Antiferromagnetic Spin Waves Research Articles

Effect of single-ion anisotropy on magnons in the [formula omitted] bilayer antiferromagnet

Spectrum of spin waves (magnons) in the two-dimensional bilayer antiferromagnet based on the VSe 2 is studied theoretically. The vanadium atoms within individual layers are coupled ferromagnetically, while the exchange coupling between V atoms located in different planes is antiferromagnetic. The magnon dispersion relation and its dependence on magnetic anisotropy is analyzed in the regime of weak external magnetic field. The spin-wave spectra are derived within the spin-wave theory of antiferromagnets in terms of the Holstein–Primakoff transformation combined with the Bogolubov diagonalization scheme. The magnon dispersion features are discussed in case of the T-type stacking of the VSe2 bilayer.

Suppression of Skyrmion Hall Motion in Antiferromagnets Driven by Circularly Polarized Spin Waves

An investigation of spin waves interacting with antiferromagnetic spin textures is meaningful for future spintronic and magnonic-based memory and logic applications. In this work, we numerically study the skyrmion dynamics driven by circularly polarized spin waves in antiferromagnets and propose a method of suppressing the Hall motion. It is demonstrated that the application of two circularly polarized spin waves with opposite chirality allows the skyrmion motion straightly along the intersection line of the two spin wave sources. The skyrmion speed depending on these parameters of the spin waves and system is estimated, and a comparison with other methods is provided. Furthermore, two depinning behaviors of the skyrmion related to the strengths of the defect are also observed in the simulations. Thus, the proposed method could be used in precisely modulating the skyrmion dynamics, contributing to skyrmion-based memory device design.

Transverse susceptibility and resonance absorption of an anti-ferromagnetic spin wave interacting with a phonon reservoir

Abstract A form of the transverse magnetic susceptibility for an anti-ferromagnetic spin system interacting with a phonon reservoir is derived employing the TCLE method (a method in which the admittance of a physical system is directly derived from time-convolutionless equations with external driving terms) in terms of the non-equilibrium thermo-field dynamics (NETFD) in the spin-wave approximation. The region valid for the lowest spin-wave approximation is numerically investigated in detail, and the transverse susceptibility for the anti-ferromagnetic system of one-dimensional infinite spins of magnitude $S = 2$ is numerically studied in that region. It is confirmed that the effects of the memory and initial correlation for the spin system and phonon reservoir, which are represented by the interference terms in the TCLE method, increase the power absorption in the resonance region. In the region valid for the lowest spin-wave approximation, it is shown that the line half-width $\Delta \omega_{\mathrm{RF}}$ of the power absorption in the resonance region for a transversely rotating magnetic field increases as the temperature $T$ rises, and decreases as the uniaxial anisotropy energy $\hbar K$ of the $z$ direction increases or as the wave number $k$ becomes large, and also that the line peak-height $H_{\mathrm{RF}}$ of the power absorption in the resonance region for the transverse magnetic field decreases as $T$ rises and increases as $\hbar K$ increases or as $k$ becomes large. According to analytic considerations, it is anticipated that $\Delta \omega_{\mathrm{RF}}$ and $H_{\mathrm{RF}}$ vary approximately as $\Delta \omega_{\mathrm{RF}} \approx \{A_k + B_k \bar{n} (\omega_{{\mathrm{R}} k} )\} \{ \bar{n}(\omega_{{\mathrm{R}} k}) + \,1\}$ and $H_{\mathrm{RF}} \propto \{A_k + B_k \bar{n}(\omega_{{\mathrm{R}} k})\}^{- 1} \{ \bar{n}(\omega_{{\mathrm{R}} k}) + 1\}^{- 1}$, with $\bar{n}(\omega_{{\mathrm{R}} k}) = \{ \exp(\hbar \omega_{{\mathrm{R}} k}/ (k_{\mathrm{B}} T)) -1 \}^{- 1}$, where $\omega_{{\mathrm{R}} k}$ is the characteristic frequency of the phonon reservoir. Here, $A_k$ is a positive quantity dependent on the Zeeman frequency, the spin magnitude, and the anisotropy energy of the spin system, and $B_k$ is a positive quantity dependent on the anisotropy energy. At low temperatures ($k_{\mathrm{B}} T \ll \hbar \omega_{{\mathrm{R}} k}$), it is anticipated that $\Delta \omega_{\mathrm{RF}}$ and $H_{\mathrm{RF}}$ vary approximately as $\Delta \omega_{\mathrm{RF}} \approx A_k \exp\{((A_k + B_k)/ A_k) \exp(- \hbar \omega_{{\mathrm{R}} k}/ (k_{\mathrm{B}} T))\}$ and $H_{\mathrm{RF}} \propto A_k^{- 1} \exp\{- ((A_k + B_k)/ A_k) \exp(- \hbar \omega_{{\mathrm{R}} k}/ (k_{\mathrm{B}} T))\}$.

