The development of spintronic emitters of broadband THz pulses relies on designing heterostructures where processes of laser-driven spin current generation and subsequent spin-to-charge current conversion are the most efficient. An interface between ferromagnetic and nonmagnetic layers in the emitter is one of the critical elements. Here, we study experimentally single-cycle THz pulse generation from a laser-pulse excited Pt/Co emitter with a composition gradient interface between Pt and Co and compare it with the emission from a conventional Pt/Co structure with an abrupt interface. We find that the gradient interface enhances the efficiency of optics-to-THz conversion by a factor of two in a wide range of optical fluences up to 3 mJ cm^-2. We reveal that this enhancement is caused by a pronounced increase in transmittance of the laser-driven spin-polarized current through the gradient interface compared to the abrupt one. Furthermore, we find that such a transmission deteriorates with laser fluence due to the spin accumulation effect.

The article focuses on the micromagnetic modeling of the nucleation, energetic stability regions, and dynamic of chiral spin textures in thin-film heavy-metal/ferromagnetic (FM) systems with Dzyaloshinskii–Moriya interaction. A simple two-layer model for analyzing ferrimagnetic (FIM) structures is proposed. The analytical description and simulation of the mechanisms of stabilization and current-induced motion of skyrmions in both FM and FIM films are carried out, depending on the parameters of the magnetic model and the external magnetic field. The influence of the magnetization, magnetic anisotropy, and damping on the parameters and velocity of skyrmions (e.g. size, stability regions etc.) in thin-film structures is estimated. The specific dynamics of skyrmion motion under the influence of spin-polarized current as a function of the magnitude and sign of the Dzyaloshinskii–Moriya interaction is also identified. The results demonstrate the promising potential of FIM films for skyrmions stabilization and investigation of its dynamics.

Here we report recent data on the phenomenon of total nonreflection of spin waves (NRSWs). Using space- and phase-resolved Brillouin light scattering we experimentally studied the amplitude, direction, and phase of the NRSW in yttrium iron garnet films with artificial defects---an antidot array. An NRSW in the form of a focused beam of high intensity with the group velocity pointing along the line of defects is observed only for a critical angle between spin-wave phase velocity and an antidot array direction in a magnetic field. It is shown that the NRSW is an independent noncollinear spin wave arising as a result of the anisotropic nature of the propagation of surface magnetostatic spin waves in a magnetic medium. The numerical simulations agree well with experimental data. Micromagnetic simulations performed let us elucidate the phenomenon of the NRSW and map the spin-wave profile of dynamic magnetization inside the NRSW. The estimated mean free path of the NRSW reaches about 1 mm in the yttrium iron garnet film with a thickness of a few micrometers. These results demonstrate a unique approach to focusing and enhancing spin waves in magnetics through the use of artificial defects.

Micromagnetic modeling of non-linear autoresonance magnetization oscillations in thin films of yttrium iron garnet (YIG) with specified growth directions is conducted. It is found that in the case of rapid frequency modulation (sweep rate of the order of 1016 Hz/sec) of 1Oe excitation magnetic field, the maximum precession angle of magnetization can achieve up to 160 degrees. For the first time, the influence of demagnetization fields, magneto-crystalline anisotropy, and Gilbert damping on autoresonance phenomena in YIG films is numerically calculated. It is shown that demagnetization fields and damping have a weak influence on parameters of autoresonance. Simultaneously, damping provides a shorter phase-locking time between the excitation field and intrinsic magnetization oscillations in the film, favoring high amplitude of magnetization oscillations. The magneto-crystalline anisotropy leads to a reduction of the threshold sweep rate of the pumping field for YIG films with [100] direction, as well as the emergence of parametric instability for [210] films. The results of the work are aimed to be applied for the experimental observation of autoresonance phenomena in thin yttrium iron garnet films.

