Magnetostatic Surface Spin Waves Research Articles

Real-time monitoring of breath biomarkers using magnonic wireless sensor based on magnetic nanoparticles

In this paper, an innovative device with gas remote-sensing capability is proposed, which is based on the interaction between magnetic nanoparticles and gases associated with exhaled breath biomarkers that can have a metabolic origin. Magnetite (Fe₃O₄) nanoparticles of around 30 nm have been used. The gas molecules adsorbed on surface modulate the magnetization of the nanoparticles and magnetostatic surface spin waves (MSSW) propagated on an yttrium iron garnet (YIG) thin film are used to detect this modulation by the induced frequency shift. The optimization of the remote gas sensor has been carried out through simulations of a magnetic model. Simulations show the feasibility of developing a high-performance remote sensor by encapsulating the nanostructures in a polytetrafluoroethylene (PTFE) tube and detecting part per million changes in their magnetization. The results show the possibility of developing new, inexpensive, reusable, contactless magnetic gas sensors employing spin waves as transductor. The developed sensor shows a high sensitivity and selectivity to concentrations as low as 50 ppm of different breath biomarkers.

Reconfigurable bandpass filter using Ce-doped YIG ferrites for wideband applications

Integration of compact, tunable, low-cost monolithic microwave integrated circuit-based bandpass filters on a semiconductor substrate has been a persistent requirement in the communication industry. In the present investigation, a compact reconfigurable bandpass filter is fabricated using a double T-transducer based on the phenomena of magnetostatic surface spin waves (MSSWs). The Ce-doped Bi0.1YIG 500 μm thick disks with cerium concentration in the range of 0.2–1.0 provide the path for the propagation of MSSWs. The operating frequency for this device extends from S to X band Rogers RT/duroid® substrate, which is used for planar microstrip filter design, fabrication, and testing for practical applications. The effect of the coupling gap between the two transducers was optimized using appropriate choice of lumped components to improve the performance of the filter. The best coupling was observed for 1.2 mm coupling gap. The simulated and experimental results confirm the lowest insertion loss of −1.24 dB. The operating frequency shows a large tunability of 132.3% with a relatively small bias field of 0.3–2.1 kOe. The promising characteristics of bandpass filters such as low passband insertion loss, narrow bandwidth, high bandpass reflection coefficient, and high band-stop rejection were achieved by varying the Ce concentration in YIG samples.

Unidirectional microwave transduction with chirality selected short-wavelength magnon excitations

Nonreciprocal magnon propagation has recently become a highly potential approach of developing chip-embedded microwave isolators for advanced information processing. However, it is challenging to achieve large nonreciprocity in miniaturized magnetic thin-film devices because of the difficulty of distinguishing propagating surface spin waves along the opposite directions when the film thickness is small. In this work, we experimentally realize unidirectional microwave transduction with sub-micrometer-wavelength propagating magnons in a yttrium iron garnet (YIG) thin-film delay line. We achieve a non-decaying isolation of 30 dB with a broad field-tunable bandpass frequency range up to 14 GHz. The large isolation is due to the selection of chiral magnetostatic surface spin waves with the Oersted field generated from the coplanar waveguide antenna. Increasing the geometry ratio between the antenna width and YIG thickness drastically reduces the nonreciprocity and introduces additional magnon transmission bands. Our results pave the way for on-chip microwave isolation and tunable delay line with short-wavelength magnonic excitations.

Reconfigurable magnon interference by on-chip dynamic wavelength conversion

Spin waves (SWs), an ultra-low power magnetic excitation in ferro or antiferromagnetic media, have tremendous potential as transport less data carriers for post-CMOS technology using their wave interference properties. The concept of magnon interference originates from optical interference, resulting in a historical taboo of maintaining an identical wavevector for magnon interference-based devices. This makes the attainment of on-chip design reconfigurability challenging owing to the difficulty in phase tuning via external fields. Breaking the taboo, this study explores a novel technique to systematically control magnon interference using asymmetric wavevectors from two different SW modes (magnetostatic surface SWs and backward volume magnetostatic SWs) in a microstructured yttrium iron garnet crossbar. Using this system, we demonstrate phase reconfigurability in the interference pattern by modulating the thermal landscape, modifying the dispersion of the interfering SW modes. Thus, we manifest that such a tunable interference can be used to implement reconfigurable logic gates operating between the XNOR and XOR modes by using symmetric and asymmetric interference, respectively.

