Luqiao Liu

In-memory computing (IMC) has emerged as a promising paradigm for data-intensive applications, as it mitigates data movement overhead and enhances computational efficiency by performing operations directly within memory arrays. Nevertheless, conventional IMC architectures face scalability and density constraints arising from the integration of complex logic circuits into memory structures. To overcome these limitations, we introduce SIMCH, an in-memory stochastic computing (IMSC) architecture that exploits stochastic magnetic tunnel junctions (sMTJs) to enable a fully digital, parallel, and probabilistic computation. By co-integrating fixed-probability sMTJs with MRAM bit cells, SIMCH achieves high-density stochastic matrix–vector multiplication without the need for analog biasing. When evaluated on hyperdimensional computing (HDC) workloads, the IMSC system demonstrates up to 6.7× higher memory density compared to an SRAM-based IMC macro and delivers a 7.7× improvement in throughput over a state-of-the-art ReRAM-based HDC accelerator, while maintaining accuracy close to the baseline.

This article presents physical modeling and benchmarking for two-terminal spin-orbit-torque magnetic random access memory (2T SOT-MRAM). The results indicate that the common spin-orbit torque (SOT) materials that provide only in-plane torque can provide little to no improvement over spin-transfer-torque (STT) MRAM in terms of write energy. However, emerging SOT materials that provide out-of-plane torques with efficiencies as small as 0.1 can result in significant improvements in the write energy for such two-terminal devices, especially when the magnet lateral dimensions are scaled down to 30 or 20 nm. Additionally, a novel 2T SOT-MRAM device is proposed that can increase the path electrons passing through the SOT layer, hence increasing the generated spin current and the energy efficiency of the device. Our benchmarking results indicate that an out-of-plane SOT efficiency of 0.051 for 20-nm-wide devices can result in write energies approaching SRAM at the 7-nm technology node.

The internal coupling of magnetic excitations (magnons) with themselves has created a new research sub-field in hybrid magnonics, i.e., magnon-magnon coupling, which focuses on materials discovery and engineering for probing and controlling magnons in a coherent manner. This is enabled by, one, the abundant mechanisms of introducing magnetic interactions, with examples of exchange coupling, dipolar coupling, Ruderman–Kittel–Kasuya–Yosida (RKKY) coupling, and Dzyaloshinskii–Moriya interaction (DMI) coupling, and two, the vast knowledge of how to control magnon band structure, including field and wavelength dependences of frequencies, for determining the degeneracy of magnon modes with different symmetries. In particular, we discuss how magnon-magnon coupling is implemented in various materials systems, with examples of magnetic bilayers, synthetic antiferromagnets, nanomagnetic arrays, layered van der Waals magnets, and (DMI spin-orbit torque materials) in magnetic multilayers. We then introduce new concept of applications for these hybrid magnonic materials systems, with examples of frequency up/down conversion and magnon-exciton coupling, and discuss what properties are desired for achieving those applications.

Correlated microwave photon sources are key enablers for technologies in quantum-limited sensing, signal amplification and communication, but the reliance on millikelvin operating temperature limits their scalability for broader applications. Here, at room temperature, we demonstrate strong correlated microwave signals emitted from a hybrid magnon-photon platform. Different from traditional parametrically induced magnons with degenerate frequencies, we achieve non-degenerate excitations by coupling magnon modes simultaneously with two cavity photon modes. Through the magnon-photon interactions in the corresponding linear and nonlinear regimes, one input microwave photon splits into a pair of magnon polaritons that possess distinct frequencies but maintain strong inter-mode correlations. The nonlinear magnon polariton dynamics empowered by this new parametric platform brings both verified true randomness and robust multi-channel correlations, from which we construct a microwave communication experiment for noise resilient signal transmission with added security. This work establishes cavity magnonics as a versatile and compact platform for generating correlated multi-mode microwave signals, opening new avenues for applications in classical and quantum domains.

Correlated microwave photon sources are key enablers for technologies in quantum-limited sensing, signal amplification and communication, but the reliance on millikelvin operating temperature limits their scalability for broader applications. Here, at room temperature, we demonstrate strong correlated microwave signals emitted from a hybrid magnon-photon platform. Different from traditional parametrically induced magnons with degenerate frequencies, we achieve non-degenerate excitations by coupling magnon modes simultaneously with two cavity photon modes. Through the magnon-photon interactions in the corresponding linear and nonlinear regimes, one input microwave photon splits into a pair of magnon polaritons that possess distinct frequencies but maintain strong inter-mode correlations. The nonlinear magnon polariton dynamics empowered by this new parametric platform brings both verified true randomness and robust multi-channel correlations, from which we construct a microwave communication experiment for noise resilient signal transmission with added security. This work establishes cavity magnonics as a versatile and compact platform for generating correlated multi-mode microwave signals, opening new avenues for applications in classical and quantum domains.