Multilayered metallic nanostructures are promising for the fabrication of spin valves based on the giant magnetoresistive effect and for studies of the nature of topological magnetism, aimed at the development of new nanoscale data storage and transfer devices, e.g. those based on magnetic skyrmions. It is still an important task to develop methods of synthesis and configuration of thin-film nanostructures and control of spin textures in those nanostructures under electric and spin currents generated as a result of the spin Hall effect in external electric fields. Thin-film polycrystalline ferromagnetic / heavy metal Ru(10nm)/Co(0,8)/Ru(2), Ru(10)/Co(0,8)/Ru(2)/W(4), Pt(5)/Co(0,8)/MgO(2)/Pt(2) and Pt(15)/Co(0,8)/MgO(2)/Pt(2) nanostructures have been synthesized using magnetron sputtering. Electric contacts and Hall structures with different conductive bridge thicknesses have been synthesized on the specimens using electron beam photolithography. Experimental vibration magnetometric data have been utilized to calculate magnetic parameters of the specimens, i.e., saturation magnetization, magnetic anisotropy energy and field and coercive force as functions of ferromagnetic and heavy metal layer types. The domain structure of the specimens has been studied using Kerr microscopy. The electrical resistivity has been simulated and the critical current and current density of the nanostructures have been assessed. We show that all the film specimens exhibit perpendicular magnetic anisotropy and can be used in the studies of current-induced phenomena and spin moment transfer processes in nanostructures.

A model for micromagnetic simulation of the magnetization of a ferrimagnetic film consisting of an alloy of ferromagnetic and rare-earth metals is proposed and discussed. It is shown that the model qualitatively replicates the experimentally observed temperature dependencies of the saturation magnetization of various ferrimagnetic alloys for different percentages of the rare-earth element and that it exhibits a similar magnetic hysteresis loop. The results of the study are of interest for the theoretical analysis of the magnetization behavior of ferromagnetic–heavy-metal film nanostructures, as well as for solving problems of applied materials science and magnetism.

We report on the spin-wave propagation along a magnonic waveguide with a local area of decreased magnetization, which is induced by heating produced with a focused laser spot. A phase-sensitive Brillouin light scattering technique is used to image how the spin wave propagates along the waveguide with a local heat landscape. Frequency-selective signal propagation along the waveguide is demonstrated. Micromagnetic simulations reveal intermodal interference variation in the region after the heated area. The proposed way to reconfigure the magnetization landscape can be used in magnonic devices with frequency-selective spin-wave transport.

Nonreciprocity, i.e. inequivalence in amplitudes and frequencies of spin waves propagating in opposite directions, is a key property underlying functionality in prospective magnonic devices. Here we demonstrate experimentally and theoretically a simple approach to induce frequency nonreciprocity in a magnetostatically coupled ferromagnetic bilayer structure with a nonmagnetic spacer by its geometrical asymmetry. Using Brillouin light scattering, we show the formation of two collective spin wave modes in Fe81Ga19/Cu/Fe81Ga19 structure with different thicknesses of ferromagnetic layers. Experimental reconstruction and theoretical modeling of the dispersions of acoustic and optical collective spin wave modes reveal that both possess nonreciprocity reaching several percent at the wavenumber of 22 × 104 rad cm-1. The analysis demonstrates that the shift of the amplitudes of counter-propagating coupled modes towards either of the layers is responsible for the nonreciprocity because of the pronounced dependence of spin wave frequency on the layers' thickness. The proposed approach enables the design of multilayered ferromagnetic structures with a given spin wave dispersion for magnonic logic gates.

Autoresonance is a new non-linear method for excitation of spin subsystem in magnetics by an extremely low magnetic field. Here, we consider the autoresonance (autophasing) process in a model of yttrium iron garnet (YIG) film possessing out-of-plane uniaxial anisotropy. As a result of simulation in MuMax3 software, the parameters of exciting field are determined and a model is proposed for successful auto-phase locking at GHz frequencies. The numerical data obtained for the model with material parameters close to ones for low-damping yttrium iron garnet films are in good agreement with theoretical predictions. It is shown that the process of phase locking leads to a soliton-like character of the excited magnetic oscillations with a high-amplitude of precession. The maximum angle of magnetization deflection reaches up to 150° at the exciting field of 1 mT sweeping with the rate 4.3 × 1016 s−2. It is presented the stability of the autoresonance in the case of low damping in the developed model. Besides, the damping could be used for adjusting the parameters of the autoresonance, which paves the way for potential experimental testing.