Spin wave dispersion relation engineering by magnonic crystals with arbitrary symmetry

The use of metasurfaces to engineer the response of magnetic materials is of utmost importance in the field of magnon-spintronics. Here, we demonstrate a method to fabricate one- and two-dimensional magnonic crystals with arbitrary symmetry and use it to engineer the amplitude-frequency characteristic of magnetostatic surface spin waves excited in a magnetic material. The technique is based on the gentle microablation of the sample surface by focused femtosecond laser pulses. Tightly focused illumination allows using modest pulse energy while achieving micrometer precision. By raster scanning the incident laser spot on the sample surface, we control the shape and size of the building blocks constituting the unit cell of the crystal along with its symmetry and lattice parameter. Remarkable and controlled changes in the measured transmission characteristics reveal the strong and complex symmetry-dependent interaction of the spin waves with Bravais and non-Bravais lattices. The described single-step microfabrication method facilitates and speeds up the realization of integrated spintronics components and provides an efficient tool to explore complex magnetic dynamics in scattering lattices.

Excitation and reception of magnetostatic surface spin waves in thin conducting ferromagnetic films by coplanar microwave antennas. Part I: Theory

A fully self-consistent model for the excitation and reception of magnetostatic surface waves in thin ferromagnetic films by a set of coplanar antennas was developed and implemented numerically. The model assumes that the ferromagnetic film is highly conducting and is interfaced with non-magnetic metallic films, but is also suitable for modeling magneto-insulating films. Perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction can be included at both interfaces of the ferromagnetic layer. The model calculates the coupling impedances between the different strips constituting the coplanar antennas. In some situations, this leads to a frequency non-reciprocity between counter-propagating waves even in the case of no asymmetry in the spin-wave dispersion relation. Several intermediate results of the model were checked numerically and the final output of the model, given as the scattering parameters, S11, S12, and S21 of the antenna system, were in good agreement with previous experimental studies.

Excitation and reception of magnetostatic surface spin waves in thin conducting ferromagnetic films by coplanar microwave antennas. Part II: Experiment

We report on propagating spin-wave spectroscopy measurements carried out on coplanar nano-antenna devices made from a Si/SiO2/Ru(5 nm)/Co(20)/Pt(5 nm) film. The measurements were analyzed in detail by employing newly developed theoretical modeling and de-embedding procedures. The magnetic parameters of the film were determined by complementary Brillouin light scattering and ferromagnetic resonance measurements. The propagating spin wave signals could be accounted for quantitatively for the range of externally applied magnetic fields investigated in this study: 130–1500 Oe.

Magnetic metasurfaces with metallic inclusions

Purpose of this paper is the development and creation of the magnetic metasurfaces with metallic inclusions operating both in the microwave and terahertz frequency ranges. Methods. The Maxwell’s equations and the expressions for the effective medium parameters are used to build the analytical models of the magnetic metasurfaces based on either a ferromagnetic (FM) or antiferromagnetic (AFM) dielectric matrix, containing a two-dimensional periodic structure of thin metal (non-magnetic) wires surrounded by insulators. Numerical simulation of such structures operating in the microwave range is carried out using the MaxLLG software package. The magnetron sputtering, liquid etching, optical lithography, and lift-off photolithography are used to create bicomponent magnetic metasurfaces, consisting of two magnetic materials with very different values of magnetization. The study of linear and nonlinear characteristics of the bicomponent magnetic metasurfaces is carried out using the methods of microwave and Brillouin spectroscopy. Results. Based on the developed analytical model of the magnetic metasurface with metallic (nonmagnetic) inclusions it is shown that the FM metasurface possesses properties of a left-handed medium in a microwave range and the AFM metasurface possesses similar properties in a terahertz range. In the last case, the material parameters of the AFM metasurface are twice negative in two frequency bands. For the magnetic metasurfaces with metallic magnetic inclusions, the formation of absorption bands in the spectrum of a traveling magnetostatic surface spin wave due to the resonant properties of the inclusions has been established. In the nonlinear regime, the effect of nonreciprocal parametric three-wave resonance was obtained. Conclusion. The results presented in the paper demonstrate a number of physical phenomena that are observed only in the magnetic metasurfaces with metallic (nonmagnetic and magnetic) inclusions.

Spectrum evolution and chirping of laser-induced spin wave packets in thin iron films

We present an experimental study of ultrafast optical excitation of magnetostatic surface spin wave (MSSW) packets and their spectral properties in thin films of pure iron. As the packets leave the excitation area and propagate in space, their spectra evolve non-trivially. Particularly, low or high frequency components are suppressed at the border of the excitation area depending on the orientation of the external magnetic field with respect to the magnetocrystalline anisotropy axes of the film. The effect is ascribed to the ultrafast local heating of the film. Furthermore, the time resolution of the implemented all-optical technique allows us to extract the chirp of the MSSW packet in the time domain via wavelet analysis. The chirp is a result of the group velocity dispersion of the MSSW and, thus, is controlled by the film's magnetic parameters, magnetization and anisotropy, and external field orientation. The demonstrated tunable modulation of MSSW wave packets with femtosecond laser pulses may find application in future magnonic-photonic hybrid devices for wave-based data processing.

