Gap electroacoustic waves inPT-symmetric piezoelectric heterostructure near the exceptional point

The spectral properties of gap electroacoustic waves in aPT-symmetric structure of piezoelectrics of symmetry class 6mm separated by a gap are theoretically investigated. The spectra were calculated for lead germanate (non-zero transverse piezoactivity) and barium titanate (symmetry class 4mm - zero transverse piezoactivity). It has been established that at a certain level of losses and gain in piezoelectrics, the symmetric and antisymmetric modes intersect. The intersection point determines the singular point of thePT-symmetric structure. Beyond this point, there is a violation of the symmetric and antisymmetric distribution of electric fields in the gap of the slotted structure of two identical piezoelectrics, which is confirmed by the calculation of the electric field profiles. It is shown that the dependence of the amplitude on the frequency at an exceptional point has an extremely narrow resonance peak, which opens up the possibility of creating supersensitive sensors based onPT-symmetric physical structures.

Hypersonic magnetoelastic waves in inhomogeneous structures

This work focuses on the propagation of surface magnetoelastic waves in a inhomogeneous structure containing ferromagnetic layer of smooth and slowly variable thickness. The dispersion relation of magnetoelastic waves and the wave attenuation due to the thickness variation in the studied structure were analytically obtained. We found that the magnetoelastic resonance frequency is changed at different positions in this structure. Then proposed the idea of modifying the magnetic field to control the magnetoelastic resonance region, and finally considered the possible applications of this structure for signal processing.

The 2021 Magnonics Roadmap

Magnonics is a budding research field in nanomagnetism and nanoscience that addresses the use of spin waves (magnons) to transmit, store, and process information. The rapid advancements of this field during last one decade in terms of upsurge in research papers, review articles, citations, proposals of devices as well as introduction of new sub-topics prompted us to present the first roadmap on magnonics. This is a collection of 22 sections written by leading experts in this field who review and discuss the current status besides presenting their vision of future perspectives. Today, the principal challenges in applied magnonics are the excitation of sub-100 nm wavelength magnons, their manipulation on the nanoscale and the creation of sub-micrometre devices using low-Gilbert damping magnetic materials and its interconnections to standard electronics. To this end, magnonics offers lower energy consumption, easier integrability and compatibility with CMOS structure, reprogrammability, shorter wavelength, smaller device features, anisotropic properties, negative group velocity, non-reciprocity and efficient tunability by various external stimuli to name a few. Hence, despite being a young research field, magnonics has come a long way since its early inception. This roadmap asserts a milestone for future emerging research directions in magnonics, and hopefully, it will inspire a series of exciting new articles on the same topic in the coming years.

Magnetic Direct-Write Skyrmion Nanolithography

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.

Manipulation of the Dzyaloshinskii-Moriya Interaction in Co/Pt Multilayers with Strain

Interfacial Dzyaloshinskii-Moriya interaction (DMI) is experimentally investigated in Pt/Co/Pt multilayer films under strain. A strong variation (from 0.1 to 0.8  mJ/m^{2}) of the DMI constant is demonstrated at ±0.1% in-plane uniaxial deformation of the films. The anisotropic strain induces strong DMI anisotropy. The DMI constant perpendicular to the strain direction changes sign, while the constant along the strain direction does not. Estimates show that the DMI can be controlled with an electric field in hybrid ferroelectric-ferromagnetic systems. So, the observed effect opens the way to control the DMI and eventually skyrmions with a voltage via a strain-mediated magnetoelectric coupling.