A new approach to describing the magnetic properties of FeRh alloys is proposed. It is based on two assumptions about the properties of 3 d and 4 d electrons in these alloys. The first is the assumption that the 4 d band is submerged under the Fermi level to a depth that ensures its complete filling at a temperature T = 0 K. The second is the assumption that there are two different spatial distributions of 3 d and 4 d electrons that are compatible with one atomic structure. The first assumption makes it possible to explain the absence of uncompensated spins of 4 d electrons in the low-temperature antiferromagnetic (AFM) phase of FeRh. The second assumption is proposed to explain the strong changes in the spin structure of the FeRh alloy upon the AFM–FM transition, while its atomic structure is almost unchanged. Attempts have been made to predict the new properties of the FeRh alloy that follow from these assumptions. Our second assumption proved to be successful and enabled us to predict the existence of local magnetic moments of 3 d electrons in the AFM phase, which are larger than the magnetic moments of Fe ions. Measurements of the magnetic susceptibility of the FeRh alloy in the AFM phase confirmed this prediction.

Numerical methods in the MuMAX3 software are used to calculate both the saturation fields and the resonance spectra of magnetization for a thin magnetic plate with perpendicular magnetic anisotropy. The optimal parameters for modeling autoresonance processes in yttrium iron garnet films are determined. The role of surface anisotropy and dipole-dipole interaction in modeling of static and dynamic magnetization in YIG film is discussed.

The strain-magnetooptical properties of single crystals of the ferrimagnetic spinel CoFe2O4, which reflect a correlation between optical properties (magnetoabsorption and magnetoreflection) and magnetostriction, have been studied in the infrared spectral range. The conditions for the observation of the strain-magnetooptics are specified and physical mechanisms responsible for these effects in the spinel are explained.

One of the main problems of magnonics is finding the ways of efficiently spin waves excitation in a magnet. This paper considers the method of nonlinear amplification by phase locking of amplitude of dynamic magnetization in yttrium-iron garnet film performed by micromagnetic modeling with MuMAX(3) software taking into account the real materials parameters. It is shown that the excited magnetization precession can be considered as a autoresonance phenomena. The intensity of the autoresonance in ferrimagnetic yttrium-iron garnet films has threshold dependence on the chirp rate of the exciting magnetic field.

ОСОБЕННОСТИ РАСПРОСТРАНЕНИЯ НЕОТРАЖЕННОЙ ВОЛНЫ В ДЕФЕКТНЫХ ПЛЕНКАХ ЖИГ

The spectral, temperature and field dependences of the Faraday effect for single crystals of ferrimagnetic spinel CoFe2O4 are studied in the infrared range. It is shown that the magneto-optical q-factor of CoFe2O4 varies from -600/dB до +200/dB in the spectral range from 1.5 μm to 10 μm and weakly depends on temperature from 200 K to 300 K. The physical mechanisms responsible for the observed Faraday effect are proposed.

The spectral, temperature, and field dependences of the Faraday effect in the IR spectral range have been studied in single crystals of the CoFe 2 O 4 ferrimagnetic spinel. It is established that the magneto-optical quality factor of CoFe 2 O 4 spinel varies from –60 to +20°/dB in a wavelength range of 1.5–10 μm and weakly depends on the temperature in a 200–300 K interval. Physical mechanisms are proposed that may be responsible for the Faraday effect.

This review presents the data of the experimental investigation of the propagation and decay of magnetostatic spin waves (MSWs) obtained when analyzing the Brillouin light scattering data in yttrium iron garnet films. It is shown that the effect of nonreflected wave connected with artificial antidots in films leads to a significant increase in the intensity and propagation length of the magnetostatic spin waves in yttrium iron garnet.

Trabajo presentado en el 21st International Conference on Magnetism (ICM2018), celebrado en San Francisco (California, EE.UU.), del 15 al 20 de julio de 2018

ДИСПЕРСИЯ СОБСТВЕННЫХ КОЛЕБАНИЙ МАГНИТНЫХ МОМЕНТОВ В ПЛЕНКЕ ПЕРМАЛЛОЯ: ДАННЫЕ МАНДЕЛЬШТАМ-