Non-resonant wave front reversal of spin waves used for microwave signal processing

It is demonstrated that non-resonant wave front reversal (WFR) of spin-wave pulses caused by pulsed parametric pumping can be effectively used for microwave signal processing. When the frequency band of signal amplification by pumping is narrower than the spectral width of the signal, the non-resonant WFR can be used for the analysis of the signal spectrum. In the opposite case the non-resonant WFR can be used for active (with amplification) filtering of the input signal.Comment: 4 pages, 3 figure

Field-induced transition from parallel to perpendicular parametric pumping for a microstrip transducer

Microstrip transducers used for the excitation of spin waves in magnetic films possess two characteristic properties: high spatial localization of the microwave magnetic field and the presence of field components parallel and perpendicular to the bias field. Here, the effects of these features on the process of parametric pumping are presented. By microwave measurements of the spin-wave instability threshold a transition from parallel pumping to perpendicular pumping at the critical field $H_{\rm c}$ with the minimal threshold is observed. This transition is accompanied by a sharp threshold increase above the critical field due to the spatial confinement of the pump region.Comment: 4 pages, 2 figure

Dispersion Characteristics of Spin-Electromagnetic Waves in Planar Multiferroic Structures with Coplanar Transmission Line

Introduction. The distinctive feature of a coplanar transmission line with thin ferrite and ferroelectric films is the absence of undesirable irregularities in dispersion for relatively low frequencies when the wavelength approaches the thickness of ferroelectric layer, in contrast to the open ferrite-ferroelectric wave-guiding structure without metallization.Aim. The purpose of this paper is twofold: (i) to develop a theory of the wave spectra in the multiferroic structures based on the coplanar lines; (ii) using this theory to find ways to enhance the electric tuning range.Materials and methods. The dispersion relation for spin-electromagnetic waves was derived through analytical solution of the full set of the Maxwell's equations utilizing a method of approximate boundary conditions.Results. A theory of spin-electromagnetic wave spectrum has been developed for the thin-film ferrite-ferroelectric structure based on a coplanar transmission line. According to this theory, dispersion characteristics of the spin-electromagnetic waves were described and analyzed for different parameters of the structure. The obtained results show that the investigated structure demonstrates a dual electric and magnetic field tunability of wave spectra. Its efficiency increases with an increase in the thicknesses of the ferrite and ferroelectric films and with a decrease in the width of the central metal strip.Conclusion. The distinctive features of the proposed coplanar waveguides are the thin-film planar topology and dual tunability of the wave spectra. All these advantages make the proposed structures perspective for development of new microwave devices.Introduction. The distinctive feature of a coplanar transmission line with thin ferrite and ferroelectric films is the absence of undesirable irregularities in dispersion for relatively low frequencies when the wavelength approaches the thickness of ferroelectric layer, in contrast to the open ferrite-ferroelectric wave-guiding structure without metallization.Aim. The purpose of this paper is twofold: (i) to develop a theory of the wave spectra in the multiferroic structures based on the coplanar lines; (ii) using this theory to find ways to enhance the electric tuning range.Materials and methods. The dispersion relation for spin-electromagnetic waves was derived through analytical solution of the full set of the Maxwell's equations utilizing a method of approximate boundary conditions.Results. A theory of spin-electromagnetic wave spectrum has been developed for the thin-film ferrite-ferroelectric structure based on a coplanar transmission line. According to this theory, dispersion characteristics of the spin-electromagnetic waves were described and analyzed for different parameters of the structure. The obtained results show that the investigated structure demonstrates a dual electric and magnetic field tunability of wave spectra. Its efficiency increases with an increase in the thicknesses of the ferrite and ferroelectric films and with a decrease in the width of the central metal strip.Conclusion. The distinctive features of the proposed coplanar waveguides are the thin-film planar topology and dual tunability of the wave spectra. All these advantages make the proposed structures perspective for development of new microwave devices

Theory for a dissipative droplet soliton excited by a spin torque nanocontact

A novel type of solitary wave is predicted to form in spin torque oscillators when the free layer has a sufficiently large perpendicular anisotropy. In this structure, which is a dissipative version of the conservative droplet soliton originally studied in 1977 by Ivanov and Kosevich, spin torque counteracts the damping that would otherwise destroy the mode. Asymptotic methods are used to derive conditions on perpendicular anisotropy strength and applied current under which a dissipative droplet can be nucleated and sustained. Numerical methods are used to confirm the stability of the droplet against various perturbations that are likely in experiments, including tilting of the applied field, non-zero spin torque asymmetry, and non-trivial Oersted fields. Under certain conditions, the droplet experiences a drift instability in which it propagates away from the nanocontact and is then destroyed by damping.Comment: 15 pages, 12 figure

Generation of spin-wave dark solitons with phase engineering

We generate experimentally spin-wave envelope dark solitons from rectangular high-frequency dark input pulses with externally introduced phase shifts in yttrium-iron garnet magnetic fims. We observe the generation of both odd and even numbers of magnetic dark solitons when the external phase shift varies. The experimental results are in a good qualitative agreement with the theory of the dark-soliton generation in magnetic films developed earlier [Phys. Rev. Lett. 82, 2583 (1999)].Comment: 6 pages, including 7 figures, submitted to Phys. Rev.