Schottky-like anomaly in the heat capacity and magnetocaloric effect of charge-ordered single-crystalline (Sm, Ca, Sr)MnO[formula omitted] compound

We present a comprehensive experimental study on the magnetic and magnetocaloric properties of a charge-ordered single-crystalline Sm0.5Ca0.25Sr0.25MnO3 compound. The studies on x-ray photoelectron spectroscopy (XPS) reveals the presence of an equal distribution of Mn3+ and Mn4+ ions in the studied system. The Oxygen, O1s-core level spectra have been simulated with three binding energies curves, which correspond to the O2− ions, O1− ions, and chemically adsorbed oxygens, Ochem. The XPS analysis of the O1s-core-level spectra and magnetic characterizations indicate the proper stoichiometry of the present sample. Considering the change of volume phase fraction in the isofield magnetization measurements during the first-order magnetic phase transition from paramagnetic state to ferromagnetic state, the isothermal magnetic entropy change (ΔS) has been estimated based on the modified Clausius–Clapeyron equation. An inverse magnetocaloric effect has also been noticed in the -ΔS vs. T plot calculated by Maxwell’s thermodynamic relation, suggesting the dominant antiferromagnetic ground state supported by a charge-ordered phase of the studied system. The high-temperature zero-field heat capacity (CP) data can be well-interpreted quantitatively using the Debye model of heat capacity. With the extracted magnetic heat capacity (Cmag) data, the temperature variation of the magnetic entropy (S(0)), as well as the adiabatic temperature change (ΔTad), have been estimated. In addition to that, the low-temperature CP data displays a Schottky-like anomaly in the temperature region between 2 K and 20 K. The experimental data points are successfully fitted by considering the various contributing factors of the low-temperature heat capacity such as the lattice-phonon vibration (Clat), antiferromagnetic spin-wave (Cmag), and the two-level Schottky function (Csch) due to the energy splitting of the Sm3+ cations.

Reinvestigation of crystal symmetry and fluctuations in La2CuO4

New surprises continue to be revealed about La$_2$CuO$_4$, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the CuO$_2$ planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in CuO$_6$ octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the "chiral" phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.

Strong Coupling of Antiferromagnetic Resonance with Subterahertz Cavity Fields

Strong coupling of electromagnetic cavity fields with antiferromagnetic spin waves in hematite ($\ensuremath{\alpha}$-${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$) is achieved above room temperature. A cube of hematite is placed in a metallic tube and the transmission is measured, using a continuous-wave terahertz spectrometer. The spectra, collected as a function of the temperature, reveal the formation of magnetic polaritons.

Theory of spin waves in a hexagonal antiferromagnet

We construct a field-theoretic description of spin waves in hexagonal antiferromagnets with three magnetic sublattices and coplanar $120^\circ$ magnetic order. The three Goldstone modes can be separated by point-group symmetry into a singlet $\alpha_{0}$ and a doublet $(\beta_x,\beta_y)$. The $\alpha_0$ singlet is described by the standard theory of a free relativistic scalar field. The field theory of the $(\beta_x,\beta_y)$ doublet is analogous to the theory of elasticity of a two-dimensional isotropic solid with distinct longitudinal and transverse "speeds of sound". The well-known Heisenberg models on the triangular and kagome lattices with nearest-neighbour exchange turn out to be special cases with accidental degeneracy of the spin-wave velocities. The speeds of sound can be readily calculated for any lattice model. We apply this approach to the compounds of the Mn$_3$X family with stacked kagome layers.