Exact equations for averaged electromagnetic field and its fluctuations at wave multiple scattering by plane periodic array of magnetic microelements

Electromagnetic wave (EM) multiple scattering by a plane periodic array of magnetic microelements in free space is considered analytically by natural subdividing of the EM wave into the averaged and fluctuation components. Each magnetic element is characterized by magnetic susceptibility tensor and shape. An exact Dyson integral equation is derived for the magnetic field Floquet-Bloch amplitude in-plane averaged over an array unit cell. The mass operator of the Dyson equation is expressed via the T-scattering operator of the array unit cell that satisfies a type of the Lippmann-Schwinger equation. We showed that magnetic field fluctuations are generated by the Bragg-Laue diffraction of an averaged magnetic field on the periodic array and are described inside the array as waves propagating with the Laue wave vectors equal to the difference between the in-plane wave vector of the incident magnetic field and the reciprocal lattice wave vector. We derived, for the first time, an exact quadrature to calculate magnetic field fluctuations from their averaged value. These general results are illustrated by a simple Born approximation. In particular, we revealed a mechanism of discrete waveguide excitation by an incident plane EM wave via the averaged EM wave Brag-Laue diffraction on the magnetic microelement array in the quasi-static approach when the wavelength of incident EM is much larger than the sizes of magnetic elements and periods of the array. The mode energy excitation coefficient at normal incidence of the plane EM wave on the array is evaluated.

Magnon Straintronics: Reconfigurable Spin-Wave Routing in Strain-Controlled Bilateral Magnetic Stripes

We observe and explain theoretically strain-induced spin-wave routing in the bilateral composite multilayer. By means of Brillouin light scattering and microwave spectroscopy, we study the spin-wave transport across three adjacent magnonic stripes, which are strain coupled to a piezoelectric layer. The strain may effectively induce voltage-controlled dipolar spin-wave interactions. We experimentally demonstrate the basic features of the voltage-controlled spin-wave switching. We show that the spin-wave characteristics can be tuned with an electrical field due to piezoelectricity and magnetostriction of the piezolayer and layered composite and mechanical coupling between them. Our experimental observations agree with numerical calculations.

Pinning and hysteresis in the field dependent diameter evolution of skyrmions in Pt/Co/Ir superlattice stacks

We have imaged Néel skyrmion bubbles in perpendicularly magnetised polycrystalline multilayers patterned into 1 µm diameter dots, using scanning transmission x-ray microscopy. The skyrmion bubbles can be nucleated by the application of an external magnetic field and are stable at zero field with a diameter of 260 nm. Applying an out of plane field that opposes the magnetisation of the skyrmion bubble core moment applies pressure to the bubble and gradually compresses it to a diameter of approximately 100 nm. On removing the field the skyrmion bubble returns to its original diameter via a hysteretic pathway where most of the expansion occurs in a single abrupt step. This contradicts analytical models of homogeneous materials in which the skyrmion compression and expansion are reversible. Micromagnetic simulations incorporating disorder can explain this behaviour using an effective thickness modulation between 10 nm grains.

Anisotropic tapered polarization-maintaining large mode area optical fibers

We demonstrate a novel type of tapered large mode area polarization-maintaining fiber. These birefringent fibers have an elliptical inner cladding and a core diameter that increases adiabatically from 8 µm to 70 µm. The polarization maintaining ability of the fiber samples was investigated by measuring the spatial distribution of polarization beat length by using optical frequency-domain reflectometry. The measurements show a clear correlation between the birefringence and the fiber core size, resulting in a modest 10-15% variation in polarization beat length along the fiber. There is no significant coupling of polarization modes or transverse modes in the tested fibers and, therefore, the linear polarization state of propagating light is preserved.

Optical theorem for electromagnetic field scattering by dielectric structures and energy emission from the evanescent wave

We present an optical theorem for evanescent (near field) electromagnetic wave scattering by a dielectric structure. The derivation is based on the formalism of angular spectrum wave amplitudes and block scattering matrix. The optical theorem shows that an energy flux is emitted in the direction of the evanescent wave decay upon scattering. The energy emission effect from an evanescent wave is illustrated in two examples of evanescent wave scattering, first, by the electrical dipole and, second, one-dimensional grating with line-like rulings. Within the latter example, we show that an emitted energy flux upon evanescent wave scattering can travel through a dielectric structure even if the structure has a forbidden gap in the transmission spectrum of incident propagating waves.