Antiferromagnetic Resonance Frequency Research Articles

Theory of tensorial Gilbert damping in antiferromagnets

Although the magnetic Gilbert damping was considered as a scalar quantity in micromagnetic and atomistic spin simulations, recent investigations show that the Gilbert damping parameter is a tensor. Here, we investigate the effect of anisotropic and chiral damping in one-sublattice ferromagnets and two-sublattice antiferromagnets. We employ linear response theory to calculate the susceptibility with the damping tensor and determine the ferromagnetic and antiferromagnetic resonance frequencies together with the effective damping. Our results show that apart from the scalar Gilbert damping, the antisymmetric chiral damping has a significant contribution to the spin dynamics that it breaks the antiparallel alignment of two sublattices in antiferromagnets even in the absence of an applied field. To this end, we also compare the tensorial damping and cross-sublattice scalar damping in antiferromagnets.

Tuning of exchange constants and magnetic anisotropy for terahertz antiferromagnetic resonance frequencies in cation-doped NiO

Tuning of exchange constants and magnetic anisotropy for terahertz antiferromagnetic resonance frequencies in cation-doped NiO

Antiferromagnetic and nutation resonance frequencies of antiferromagnets at an arbitrary strength of the applied dc field

Nutation and precession resonances in an antiferromagnet subjected to a dc magnetic field are investigated by employing coupled linearized inertial Landau–Lifshitz–Gilbert equations describing the dynamics of magnetizations of antiferromagnet sublattices with uniaxial magnetocrystalline anisotropy. Analytical expressions for the eigenfrequencies of such an antiferromagnet are obtained for the longitudinal and transverse directions of the external dc field and for different ranges of its strength. The effect of inertia on the values of the resonant frequencies is shown for all possible states of the antiferromagnet in both the longitudinal and transverse directions of the external field. The estimated resonant frequencies are compared with those obtained from the numerical solution of the system of undamped inertial Landau–Lifshitz–Gilbert equations for closed trajectories of sublattice magnetizations. The good agreement of both independent estimations is demonstrated.

Coupling of Ferromagnetic and Antiferromagnetic Spin Dynamics in Mn_{2}Au/NiFe Thin Film Bilayers.

We investigate magnetization dynamics of Mn_{2}Au/Py (Ni_{80}Fe_{20}) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40GHz, far above the single-layer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn_{2}Au. The coupling strength is in the order of 1.6T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn_{2}Au/Py interface.

Magneto-optical detection of terahertz cavity magnon-polaritons in antiferromagnetic HoFeO3

An intense THz pulse excites a high-Q magnetic resonance mode in the antiferromagnetic insulator HoFeO3 by the THz Zeeman torque. By using magneto-optical detection and sweeping the temperature, we observed an anomalous beating in the magnon dynamics for certain temperatures. The beating originates from the formation of cavity magnon-polaritons upon the intersection of the antiferromagnetic resonance frequency with the frequencies of the Fabry–Pérot modes inside the etalon formed by the sample cavity in the weak coupling limit. The validity of this idea is demonstrated by simulations using Maxwell's equations. Furthermore, the observed beating pattern depends on the polarization of the probe pulse. This dependence can be reproduced in the simulations by considering an imaginary Verdet constant, which could be a result of an interplay between the magneto-optical Faraday effect and static linear birefringence.

Multiferroic antiferromagnetic artificial synapse

Artificial intelligence frameworks utilizing unsupervised learning techniques can avoid the bottleneck of labeled training data required in supervised machine learning systems, but the programming time of these systems is inherently limited by their hardware implementations. Here, a finite-element model coupling micromagnetics and dynamic strain is used to investigate a multiferroic antiferromagnet as a high-speed artificial synapse in artificial intelligence applications. The stability of strain-induced intermediate antiferromagnetic magnetization states (non-uniform magnetization states between a uniform 0 or 1), along with the minimum time scale at which these states can be programmed is investigated. Results show that due to the antiferromagnetic material's magnetocrystalline anisotropy, two intermediate states (Néel vector 1/3z, 2/3x, and Néel vector 2/3z, 1/3x) between fully x and fully z Néel vector orientations can be successfully programmed using 375 με strain pulses, and that the time associated with this programming is limited to ∼0.3 ns by the material's antiferromagnetic resonance frequency.

