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While being electrically insulating, magnetic insulators can behave as good spin conductors by carrying spin current with excited spin waves. So far, magnetic insulators are utilized in multilayer heterostructures for optimizing spin transport or to form magnon spin valves for reaching controls over the spin flow. In these studies, it remains an intensively visited topic as to what the corresponding roles of coherent and incoherent magnons are in the spin transmission. Meanwhile, understanding the underlying mechanism associated with spin transmission in insulators can help to identify new mechanisms that can further improve the spin transport efficiency. Here, by studying spin transport in a magnetic-metal/magnetic-insulator/platinum multilayer, it is demonstrated that coherent magnons can transfer spins efficiently above the magnon bandgap of magnetic insulators. Particularly the standing spin-wave mode can greatly enhance the spin flow by inducing a resonant magnon transmission. Furthermore, within the magnon bandgap, a shutdown of spin transmission due to the blocking of coherent magnons is observed. The demonstrated magnon transmission enhancement and filtering effect provides an efficient method for modulating spin current in magnonic devices.

Recently discovered magnetic Weyl semimetals (MWSM), with enhanced Berry curvature stemming from the topology of their electronic band structure, have gained much interest for spintronics applications. In this category, Co2MnGa, a room temperature ferromagnetic Heusler alloy, has garnered special interest as a promising material for topologically driven spintronic applications. However, until now, the structural-order dependence of spin current generation efficiency through the spin Hall effect has not been fully explored in this material. In this paper, we study the evolution of magnetic and transport properties of Co2MnGa thin films from the chemically disordered B2 to ordered L21 phase. We also report on the change in spin generation efficiency across these different phases, using heterostructures of Co2MnGa and ferrimagnet Co xTb1− x with perpendicular magnetic anisotropy. We measured large spin Hall angles in both the B2 and L21 phases, and within our experimental limits, we did not observe the advantage brought by the MWSM ordering in generating a strong spin Hall angle over the disordered phases, which suggests more complicated mechanisms over the intrinsic, Weyl-band structure-determined spin Hall effect in these material stacks.

© 2020 American Physical Society. Ferromagnetic metals and insulators are widely used for generation, control, and detection of magnon spin signals. Most magnonic structures are based primarily on either magnetic insulators or ferromagnetic metals, while heterostructures integrating both of them are less explored. Here, by introducing a Pt/yttrium iron garnet (YIG)/permalloy (Py) hybrid structure grown on a Si substrate, we study the magnetic coupling and magnon transmission across the interface of the two magnetic layers. We find that within this structure, Py and YIG exhibit an antiferromagnetic coupling field as strong as 150 mT, as evidenced by both magnetometry and polarized neutron reflectometry measurements. By controlling individual layer thicknesses and external fields, we realize parallel and antiparallel magnetization configurations, which are further utilized to control the magnon current transmission. We show that a magnon spin valve with an on:off ratio of approximately 130% can be realized out of this multilayer structure at room temperature through both spin pumping and spin-Seebeck-effect experiments. Owing to the efficient control of magnon current and the compatibility with Si technology, the Pt/YIG/Py hybrid structure could potentially find applications in magnon-based logic and memory devices.

© 2019 American Physical Society. We report the observation of antiferromagnetic insulator-mediated spin-orbit-torque switching in Pt/NiO/Co1-xTbx heterostructures. By measuring the current-induced shift in the magnetic hysteresis loops and the second-harmonic anomalous Hall resistance, we quantitatively determine the spin-orbit-torque efficiency in Pt/NiO/Co1-xTbx samples with different NiO thicknesses, uncovering a systematic evolution of the magnetic switching behavior. The measured spin-orbit-torque efficiency is enhanced in the low NiO thickness regime (1-2 nm), highlighting the efficient spin manipulation across a disordered antiferromagnetic insulator.

© 2019 American Physical Society. We report broadband microwave absorption spectroscopy of the layered antiferromagnet CrCl3. We observe a rich structure of resonances arising from quasi-two-dimensional antiferromagnetic dynamics. Because of the weak interlayer magnetic coupling in this material, we are able to observe both optical and acoustic branches of antiferromagnetic resonance in the GHz frequency range and a symmetry-protected crossing between them. By breaking rotational symmetry, we further show that strong magnon-magnon coupling with large tunable gaps can be induced between the two resonant modes.

© 2019 Author(s). We study the magnon transport in the nonlocal configuration composed of two Pt strips on top of a yttrium iron garnet, with and without the presence of RF microwaves generated by an on-chip antenna. We find that the spin-Hall induced thermal magnon heating/cooling, the Oersted field, and the Joule heating generated by the alternating current in the Pt injector can significantly influence the spin-pumping signal measured by the Pt detector in the presence of RF microwaves, forcing the spin-pumping voltage to show up in the first and second harmonic signals in the nonlocal magnon transport measurement. These results indicate that the nonlocal magnon transport configuration can serve as a structure to electrically detect and manipulate the spin-pumping signal. Furthermore, certain caution is needed when studying the interplay between incoherent magnon and coherent magnon spin transport in the nonlocal transport configuration, since the change in microwave-induced spin-pumping voltage can overwhelm the incoherent magnon transport signals.

