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.

Electrically tunable THz-detector based on an antiferromagnet

Diagram of a structure with layered Pt and AFM materials showing input current (DC+AC), spin current flow (jSH, jsp), and voltage output (Vout), with axes labeled ex, ey, and ez.

A concept of an electrically tunable resonance detector of THz-frequency signals based on an antiferromagnetic/heavy metal (AFM/HM) heterostructureis proposed. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in an (AFM/HM) bilayer. An additional bias DC in the HM layer can be used to vary the effective anisotropy of the AFM and, therefore, to tune the antiferromagnetic resonance (AFMR) frequency. The proposed AFM/HM heterostructure works as a resonance-type quadratic detector, which can be tuned by the bias current in the range of at least 10% of the AFMR frequency, and our estimations show that the sensitivity of this detector could be comparable to that of modern detectors based on the Schottky, Gunn, or graphene-based diodes. This work has been published in Applied Physics Letters and can be accessed here.

Using NV two-magnon relaxometry to detect high frequency, high wave-vector magnons generated by antiferromagnetic Néel Order switching

Six-panel scientific graph depicting Microwave (MW) power and magnetic field relationships. Panels a-c show MW transmission percentages, and panels d-f highlight uniform mode FMR frequency.

Identification of magnon populations in antiferromagnets and understanding of the associated magnon-magnon interactions is essential for the development of AF spintronic applications. A single NV spin is a powerful, nanoscale probe for local detection of magnons.  As we and others have shown, the relaxation rate of the NV spin is sensitive to fluctuations of the magnetic dipolar fields generated by magnons.  However, this detection technique was long thought to be limited to fluctuations at the NV resonance frequency (~2.9 GHz).  We recently showed the effectiveness of two-magnon processes in relaxing NV moments.  These two-magnon processes involve very high frequency/wavevector magnons whose difference frequency is constrained to match the NV frequency.  Magnon systems in antiferromagnets remain poorly understood in large part due to the technical challenge of detecting high frequency magnetics.  Our recent advance opens a route to NV detection and study of very high frequency magnon dynamics. This work has been published in nature communications and can be accessed here.

Nonlocal Uniform-Mode Ferromagnetic Resonance Spin Pumping

Graph showing magnetic properties of YIG and Pt/YIG bilayers: (a) dI_FMR/dH vs. H, (b) schematic of spin current flow in Pt/YIG, (c) V_ISHE vs. H - H_res.

Magnonic transport is an important component in realizing THz antiferromagnetic spintronic devices.  Due to the fundamental similarities between ferrimagnetic and antiferromagnetic insulators, magnonic transport studies in ferrimagnetic insulator heterostructures can provide valuable insights for their antiferromagnetic counterparts.  Nonlocal spin transport using lateral structures is ideal for investigating lateral magnonic transport.  Typically, a spin current is generated by a ferro-/ferri-magnetic (FM) or a heavy metal (HM) electrode in a nonlocal structure, which can be detected by another FM or HM electrode.  The Yang and Hammel groups reported a new nonlocal spin injection scheme using uniform-mode ferromagnetic resonance spin pumping in Pt/Y3Fe5O12 (YIG) lateral structures. This scheme is enabled by well-separated resonant fields of Pt/YIG and bare YIG due to substantial change of anisotropy in YIG films induced by a Pt overlayer, allowing for clearly distinguishable local and nonlocal spin pumping.  These results show that the spin decay length of nonlocal uniform-mode spin pumping in 20 nm YIG films is 2.1 μm at room temperature, opening up a new path for spin current generation, propagation, and detection for future spintronic application.

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