Theory of Antiferromagnet-Based Detector of Terahertz Frequency Signals

We present a theory of a detector of terahertz-frequency signals based on an antiferromagnetic (AFM) crystal. 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 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 of the antiferromagnetic resonance (AFMR).

Influence of intersublattice coupling on the terahertz nutation spin dynamics in antiferromagnets

Spin nutation resonance has been well-explored in one-sublattice ferromagnets. Here, we investigate the spin nutation in two-sublattice antiferromagnets as well as, for comparison, ferrimagnets with inter-and intra-sublattice nutation coupling. In particular, we derive the susceptibility of the two-sublattice magnetic system in response to an applied external magnetic field. To this end, the antiferromagnetic and ferrimagnetic (sub-THz) precession and THz nutation resonance frequencies are calculated. Our results show that the precession resonance frequencies and effective damping decrease with intra-sublattice nutation coupling, while they increase with inter -sublattice nutation in an antiferromagnet. However, we find that the THz nutation resonance frequencies decrease with both the intra-and inter-sublattice nutation couplings. For ferrimagnets, conversely, we calculate two nutation modes with distinct frequencies, unlike antiferromagnets. The exchange-like precession resonance frequency of ferrimagnets decreases with intra-sublattice nutation coupling and increases with inter-sublattice nutation coupling, like antiferromagnets, but the ferromagnetic-like precession frequency of ferrimagnets is practically invariant to the intra and inter-sublattice nutation couplings.

Electrically tunable detector of THz-frequency signals based on an antiferromagnet

A concept of an electrically tunable resonance detector of THz-frequency signals based on an antiferromagnetic/heavy metal (AFM/HM) heterostructure is 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.

Magnetic field-dependent low-energy magnon dynamics in α–RuCl3

Revealing the spin excitations of complex quantum magnets is key to developing a minimal model that explains the underlying magnetic correlations in the ground state. We investigate the low-energy magnons in $\alpha$-RuCl$_3$ by combining time-domain terahertz spectroscopy under an external magnetic field and model Hamiltonian calculations. We observe two absorption peaks around 2.0 and 2.4 meV, which we attribute to zone-center spin waves. Using linear spin-wave theory with only nearest-neighbor terms of the exchange couplings, we calculate the antiferromagnetic resonance frequencies and reveal their dependence on an external field applied parallel to the nearest-neighbor Ru-Ru bonds. We find that the magnon behavior in an applied magnetic field can be understood only by including an off-diagonal $\Gamma$ exchange term to the minimal Heisenberg-Kitaev model. Such an anisotropic exchange interaction that manifests itself as a result of strong spin-orbit coupling can naturally account for the observed mixing of the modes at higher fields strengths.

Features of Dispersion Characteristics of Surface Spin Waves in Coupled Antiferromagnetic Films with Easy-Axis Anisotropy

Propagation of magnetostatic spin waves in a layered structure consisting of two vertically coupled via a nonmagnetic dielectric layer antiferromagnetic (AFM) films has been studied. The results of calculation of the dispersion dependence for the AFM films with an easy-axis anisotropy are presented. It has been shown that splitting of coupled modes occurs in the region of AFM resonance frequencies. The behavior of coupled modes as a function of the parameter of coupling between films has been studied.

Tunable long-distance spin transport in a crystalline antiferromagnetic iron oxide.

