acoustic phonons to form AFM magnon polarons, which can also be probed by the spin Seebeck effect. The spin Seebeck signal is founded to be enhanced due to the magnon polarons at low temperatures.

Highly Efficient Spin-Orbit Torque and Switching of Layered Ferromagnet Fe3GeTe2.

Among van der Waals (vdW) layered ferromagnets, Fe3GeTe2 (FGT) is an excellent candidate material to form FGT/heavy metal heterostructures for studying the effect of spin-orbit torques (SOT). Its metallicity, strong perpendicular magnetic anisotropy built in the single atomic layers, relatively high Curie temperature (Tc ∼ 225 K), and electrostatic gate tunability offer a tantalizing possibility of achieving the ultimate high SOT limit in monolayer all-vdW nanodevices. In this study, we fabricate heterostructures of FGT/Pt with 5 nm of Pt sputtered onto the atomically flat surface of ∼15-23 nm exfoliated FGT flakes. The spin current generated in Pt exerts a damping-like SOT on FGT magnetization. At ∼2.5 × 1011 A/m2 current density, SOT causes the FGT magnetization to switch, which is detected by the anomalous Hall effect of FGT. To quantify the SOT effect, we measure the second harmonic Hall responses as the applied magnetic field rotates the FGT magnetization in the plane. Our analysis shows that the SOT efficiency is comparable with that of the best heterostructures containing three-dimensional (3D) ferromagnetic metals and much larger than that of heterostructures containing 3D ferrimagnetic insulators. Such large efficiency is attributed to the atomically flat FGT/Pt interface, which demonstrates the great potential of exploiting vdW heterostructures for highly efficient spintronic nanodevices.

Spin Seebeck Effect from Antiferromagnetic Magnons and Critical Spin Fluctuations in Epitaxial FeF_{2} Films.

We report a longitudinal spin Seebeck effect (SSE) study in epitaxially grown FeF_{2}(110) antiferromagnetic (AFM) thin films with strong uniaxial anisotropy over the temperature range of 3.8-250 K. Both the magnetic-field and temperature-dependent SSE signals below the Néel temperature (T_{N}=70 K) of the FeF_{2} films are consistent with a theoretical model based on the excitations of AFM magnons without any net induced static magnetic moment. In addition to the characteristic low-temperature SSE peak associated with the AFM magnons, there is another SSE peak at T_{N} which extends well into the paramagnetic phase. All the SSE data taken at different magnetic fields up to 12 T near and above the critical point T_{N} follow the critical scaling law very well with the critical exponents for magnetic susceptibility of 3D Ising systems, which suggests that the AFM spin correlation is responsible for the observed SSE near T_{N}.

Freezing-enhanced chlorination of organic pollutants for water treatment.

Freezing has been reported to accelerate chemical reactions and thus affect the fate of pollutants in the environment. However, little research has been conducted on the potential effects of freezing on the chlorination process. This study aimed to explore the freezing-enhanced chlorination process by comparing the oxidation of clofibric acid (CA) by chlorine in ice (at -20 °C) to the same reaction in water (at 25 °C). The degradation of CA, which was negligible in water, was significantly accelerated in ice. This acceleration can be attributed to the freeze concentration effect that occurs during freezing, which excludes solutes such as chlorine, CA and protons from the ice crystals, leading to their accumulated concentration in the liquid brine. The increased concentration of chlorine and protons in the liquid brine leads to higher rates of CA oxidation, supporting the freeze concentration effect as the underlying cause for the accelerated chlorination of CA in ice. Moreover, the chlorine/freezing system was also effective in the degradation of other organic pollutants. This highlights the environmental relevance and significance of freezing-enhanced chlorination in cold regions, particularly for the treatment of organic contaminants.

