Hybrid magnonics for short-wavelength spin waves facilitated by a magnetic heterostructure

Recent research on hybrid magnonics has been restricted by the long magnon wavelengths of the ferromagnetic resonance modes. We present an experiment on the hybridization of 250-nmwavelength magnons with microwave photons in a multimode magnonic system consists of a planar cavity and a magnetic bilayer. The coupling between magnon modes in the two magnetic layers, i.e., the uniform mode in Permalloy (Py) and the perpendicular standing spin waves (PSSWs) in YIG, serves an effective means for exciting short-wavelength PSSWs, which is further hybridized with the photon mode of the microwave resonator. The demonstrated magnon-photon coupling approaches the superstrong coupling regime and can even be achieved near zero bias field.

This article was published in Physical Review Applied and is available here.

Artificial neurons based on antiferromagnetic auto-oscillators as a platform for neuromorphic computing

Spiking artificial neurons emulate the voltage spikes of biological neurons and constitute the building blocks of a new class of energy efficient, neuromorphic computing systems. Antiferromagnetic materials can, in theory, be used to construct spiking artificial neurons. When configured as a neuron, the magnetization in antiferromagnetic materials has an effective inertia that gives them intrinsic characteristics that closely resemble biological neurons, in contrast with conventional artificial spiking neurons. It is shown here that antiferromagnetic neurons have a spike duration on the order of picoseconds, a power consumption of about 10−3 pJ per synaptic operation, and built-in features that directly resemble biological neurons, including response latency, refraction, and inhibition. It is also demonstrated that antiferromagnetic neurons interconnected into physical neural networks can perform unidirectional data processing even for passive symmetrical interconnects. The flexibility of antiferromagnetic neurons is illustrated by simulations of simple neuromorphic circuits realizing Boolean logic gates and controllable memory loops.

This article was published in AIP Advances and is available here.

Nonreciprocity of Phase Accumulation and Propagation Losses of Surface Acoustic Waves in Hybrid Magnetoelastic Heterostructures

Lack of nonreciprocity-in particular, nonreciprocity of phase accumulation—is one of the major drawbacks of microwave solid-state acoustic devices, which has prevented the development of acoustic isolators and circulators. Here we report the observation of the phase nonreciprocity of hybridized surface acoustic waves (SAWs) and spin waves in a magnetoelastic heterostructure containing an artificial antiferromagnetic (AFM) bilayer. Our system consists of a Fe-Ga-B/Al2O3/Fe-Ga-B multilayer on top of a LiNbO3 crystal.  Magnetic layers are ordered antiferromagnetically. Maximum values of the observed nonreciprocal phase accumulation easily exceed π radians over a broad range of field conditions, which is necessary for the development of an effective circulator. In addition, under the application of bias magnetic field, the structure demonstrates tunable giant nonreciprocity of propagation losses with isolation as high as 48 dB, necessary for the development of isolators. Theoretical calculations provide an insight into the observed phenomena and demonstrate a pathway for further improvement of nonreciprocal SAW devices based on magnetoelastic coupling.

This article was published in Physical Review Applied and is available here.

Bright and Dark States of Two Distant Macrospins Strongly Coupled by Phonons

Slavin and Tyberkevych studied the collective dynamics of two distant magnets coherently coupled by acoustic phonons that are transmitted through a nonmagnetic spacer. By tuning the ferromagnetic resonances of the two magnets to an acoustic resonance of the intermediate, we control a coherent three-level system. We show that the parity of the phonon mode governs the indirect coupling between the magnets: the resonances with odd (even) phonon modes correspond to out-of-phase (in-phase) lattice displacements at the interfaces, leading to bright (dark) states in response to uniform microwave magnetic fields, respectively. The experimental sample is a trilayer garnet consisting of two thin magnetic films epitaxially grown on both sides of a half-millimeter-thick nonmagnetic single crystal. In spite of the relatively weak magnetoelastic interaction, the long lifetimes of the magnon and phonon modes are the key to unveil strong coupling over a macroscopic distance, establishing the value of garnets as a platform to study multipartite hybridization processes at microwave frequencies.

This article was published in Physical Review X and is available here.

Surface state mediated ferromagnetism in Mn0.14Bi1.86Te3 thin films

A spontaneous ferromagnetic moment can be induced in Bi2Te3 thin films below a temperature T≈16 K by the introduction of Mn dopants. We demonstrate that films grown via molecular beam epitaxy with the stoichiometry Mn0.14Bi1.86Te3 maintain the crystal structure of pure Bi2Te3. The van der Waals nature of inter-layer forces in the Mn0.14Bi1.86Te3 crystal causes lattice mismatch with the underlayer to have a limited effect on the resulting crystal structure, as we demonstrate by thin film growth on tetragonal MgF2 and the antiferromagnet NiF2 (110). Magnetic heterostructures consisting of Mn0.14Bi1.86Te3 grown on thin film antiferromagnetic NiF2 show magnetic behavior consistent with a coexistence of two decoupled magnetic layers. Electronic transport measurements in the Mn0.14Bi1.86Te3 films exhibit the onset of the anomalous Hall effect at low temperatures. An inverse correlation between the magnitude of the anomalous Hall effect and the electron carrier density is observed in the samples. This correlation demonstrates that as the Fermi level is lowered and approaches the bulk band gap, the magnetic moment of the film increases, suggesting that topological surface states play a role in the development of ferromagnetism.

This article was published in Physical Review Materials and is available here.

