Efficient Spin Torques in Antiferromagnetic CoO/Pt Quantified by Comparing Field- and Current-Induced Switching

Emerging Antiferromagnets for Spintronics

Antiferromagnets constitute promising contender materials for next-generation spintronic devices with superior stability, scalability, and dynamics. Nevertheless, the perception of well-established ferromagnetic spintronics underpinned by spontaneous magnetization seemed to indicate the inadequacy of antiferromagnets for spintronics-their compensated magnetization has been perceived to result in uncontrollable antiferromagnetic order and subtle magnetoelectronic responses. However, remarkable advancements have been achieved in antiferromagnetic spintronics in recent years, with consecutive unanticipated discoveries substantiating the feasibility of antiferromagnet-centered spintronic devices. It is emphasized that, distinct from ferromagnets, the richness in complex antiferromagnetic crystal structures is the unique and essential virtue of antiferromagnets that can open up their endless possibilities of novel phenomena and functionality for spintronics. In this Perspective, the recent progress in antiferromagnetic spintronics is reviewed, with a particular focus on that based on several kinds of antiferromagnets with special antiferromagnetic crystal structures. The latest developments in efficiently manipulating antiferromagnetic order, exploring novel antiferromagnetic physical responses, and demonstrating prototype antiferromagnetic spintronic devices are discussed. An outlook on future research directions is also provided. It is hoped that this Perspective can serve as guidance for readers who are interested in this field and encourage unprecedented studies on antiferromagnetic spintronic materials, phenomena, and devices.

Mechanisms of Electrical Switching of Ultrathin CoO/Pt Bilayers

We study current-induced switching of the Néel vector in CoO/Pt bilayers to understand the underlying antiferromagnetic switching mechanism. Surprisingly, we find that for ultrathin CoO/Pt bilayers electrical pulses along the same path can lead to an increase or decrease of the spin Hall magnetoresistance signal, depending on the current density of the pulse. By comparing these results to XMLD-PEEM imaging of the antiferromagnetic domain structure before and after the application of current pulses, we reveal the details of the reorientation of the Néel vector in ultrathin CoO(4 nm). This allows us to understand how opposite resistance changes can result from a thermomagnetoelastic switching mechanism. Importantly, our spatially resolved imaging shows that regions where the current pulses are applied and regions further away exhibit different switched spin structures, which can be explained by a spin-orbit torque-based switching mechanism that can dominate in very thin films.

The effects of protein-rich extract from Rhizoma Gastrodiae against cerebral ischemia/reperfusion injury via regulating MAPK and PI3K/AKT signaling pathway

BACKGROUND

Rhizoma Gastrodiae is a highly valuable traditional Chinese medicine and functional health food that has been used in China to treat neurological disorders for thousands of years. Rhizoma Gastrodiae contains various of biological activities, such as antioxidative, neuroprotective, learning improvement, anxiolytic, and antidepressant effects. However, no studies have been conducted to explore the effects of the protein components in Rhizoma Gastrodiae (GEPS) and its potential protective effects against ischemic stroke.Our main goal was to investigate the effects of GEPS on ischemia/reperfusion (I/R) injury and its possible mechanisms.

METHODS

A middle cerebral artery occlusion (MCAO) induced focal cerebral ischemia mouse model and an oxygen-glucose deprivation (OGD/R) injury model in HT22 cells were established. A neurobehavioral test was performed 24 h after MCAO, and brain infarction was measured. A Morris water maze experiment was conducted on Day 14 after reperfusion in mice. Hematoxylin and eosin (HE) and TUNEL staining were performed to assess apoptotic neuronal death. Immunohistochemical analysis was used to detect BDNF and GAP43 expression. The content of SOD, MDA, GSH-PX and ROS were detected. The protein expression was analyzed using Western blotting. Cell viability was determined by MTT assay. Cell apoptosis was examined by flow cytometry.

RESULTS

GEPS reduced apoptosis, decreased cerebral infarction, improved neurological defects, and ameliorated oxidative stress in the ischemic penumbra. In addition, GEPS increased the expression of BDNF and GA43 in the penumbra. Mechanistically, GEPS counteracted MCAO-induced PI3K/AKT inhibition and activation of MAPK signaling pathways.