Spintronics uses spins, the intrinsic angular momentum of electrons, as an alternative for the electron charge. Its long-term goal is to develop beyond-Moore, low-dissipation technology devices, recently demonstrating long-distance transport of spin signals across ferromagnetic insulators1. Antiferromagnetically ordered materials, the most common class of magnetic materials, have several crucial advantages over ferromagnetic systems2. Antiferromagnets exhibit no net magnetic moment, rendering them stable and impervious to external fields. Additionally, they can be operated at THz frequencies3. Although their properties bode well for spin transport4–7, previous indirect observations indicate that spin transmission through antiferromagnets is limited to only a few nanometers8–10. Here we demonstrate the long-distance propagation of spin-currents through single-crystalline hematite (α-Fe2O3)11, the most common antiferromagnetic iron oxide, exploiting the spin Hall effect for spin injection. We control the spin-current flow by the interfacial spin-bias, tuning the antiferromagnetic resonance frequency with an external magnetic field12. This simple antiferromagnetic insulator conveys spin information parallel to the Néel order over distances exceeding tens of micrometers. This newly-discovered mechanism transports spin as efficiently as the net magnetic moments in the best-suited complex ferromagnets1. Our results pave the way to ultra-fast, low-power antiferromagnet-insulator-based spin-logic devices6,13 that operate, without magnetic fields, at room temperature.

Spin pumping and inverse spin Hall voltages from dynamical antiferromagnets

Dynamical antiferromagnets pump spins efficiently into adjacent conductors as ferromagnets. The high antiferromagnetic resonance frequencies represent a challenge for experimental detection, but magnetic fields can reduce these resonance frequencies. We compute the inverse spin Hall voltages resulting from dynamical spin excitations as a function of a magnetic field along the easy axis and the polarization of the driving AC magnetic field perpendicular to the easy axis. We consider the insulating antiferromagnets MnF$_2$, FeF$_2$, and NiO. Near the spin-flop transition, there is a significant enhancement of the DC spin pumping and inverse spin Hall voltage for the uniaxial antiferromagnets MnF$_2$ and FeF$_2$. In the biaxial NiO, the voltages are much weaker, and there is no spin-flop enhancement of the DC component.

Optical second harmonic generation induced by picosecond terahertz pulses in centrosymmetric antiferromagnet NiO

Optical second harmonic generation at the photon energy of 2ℏω = 2eV in the model centrosymmetric antiferromagnet NiO irradiated with picosecond terahertz pulses (0.4–2.5 THz) at room temperature is detected. The analysis of experimental results shows that induced optical second harmonic generation at the moment of the impact of a terahertz pulse arises through the electric dipole mechanism of the interaction of the electric field of a pump pulse with the electron subsystem of NiO. Temporal changes in optical second harmonic generation during 7 ps after the action of the pulse are also of an electric dipole origin and are determined by the effects of propagation of the terahertz pulse in a NiO platelet. Coherent oscillations of spins at the antiferromagnetic resonance frequency induced by the magnetic component of the terahertz pulse induce a relatively weak modulation of magnetic dipole optical second harmonic generation.

Numeric Calculation of Antiferromagnetic Resonance Frequencies for the Noncollinear Antiferromagnet

We present an algorithm for the numeric calculation of antiferromagnetic resonance frequencies for the non-collinear antiferromagnets of general type. This algorithm uses general exchange symmetry approach \cite{andrmar} and is applicable for description of low-energy dynamics of an arbitrary noncollinear spin structure in weak fields. Algorithm is implemented as a MatLab and C++ program codes, which are available for download. Program codes are tested against some representative analytically solvable cases.

Left-handed metastructures with selective frequency transmission window for gigahertz shielding applications

Experimental evidence of left-handed properties of the Fe-, CoFe-, and Co-based glass-covered amorphous microwires in free standing systems is demonstrated. A new broadband frequency shielding metastructure with selective transmission frequency in the microwave range is presented. The X-band microwave-guide and the free-space methods were used as experimental techniques in the frequency range from 1 to 12 GHz. The X-band experimental results show that the mixed metastructure presents left-handed behavior between 8.5 and 10.5 GHz. The mixed metastructure for large areas presents broadband frequency domains, with left-handed properties between 6.5 and 10.5 GHz (the sum of intrinsic ferromagnetic and antiferromagnetic resonance frequencies domains of individual microwires) and a transmission window between 7.6 and 10.5 GHz. Double negative characteristics of the metastructures are in agreement with the computed results calculated based on Landau–Lifshitz–Gilbert equations and Nicolson-Ross-Weir analytical method.