Surface Spin-Wave Propagation in the Orthogonal Transverse Junction of YIG-Based Magnonic Stripes

In this article, we experimentally demonstrate the propagation of magnetostatic surface spin wave (MSSW) in the orthogonally joined yttrium iron garnet stripes in the form of T-shaped connection. By the means of microwave spectroscopy technique, we show how the transmission is affected by the distance between orthogonal yttrium iron garnet stripes. Micromagnetic modeling demonstrates the presence of nonreciprocal propagation of spin waves (SWs) between all three ports of the T-junction. A mechanism for controlling the spatial-frequency selection of a propagating signal by variation of the air gap in the area of connected orthogonal sections is shown. The proposed structure can act as a magnonic splitter for 3-D topology of multilevel magnonic networks (MNs). The transition to a multilayer topology of MNs opens the possibility of increase of the packing density of computational units based on the principles of magnonics.

Особенности распространения спиновых волн в магнонных кристаллах с неоднородным распределением намагниченности по толщине

With the help of micromagnetic modeling, we considered particularities of dispersions and amplitude-frequency response of spin waves in a magnonic crystal (MC) formed by etching an array of grooves in the surface of yttrium iron garnet film having linear distribution of magnetization across the thickness from 1.7 kG at the upper surface till 2.02 kG at the bottom. For the geometry of surface magnetostatic spin waves (MSSW), it is shown that nonuniformity of magnetization distribution across the thickness leads to the appearance of frequency regions in the MC spectrum where MSSW propagation is unidirectional and, as the consequence, does not have Bragg resonances. It was also demonstrated that the MC spectrum is determined by the choice of surface used for the formation of the array of grooves.

Particularities of spin wave propagation in magnonic crystals with nonuniform magnetization distribution across the thickness

With the help of micromagnetic modeling, we considered particularities of dispersions and amplitude-frequency response of spin waves in a magnonic crystal (MC) formed by etching an array of grooves in the surface of yttrium iron garnet film having linear distribution of magnetization across the thickness from 1.7 kG at the upper surface till 2.02 kG at the bottom. For the geometry of surface magnetostatic spin waves (MSSW), it is shown that nonuniformity of magnetization distribution across the thickness leads to the appearance of frequency regions in the MC spectrum where MSSW propagation is unidirectional and, as the consequence, does not have Bragg resonances. It was also demonstrated that the MC spectrum is determined by the choice of surface used for the formation of the array of grooves. Keywords: spin wave, micromagnetic modelling, film of yttrium iron garnet, magnonic crystal.

Magnonic Crystal with Strips of Magnetic Nanoparticles: Modeling and Experimental Realization via a Dip-Coating Technique

In this article, we describe a magnonic crystal formed by magnetite nanoparticles. The periodic strip-like structure of the nanoparticles was fabricated on the surface of thin yttrium iron garnet single-crystal film grown on a gallium–gadolinium garnet substrate via dip-coating techniques. It was shown that such periodic structure induces the formation of the bandgaps in the transmission spectra of magnetostatic surface spin-waves (MSSW). The structure was simulated by the transfer matrix method. Spin-wave detection has been carried out by using a pair of microwave antennas and a vector network analyzer.

Space-Quasiperiodic and Time-Chaotic Parametric Patterns in a Magnonic Quasicrystal Active Ring Resonator

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.

Spin current and spin waves at a platinum/yttrium iron garnet interface: impact of microwave power and temperature

The spectra of ferromagnetic resonance and spin current of the heterostructure consisting of an epitaxial film of yttrium iron garnet Y3Fe5O12 (YIG) grown on a gallium gadolinium garnet (Gd3Ga5O12) substrate and with 10 nm thick platinum (Pt) film on top were studied at frequencies f = 2–18 GHz under applied power in the range of 20 μW–50 mW as well as temperatures T = 77–300 K. The standing spin-wave resonances (SWRs) with both magnetostatic backward volume spin waves and magnetostatic surface spin waves (MSSWs) were detected. The SWR evolution was investigated with change of microwave power. Due to generation of spin current perpendicular to the Pt/YIG interface and the inverse spin Hall effect the charge current on the Pt film was registered and analyzed. The SWR resulted in deviations of spin current spectral lines from a Lorentz line form, indicating that at high microwave power the main contribution is made by MSSW. As the temperature decreases, the amplitude of the spin current decreases, although the width of the spectral line remains the same while the magnetization increases at lower temperatures. A decrease in spin Hall angle with cooling may explain this behavior of the spin current at low temperatures.