Antichiral spin order, its soft modes, and their hybridization with phonons in the topological semimetal .

We report the magnetic structure and spin excitations of , a breathing kagome antiferromagnet with transport anomalies attributed to Weyl nodes. Using polarized neutron diffraction, we show the magnetic order is a coplanar state belonging to a irreducible representation, which can be described as a perfect 120° antichiral structure with a moment of , superimposed with weak collinear ferromagnetism. Inelastic neutron scattering shows three collective excitations at , , and . A field theory of spin waves in triangular antiferromagnets with a 120° spin structure was used to classify these modes. The in-plane mode is gapless, is the gap to a doublet of out-of-plane spin excitations ( , ), and , result from hybridization of optical phonons with magnetic excitations. While a phenomenological spin Hamiltonian including exchange interactions, Dzyaloshinskii-Moriya interactions, and single-ion crystal field terms can describe aspects of the Mn-based magnetism, spin-wave damping [ ] and the extended range of magnetic interactions indicate itinerant magnetism consistent with the transport anomalies.

Distinct handedness of spin wave across the compensation temperatures of ferrimagnets.

Antiferromagnetic spin waves have been predicted to offer substantial functionalities for magnonic applications due to the existence of two distinct polarizations, the right-handed and left-handed modes, as well as their ultrafast dynamics. However, experimental investigations have been hampered by the field-immunity of antiferromagnets. Ferrimagnets have been shown to be an alternative platform to study antiferromagnetic spin dynamics. Here we investigate thermally excited spin waves in ferrimagnets across the magnetization compensation and angular momentum compensation temperatures using Brillouin light scattering. Our results show that right-handed and left-handed modes intersect at the angular momentum compensation temperature where pure antiferromagnetic spin waves are expected. A field-induced shift of the mode-crossing point from the angular momentum compensation temperature and the gyromagnetic reversal reveal hitherto unrecognized properties of ferrimagnetic dynamics. We also provide a theoretical understanding of our experimental results. Our work demonstrates important aspects of the physics of ferrimagnetic spin waves and opens up the attractive possibility of ferrimagnet-based magnonic devices.

Enhancement of spin wave group velocity in ferrimagnets with angular momentum compensation

In this study, we theoretically investigated properties of magnetostatic spin waves in rare-earth and transition-metal ferrimagnets with various angular momentum. The resonant frequency of high- (low-) frequency mode has the lowest (highest) value at angular momentum compensation temperature (TA). It was found that both resonance modes show a maximum group velocity at TA. Furthermore, the low-frequency mode shows higher group velocity than the high-frequency mode at any temperature. Our findings are the key to advance both the physics and the technology to develop research field of magnonics based on the antiferromagnetic spin waves.

Switchable giant nonreciprocal frequency shift of propagating spin waves in synthetic antiferromagnets

The nonreciprocity of propagating spin waves, i.e., the difference in amplitude and/or frequency depending on the propagation direction, is essential for the realization of spin wave-based logic circuits. However, the nonreciprocal frequency shifts demonstrated so far are not large enough for applications because they originate from interfacial effects. In addition, switching of the spin wave nonreciprocity in the electrical way remains a challenging issue. Here, we show a switchable giant nonreciprocal frequency shift of propagating spin waves in interlayer exchange-coupled synthetic antiferromagnets. The observed frequency shift is attributed to large asymmetric spin wave dispersion caused by a mutual dipolar interaction between two magnetic layers. Furthermore, we find that the sign of the frequency shift depends on relative configuration of two magnetizations, based on which we demonstrate an electrical switching of the nonreciprocity. Our findings provide a route for switchable and highly nonreciprocal spin wave-based applications.