ABSTRACT The interactions between magnons and domain walls provide opportunities for extending the functionality of magnonic and spintronic devices. Building on our previous demonstration of field‐free magnon‐induced domain wall motion in low‐damping bismuth‐substituted yttrium iron garnet, this work explores the underlying dynamics and the effects of small magnetic fields on magnon‐driven domain wall motion. Time‐resolved imaging unveils the dynamical response of the garnet track to the RF signal, as well as the excitation of oscillations in a domain wall placed in the track. Magnon transmission is detected over distances of 70 µm from which a damping parameter is extracted. The combined influence of small out‐of‐plane fields (≤ 2 mT) and magnons on domain wall depinning and motion enables direction control. The findings culminate in the demonstration of robust, bidirectional domain wall motion up to 40 µm, establishing a pathway toward field‐tunable, low‐loss magnonic devices.

Analog in-memory computing based on crossbar arrays offers a path to energy-efficient AI hardware, but has been limited by reliance on bulky, power-hungry analog-to-digital converters. This study introduces stochastic nanomagnets driven by spin-orbit torque as intrinsic analog-to-digital interfaces, enabling compact, fast, low-power digitization while maintaining high computational accuracy. These results suggest a promising direction for energy-efficient AI hardware accelerators and significant advances in next-generation machine-learning hardware.

The interplay between magnetic order and electronic band structure in antiferromagnets has garnered increasing interest due to its potential for spintronic applications. While magnetic transitions have been shown to induce substantial band structure modifications in optical measurements, their influence on electronic transport remains poorly understood. In this work, we investigate the transport properties of CrSBr, a van der Waals antiferromagnetic semiconductor, over a wide range of carrier densities modulated by gate voltage. We observe a drastic contrast in magnetoresistance behavior between the low- and high-carrier density regimes. Through a combination of experiment and modeling, we identify magnetically driven carrier concentration modulation and mobility modulation as the dominant mechanisms governing magnetoresistance in the respective regimes. These findings advance the understanding of magnetoelectric transport in antiferromagnets and suggest promising routes for energy-efficient spintronic technologies in memory, logic, and sensing applications.

We experimentally explore the interfacial coherent spin pumping in the magnon-magnon hybridized regime of a perpendicular anisotropy ferrimagnetic insulator (Bi-substituted yttrium iron garnet, or BiYIG) and ferromagnetic metal (permalloy, or Py) bilayer. In addition to the interfacial exchange coupling-induced avoided crossings between the uniform modes of BiYIG and Py, we observe that the gap opening can be tailored by changing the relative orientations of the BiYIG and Py magnetizations due to a purely dynamic interaction. The avoided-crossing gap opening is small (large) when BiYIG and Py magnetizations are nearly perpendicular (partially collinear) to each other, indicating that interfacial fieldlike torque from spin pumping plays a dominant role. Macrospin simulations with a sizable fieldlike spin-pumping torque agree with the gap opening trend observed in the experiments.

Physical devices exhibiting stochastic functions with low energy consumption and high device density have the potential to enable complex probability-based computing algorithms, accelerate machine learning, and enhance hardware security. Recently, superparamagnetic tunnel junctions (sMTJs) have been widely explored for such purposes, leading to the development of sMTJ-based systems; however, the reliance on nanoscale ferromagnets limits scalability and reliability, making sMTJs sensitive to external perturbations and prone to significant device variations. Here, we present an experimental demonstration of closed loop three-terminal sMTJs as reliable and potentially scalable sources of true randomness, in the absence of external magnets. By leveraging dual-current controllability and incorporating feedback, we stabilize the switching operation of superparamagnets and reach cryptographic-quality random bitstreams. The realization of controllable and robust true random sMTJs underpins a general hardware platform for computing schemes exploiting the stochasticity in the physical world, as demonstrated by the generative artificial intelligence example in our experiment.