The charge-to-spin conversion efficiency is a crucial parameter in determining the performance of many useful spintronic materials. Usually, this conversion efficiency is predetermined by the intrinsic nature of solid-state materials, which cannot be easily modified without invoking chemical or structural changes in the underlying system. Here we report on successful modulation of charge-spin conversion efficiency via the metal-insulator transition in a quintessential strongly correlated electron compound vanadium dioxide (VO2). By employing ferromagnetic resonance driven spin pumping and the inverse spin Hall effect measurement, we find a dramatic change in the spin pumping signal (decrease by > 80%) and charge-spin conversion efficiency (increase by five times) upon insulator to metal transition. The abrupt change in the structural and electrical properties of this material therefore provides useful insights on the spin related physics in a strongly correlated material undergoing a phase transition., United States. Office of Naval Research (Award FA9550-19-1-0048), National Science Foundation (U.S.) (Award DMR 14-19807)

Coupled microwave photon-magnon hybrid systems offer promising applications by harnessing various magnon physics. At present, in order to realize high coupling strength between the two subsystems, bulky ferromagnets with large spin numbers are utilized, which limits their potential applications for scalable quantum information processing. By enhancing single spin coupling strength using lithographically defined superconducting resonators, we report high cooperativities between a resonator mode and a Kittel mode in nanometer thick Permalloy wires. The on-chip, lithographically scalable, and superconducting quantum circuit compatible design provides a direct route towards realizing hybrid quantum systems with nanomagnets, whose coupling strength can be precisely engineered and dynamic properties can be controlled by various mechanisms derived from spintronic studies. ©2019 American Physical Society, NSF (Grant No. ECCS-1653553), AFOSR (Grant No. FA9550-19-1-0048)

Magnetic domain walls are information tokens in both logic and memory devices and hold particular interest in applications such as neuromorphic accelerators that combine logic in memory. Here, we show that devices based on the electrical manipulation of magnetic domain walls are capable of implementing linear, as well as programmable nonlinear, functions. Unlike other approaches, domain-wall-based devices are ideal for application to both synaptic weight generators and thresholding in deep neural networks. Prototype micrometer-size devices operate with 8 ns current pulses and the energy consumption required for weight modulation is ≤16 pJ. Both speed and energy consumption compare favorably to other synaptic nonvolatile devices, with the expected energy dissipation for scaled 20 nm devices close to that of biological neurons., National Science Foundation (U.S.) (Award 1639921)

The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show a large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. We report spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Our results demonstrate robust charge-spin conversion in TI and provide a direct avenue towards applicable TI-based spintronic devices., National Science Foundation (U.S.) (Grant DMR-141980), National Science Foundation (U.S.) (Grant 1639921)

Owing to the difficulty in detecting and manipulating the magnetic states of antiferromagnetic materials, studying their switching dynamics using electrical methods remains a challenging task. By employing heavy-metal–rare-earth–transition-metal alloy bilayers, we experimentally study current-induced domain wall dynamics in an antiferromagnetically coupled system. We show that the current-induced domain wall mobility reaches a maximum at the angular momentum compensation point. With experiment and modeling, we further reveal the internal structures of domain walls and the underlying mechanisms for their fast motion. We show that the chirality of the ferrimagnetic domain walls remains the same across the compensation points, suggesting that spin orientations of specific sublattices rather than net magnetization determine Dzyaloshinskii-Moriya interaction in heavy-metal–ferrimagnet bilayers. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnetic material opens new opportunities for high-speed spintronic devices., National Science Foundation (U. S.) (Grant No. 1639921), Nanoelectronics Research Corporation, Nanoelectronics Research Initiative Center (Research Task ID No. 2700.001)

Despite the potential advantages of information storage in antiferromagnetically coupled materials, it remains unclear whether one can control the magnetic-moment orientation efficiently because of the canceled magnetic moment. Here, we report spin-orbit-torque-induced magnetization switching of ferrimagnetic Co[subscript 1−x]Tb[subscript x] films with perpendicular magnetic anisotropy. Current-induced switching is demonstrated in all of the studied film compositions, including those near the magnetization compensation point. The spin-orbit-torque-induced effective field is further quantified in the domain-wall motion regime. A divergent behavior that scales with the inverse of magnetic moment is confirmed close to the compensation point, which is consistent with angular momentum conservation. Moreover, we also quantify the Dzyaloshinskii-Moriya interaction energy in the Ta/Co[subscript 1−x]Tb[subscript x] system and we find that the energy density increases as a function of the Tb concentration. The demonstrated spin-orbit-torque switching, in combination with the fast magnetic dynamics and minimal net magnetization of ferrimagnetic alloys, promises spintronic devices that are faster and with higher density than traditional ferromagnetic systems.