Analytical expression of the magneto-optical Kerr effect and Brillouin light scattering intensity arising from dynamic magnetization

Time-resolved magneto-optical Kerr effect (MOKE) and Brillouin light scattering (BLS) spectroscopy are important techniques for the investigation of magnetization dynamics. Within this article, we calculate analytically the MOKE and BLS signals from prototypical spin-wave modes in the ferromagnetic layer. The reliability of the analytical expressions is confirmed by optically exact numerical calculations. Finally, we discuss the dependence of the MOKE and BLS signals on the ferromagnetic layer thickness

Аналитическая теория дисперсии оптических волн регулярных микроволноводов

A method for analysis of dispersion characteristics of guided optical modes propagating in the optical waveguides with small cross-sections is proposed. The method is based on introduction of a correction factor for a longitudinal wavenumber of propagating modes. The correction factor arises when a cross-section of the basic rectangular waveguide is subjected to perturbation. The electromagnetic field distributions along with the mode longitudinal wavenumber are found by means of variable separation method. The longitudinal wavenumber correction factor is analytically calculated in terms of coupled mode theory. The combined use of the complete set of equations of electrodynamics together with the concept of effective sources gives rise to the correction factor in the form of an intermodal coupling coefficient. It is pointed out that the coupling coefficient consists of two components, namely bulk and surface, owing to accurate account of the electrodynamics boundary conditions. Using the method proposed, the dispersion characteristics of the fundamental modes propagating in the practically employed optical waveguides having a trapezoidal cross-section are calculated. An impact of the waveguide cross-section shape to cladding dielectric constant ratio on the mode dispersion characteristics is analyzed. The necessity to take into consideration an imperfection of the waveguide cross-section in a wide range of operating wavelengths is demonstrated.Разработан метод анализа дисперсионных характеристик направляемых мод в регулярных оптических микроволноводах малого поперечного сечения. Метод основан на введении поправок к продольному волновому числу мод прямоугольного волновода, выбранного в качестве базового, при искажении формы его поперечного сечения. Распределения электромагнитного поля и продольного волнового числа базового волновода рассчитываются методом разделения переменных. Поправка к продольному волновому числу рассчитывается аналитически в терминах теории связанных мод. Указанная поправка в виде коэффициента межмодовой связи возникает на основании совместного использования полной системы уравнений Максвелла при введении понятия об эффективных источниках. Показано, что последовательный учет граничных условий электродинамики приводит к форме коэффициента связи, включающей объемную и поверхностную составляющие. Разработанный метод применен для расчета дисперсионных характеристик низших волноводных мод, распространяющихся в микроволноводах трапециевидного сечения, применяемых на практике. Продемонстрировано влияние поперечного сечения микроволновода на дисперсионные характеристики мод в зависимости от соотношения сторон, а также от отношения значений диэлектрических проницаемостей сердцевины микроволновода и его оболочки. Показана необходимость учета влияния формы микроволновода на дисперсионные характеристики мод в широком диапазоне значений рабочих длин волн и при различных распределениях диэлектрической проницаемости волноведущей структуры

Brillouin light scattering study of Co2_{2}Cr0.6_{0.6}Fe0.4_{0.4}Al and Co2_{2}FeAl Heusler compounds

The thermal magnonic spectra of Co$_{2}$Cr$_{0.6}$Fe$_{0.4}$Al (CCFA) and Co$_2$FeAl were investigated using Brillouin light scattering spectroscopy (BLS). For CCFA, the exchange constant A (exchange stiffness D) is found to be 0.48 $\mu$erg/cm (203 meV A$^2$), while for Co$_2$FeAl the corresponding values of 1.55 $\mu$erg/cm (370 meV A$^2$) were found. The observed asymmetry in the BLS spectra between the Stokes and anti-Stokes frequencies was assigned to an interplay between the asymmetrical profiles of hybridized Damon-Esbach and perpendicular standing spin-wave modes, combined with the optical sensitivity of the BLS signal to the upper side of the CCFA or Co$_2$FeAl film