MnF2/SiO2 Transport Properties of Quasiperiodic Photonic Crystals for Potential Catalytic Applications

Magnetically recyclable materials should be ideal support in photocatalytic system because they permit the photocatalysts to be recovered rapidly and efficiently by applying an external magnetic field such as, MnF2. In this paper, MnF2 and SiO2 layers constitute a one‐dimensional quasiperiodic photonic crystal according to Fibonacci. When the electromagnetic wave irradiates obliquely, the transmission peak moves to higher frequency direction with the angle increasing. Both the number of transmission peaks and the transmission peaks of double‐forked structure increase with the increase of structural progression. We also found that the polarization of electromagnetic waves has influence on the transmission properties; TM wave transmission peak half wide is significantly greater than TE wave transmission peak half wide. The band gap near antiferromagnetic (AF) resonance frequency becomes narrow as the intensity of the applied static magnetic field increases. The as‐prepared photonic crystal has tremendous potential practical use to eliminate organic pollutants from wastewater.

Magnetoelectric effect and magnetic dynamics in the Gd2CuO4 antiferromagnet

The magnetoelectric effect and the magnetic dynamics in Gd2CuO4 have been studied using a phenomenological approach and group-theory methods. Vector order parameters are introduced based on four magnetic sublattices. Invariant products of the order parameters are determined, from which the thermodynamic potential density is constructed. Using the spin-wave representation, the calculations can be significantly simplified and the ground orientation magnetic state can be presumably determined. The magnetic dynamics is described by the Landau-Lifshitz equations, from which the antiferromagnetic resonance frequency and the dynamic susceptibilities, namely, magnetic, antiferromagnetic, magnetoelectric, and antiferroelectric susceptibilities are found. The frequency and the susceptibility are shown to be controlled by applied electric field.

Observation of Antiferromagnetic Magnons and Magnetostriction in Manganese Oxide Using Terahertz Time-Domain Spectroscopy

We observed antiferromagnetic magnons in manganese oxide (MnO) using terahertz time-domain spectroscopy (THz-TDS). The antiferromagnetic resonance (AFMR) frequency shows the behavior of a first-order phase transition near the Neel temperature (116 K). The temperature dependence of the AFMR frequency is explained well by a molecular field approximation including biquadratic exchange. It was found that the relaxation rate of the AFMR signal shows an abrupt increase near the Neel temperature, where the antiferromagnetic long-range order disappears. The change \(\Delta n\) in refractive index was obtained from the peak shift in the THz electric field. The temperature dependence of \(\Delta n\) can be explained by the lattice and magnetostrictive contributions to this refractive index change. The measurement of refractive index by THz-TDS is very useful for optically opaque materials such as MnO.

Optical reflectivity and magnetoelectric effects on resonant plasmon modes in composite metal-multiferroic systems

The role of the magnetoelectric effect upon optical reflectivity is studied by adapting an electrodynamic-based model for a system composed by a 2D metallic film in contact with an extended multiferroic material exhibiting weak ferromagnetism. The well-known Nakayama's boundary condition is reformulated by taking into account the magnetoelectric coupling as well as an externally applied magnetic field B in an arbitrary direction. It is found that the reflectance shows strong fluctuations for incident radiation close to the characteristic antiferromagnetic resonance frequency associated with the multiferroic material in the THz regime. These results were verified for a 10nm metallic foil by using a finite element method (FEM) and the Rouard's approach, for a wide range of wavelengths (0.1–5mm), showing good agreement with respect to Nakayama's outcome, for the particular material BaMnF4.