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The possibility of using implantation of 20-keV He+ ions for modification of the domain structure and magnetic properties of CoPt films (formed by the electron-beam evaporation method) with different cobalt contents (Co0.45Pt0.55 and Co0.35Pt0.65) has been investigated. A decrease in the coercive field (narrowing of the hysteresis loop in the magnetic-field dependences of the Faraday angle and magnetization) with an increase in the He+ ion fluence from 2 × 1014 to 4 × 1014 cm–2 has been found for the irradiated CoPt samples of both compositions. The residual magnetization of the Co0.35Pt0.65 films coincides with the saturation magnetization, and a decrease in the residual magnetization is observed for Co0.45Pt0.55. It is shown by magnetic force microscopy that the largest number of round isolated domains (skyrmions) for the Co0.45Pt0.55 alloy are formed when the ion fluence increases to 3 × 1014 cm–2, while 360-degree domain walls (1D skyrmions) are observed along with round isolated domains for the Co0.35Pt0.65 films irradiated with a He+ fluence of 4 × 1014 cm–2. An analysis of CoPt films by Mandelstam–Brillouin spectroscopy revealed an increase in the shift between the Stokes and anti-Stokes components of the spectrum and a significant enhancement of the Dzyaloshinski–Moriya interaction in the irradiated samples. Model calculations performed using the SRIM program showed that the ion irradiation facilitates asymmetric mixing of Co and Pt atoms in CoPt films, which may underlie the mechanism of the observed effects of ion irradiation on their magnetic properties and domain structure.

In this work we present a model explaining the properties of magnetoelastic waves propagation in the heterostructure containing an antiferromagnetic layer on a non-magnetic elastic substate. Horizontally polarized shear surface waves (SH-waves) propagating in thin film are Love waves. The dispersion characteristic of magnetoelastic waves in such structure was obtained, and the effect of variation of the thickness of the antiferromagnetic layer and the external magnetic field on the frequency of the magnetoelastic resonance was also studied. It was found that an increase in the magnetic field magnitude leads to the increase in the magnetoelastic resonance frequency, and, on the contrary, with an increase in the thickness of the AFM layer the magnitude of the magnetoelastic resonance frequency decreases. The obtained results can be used to develop devices for generating and processing signals in the GHz and THz frequency ranges.

Magnetic skyrmions are stable spin textures with quasi-particle behavior and attract significant interest in fundamental and applied physics. The metastability of magnetic skyrmions at zero magnetic field is particularly important to enable, for instance, a skyrmion racetrack memory. Here, the results of the nucleation of stable skyrmions and formation of ordered skyrmion lattices by magnetic force microscopy in (Pt/CoFeSiB/W)n multilayers, exploiting the additive effect of the interfacial Dzyaloshinskii-Moriya interaction, are presented. The appropriate conditions under which skyrmion lattices are confined with a dense two-dimensional liquid phase are identified. A crucial parameter to control the skyrmion lattice characteristics and the number of scans resulting in the complete formation of a skyrmion lattice is the distance between two adjacent scanning lines of a magnetic force microscopy probe. The creation of skyrmion patterns with complex geometry is demonstrated, and the physical mechanism of direct magnetic writing of skyrmions is comprehended by micromagnetic simulations. This study shows a potential of a direct-write (maskless) skyrmion (topological) nanolithography with sub-100 nm resolution, where each skyrmion acts as a pixel in the final topological image.

State-of-the-art studies of dielectric magnonics and magnon spintronics are reviewed. Theoretical and experimental approaches to exploring physical processes in and calculations of the parameters of magnonic micro- and nanostructures are described. We discuss the basic concepts of magnon spintronics, the underlying physical phenomena, and the prospects for applying magnon spintronics for data processing, transmission, and reception. Special attention is paid to the feasibility of boosting the operating frequencies of magnonic devices from the gigahertz to terahertz frequency range. We also discuss specific implementations of the component base of magnonics and ways to further develop it.