CONCLUSION

GEPS has a clear neuroprotective effect on I/R injury, and its mechanism may be linked to the PI3K/AKT and MAPK signaling pathways.

Terahertz Spin Current Dynamics in Antiferromagnetic Hematite

An important vision of modern magnetic research is to use antiferromagnets (AFMs) as controllable and active ultrafast components in spintronic devices. Hematite (α-Fe₂ O₃ ) is a promising model material in this respect because its pronounced Dzyaloshinskii-Moriya interaction leads to the coexistence of antiferromagnetism and weak ferromagnetism. Here, femtosecond laser pulses are used to drive terahertz (THz) spin currents from α-Fe₂ O₃ into an adjacent Pt layer. Two contributions to the generation of the spin current with distinctly different dynamics are found: the impulsive stimulated Raman scatting that relies on the AFM order and the ultrafast spin Seebeck effect that relies on the net magnetization. The total THz spin current dynamics can be manipulated by a medium-strength magnetic field below 1 T. The control of the THz spin current achieved in α-Fe₂ O₃ opens the pathway toward tailoring the exact spin current dynamics from ultrafast AFM spin sources.

Probing the Néel-Type Antiferromagnetic Order and Coherent Magnon-Exciton Coupling in Van Der Waals VPS3

2D van der Waals (vdW) antiferromagnets have received intensive attention due to their terahertz resonance, multilevel magnetic-order states, and ultrafast spin dynamics. However, accurately identifying their magnetic configuration still remains a challenge owing to the lack of net magnetization and insensitivity to external fields. In this work, the Néel-type antiferromagnetic (AFM) order in 2D antiferromagnet VPS₃ with the out-of-plane anisotropy, which is demonstrated by the temperature-dependent spin-phonon coupling and second-harmonic generation (SHG), is experimentally probed. This long-range AFM order even persists at the ultrathin limit. Furthermore, strong interlayer exciton-magnon coupling (EMC) upon the Néel-type AFM order is detected based on the monolayer WSe₂ /VPS₃ heterostructure, which induces an enhanced excitonic state and further certifies the Néel-type AFM order of VPS₃ . The discovery provides optical routes as the novel platform to study 2D antiferromagnets and promotes their potential applications in magneto-optics and opto-spintronic devices.

Control of Néel Vector with Spin-Orbit Torques in an Antiferromagnetic Insulator with Tilted Easy Plane

Injecting spin currents into antiferromagnets and realizing efficient spin-orbit-torque switching represents a challenging topic. Because of the diminishing magnetic susceptibility, current-induced antiferromagnetic dynamics remain poorly characterized, complicated by spurious effects. Here, by growing a thin film antiferromagnet, α-Fe_{2}O_{3}, along its nonbasal plane orientation, we realize a configuration where the spin-orbit torque from an injected spin current can unambiguously rotate and switch the Néel vector within the tilted easy plane, with an efficiency comparable to that of classical ferrimagnetic insulators. Our study introduces a new platform for quantitatively characterizing switching and oscillation dynamics in antiferromagnets.

Spin Hall Magnetoresistance Effect from a Disordered Interface

The spin Hall magnetoresistance (SMR) emerged as a reference tool to investigate the magnetic properties of materials with an all-electrical setup. Its sensitivity to the magnetization of thin films and surfaces may turn it into a valuable technique to characterize van der Waals magnetic materials, which support long-range magnetic order in atomically thin layers. However, realistic surfaces can be affected by defects and disorder, which may result in unexpected artifacts in the SMR, rather than the sole appearance of electrical noise. Here, we study the SMR response of heterostructures combining a platinum (Pt) thin film with the van der Waals antiferromagnet MnPSe₃ and observe a robust SMR-like signal, which turns out to originate from the presence of strong interfacial disorder in the system. We use transmission electron microscopy (TEM) to characterize the interface between MnPSe₃ and Pt, revealing the formation of a few nanometer-thick platinum-chalcogen amorphous layer. The analysis of the transport and TEM measurements suggests that the signal arises from a disordered magnetic system formed at the Pt/MnPSe₃ interface, washing out the interaction between the spins of the Pt electrons and the MnPSe₃ magnetic lattice. Our results show that the damaged interfaces can yield an important contribution to SMR, questioning a widespread assumption on the role of disorder in such measurements.