Unidirectional Spin Hall Magnetoresistance in Antiferromagnetic Heterostructures

In collaboration with Prof. Kang Wang’s group at UCLA, Cheng, Yang and collaborators reported observation of unidirectional spin Hall magnetoresistance in Pt/α-Fe2O3 bilayers grown on α-Fe2O3 (0001). Spin Hall magnetoresistance (SMR) has been extensively used to probe magnetic and spin configurations in both ferromagnetic and antiferromagnetic heterostructures in the past decade. Recently, a new type of magnetoresistance, unidirectional SMR (USMR), has been observed in in HM/FM heterostructures. Compared with conventional SMR, the USMR is a non-linear magnetoresistance where the measured voltage depends quadratically on the applied electrical current. The magnitude of USMR unidirectionally depends on the angle between spin polarization and the FM magnetization, providing a more precise way to probe the spin state of the FM. The USMR in Pt/α-Fe2O3 bilayers has been observed through systematic field and temperature dependent measurements, which confirm the magnonic origin of the USMR. The appearance of USMR in Pt/α-Fe2O3 is driven by the imbalance of creation and annihilation of antiferromagnetic magnons by spin-orbit torque due to the thermal random field. Theoretical modeling reveals that the USMR in Pt/hematite, unlike its ferromagnetic counterpart, is determined by the antiferromagnetic magnon number with an apparent non-monotonic field dependence. The theoretical result can well explain the experimental observation, paving new ways for the highly sensitive detection of AFM spin states. This finding extends the generality of the USMR which pave the ways for the highly sensitive detection of AFM spin state.

This article was published in Physical Review Letters and is available here.

A 220GHz-1.1THz continuous frequency and polarization tunable high frequency quasi-optical spectroscopic system

The existence of the THz gap in the electromagnetic spectrum is not only preventing the advancement of technologies but also hindering research and developmental activities due to a lack of research facilities operating in the gap region. There are many material candidates with their dynamics lying in the THz gap region whose study could potentially lead to the development of new technologies for the generation, detection and processing of THz signals. This would lead to the transformation of currently existing GHz technologies by increasing their operational speeds by at least 3 orders of magnitude faster. For such a paradigm shift in technological development to occur it is necessary to have a well-equipped and easily accessible research facility where one can perform careful studies of potential candidates which could be used in future THz technology. Here we describe a unique custom-designed quasi-optical system continuously operating in the frequency range 220GHz to 1.1THz with a temperature range of 5-300K and magnetic fields up to 9T. Initial results from test measurements on antiferromagnetic MnF2 single crystals are included to verify operation of the system.

This article was published in Review of Scientific Instruments and is available here.

Quantum interference in a superconductor-MnBi2Te4-superconductor Josephson junction

The antiferromagnetic topological insulator MnBi2Te4 features lots of exotic physical phenomena at low temperatures, forging a new direction that joins topological materials with magnetic materials. Yet a missing ingredient in the grand inquiry into antiferromagnetic topological insulators is how the antiferromagnetic behavior can manifest in the quantum tunneling, which has wide applications in quantum computing and related applications. To this end, PI-Cheng’s group studied the transport properties of a Josephson junction consisting of two identical s-wave superconductors separated by an even-layer MnBi2Te4. By numerically calculating the supercurrent in the presence of a perpendicular magnetic field, PI-Cheng found that the quantum interference in such a device setup exhibits distinct patterns when the MnBi2Te4 is in different magnetic states. In its antiferromagnetic state, the MnBi2Te4 is an axion insulator supporting an extended “hinge” supercurrent, which leads to a sinusoidal interference pattern decaying with the field strength. In the ferromagnetic state, on the other hand, the MnBi2Te4 is a Chern insulator and the unbalanced chiral supercurrents on opposite edges give rise to a highly asymmetric interference pattern. If the MnBi2Te4 turns into a metal as the Fermi level is tuned into the conduction band, the interference exhibits a Fraunhofer pattern due to the uniformly distributed bulk supercurrent. This work unravels a strong indicator to identify different phases in MnBi2Te4 and can be verified directly by experiments.

This article was published in Physical Review Research and is available here.

Robust spin injection via thermal magnon pumping in antiferromagnet/ferromagnet hybrid systems

Robust spin injection and detection in antiferromagnetic thin films is a prerequisite for the exploration of antiferromagnetic spin dynamics and the development of nanoscale antiferromagnet-based spintronic devices. Existing studies have shown spin injection and detection in antiferromagnet-nonmagnetic metal bilayers; however, spin injection in these systems has been found effective at cryogenic temperatures only. A recent experiment done by Prof. Igor Barsukov at UC-Riverside experimentally demonstrated sizable interfacial spin transport in a hybrid antiferromagnet-ferromagnet system, consisting of Cr2O3 and permalloy, which remains robust up to the room temperature. In collaboration with Prof. Igor Barsukov, PI-Cheng well explained the experimental data using a spin diffusion model and find evidence for the important role of interfacial magnon pumping in the signal generation. The experimental results, along with the intuitive theoretical model, bridge spin-orbitronic phenomena of ferromagnetic metals with antiferromagnetic spintronics and demonstrate an advancement toward the realization of room temperature antiferromagnetic spintronics devices.

This article was published in Physical Review Research and is available here.

Identifying Axion Insulator by Quantized Magnetoelectric Effect in Antiferromagnetic Tunnel Junction

The intrinsic magnetic topological insulator MnBi2Te4 is believed to be an axion insulator in its antiferromagnetic ground state (with even number of septuple layers). However, direct identification of axion insulators remains experimentally elusive because the observed vanishing Hall resistance, while indicating the onset of the axion field, is inadequate to distinguish the system from a trivial normal insulator. Using numerical Green’s functions, PI-Cheng’s group theoretically demonstrated the quantized magnetoelectric current in a tunnel junction of atomically thin MnBi2Te4 sandwiched between two metallic contacts, which is a smoking-gun signal that unambiguously confirms antiferromagnetic MnBi2Te4 as an axion insulator.

This article has been published in Physical Review Research and is available here.