Dynamics of a filter-free synthesizer of discrete frequency spectrum based on the phase-locked system of an antiferromagnetic spintronic oscillator, which generates THz oscillations in a wide frequency range under dc current, was investigated. The dependence of the phase-locked-loop gain on the difference between the frequencies of phase-locked oscillations at different feedback division factors is obtained by the method of slowly varying amplitudes. It is shown that an increase in the division factor narrows the band of phase-locked oscillations at invariable parameters of the antiferromagnetic spintronic oscillator.

Taking into account the electron spin mobility, the paper considers the numerical solution to the equation for nonequilibrium spin polarization at the boundary of a magnetic contact with a current formed by two ferromagnets. The frequencies of spin injection radiation are calculated. It is shown that at a certain current density exceeding the threshold value, these frequencies lie in the terahertz frequency range. It was found that for the same terahertz frequency range, even for small values of the difference in the mobility of electrons with spin up and down, the current density can be two orders of magnitude lower than the current density with the same mobility.

Taking into account the difference in the mobilities of electrons with different spin projections, the frequencies of spin-injection radiation in the current-carrying magnetic contact formed by two ferromagnets are calculated. It is shown that these frequencies are in the terahertz range at a certain current density exceeding the threshold value, and the current density required to generate terahertz radiation can be, even at small values of the electron mobility difference, two orders of magnitude less than that in the absence of the electron mobility difference. The effect of the spin polarization ratio of ferromagnets that form a magnetic junction on the radiation frequency is analyzed.

The effect is studied of tensile deformations on the spectrum of the ferromagnetic resonance of submicron yttrium–iron garnet polycrystalline films manufactured by means of ion-beam sputtering onto silicon and gallium arsenide substrates. Magnetoelastic constants of films on both substrates, the values of which did not exceed 16% of that of the bulk polycrystalline garnet, were calculated from the magnitude of the frequency shift of the absorption maximum in the ferromagnetic resonance spectrum. An approach is proposed to increase the efficiency of electrical frequency tuning in composite multiferroid structures by means of combining static and dynamic (caused by piezoelectric effect) tensile deformations.

A method for the measurement of the complex permittivity of insulators using a microwave coaxial photonic crystal is proposed and implemented. The method is based on a solution to an inverse problem using minimization of the difference of experimental and calculated frequency dependences of the transmission and reflection coefficients at the defect-mode frequency in the band gap of a photonic microwave crystal containing a structure with the parameters that must be determined. An unambiguous solution to the inverse problem can be obtained using a defect mode in the band gap at the frequency of which a maximum of the standing wave is obtained in the region of the structure under study.

Simultaneous quantum correlations between two spins in magnetic nanostructures are considered in the model of a linear chain of a finite number of atoms with exchange interaction between electron spins of neighboring atoms in the framework of the Heisenberg ferromagnetism theory. We assume that in the initial state, the spins of all chain atoms except the first two are oriented along the same direction. The spins of the first two atoms are flipped. Due to the exchange interaction, this initial state generates a spin flip wave along the chain. The expressions obtained for nonstationary quantum amplitudes of the flip probability waves for an even number of spins can be used for calculating quantum correlations between two spins separated by a large distance in a chain. Numerical calculations of the spin correlator reveal that the correlation between two spins in the chain occurs with a delay on the order of the time of propagation of the exchange interaction along the spin chain. After the delay, the spin correlation amplitude abruptly increases followed by subsequent oscillatory temporal behavior.

A two-sublattice ferrimagnet undergoes a transition from a collinear to canted magnetic phase at magnetic field oriented along an easy magnetization direction. In this work, we study the transition by means of the magneto-optical Faraday effect in a thin film of compensated iron garnet (Lu$_{3-{\rm{x}}}$Bi$_{\rm{x}}$)(Fe$_{5-{\rm{y}}-{\rm{z}}}$Ga$_{\rm{y}}$Al$_{\rm{z}}$)O$_{12}$ grown on Gd$_3$Ga$_5$O$_{12}$ substrate. In the immediate vicinity of the compensation temperature a precursor of the transition with a complex shape was observed. Using a special sample with variable thickness we demonstrate an interfacial origin of the precursor. Diffusion of gadolinium from the substrate into the film forms a thin intermixed layer with enhanced magnetization. It induces an extended inhomogeneous magnetic structure in the film. A two-step shape of the precursor appears due to an easy-plane anisotropy of the intermixed magnetic layer. We emphasize that an effective width of the inhomogeneous magnetization distribution in the film grows enormously while approaching the compensation temperature., Comment: 10 pages, 5 figures