Strain-Tunable Interfacial Dzyaloshinskii-Moriya Interaction and Spin-Hall Topological Hall Effect in Pt/Tm3Fe5O12 Heterostructures.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) in heavy-metal/ferromagnet heterostructures enables to stabilize and manipulate novel topological spin textures, such as skyrmions, which arise as potential logic and memory devices for future information technology. Along these lines, we study in this work the topological spin textures in the films of magnetic insulators by detecting the spin-Hall topological Hall effect (SH-THE). The SH-THE presents obvious dependence of epitaxial strain in Pt/Tm3Fe5O12 (TmIG) bilayers deposited on a series of (111)-oriented garnet substrates, indicating that the topological spin textures can be tuned by epitaxial strain in this system. It is interesting to note that the room-temperature and low-field peak of SH-THE is also recorded within the Pt/TmIG bilayer configuration. We have also examined the interfacial DMI in the Pt/TmIG bilayers by an extended droplet model. The results indicate that the epitaxial strain can effectively change the interfacial DMI in this system, suggesting that the strain-induced modification of the interfacial DMI is the driving force for the SH-THE and topological spin textures in the Pt/TmIG bilayers. Our outcomes open new exciting avenues and opportunities in engineering chiral magnetism and examining the future skyrmion technology in magnetic insulators through the application of epitaxial strain.

Gender-specific prevalence and risk factors of mild cognitive impairment among older adults in Chongming, Shanghai, China.

Objective: This study explores the gender differences in the prevalence of mild cognitive impairment (MCI) and the correlation between multiple influencing factors. Materials and methods: The sample was comprised of 1325 relatively healthy participants aged ≥ 60 years in a Shanghai community-dwelling (557 males and 768 females). Cognitive function was assessed by Mini-Mental State Examination (MMSE). The Instrumental Activities of Daily Living (IADL) scale was used to assess the activities of daily living. Results: The overall prevalence of MCI was 15.2%, with 10.2% in men and 18.9% in women. In older male subjects, those with higher the Geriatric Depression Scale (GDS) scores [odds ratio (OR) = 1.07, 95% confidence interval (CI) = 1.01-1.14] and hypertension (OR = 2.33, 95% CI = 1.15-4.73) had a higher risk of MCI. female subjects who were illiterate (OR = 2.95, 95% CI = 1.82-4.78), had a farming background (OR = 1.69, 95% CI = 1.05-2.72), and a history of stroke (OR = 1.96, 95% CI = 1.07-3.59) had a higher risk of MCI, but this was not true for males. However, Male subjects who never smoked were less likely to have MCI (OR = 0.22, 95% CI = 0.09-0.54). Additionally, the prevalence of MCI was lower in older women with high grip strength (OR = 0.96, 95% CI = 0.92-0.99) and hyperlipidemia (OR = 0.45, 95% CI = 0.22-0.96). Conclusion: The prevalence of MCI was higher in the population of elderly women compared to men. Moreover, it was found that members with MCI tended to having higher GDS scores, smoking, and hypertension; whereas a history of farming, illiteracy, stroke, grip strength, and hyperlipidemia were correlated with MCI in women.

Poor Physical Capacity Combined With High Body Fat Percentage as an Independent Risk Factor for Incident Hypertension in Chinese Suburb-Dwelling Older Adults.

Background: This study examined the effects of poor physical capacity and high body fat percentage (BF%) on the incidence of hypertension in Chinese suburb-dwelling older adults. Methods: This study was conducted on 368 Chinese suburb-dwelling participants aged ≥ 60 years without hypertension (mean age: 66.74 ± 5.59 years, 48.9% men). Poor physical capacity is defined by the Asian Working Group for Sarcopenia (AWGS) criteria as grip strength < 26 kg for men and < 18 kg for women or walking speed <0.8 m/s. High BF% was defined as values that are greater than the upper tertile for BF% as stratified by sex. The outcome was the incidence of hypertension. Results: Overall, 5.7% of subjects had both poor physical capacity and high BF%. After the average follow-up duration of 2 years, the incidence of hypertension was 39.7%, and those experiencing both poor physical capacity and high BF% had the highest incidence (81.0%). After multivariate adjustments, the incidence of hypertension was associated with the combination of poor physical capacity and high BF% [odds ratio (OR) = 6.43, 95% CI = 1.91-21.64] but not solely with poor physical capacity (OR = 1.11, 95% CI = 0.55-2.25) or only high BF% (OR = 1.37, 95% CI = 0.80-2.34). Conclusion: The combination of poor physical capacity and high BF% can significantly increase the incidence of hypertension in Chinese suburb-dwelling older adults. For hypertension prevention, ideally, we should strive toward decreasing body fat mass while simultaneously improving physical capacity.