Full 3D + 1 modeling of tilted-pulse-front setups for single-cycle terahertz generation

The tilted-pulse-front setup utilizing a diffraction grating is one of the most successful methods to generate single- to few-cycle terahertz pulses. However, the generated terahertz pulses have a large spatial inhomogeneity, due to the noncollinear phase-matching condition and the asymmetry of the prism-shaped nonlinear crystal geometry, especially when pushing for high optical-to-terahertz conversion efficiency. A 3 D + 1 ( x , y , z , t ) numerical model is necessary in order to fully investigate the terahertz generation problem in the tilted-pulse-front scheme. We compare in detail the differences among 1 D + 1 , 2 D + 1 , and 3 D + 1 models. The simulations show that the size of the optical beam in the pulse-front-tilt plane sensitively affects the spatiotemporal properties of the terahertz electric field. The terahertz electric field is found to have a strong spatial dependence such that a few-cycle pulse is generated only near the apex of the prism. Even though the part of the beam farther from the apex can contain a large fraction of the energy, the terahertz waveform shows less few-cycle character. This strong spatial dependence must be accounted for when using the terahertz pulses for strong-field physics and carrier-envelope-phase sensitive experiments such as terahertz acceleration, coherent control of antiferromagnetic spin waves, and terahertz high-harmonic generation.

Synthetic Antiferromagnets: Optically Inspired Nanomagnonics with Nonreciprocal Spin Waves in Synthetic Antiferromagnets (Adv. Mater. 9/2020)

Synthetic Antiferromagnets: Optically Inspired Nanomagnonics with Nonreciprocal Spin Waves in Synthetic Antiferromagnets (Adv. Mater. 9/2020)

Effect of inhomogeneous Dzyaloshinskii-Moriya interaction on antiferromagnetic spin-wave propagation

We investigate the effect of inhomogeneous Dzyaloshinskii-Moriya interaction (DMI) on antiferromagnetic spin-wave propagation theoretically and numerically. We find that antiferromagnetic spin waves can be amplified at a boundary where the DMI varies. The inhomogeneous DMI also provides a way to construct a magnonic crystal with forbidden and allowed antiferromagnetic spin-wave bands in terahertz frequency ranges. In contrast to ferromagnetic spin waves, antiferromagnetic spin waves experience a polarization-dependent phase shift when passing through the inhomogeneous DMI, offering a magnonic crystal that also serves as a spin-wave polarizer.

Optically Inspired Nanomagnonics with Nonreciprocal Spin Waves in Synthetic Antiferromagnets.

Integrated optically inspired wave-based processing is envisioned to outperform digital architectures in specific tasks, such as image processing and speech recognition. In this view, spin waves represent a promising route due to their nanoscale wavelength in the gigahertz frequency range and rich phenomenology. Here, a versatile, optically inspired platform using spin waves is realized, demonstrating the wavefront engineering, focusing, and robust interference of spin waves with nanoscale wavelength. In particular, magnonic nanoantennas based on tailored spin textures are used for launching spatially shaped coherent wavefronts, diffraction-limited spin-wave beams, and generating robust multi-beam interference patterns, which spatially extend for several times the spin-wave wavelength. Furthermore, it is shown that intriguing features, such as resilience to back reflection, naturally arise from the spin-wave nonreciprocity in synthetic antiferromagnets, preserving the high quality of the interference patterns from spurious counterpropagating modes. This work represents a fundamental step toward the realization of nanoscale optically inspired devices based on spin waves.

Terahertz emission spectrum of polar antiferromagnet Fe<sub>2</sub>Mo<sub>3</sub>O<sub>8</sub>

In polar materials, the transition of electrons in momentum space will change the spontaneous polarization. When excited by femtosecond pulse laser, the transient modulation of the electric polarization will radiate electromagnetic wave at terahertz frequency. In a magnetic ordered system, the coherent excited spin wave radiates electromagnetic waves of the same frequency in the process of precession and relaxation. The investigation of the terahertz emission spectra of these materials not only helps us to understand the ferroelectric and magnetic ordered dynamic processes of materials, but also provides a reference for searching for new terahertz sources. We study the terahertz emission spectrum of the polar antiferromagnet Fe<sub>2</sub>Mo<sub>3</sub>O<sub>8</sub>. Under the pumping of 800 nm laser, electrons in the material are excited across the band gap leading the electric polarization to be ultra-fast modulated. The broadband terahertz excitation spectrum from 0.1 to 3.5 THz is observed, and the direction of the terahertz electric field is along the inherent electric polarization direction of the material. After entering into the magnetic order state, two new single-frequency terahertz oscillations are observed, located at 1.25 THz and 2.7 THz respectively, which correspond to the excitation of the two antiferromagnetic spin waves of Fe<sub>2</sub>Mo<sub>3</sub>O<sub>8</sub>.