An original equation for nonequilibrium spin polarization in magnetic junctions based on dynamic equations for magnetic moment was obtained. Spatial nonuniformity of the distribution of carriers is taken into account in the equations, and the magnetic moment is averaged over the ensemble of nonequilibrium spin-injected electrons. The solution to the dynamic equations for the magnetic moment is used to estimate the probability of spin-flip electron transitions upon spin injection, and the solution to the equation for the nonequilibrium spin polarization in magnetic junctions is used to calculate frequencies of photon emission or absorption at energies corresponding to the energy of effective exchange splitting of spin subbands.

The influence of strains on the ferromagnetic resonance (FMR) spectrum of submicron yttrium iron garnet (YIG) films produced by the ion beam sputtering on gadolinium-gallium garnet (GGG) and silicon (Si) substrates is explored. It is shown that the strain influence is displayed as a frequency shift of the absorption maximum in the FMR spectrum. The results indicate that the studied YIG/GGG and YIG/Si films have an efficient magnetoelastic coupling of the spin and elastic subsystems, which suggests that ion beam sputtering of YIG films on GGG and Si substructures can be a promising technique for production of straintronic devices.

We demonstrate the dissipative spatiotemporal patterns that are generated by an active ring resonator based on a magnonic quasicrystal (MQC) with Fibonacci-type structure. The dissipative patterns are formed through the magnetostatic surface spin-wave (MSSW) parametric decay into exchange spin waves (SWs), temporal dispersion of the ring resonator and amplification. In the spatial domain, the MSSW and SW parametric patterns measured with the help of Brillouin light spectroscopy have the quasiperiodic spatial localization in crests and grooves of the MQC, respectively. In contrast to the optical quasiperiodic parametric spatial conservative solitons, the amplitude profiles of the magnonic quasiperiodic parametric spatial dissipative patterns correspond to the profiles of the crests and grooves of the MQC. In the time domain, the packets of the chaotic parametric pulses, which are analogs of temporal solitons, are generated at each point of the spatial patterns. The chaotic nature of such pulses is confirmed by an estimate of a highest Lyapunov exponent from the experimental time series. The pulse packets are formed through the time-filtering technique using the external microwave (MW) pulses to control a ring gain. It is shown that at a certain duty factor of the external MW pulses, the parametric MSSW pulses have the amplitude and phase profiles corresponding to the profiles of a conservative bright envelope soliton.

A concept of a subterahertz (sub-THz) frequency range signals detector based on a ferrimagnet/normal metal (FiM/NM) heterostructure is proposed. An external linearly polarized electromagnetic (EM) wave excites precession in the FiM and spin pumping at the FiM/NM interface. The excited spin current in the NM converts into a direct current via the inverse spin Hall effect. We show that non-zero uncompensation between magnetic spins in FiM gives a non-zero output rectified voltage for both “easy axis” and “easy plane” uniaxial anisotropies. The increase in the uncompensation parameter removes degeneracy from two normal resonance modes and changes the sensitivity to the EM wave. Namely, the sensitivity of the higher mode increases, and the lower mode decreases with increasing the uncompensation parameter. We believe that our results can be helpful for the practical development of sub-THz frequency tunable resonance detector based on the FiM.

The examples of application of pulse force nanolithography carried out using an atomic-force microscope probe for formation of magnetic nanowires, nanocontacts, one- and two-dimensional gratings with typical sizes of about 50–100 nm have been considered.

We have analyzed the self-action effects in the propagation of surface magnetostatic wave (SMSW) pulses in a 1D magnonic crystal–dielectric–metal structure in which the region of anomalous dispersion controlled by the selection of thickness h of the insulator and ensuring the fulfillment of the Lighthill criterion in the generation of SMSW solitons is formed. It is shown that when the metallization-induced region of anomalous dispersion coincides with the Bragg resonance frequency range, no SMSW solitons form.