Both Short and Long Sleep Durations are Risk Factors for Sarcopenia in Suburban-Dwelling Older Chinese Individuals: A 3-Year Longitudinal Study.

Background: Few longitudinal studies have explored exploring the relationship between sleep duration and sarcopenia. Evidence concerning the relationship between sleep duration and sarcopenia is limited and inconsistent. The purpose of this 3-year prospective study was to explore whether sleep duration was associated with sarcopenia onset in suburb-dwelling older Chinese individuals. Methods: This was a prospective study that included 754 Chinese suburb-dwelling men and women aged ≥60 years (men n=327, mean age 65.24± 4.87 years) who were not initially diagnosed with sarcopenia. We defined sarcopenia using the diagnostic algorithm recommended by the Asian Working Group for Sarcopenia. Self-reported sleep duration was a component of the interview measured by trained interviewers. Subjects were categorized into 3 groups at baseline [short: <6 h, medium: 6-8 h, and long: >8 h]. Results: The incidence of sarcopenia during the 3-year follow-up was 12.2%. Multivariate logistic regression analyses showed that after adjustments for potential confounders long sleep duration was independently associated with sarcopenia incidence from baseline through the 3-year follow-up: when using the 6-8 h sleep duration group as a reference, the adjusted ORs for sarcopenia of the groups who slept <6 and >8 hours were 2.74 (95% CI 1.05-7.13) and 1.84 (95% CI 1.07-3.14). Conclusion: Both short and long sleep durations were associated with a greater incidence of sarcopenia. Thus, sleep duration should be considered when developing prevention and management strategies for sarcopenia.

Proximity-magnetized quantum spin Hall insulator: monolayer 1 T' WTe2/Cr2Ge2Te6.

Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T' WTe2, a two-dimensional quantum spin Hall insulator, and few-layer Cr2Ge2Te6, an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe2 manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.

Role of dimensional crossover on spin-orbit torque efficiency in magnetic insulator thin films.

Magnetic insulators (MIs) attract tremendous interest for spintronic applications due to low Gilbert damping and the absence of Ohmic loss. Spin-orbit torques (SOTs) on MIs are more intriguing than magnetic metals since SOTs cannot be transferred to MIs through direct injection of electron spins. Understanding of SOTs on MIs remains elusive, especially how SOTs scale with the MI film thickness. Here, we observe the critical role of dimensionality on the SOT efficiency by studying the MI layer thickness-dependent SOT efficiency in tungsten/thulium iron garnet (W/TmIG) bilayers. We show that the TmIG thin film evolves from two-dimensional to three-dimensional magnetic phase transitions as the thickness increases. We report the significant enhancement of the measured SOT efficiency as the TmIG thickness increases, which is attributed to the increase of the magnetic moment density. We demonstrate the current-induced SOT switching in the W/TmIG bilayers with a TmIG thickness up to 15 nm.

Observation of magnon-mediated current drag in Pt/yttrium iron garnet/Pt(Ta) trilayers.

Pure spin current, a flow of spin angular momentum without flow of any accompanying net charge, is generated in two common ways. One makes use of the spin Hall effect in normal metals (NM) with strong spin-orbit coupling, such as Pt or Ta. The other utilizes the collective motion of magnetic moments or spin waves with the quasi-particle excitations called magnons. A popular material for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we demonstrate in NM/MI/NM trilayers that these two types of spin currents are interconvertible across the interfaces, predicated as the magnon-mediated current drag phenomenon. The transmitted signal scales linearly with the driving current without a threshold and follows the power-law T(n) with n ranging from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can serve as a scalable pure spin current valve device which is an essential ingredient in spintronics.