Propagation Length of Antiferromagnetic Magnons Governed by Domain Configurations.

The compensated magnetic order and characteristic terahertz frequencies of antiferromagnetic materials make them promising candidates to develop a new class of robust, ultrafast spintronic devices. The manipulation of antiferromagnetic spin-waves in thin films is anticipated to lead to new exotic phenomena such as spin-superfluidity, requiring an efficient propagation of spin-waves in thin films. However, the reported decay length in thin films has so far been limited to a few nanometers. In this work, we achieve efficient spin-wave propagation over micrometer distances in thin films of the insulating antiferromagnet hematite with large magnetic domains while evidencing much shorter attenuation lengths in multidomain thin films. Through transport and magnetic imaging, we determine the role of the magnetic domain structure and spin-wave scattering at domain walls to govern the transport. We manipulate the spin transport by tailoring the domain configuration through field cycle training. For the appropriate crystalline orientation, zero-field spin transport is achieved across micrometers, as required for device integration.

Current-controlled propagation of spin waves in antiparallel, coupled domains.

Spin waves may constitute key components of low-power spintronic devices. Antiferromagnetic-type spin waves are innately high-speed, stable and dual-polarized. So far, it has remained challenging to excite and manipulate antiferromagnetic-type propagating spin waves. Here, we investigate spin waves in periodic 100-nm-wide stripe domains with alternating upward and downward magnetization in La0.67Sr0.33MnO3 thin films. In addition to ordinary low-frequency modes, a high-frequency mode around 10 GHz is observed and propagates along the stripe domains with a spin-wave dispersion different from the low-frequency mode. Based on a theoretical model that considers two oppositely oriented coupled domains, this high-frequency mode is accounted for as an effective antiferromagnetic spin-wave mode. The spin waves exhibit group velocities of 2.6 km s-1 and propagate even at zero magnetic bias field. An electric current pulse with a density of only 105 A cm-2 can controllably modify the orientation of the stripe domains, which opens up perspectives for reconfigurable magnonic devices.

Nonabelian magnonics in antiferromagnets

We present a semiclassical formalism for antiferromagnetic (AFM) magnonics which promotes the central ingredient of spin wave chirality, encoded in a quantity called magnonic isospin, to a first-class citizen of the theory. We use this formalism to unify results of interest from the field under a single chirality-centric formulation. Our main result is that the isospin is governed by unitary time evolution, through a Hamiltonian projected down from the full spin wave dynamics. Because isospin is SU(2)-valued, its dynamics on the Bloch sphere are precisely rotations - which, in general, do not commute. Consequently, the induced group of operations on AFM spin waves is nonabelian. This is a paradigmatic departure from ferromagnetic magnonics, which operates purely within the abelian group generated by spin wave phase and amplitude. Our investigation of this nonabelian magnonics in AFM insulators focuses on studying several simple gate operations, and offering in broad strokes a program of study for interesting new logic families in antiferromagnetic spin wave systems

Polarization-selective spin wave driven domain-wall motion in antiferromagnets

The control of magnetic domain walls is essential for the magnetic-based memory and logic applications. As an elementary excitation of magnetic order, spin wave is capable of moving magnetic domain walls just as the conducting electric current. Ferromagnetic spin waves can only be right-circularly polarized. In contrast, antiferromagnetic spin waves have full polarization degree of freedom, including both left- and right-circular polarizations, as well as all possible linear or elliptical ones. Here we demonstrate that, due to the Dzyaloshinskii-Moriya interaction, the spin wave driven domain wall motion in antiferromagnets strongly depends on the linear polarization direction of the injected spin waves. Steering domain wall motion by simply tuning the polarization of spin waves offers new designing principles for domain-wall based information processing devices.