The frequency characteristics of spin waves excited in the chain of cylindrical ferromagnetic pillars are investigated. The presence of a boundary in the chain and the finite height of the pillars lead to the existence of inhomogeneity of magnetization and effective magnetic fields. Using methods of finite-difference discretization simulation, we demonstrated the existence of additional resonant frequencies for the edge modes of spin waves due to the demagnetization of the pillars and dipole-dipole interaction between the pillars of the chain.

The effect of ion irradiation on magnetic properties of films of the ferromagnetic CoPt alloy fabricated by electron-beam evaporation has been studied. It is found that the coercive force decreases and the lateral component of the easy-magnetization axis increases as the He+ ion fluence increases from 1 × 1013 to 1 × 1016 cm–2. It is shown by magnetic-force microscopy and the Mandelstam–Brillouin spectroscopy that the formation of isolated circle domains which are magnetic skirmions in the CoPt layer is activated at certain ion-irradiation fluence (3 × 1014 cm–2).

The equation of the dynamics of the magnetic moment motion that has been averaged over the ensemble of nonequilibrium spin-injected electrons in a ferromagnetic transition in the presence of exchange interaction, interaction with an external electromagnetic field, and a thermostat has been obtained taking into account the spatial inhomogeneity of the charge carrier distribution. It has been shown that taking into account the spatial inhomogeneity of the charge carrier distribution in the equation of the dynamics of the magnetic moment motion allows for describing the various processes of electron transfer in the magnetic transition. The probability of quantum transitions of electrons with opposite spin directions, which determine spin relaxation in interaction with a thermostat, has been estimated. It has been shown that the anisotropy of the radiation medium is determined not only by the anisotropy of the sd-exchange tensor, but also by the additional anisotropy that is caused by the electron impulse derivatives of this tensor. The considered phenomena have a great potential for the detection of new effects and development of new devices based on them, including compact tunable radiation sources in the terahertz frequency range, which is obviously difficult to generate.

We present three-dimensional (3D) model of periodic meander-shaped ferromagnetic films. Spin wave propagation in such films and vertically coupled structures was studied using micromagnetic modeling and theoretical plane wave method. Spin waves in these structures essentially propagate in film’s segments located at right angles with respect to each other. This makes really possible for the wave to propagate in three dimensions. We calculated internal effective magnetic fields and obtained spin wave dispersion in single and vertically coupled structures. Additionally, comparison of surface and volume spin waves propagation in such meander films was provided. Our results can be useful for magnonic logic elements development.

We present results of studying properties of surface spin waves in vertically coupled antiferromagnetic films with easy-axis anisotropy. The dispersion curves and frequency region of the existence of the surface magnetostatic waves are obtained using theoretical model for different antiferromagnetic materials. This study was carried out by solving the dispersion relation for the surface magnetostatic waves. Behavior of two types of dispersion curves on the value of coupling parameter is investigated. We conclude, that results have potential applications for antiferromagnetic THz spintronics.

We study nonlinear dynamics of the spintronic nanosized antiferromagnetic terahertz oscillator consisting of an antiferromagnetic layer with easy-plane anisotropy (hematite) and a normal-metal (platinum) layer. Normal oscillation frequencies, namely, ferromagnetic and antiferromagnetic (terahertz) ones, are found. Their dependence on the value of a static magnetic field parallel to the sample plane is obtained. An approximate mathematical model in the form of the equations for the Neel-vector rotation angle in the azimuthal plane is developed for describing the oscillator dynamics. The adjustment characteristic, i.e., the dependence of the antiferromagnetic-mode frequency on the value of the direct current flowing in the platinum layer is obtained.

A feasible design of a system with an allowed band with near-unity frequency-independent transmission utilizing the reflective properties of photonic crystals with dielectric filling and resonant irises in a setup with a Y-circulator is demonstrated. The existence of impurity modes of oscillation damping in the allowed band of this photonic crystal with disturbed periodicity is confirmed. Bandstop filters with parameters controlled by n–i–p–i–n diodes are constructed based on photonic crystals with resonant irises. These filters have a rejection level of >43.0 dB in the stop band and losses below 0.8 dB outside of it.