Implementing a Magnonic Reservoir Computer Model Based on Time-Delay Multiplexing

Recently we demonstrated experimentally that microwave oscillators based on the time delay feedback provided by traveling spin waves could operate as reservoir computers. In the present paper, we extend this concept by adding the feature of time multiplexing made available by the large propagation times/distances of traveling spin waves. The system utilizes the nonlinear behavior of propagating magnetostatic surface spin waves in a yttrium-iron garnet thin film and the time delay inherent in the active ring configuration to process time dependent data streams. Higher reservoir dimensionality is obtained through the time-multiplexing method, emulating "virtual" neurons as temporally separated spin-wave pulses circulating in the active ring below the auto-oscillation threshold. To demonstrate the efficacy of the concept, the active ring reservoir computer is evaluated on the short-term memory and parity check benchmark tasks, and the physical system parameters are tuned to optimize performance. By incorporating a reference line to mix the input signal directly onto the active ring output, both the amplitude and phase nonlinearity of the spin waves can be exploited, resulting in significant improvement on the nonlinear parity check task. We also find that the fading memory capacity of the system can be easily tuned by controlling the active ring gain. Finally, we show that the addition of a second spin-wave delay line configured to transmit backward volume spin waves can partly compensate dispersive pulse broadening and enhance the fading memory capacity of the active ring.

Local heat emission due to unidirectional spin-wave heat conveyer effect observed by lock-in thermography

Lock-in thermography measurements were performed to reveal heat source distribution induced by the unidirectional spin-wave heat conveyer effect (USHCE) of magnetostatic surface spin waves. When the magnetostatic surface spin waves are excited in an yttrium iron garnet slab, the lock-in thermography images show spatially biased sharp and complicated heating patterns, indicating the importance of edge spin-wave dynamics for USHCE. The accessibility to the local heat emission properties allows us to clarify a capability of remote heating realized by USHCE; it can transfer energy for heating even through a macro-scale air gap between two magnetic materials owing to the long-range dipole–dipole coupling.

Dispersion relation of nutation surface spin waves in ferromagnets

Inertia effects in magnetization dynamics are theoretically shown to result in a different type of spin waves, i.e. nutation surface spin waves, which propagate at terahertz frequencies in in-plane magnetized ferromagnetic thin films. Considering the magnetostatic limit, i.e. neglecting exchange coupling, we calculate dispersion relation and group velocity, which we find to be slower than the velocity of conventional (precession) spin waves. In addition, we find that the nutation surface spin waves are backward spin waves. Furthermore, we show that inertia causes a decrease of the frequency of the precession spin waves, namely magnetostatic surface spin waves and backward volume magnetostatic spin waves. The magnitude of the decrease depends on the magnetic properties of the film and its geometry.

Ferromagnetic-waveguide width dependence of propagation properties for magnetostatic surface spin waves

The width dependence of propagation properties for magnetostatic surface spin waves (MSSWs) in Permalloy (Py) waveguides was investigated experimentally and numerically. When both the short-side width of a Py waveguide (WPy) and the magnetic field (μ0H) were large enough, the spin wave (SW) intensity almost linearly decreased with decreasing WPy, and the resonant frequency fr was almost independent of WPy. However, when both WPy and μ0H were small, the SW intensity rapidly reduced, and fr decreased with decreasing WPy. Micromagnetic simulations revealed that when both WPy and μ0H are large enough, the magnetization direction of Py is almost parallel to the magnetic field direction, and the SW mode becomes the MSSW mode. In contrast, when both WPy and μ0H are small, magnetization direction tilts to the long-side direction of the Py strip from the magnetic field direction due to a strong demagnetizing field, which is the origin of the unexpected reduction in the intensity and fr. When miniaturized ferromagnetic waveguides are used for shrinking MSSW devices, a sufficiently large magnetic field is necessary because the influence of the shape magnetic anisotropy cannot be ignored.

2D Surface Spin Waves in Dynamic Magnonic Crystals Created by a Surface Acoustic Wave in YIG Films

Some results of the study of 2D propagation of surface magnetostatic spin waves (SMSW) in dynamic magnonic crystals (DMC) created by a surface acoustic wave (SAW) in a structure with a yttrium-iron garnet (YIG) film are presented. Such studies are interesting from the practical point of view of creating real relatively complex devices on spin waves when they propagate in different directions, and not only in the 1D case. The methods of experimental research are presented, in particular, the features of the method of excitation of SAW in the structure that do not lead to the appearance of additional magnetic anisotropy in it, a method for measuring the angular dependencies of SMSW using mobile antenna–probes is presented. The angular dependences of the magnonic band gap frequencies are measured. It is established that the transmission bands with the transformation of the reflected SMSW into other types of magnetostatic waves (MSW) exist at any angle values, while the intervals in which there are no SMSW transformations during reflections occur in a certain narrower range of angles. The angles of the directions of the wave vectors and the Poynting vector of the reflected SMSW were also measured. A satisfactory agreement was obtained with the calculation performed using the method of isofrequency curves and the laws of inelastic scattering SMSW on SAW.