Oxide superlattices reveal a rich array of emergent properties derived from the composition modulation and the resulting lattice distortion, charge transfer, and symmetry reduction that occur at the interfaces between the layers. The great majority of studies have focused on perovskite oxide superlattices, revealing, for example, the appearance of an interfacial 2D electron gas, magnetic moment, or improper ferroelectric polarization that is not present in the parent phases. Garnets possess greater structural complexity than perovskites: the cubic garnet unit cell contains 160 atoms with the cations distributed between three different coordination sites, and garnets exhibit a wide range of useful properties, including ferrimagnetism and ion transport. However, there have been few reports of the synthesis or properties of garnet superlattices, with layer thicknesses approaching the unit cell dimension of 1.2 nm. Here, we describe superlattices made from Bi and rare earth (RE = Tm, Tb, Eu, Lu) iron garnets (IGs) grown by pulsed laser deposition. Atom probe tomography and transmission electron microscopy reveal the composition modulation without dislocations and layer thicknesses as low as 0.45 nm, less than half a unit cell. TmIG/TbIG superlattices exhibit perpendicular magnetic anisotropy that is qualitatively different from the in-plane anisotropy of the solid solution, and BiIG/LuIG superlattices exhibit ferromagnetic resonance linewidth characteristics of the end-members rather than the solid solution. Garnet superlattices provide a playground for exploring interface physics within the vast parameter space of cation coordination and substitution.

Magnetic sensing beyond the linear regime could broaden the frequency range of detectable magnetic fields, which is crucial to various microwave and quantum applications. Recently, nonlinear interactions in diamond nitrogen-vacancy (NV) centers are proposed to realize magnetic sensing across arbitrary frequencies. In this work, we enhanced these capabilities by exploiting the nonlinear spin dynamics in hybrid systems of NV centers and ferri- or ferromagnetic (FM) thin films. We studied the frequency mixing effect in the hybrid systems and demonstrated that the introduction of FM films not only amplifies the intensity of nonlinear resonance signals that are intrinsic to NV spins but also enables novel frequency mixing through parametric pumping and nonlinear magnon scattering effects. The discovery and understanding of the magnetic nonlinearities in hybrid NV/magnon systems position them as a prime candidate for magnetic sensing with a broad frequency range and high tunability, particularly meaningful for nanoscale, dynamical, and noninvasive materials characterization.

Magnetic Weyl semimetals present particular promise for spintronic applications, as their band-structure topology can be tuned by a magnetic field. Systematic experimental investigation of magnetic tunnel junctions (MTJs) with electrodes of ferromagnetic Weyl semimetal is needed. This study develops fully epitaxial single-crystalline MTJs featuring Co${}_{2}$MnGa, a clean material system for investigating Weyl physics in the devices. Along the way, the authors establish the relationship between tunneling magnetoresistance and the degree of chemical and topological ordering of Co${}_{2}$MnGa.

Efficient detection of the magnetic state is a critical step towards useful antiferromagnet-based spintronic devices. Recently, finite tunneling magnetoresistance (TMR) has been demonstrated in tunnel junctions with antiferromagnetic electrodes, however, these studies have been mostly limited to junctions with two identical antiferromagnet (AFM) electrodes, where the matching of the spin-split Fermi surfaces played critical role. It remains unclear if AFMs can provide a finite net spin polarization, and hence be used as a spin polarizer or detector. In this work, we experimentally fabricate single-sided antiferromagnetic tunnel junctions consisting of one AFM electrode (Mn3Sn) and one ferromagnet (FM) electrode (CoFeB), where the spin polarized tunneling transport from AFM is detected by the FM layer. We observe a high TMR at cryogenic temperature (>100% at 10 K) in these asymmetric AFM tunnel junctions, suggesting a large effective spin polarization from Mn3Sn despite its nearly vanishing magnetization. The large TMR is consistent with recent theoretical studies where the broken symmetry in non-collinear AFMs is predicted to lift the spin degeneracy in the band structure. Our work provides strong evidence that spin polarized electrical transport can be achieved from AFMs. Antiferromagnets have both negligible stray fields, and fast dynamics, making them ideal for fast and densely packed spintronic devices. However, readout of the antiferromagnet state is challenging. Here, Chou et al detect the spin-polarized current emanating from a noncollinear AFM, Mn3Sn.

Physical devices exhibiting stochastic functions with low energy consumption and high device density have the potential to enable complex probability-based computing algorithms, accelerate machine learning tasks, and enhance hardware security. Recently, superparamagnetic tunnel junctions (sMTJs) have been widely explored for such purposes, leading to the development of sMTJ-based systems; however, the reliance on nanoscale ferromagnets limits scalability and reliability, making sMTJs sensitive to external perturbations and prone to significant device variations. Here, we present an experimental demonstration of closed loop three-terminal sMTJs as reliable and potentially scalable sources of true randomness in the field-free regime. By leveraging dual-current controllability and incorporating feedback, we stabilize the switching operation of superparamagnets and reach cryptographic-quality random bitstreams. The realization of controllable and robust true random sMTJs underpin a general hardware platform for computing schemes exploiting the stochasticity in the physical world, as demonstrated by the generative artificial intelligence example in our experiment.

