Theoretical investigation of ferrimagnets functioning as THz antiferromagnets

Ferrimagnetic materials (FIMs) can function as high-frequency antiferromagnets while being easy to detect as ferromagnets, offering unique opportunities for ultrafast device applications. While the physical behavior of FIMs near the compensation point has been widely studied, there lacks a generic understanding of FIMs where the ratio of sublattice spins can vary continuously between the ferromagnetic and antiferromagnetic limits. PI-Cheng’s group investigated a series of physical properties of two-sublattice FIMs driven by magnetic fields and current-induced torques. By varying the ratio of sublattice spins, PI-Cheng’s group clarified how the dynamical chiral modes in a FIM are physically connected to their ferromagnetic and antiferromagnetic counterparts, based on which unique features not visible near the compensation point are demonstrated. It is found that current-induced torques can trigger spontaneous oscillation of the terahertz exchange mode (i.e. mimicking a THz antiferromagnet). Compared with its realization in antiferromagnets, a spin-torque oscillator using FIMs not only has a reduced threshold current density but also can be self-stabilized, obviating the need for dynamic feedback.

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

Harmonic Hall Analysis of Spin Torques in Antiferromagnets

Harmonic analysis is a powerful tool to characterize and quantify current-induced torques acting on magnetic materials, but so far it remains an open question in studying antiferromagnets. Inspired by the collaborative work with PI-Kent and PI-Yang, PI-Cheng’s group formulated a general theory of harmonic Hall responses of collinear antiferromagnets driven by current-induced torques including both field-like and damping-like components. By scanning a magnetic field of variable strength in three orthogonal planes, one is able to distinguish the contributions from field-like torque, damping-like torque, and concomitant thermal effects by analyzing the second harmonic signals in the Hall voltage. The analytical expressions of the first and second harmonics as functions of the magnetic field direction and strength are confirmed by numerical simulations with good agreement. The theory has been utilized directly to explain experimental observations of PI-Kent’s group, and has also been generalized to NiO, providing general guidance to future experiments.

This article has been published in the Journal of Magnetism and Magnetic Materials and is available here.

Diffusive Magnon Spin Nernst Effect in Antiferromagnets

Magnon spin Nernst effect (SNE) was recently proposed as a magnonic analog of the spin Hall effect, which takes place in layered collinear antiferromagnets such as MnPS3. The magnon SNE extends many topological properties of electrons to magnonic systems where the bosonic statistics of magnons introduces unexpected physical consequences, opening a versatile platform for antiferromagnetic magnonics. However, existing studies all treated the magnon SNE as an intrinsic bulk effect, where spin diffusion and boundary spin transmission have been ignored. In real experiments, diffusion processes are essential to convert a bulk spin current into boundary spin accumulation, which then determines the spin injection into detectors through imperfect spin transmission. PI-Cheng’s group formulated a diffusive theory of the magnon SNE with boundary conditions reflecting real device geometry. It is found that in both electronic and optical detection, the actual output signals grow rapidly with an increasing system size in the transverse dimension until it eventually saturates. This provides crucial knowledge to experimentally benchmark and verify the magnon SNE in real antiferromagnetic materials. The diffusive theory also revealed that concerning symmetry properties, optical detection should be more reliable than electronic detection.

This work is published in Physical Review Applied, available here, and as an invited paper review in Applied Physics Letters, here.

Theoretical study of coherent spin pumping and THz spin-torque oscillator in a series of easy-plane antiferromagnets

Coherent spin pumping at THz frequency range was recently reported in antiferromagnets with easy-axis anisotropy such as MnF2 and Cr2O3. However, easy-plane antiferromagnets such as NiO, which are much easier to make, were considered to be bad candidates because the Néel vector is linearly polarized, placing a major restriction on the choice of materials for real applications. Cheng’s group challenged this seemingly established conclusion by showing that easy-plane antiferromagnets can pump dc spin currents with the help of either a magnetic field or the Dzyaloshinskii-Moriya interaction (DMI), and the amplitude can be as strong as their easy-axis counterparts. Collaborating with the Nanoscale Quantum Sensing Lab at UCSD, it has been demonstrated both theoretically and experimentally that bulk single-crystal hematite ( -Fe2O3) can pump dc spin current at room temperature thanks to the DMI. Furthermore, it has been predicted that NiO (and similar oxides) can pump dc spin current via the resonance of the acoustic antiferromagnetic mode (around 0.1 THz) when a magnetic field below 5T is applied along the easy axis. As a reciprocal effect, Cheng’s group predicted that the threshold current density required to excite auto-oscillation of NiO can be suppressed by 1~2 order of magnitude by a moderate magnetic field below the spin-flop transition. The reduced threshold does not come at the price of compromised frequency, paving the way to ultrafast spin-torque oscillator with low energy consumption.

