Author: Gregory Fritjofson

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.

The advantage of an ultrafast frequency-tunability of spin-torque nano-oscillators (STNOs) that have a large (>100 MHz) relaxation frequency of amplitude fluctuations is exploited to realize ultrafast wide-band time-resolved spectral analysis at nanosecond time scale with a frequency resolution limited only by the “bandwidth” theorem. The demonstration is performed with an STNO generating in the 9 GHz frequency range and comprised of a perpendicular polarizer and a perpendicularly and uniformly magnetized “free” layer. It is shown that such a uniform-state STNO-based spectrum analyzer can efficiently perform spectral analysis of frequency-agile signals with rapidly varying frequency components.

This article has been published by ACS Nano Letters and is available here.

A theory of a detector of terahertz-frequency signals based on an anisotropic antiferromagnetic (AFM) crystal is developed. The conversion of a THz-frequency electromagnetic signal into the DC voltage is realized using the inverse spin Hall effect in an antiferromagnet/heavy metal bilayer. An additional bias DC magnetic field can be used to tune the antiferromagnetic resonance frequency. We show that if a uniaxial AFM is used, the detection of linearly polarized signals is possible only for a non-zero DC magnetic field, while circularly polarized signals can be detected in a zero DC magnetic field. In contrast, a detector based on a biaxial AFM can be used without a bias DC magnetic field for the rectification of both linearly and circularly polarized signals. The sensitivity of a proposed AFM detector can be increased by increasing the magnitude of the bias magnetic field, or by decreasing the thickness of the AFM layer. We believe that the presented results will be useful for the practical development of tunable, sensitive and portable spintronic detectors of THz-frequency signals based on the antiferromagnetic resonance (AFMR).

This article has been published in Magnetochemistry and is available here.

Previously, it has been shown that rapid cooling of yttrium-iron-garnet–platinum nanostructures, preheated by an electric current sent through the Pt layer, leads to overpopulation of a magnon gas and to subsequent formation of a Bose-Einstein condensate (BEC) of magnons. The spin Hall effect (SHE), which creates a spin-polarized current in the Pt layer, can inject or annihilate magnons depending on the electric current and applied field orientations. Here we demonstrate that the injection or annihilation of magnons via the SHE can prevent or promote the formation of a rapid cooling-induced magnon BEC. Depending on the current polarity, a change in the BEC threshold of −8% and +6% was detected. These findings demonstrate a new method to control macroscopic quantum states, paving the way for their application in spintronic devices.

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

Magnetoelastic coupling is considered as one of the most reliable methods to induce nonreciprocity of propagation losses of microwave-frequency surface acoustic waves (SAW) and other acoustic modes propagating in nonmagnetic-ferromagnetic heterostructures. Here, it is demonstrated theoretically that magnetoelastic coupling can also induce phase nonreciprocity of SAW, which is necessary for the development of SAW circulators and other nonreciprocal solid-state-acoustic devices. In contrast to previous studies, induction of the phase nonreciprocity requires the coupling of SAW to a strongly nonreciprocal spin wave (SW), having the nonreciprocal splitting of the SW spectrum much larger than the strength of the magnetoelastic coupling, which, in turn, should be much larger than the geometric mean of the SW and SAW damping rates. In this case, the hybridized SAW in the spectral region between the magnetoelastic gaps demonstrate significant phase nonreciprocity, retaining, at the same time, propagation losses that are close to those of unhybridized SAW. Possible practical realization of nonreciprocal SAW phase shifters and SAW-ring-based circulators based on hybridized waves in acoustic crystal and synthetic antiferromagnetic heterostructures is discussed.

This article has been published in Advanced Electronics Materials and is available here.

Two-dimensional (2D) magnetic semiconductors feature both tightly-bound excitons with large oscillator strength and potentially long-lived coherent magnons due to the presence of bandgap and spatial confinement. While magnons and excitons are energetically mismatched by orders of magnitude, their coupling can lead to efficient optical access to spin information. Here we report strong magnon-exciton coupling in the 2D van der Waals (vdW) antiferromagnetic (AFM) semiconductor CrSBr. Coherent magnons launched by above-gap excitation modulate the interlayer hybridization, which leads to dynamic modulation of excitonic energies. Time-resolved exciton sensing reveals magnons that can coherently travel beyond 7 micrometer, with coherence time above 5 ns. We observe this exciton-coupled coherent magnons in both even and odd number of layers, with and without compensated magnetization, down to the bilayer limit. Given the versatility of vdW heterostructures, these coherent 2D magnons may be basis for optically accessible magnonics and quantum interconnects.