Magnetic sensing beyond linear regime could broaden the frequency range of detectable magnetic fields, which is crucial to various microwave and quantum applications. Recently, nonlinear interactions in diamond nitrogen-vacancy (NV) centers, one of the most extensively studied quantum magnetic sensors, are proposed to realize magnetic sensing across arbitrary frequencies. In this work, we enhance these capabilities by exploiting the nonlinear spin dynamics in hybrid systems of NV centers and ferri- or ferro-magnetic (FM) thin films. We study the frequency mixing effect in the hybrid NV/magnon systems, and demonstrate that the introduction of FM not only amplifies the intensity of nonlinear resonance signals that are intrinsic to NV spins, but also enables novel frequency mixings through parametric pumping and nonlinear magnon scattering effects. The discovery and understanding of the magnetic nonlinearities in hybrid NV/magnon systems position them as a prime candidate for magnetic sensing with a broad frequency range and high tunablity, particularly meaningful for nanoscale, dynamical, and non-invasive materials characterization.

Nonlinear phenomena represent one of the central topics in the study of wave-matter interactions and constitute the key blocks for various applications in optical communication, computing, sensing, and imaging. In this work, we show that by employing the interactions between microwave photons and electron spins of nitrogen-vacancy ($\mathrm{N}$-$V$) centers, one can realize a variety of nonlinear effects, ranging from the resonance at the sum or difference frequency of two or more waves to electromagnetically induced transparency from the interference between spin transitions. We further verify the phase coherence through two-photon Rabi-oscillation measurements. The highly sensitive optically detected $\mathrm{N}$-$V$--center dynamics not only provides a platform for studying magnetically induced nonlinearities but also promises novel functionalities in quantum control and quantum sensing.

Nonlinear phenomena represent one of the central topics in the study of wave-matter interactions and constitute the key blocks for various applications in optical communication, computing, sensing, and imaging. In this work, we show that by employing the interactions between microwave photons and electron spins of nitrogen-vacancy (NV) centers, one can realize a variety of nonlinear effects, ranging from the resonance at the sum or difference frequency of two or more waves to electromagnetically induced transparency from the interference between spin transitions. We further verify the phase coherence through two-photon Rabi-oscillation measurements. The highly sensitive, optically detected NV-center dynamics not only provides a platform for studying magnetically induced nonlinearities but also promises novel functionalities in quantum control and quantum sensing.

In this work, we theoretically study the switching and oscillation dynamics in strained non-collinear antiferromagnet (AFM) Mn3X (X = Sn, Ge, etc.). Using the perturbation theory, we identify three separable dynamic modes—one uniform and two optical modes, for which we analytically derive the oscillation frequencies and effective damping. We also establish a compact, vector equation for describing the dynamics of the uniform mode, which is in analogy to the conventional Landau–Lifshitz–Gilbert (LLG) equation for ferromagnet but captures the unique features of the cluster octuple moment. Extending our model to include spatial inhomogeneity, we are able to describe the excitations of dissipative spin wave and spin superfluidity state in the non-collinear AFM. Furthermore, we carry out numerical simulations based on coupled LLG equations to verify the analytical results, where good agreements are reached. Our treatment with the perturbative approach provides a systematic tool for studying the dynamics of non-collinear AFM and is generalizable to other magnetic systems in which the Hamiltonian can be expressed in a hierarchy of energy scales.

Hybrid dynamic systems combine advantages from different subsystems for realizing information processing tasks in both classical and quantum domains. However, the lack of controlling knobs in tuning system parameters becomes a severe challenge in developing scalable, versatile hybrid systems for useful applications. Here, we report an on-chip microwave photon–magnon hybrid system where the dissipation rates and the coupling cooperativity can be electrically influenced by the spin Hall effect. Through magnon–photon coupling, the linewidths of the resonator photon mode and the hybridized magnon polariton modes are effectively changed by the spin injection into the magnetic wires from an applied direct current, which exhibit different trends in samples with low and high coupling strengths. Moreover, the linewidth modification by the spin Hall effect shows strong dependence on the detuning of the two subsystems, in contrast to the classical behavior of a standalone magnonic device. Our results point to a direction of realizing tunable, on-chip, scalable magnon-based hybrid dynamic systems, where spintronic effects provide useful control mechanisms.