This work has been published as two papers, in Physical Review Letters, available here, and in Physical Review Applied, here.

Theory of Electric-Field-Controlled Antiferromagnetic Spin-Hall Oscillator and Detector

Four graphs show relationships between electric field, current density, and frequency in a theoretical study. Each graph contains multiple plotted lines with different variables labeled in the legend.

A theory of electrically controlled THz-frequency auto-oscillator, based on a trilayer heterostructure comprised  of piezoelectric (PZ) ceramics, an NiO-based antiferromagnet, and a heavy metal (HM), is developed in the framework of the well-established antiferromagnetic (AFM) sigma model. It is assumed that the magnetostrictive antiferromagnet is monocrystalline and monodomain, and has mixed biaxial and cubic anisotropy. The frequency of the antiferromagnetic resonance (AFMR) of the heterostructure in a passive subcritical regime is calculated as a function of the following parameters: the choice of the ceramic PZ material and of its poling direction, modulus and orientation of the static electric field applied to the PZ layer, and the magnitude of the driving electric current injected into the HM layer. It is shown that the AFMR frequency of the heterostructure and the threshold value of the driving current for THz-frequency generation depend on the total AFM anisotropy, which can be substantially reduced by the bias electric field in the case when this field is collinear to the PZ poling direction. It is also shown that the variation of the PZ poling direction in respect to the bias electric field provides an additional degree of freedom that can be used to optimize the performance of AFM-based generators and detectors of THz-frequency signals. 

This work has been published in PRB and can be accessed here.

Direct imaging of electrical switching of antiferromagnetic Néel order in α-Fe2O3 epitaxial films

This image shows a series of heat maps. Panel (a) displays states A and B under different pulse conditions. Panel (b) illustrates the differences between various states using a color scale.

X rays have been used to image the spin directions in an antiferromagnetic thin film of hematite (α-Fe2O3) and show how they reorient in response to electrical pulses. Spin switching is shown to depend on the current direction and occurs even outside the current path. The results here show that, in addition to any direct interaction between the electrical current and the spins, thermal effects lead to spin reorientation, highlighting the delicate balance of interactions important to understanding the physics of current-induced switching in antiferromagnets.

Specifically, we reported the direct observation of switching of the Néel vector of antiferromagnetic (AFM) domains in response to electrical pulses in micron-scale Pt/α−Fe2O3 Hall bars using photoemission electron microscopy. Current pulses lead to reversible and repeatable switching with the current direction determining the final state, consistent with Hall effect experiments that probe only the spatially averaged response. Current pulses also produce irreversible changes in domain structure, in and even outside the current path. In both cases only a fraction of the domains switch in response to pulses. Furthermore, the analysis of images taken with different x-ray polarizations shows that the AFM Néel order has an out-of-plane component in equilibrium that is important to consider in analyzing the switching data. These results show that—in addition to effects associated with spin-orbit torques from the Pt layer, which can produce reversible switching—changes in AFM order can be induced by purely thermal effects.

This work has been published in PRB and can be accessed here.

RF signal detector and energy harvester based on a spin-torque diode with perpendicular magnetic anisotropy

Two plots: (a) Output DC voltage vs. input RF power showing theory, simulations, and experiment at 250 MHz. (b) Output DC voltage vs. input signal frequency showing theory, simulations, and experiment at 3.2 µW.