Highly Tunable Magnetic and Magnetotransport Properties of Exchange Coupled Ferromagnet/Antiferromagnet-Based Heterostructures

Antiferromagnets (AFMs) with zero net magnetization are proposed as active elements in future spintronic devices. Depending on the critical film thickness and measurement temperature, bimetallic Mn-based alloys and transition-metal oxide-based AFMs can host various coexisting ordered, disordered, and frustrated AFM phases. Such coexisting phases in the exchange coupled ferromagnetic (FM)/AFM-based heterostructures can result in unusual magnetic and magnetotransport phenomena. Here, we integrate chemically disordered AFM γ-IrMn₃ thin films with coexisting AFM phases into complex exchange coupled MgO(001)/γ-Ni₃Fe/γ-IrMn₃/γ-Ni₃Fe/CoO heterostructures and study the structural, magnetic, and magnetotransport properties in various magnetic field cooling states. In particular, we unveil the impact of rotating the relative orientation of the thermally disordered and reversible AFM moments with respect to the irreversible AFM moments on the magnetic and magnetotransport properties of the exchange coupled heterostructures. We further reveal that the persistence of thermally disordered and reversible AFM moments is crucial for achieving highly tunable magnetic properties and multilevel magnetoresistance states. We anticipate that the presented approach and the heterostructure architecture can be utilized in future spintronic devices to manipulate the thermally disordered and reversible AFM moments at the nanoscale.

Myrtenol improves brain damage and promotes angiogenesis in rats with cerebral infarction by activating the ERK1/2 signalling pathway

CONTEXT

Cerebral ischaemia/reperfusion (I/R) injury has a high disability and fatality worldwide. Myrtenol has protective effects on myocardial I/R injury through antioxidant and anti-apoptotic effects.

OBJECTIVE

This study investigated the effect of myrtenol on cerebral ischaemia/reperfusion (I/R) injury and the underlying mechanism.

MATERIALS AND METHODS

Cerebral I/R injury was induced in adult Sprague-Dawley rats by middle cerebral artery occlusion (MCAO) for 90 min. MCAO rats were treated with or without myrtenol (10, 30, or 50 mg/kg/day) or/and U0126 (10 μL) intraperitoneally for 7 days.

RESULTS

In the present study, myrtenol had no toxicity at concentrations up to 1.3 g/kg. Myrtenol treatment improved neurological function of MCAO rats, with significantly (p < 0.05) improved neurological deficits (4.31 ± 1.29 vs. 0.00) and reduced brain edoema (78.95 ± 2.27% vs. 85.48 ± 1.24%). Myrtenol extenuated brain tissue injury and neuronal apoptosis, with increased Bcl-2 expression (0.48-fold) and decreased Bax expression (2.02-fold) and caspase-3 activity (1.36-fold). Myrtenol promoted angiogenesis in the brain tissues of MCAO rats, which was reflected by increased VEGF (0.86-fold) and FGF2 (0.51-fold). Myrtenol promoted the phosphorylation of MEK1/2 (0.80-fold) and ERK1/2 (0.97-fold) in MCAO rats. U0126, the inhibitor of ERK1/2 pathway, reversed the protective effects of myrtenol on brain tissue damage and angiogenesis in MCAO rats.

DISCUSSION AND CONCLUSIONS

Myrtenol reduced brain damage and angiogenesis through activating the ERK1/2 signalling pathway, which may provide a novel alternative strategy for preventing cerebral I/R injury. Further in vitro work detailing its mechanism-of-action for improving ischaemic cerebral infarction is needed.

Programmable Dynamics of Exchange-Biased Domain Wall via Spin-Current-Induced Antiferromagnet Switching

Magnetic domain wall (DW) motion in perpendicularly magnetized materials is drawing increased attention due to the prospect of new type of information storage devices, such as racetrack memory. To augment the functionalities of DW motion-based devices, it is essential to improve controllability over the DW motion. Other than electric current, which is known to induce unidirectional shifting of a train of DWs, an application of in-plane magnetic field also enables the control of DW dynamics by rotating the DW magnetization and consequently modulating the inherited chiral DW structure. Applying an external bias field, however, is not a viable approach for the miniaturization of the devices as the external field acts globally. Here, the programmable exchange-coupled DW motion in the antiferromagnet (AFM)/ferromagnet (FM) system is demonstrated, where the role of an external in-plane field is replaced by the exchange bias field from AFM layer, enabling the external field-free modulations of DW motions. Interestingly, the direction of the exchange bias field can also be reconfigured by simply injecting spin currents through the device, enabling electrical and programmable operations of the device. Furthermore, the result inspires a prototype DW motion-based device based on the AFM/FM heterostructure, that could be easily integrated in logic devices.