Abstract Iron garnets that combine robust perpendicular magnetic anisotropy (PMA) with low Gilbert damping are desirable for studies of magnetization dynamics as well as spintronic device development. This paper reports the magnetic properties of low‐damping bismuth‐substituted iron garnet thin films (Bi 0.8 Y 2.2 Fe 5 O 12 ) grown on a series of single‐crystal gallium garnet substrates. The anisotropy is dominated by magnetoelastic and growth‐induced contributions. Both stripe and triangular domains form during field cycling of PMA films, with triangular domains evident in films with higher PMA. Ferromagnetic resonance measurements show damping as low as 1.3 × 10 −4 with linewidths of 2.7 to 5.0 mT. The lower bound for the spin‐mixing conductance of BiYIG/Pt bilayers is similar to that of other iron garnet/Pt bilayers.

Advancing the development of spin-wave devices requires high-quality low-damping magnetic materials where magnon spin currents can efficiently propagate and effectively interact with local magnetic textures. Here we show that magnetic domain walls can modulate spin-wave transport in perpendicularly magnetized channels of Bi-doped yttrium iron garnet. Conversely, we demonstrate that the magnon spin current can drive domain-wall motion in the Bi-doped yttrium iron garnet channel device by means of magnon spin-transfer torque. The domain wall can be reliably moved over 15–20 µm distances at zero applied magnetic field by a magnon spin current excited by a radio-frequency pulse as short as 1 ns. The required energy for driving the domain-wall motion is orders of magnitude smaller than those reported for metallic systems. These results facilitate low-switching-energy magnonic devices and circuits where magnetic domains can be efficiently reconfigured by magnon spin currents flowing within magnetic channels. Spin waves are excited in a thin film of bismuth-doped yttrium iron garnet using radio-frequency pulses and interact with magnetic domain walls. Pulses as short as 1 ns translate a domain wall over 15 µm distances, offering control over domain-wall dynamics.

Abstract Unidirectional magnetoresistance (UMR) has been observed in a variety of stacks with ferromagnetic/spin Hall material bilayer structures. In this work, UMR in antiferromagnetic insulator Fe 2 O 3 /Pt structure is reported. The UMR has a negative value, which is related to interfacial Rashba coupling and band splitting. Thickness‐dependent measurement reveals a potential competition between UMR and the unidirectional spin Hall magnetoresistance (USMR). This work reveals the existence of UMR in antiferromagnetic insulators/heavy metal bilayers and broadens the way for the application of antiferromagnet‐based spintronic devices.

Spin-orbit torque (SOT) has been utilized for efficient electrical control of magnetic ordering structure. When discussing the SOT-driven dynamics of magnetic ordering in magnets with collinear spin orientations, the overall magnetic order parameters such as net magnetization and Néel vector are governed by the same law of SOT on individual magnetic moments, and this treatment has been naively extended to the magnets with non-collinear system. Here, we reveal unconventional dynamics of the overall magnetic order parameter of a non-collinear antiferromagnet, i.e., the octupole moment, which is in sharp contrast to those of previously studied collinear systems. Using ac harmonic and dc measurements on a Mn<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>Sn/Pt heterostructure, we reveal that the octupole moment undergoes an opposite rotation with respect to the individual magnetic moments, which stems from the interplay between the SOT and the chiral-spin structure of Mn3Sn. Moreover, we quantify the SOT efficiency in the heterostructure. Our study provides a guideline for understanding the electrical manipulation of non-collinear antiferromagnets, which substantially differs from the well-established collinear systems.

Hybrid magnonic systems are a newcomer for pursuing coherent information processing owing to their rich quantum engineering functionalities. One prototypical example is hybrid magnonics in antiferromagnets with an easy-plane anisotropy that resembles a quantum-mechanically mixed two-level spin system through the coupling of acoustic and optical magnons. Generally, the coupling between these orthogonal modes is forbidden due to their opposite parity. Here we show that the Dzyaloshinskii-Moriya-Interaction (DMI), a chiral antisymmetric interaction that occurs in magnetic systems with low symmetry, can lift this restriction. We report that layered hybrid perovskite antiferromagnets with an interlayer DMI can lead to a strong intrinsic magnon-magnon coupling strength up to 0.24 GHz, which is four times greater than the dissipation rates of the acoustic/optical modes. Our work shows that the DMI in these hybrid antiferromagnets holds promise for leveraging magnon-magnon coupling by harnessing symmetry breaking in a highly tunable, solution-processable layered magnetic platform.