The main features of the nonlinear spin wave propagating in the magnonic crystal/semiconductor (SC) layered heterostructure are studied. We demonstrate the possibility of double electric and magnetic control of the gap solitons parameters (their number, threshold of formation, velocity), bistability of spin waves, switching powers and nonlinear band gap shift in investigated structure. Our results show the possibility of nonlinear magnonics and SC electronics integration based on the proposed structure.

We present a model of excitation of nonlinear spin oscillations in an antiferromagnet under the action of terahertz pulses of an electromagnetic field. It is shown that, with increasing pumping pulse amplitude, the spin system response increases linearly in the fundamental (resonant) quasi-ferromagnetic mode and quadratically in the second harmonic. We have compared the theoretically obtained dependences of the intensities of the sample response amplitudes for the fundamental mode and the second harmonic with the experimental ones. The comparison demonstrates the applicability of the developed method of analyzing dynamic processes in antiferromagnets occurring under the action of terahertz electromagnetic pulses.

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.

The dynamics of the motion of the magnetic moment averaged over an ensemble of nonequilibrium spin-injected electrons in a ferromagnetic junction is considered with allowance for the exchange interaction, as well as the interaction with an external electromagnetic field and a thermostat. The solution of this problem is important for the experimental development of compact terahertz-band radiation sources. The rate of quantum transitions of electrons with opposite spins, which determine the spin relaxation under the interaction with a thermostat, is calculated within the density matrix formalism. It is shown that two spin-relaxation modes can be implemented that correspond to low- and high-Q precession of spin-nonequilibrium injected electrons. The effect of the characteristic features of spin-flip transitions under the relaxation of the magnetic moment on the emission and absorption of photons with-energy corresponding to the energy of effective exchange splitting of spin subbands is discussed.

Propagation of volume spin magnetostatic waves in meander-shaped ferromagnetic films with vertically and horizontally coupled segments is studied. Conditions under which forward and backward volume magnetostatic waves simultaneously exist in the films are determined. Micromagnetic simulation and expansion in terms of plane waves are used to calculate internal effective magnetic fields and dispersion characteristics of the waves in such structures. Spectra of coupled waves in meander-shaped vertically coupled ferromagnetic films are studied. Prospects for application of the above structures in the development of magnonic elements are analyzed.

A theory of an electrically controlled terahertz-frequency auto-oscillator is developed here within the framework of the antiferromagnetic sigma model. The structure is based on a trilayer heterostructure: a piezoelectric ceramic, NiO-based antiferromagnet (AFM), and a heavy metal. The authors demonstrate that the frequency of the AFM resonance of the layered structure in a passive subcritical regime can be controlled by the external electric field applied to the piezoelectric, by choosing a proper piezoelectric material, and by the magnitude of the electric current flowing in the heavy metal.

A frequency-tunable low-phase-noise magnetoacoustic resonator is developed on the base of a parallel-plate straight-edge bilayer consisting of an yttrium-iron-garnet (YIG) layer grown on a substrate of a gallium gadolinium garnet (GGG). When a YIG-GGG sample forms an ideal parallel plate, it supports a series of high-quality-factor acoustic modes standing along the plate thickness. Due to the magnetostriction of the YIG layer the ferromagnetic resonance (FMR) mode of the YIG layer can strongly interact with the acoustic thickness modes of the YIG-GGG structure, when the modes' frequencies match. A particular acoustic thickness mode used for the resonance excitations of the hybrid magnetoacoustic oscillations in a YIG-GGG bilayer is chosen by the YIG-layer FMR frequency, which can be tuned by the variation of the external bias magnetic field. A composite scheme of a magnetoacoustic oscillator, which includes a FMR-based resonance preselector, is developed to guarantee satisfaction of the Barkhausen criteria for a single-acoustic-mode oscillation regime. The developed low-phase-noise composite magnetoacoustic oscillator can be tuned from 0.84 to 1 GHz with an increment of about 4.773 MHz (frequency distance between the adjacent acoustic thickness modes in a YIG-GGG parallel plate), and demonstrates the phase noise of $\ensuremath{-}116$ dBc/Hz at the offset frequency of 10 kHz.