Ultrafast Amplification and Nonlinear Magnetoelastic Coupling of Coherent Magnon Modes in an Antiferromagnet

We investigate the role of domain walls in the ultrafast magnon dynamics of an antiferromagnetic NiO single crystal in a pump-probe experiment with variable pump photon energy. Analyzing the amplitude of the energy-dependent photoinduced ultrafast spin dynamics, we detect a yet unreported coupling between the material's characteristic terahertz- and gigahertz-magnon modes. We explain this unexpected coupling between two orthogonal eigenstates of the corresponding Hamiltonian by modeling the magnetoelastic interaction between spins in different domains. We find that such interaction, in the nonlinear regime, couples the two different magnon modes via the domain walls and it can be optically exploited via the exciton-magnon resonance.

Observation of current-induced switching in non-collinear antiferromagnetic IrMn3 by differential voltage measurements

There is accelerating interest in developing memory devices using antiferromagnetic (AFM) materials, motivated by the possibility for electrically controlling AFM order via spin-orbit torques, and its read-out via magnetoresistive effects. Recent studies have shown, however, that high current densities create non-magnetic contributions to resistive switching signals in AFM/heavy metal (AFM/HM) bilayers, complicating their interpretation. Here we introduce an experimental protocol to unambiguously distinguish current-induced magnetic and nonmagnetic switching signals in AFM/HM structures, and demonstrate it in IrMn₃/Pt devices. A six-terminal double-cross device is constructed, with an IrMn₃ pillar placed on one cross. The differential voltage is measured between the two crosses with and without IrMn₃ after each switching attempt. For a wide range of current densities, reversible switching is observed only when write currents pass through the cross with the IrMn₃ pillar, eliminating any possibility of non-magnetic switching artifacts. Micromagnetic simulations support our findings, indicating a complex domain-mediated switching process.

Anti-ferromagnetic based memories have a wide range of advantages over their ferromagnetic counterparts, however, their electrical signatures of switching are complicated by spurious signals. Here, Arpaci et al demonstrate an experimental method to distinguish between anti-ferromagnetic switching, and such spurious signatures.

Nonclassical Spin Transfer Effects in an Antiferromagnet

We simulate scattering of electrons by a chain of antiferromagnetically coupled quantum Heisenberg spins, to analyze spin-transfer effects not described by the classical models of magnetism. Our simulations demonstrate efficient excitation of dynamical states that would be forbidden by the semiclassical symmetries, such as generation of multiple magnetic excitation quanta by a single electron. Furthermore, quantum interference of spin wave functions enables generation of magnetization dynamics with amplitudes exceeding the transferred magnetic moment. The efficiency of excitation is almost independent of the electron spin polarization, and is governed mainly by the transfer of energy. Nonclassical spin transfer may thus enable efficient electronic control of antiferromagnets not limited by the classical constraints.

Direct Imaging of Current-Induced Antiferromagnetic Switching Revealing a Pure Thermomagnetoelastic Switching Mechanism in NiO

We unravel the origin of current-induced magnetic switching of insulating antiferromagnet/heavy metal systems. We utilize concurrent transport and magneto-optical measurements to image the switching of antiferromagnetic domains in specially engineered devices of NiO/Pt bilayers. Different electrical pulsing and device geometries reveal different final states of the switching with respect to the current direction. We can explain these through simulations of the temperature-induced strain, and we identify the thermomagnetoelastic switching mechanism combined with thermal excitations as the origin, in which the final state is defined by the strain distributions and heat is required to switch the antiferromagnetic domains. We show that such a potentially very versatile noncontact mechanism can explain the previously reported contradicting observations of the switching final state, which were attributed to spin-orbit torque mechanisms.