We demonstrate theoretically that in a spintronic diode (SD), having a free magnetic layer with perpendicular magnetic anisotropy of the first and second order and no external bias magnetic field, the out-of-plane regime of magnetization precession can be excited by sufficiently large (exceeding a certain threshold) RF signals with the frequencies ≲250 MHz. We also show that such a device can operate as a broadband energy harvester capable of converting incident RF power into a DC power with a conversion efficiency of ∼5%. The developed analytical theory of the bias-free SD operation can be used for the optimization of high-efficiency RF detectors and energy harvesters based on SDs. This work has been published in AIP advances and can be accessed here.

Precessional spin-torque dynamics in biaxial antiferromagnets

Four contour plots comparing uniaxial (left column) and biaxial (right column) behaviors. The top row shows plots of (omega_0(Theta, Phi)), while the bottom row displays (Gamma_{0,Lambda h}(Theta, Phi)).

The Néel order of an antiferromagnet subject to a spin torque can undergo precession in a circular orbit about any chosen axis. The direction of the axis uniquely determines the component of spin polarization in the orbital plane required for that particular axis orientation as well as the perpendicular component of spin polarization required for stability of the precessional motion. For biaxial antiferromagnets, the orbital angular motion reduces to equation of a damped-driven pendulum, which has hysteresis versus the spin-current drive with a critical value where the period diverges. The fundamental frequency of the motion varies inversely with the damping. The stability threshold for spin current has an approximate closed-form result, which depends on the minimum cutoff frequency the orbit can support, and the damping-anisotropy parameters. The direction along the hard axis has zero cutoff frequency and the lowest threshold, and the easy axis has the highest cutoff frequency. A device setup is proposed for electrical control and detection of the dynamics, which is promising for terahertz nano-oscillators. This work has been published in PRB and can be accessed here.

Theory of three-magnon interaction in a vortex-state magnetic nanodot

Graphical representations of frequencies, azimuthal numbers, and magnetic fields. Diagrams include a disk with magnetic vectors, a plot of frequency vs. azimuthal number, and circular patterns showing different modes.

We use vector Hamiltonian formalism (VHF) to study theoretically three-magnon parametric interaction (or three-wave splitting) in a magnetic disk existing in a magnetic vortex ground state. The three-wave splitting in a disk is found to obey two selection rules: (i) conservation of the total azimuthal number of the interacting spin-wave modes, and (ii) inequality of the radial numbers of the resultant modes, if the directly excited original mode is radially symmetric (i.e., if the azimuthal number of the directly excited mode is m = 0). The selection rule (ii), however, is relaxed in sufficiently small magnetic disks, due to the influence of the vortex core. We also found that the efficiency of the three-wave splitting of the directly excited mode strongly depends on the azimuthal and radial mode numbers of the resultant modes. This property becomes qualitatively important in the case when several different splitting channels (several pairs of resultant modes) approximately satisfy the resonance condition for the splitting. The good agreement of the VHF analytic calculations with the experiment and micromagnetic simulations proves the capability of the VHF formalism to predict the actual experimentally realized splitting channels, and the magnitude of the driving field thresholds for the three-wave splitting. This work has been published in PRB and can be accessed here.

Magnon thermal Edelstein effect in antiferromagnets

Two graphs labeled (a) and (b) showing different magnetic scenarios above a 3D diagram labeled (c) illustrating spin accumulation and current flow on a non-magnetic material under a temperature gradient.

Efficient control of antiferromagnetic magnons in spin-dependent transport is essential for magnons to function as electrons to carry and deliver spin-based information. In an easy-plane antiferromagnet with the Dzyaloshinskii–Moriya interaction (DMI), magnons are subject to spin-momentum locking. An in-plane temperature gradient can then generate interfacial accumulation of magnons with a specified spin polarization, realizing the magnon thermal Edelstein effect. Cheng’s group theoretically investigated the injection and detection of this thermally driven spin polarization through an adjacent heavy metal. It is found that the detectable inverse spin Hall voltage has a sign determined not only by the thermal gradient but also by the DMI, which is in principle controllable by a gate voltage. This allows for the electrical and thermal control of the magnon spin polarization in an antiferromagnet, enabling new device functionalities.

This work has been published in Applied Physics Letters and can be accessed here.