Ferrimagnetic materials represent unique systems where the ease of manipulating the spins with applied magnetic fields is combined with exchange-driven acceleration of the internal spin dynamics. Of particular interest is the temperature range around the magnetic and spin compensation points, finely balancing both magnetic moment and angular momentum of the system, and leading to a very particular character of magnetic switching by the domain wall motion. Here we present our studies of the temperature-dependent domain wall dynamics in the temperature range covering both angular momentum and magnetization compensation points in garnet film, and reaching up to the Curie temperature. Drastic difference in the domain wall mobility and maximum achievable velocity in the vicinities of these two compensation points is demonstrated. Also a remarkably high mobility in weak applied magnetic fields is indicated.

In this work, we provide investigations of nonlinear spin wave propagation in magnonic superlattice. Magnonic superlattice is a ferromagnetic film loaded on a dielectric ceramic substrate with the copper grating with two periods. It is shown that a superlattice with two periods allows the formation of narrower bandgaps as compared to magnonic crystal with one period. Nonlinear effects, such as the generation of spin wave excitations at half-frequency, caused by first-order parametric processes, and a nonlinear shift of the bandgap, caused by a change in the longitudinal component of magnetization with an increase in the input signal power, are revealed. For the experimental study, the Brillouin light scattering spectroscopy was used, and the theoretical model was built using the transfer matrix method.

International audience; We report on the investigations of the magnetic properties and ferromagnetic resonance in the het-erostructure consisting of an epitaxial yttrium iron garnet (Y 3 Fe 5 O 12) film and a nanometer rare-earth inter-metallic superlattice comprised of the (TbCo 2 /FeCo) n exchange-coupled layers. The (TbCo 2 /FeCo) n super-lattice exhibits the giant magnetostriction and the controlled magnetic anisotropy induced by a magnetic field or elastic stresses. The magnetic interaction of the films in the (TbCo 2 /FeCo) n /Y 3 Fe 5 O 12 heterostructure has been experimentally established and the spin current flowing through their interface has been detected.

International audience; Results are presented from an experimental study of the equilibrium distribution of magnetization and processes of magnetization reversal of magnetoelastic nanostructures in the form of strips with constant or variable width. It is shown that the symmetry of the stable magnetization states in the nanostrip can be broken by a static magnetic field applied perpendicular to the ferromagnet's easy axis. A further change in the states of magnetization allows their manipulation (e.g., moving the domain wall) using the homogeneous mechanical effects induced by applying an electric field to the piezoelectric substrate.

It is shown theoretically that in a layered heterostructure comprising piezoelectric, dielectric antiferromagnetic crystal, and heavy metal (PZ/AFM/HM), it is possible to control the anisotropy of the AFM layer by applying a dc voltage across the PZ layer. In particular, we show that by varying the dc voltage across the heterostructure and/or the dc current in the HM, it is possible to vary the frequency of the antiferromagnetic resonance of the AFM in a passive (subcritical) regime and, also, to reduce the threshold of the current-induced terahertz-frequency generation. Our analysis also shows that, unfortunately, the voltage-induced reduction of the generation threshold leads to the proportional reduction of the amplitude of the terahertz-frequency signal generated in the active (supercritical) regime. The general results are illustrated by a calculation of the characteristics of experimentally realizable PZT-5H/$\mathrm{Ni}\mathrm{O}$/$\mathrm{Pt}$.

The theoretical and experimental studies of the excitation, detection, and propagation of magnons in antiferromagnetic nanostructures have been reviewed. The properties of antiferromagnetic materials such as the absence of a macroscopic magnetization, the presence of strong exchange interactions, and a complex magnetocrystalline structure make it possible to implement new types of memory and functional electronic devices. The study of possible magnon effects in antiferromagnetic materials in micro- and nanoscales requires new experimental and theoretical approaches. In this review, the recent results on the excitation of magnetic oscillations—magnons—in antiferromagnetic materials induced by the current and optical radiation are described and systematized. The main theoretical results on antiferromagnets and multilayer antiferromagnetic heterostructures are presented. Models for description of phenomena induced by the current and optical pulses in nanoheterostructures including antiferromagnets are considered. Methods for studying antiferromagnetic micro- and nanostructures by means of Brillouin scattering, as well as prospects of the application of antiferromagnetic spintronics and magnonics, are briefly discussed.