This article has been published in Nature and is available here.

Electrical switching of antiferromagnets is an exciting recent development in spintronics, which promises active antiferromagnetic devices with high speed and low energy cost. In this emerging field, there is an active debate about the mechanisms of current-driven switching of antiferromagnets. Harmonic characterization is a powerful tool to quantify current-induced spin-orbit torques and spin Seebeck effect in heavy-metal/ferromagnet systems. However, the harmonic measurement technique has never been verified in antiferromagnetic heterostructures. Here, we report for the first time harmonic measurements in Pt/α Fe₂O₃ bilayers, which are explained by our modeling of higher-order harmonic voltages. As compared with ferromagnetic heterostructures where all current-induced effects appear in the second harmonic signals, the damping-like torque and thermally-induced magnetoelastic effect contributions in Pt/α- Fe₂O₃ emerge in the third harmonic voltage. Our results provide a new path to probe the current-induced magnetization dynamics in antiferromagnets, promoting the application of antiferromagnetic spintronic devices.

This article has been published in Nature Communications and is available here.

The effect of spin currents on the magnetic order of insulating antiferromagnets (AFMs) is of fundamental interest and can enable new applications. Toward this goal, characterizing the spin-orbit torques (SOTs) associated with AFM–heavy-metal (HM) interfaces is important. Here we report the full angular dependence of the harmonic Hall voltages in a predominantly easy-plane AFM, epitaxial c-axis oriented α-Fe₂O₃ films, with an interface to Pt. By modeling the harmonic Hall signals together with the α-Fe₂O₃ magnetic parameters, we determine the amplitudes of fieldlike and dampinglike SOTs. Out-of- plane field scans are shown to be essential to determining the dampinglike component of the torques. In contrast to ferromagnetic–heavy-metal heterostructures, our results demonstrate that the fieldlike torques are significantly larger than the dampinglike torques, which we correlate with the presence of a large imaginary component of the interface spin-mixing conductance. Our work demonstrates a direct way of characterizing SOTs in AFM-HM heterostructures.

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

Recent developments in magnetic topological insulators (MTI) such as antiferromagnetic MnBi₂Te₄ opened new arenas for electric control of magnetization dynamics. PI-Cheng’s group formulated the spin-orbit torque (SOT) arising from the topological electrons in antiferromagnetic MTI as well as its reciprocal effect—topological charge pumping. For undoped MnBi₂Te₄, the SOT originates from the adiabatic current of valence electrons, which does not incur Joule heating as ordinary current-induced torques. The proposed SOT acts oppositely on two neighboring magnetic layers, whereby an applied AC electric field can drive the resonance of the exchange mode with a frequency lying in the sub-THz regime. The SOT-induced exchange resonance manifests as a sharp peak in the effective optical conductivity, entailing a remarkably weaker energy dissipation compared to ordinary ferromagnetic and antiferromagnetic resonances driven by conventional SOT. In trilayer antiferromagnetic MnBi₂Te₄, a hitherto unknown exchange resonance lying in the sub-THz regime is predicted, which does not react to microwave radiations but can only be driven by the unconventional SOT in MTI. This works opens new pathways to manipulate MnBi₂Te₄ and similar antiferromagnetic MTIs by electrical means.

This theoretical work has been published in Physical Review B and is available here.

Owing to the extremely large exchange energy barriers, it has been a fundamental challenge to switch the Neel order of AFM systems using energy-efficient current-induced torques. In FeRh alloy and similar AFM metals, however, there exists an AFM-FM phase transition at about room temperature. PI-Cheng’s group has theoretically predicted and numerically verified that this phase transition can facilitate the Neel order switching process and reduce the threshold current density. When a current is applied to FeRh, a transient heating triggers the AFM-FM phase transition such that FeRh temporarily becomes a ferromagnet which can be switched by a much lower current density. Once the current is turned off, the material quickly retrieves the AFM state but with a 90-degree rotation of its Neel vector. The physical picture has been qualitatively confirmed by an experiment conducted by the Device Research Lab at UCLA.

This article has been published in Nature Communications and is available here.