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Scanning transmission x-ray microscopy is employed to investigate experimentally the reversal of the magnetic vortex core polarity in cylindrical Ni81Fe19 nanodisks triggered by two orthogonal monopolar magnetic field pulses with peak amplitude B0, pulse length κ = 60 ps, and delay time Δt in the range from -400 ps to +400 ps between the two pulses. The two pulses are oriented in-plane in the x- and y-directions. We have experimentally studied vortex core reversal as a function of B0 and Δt. The resulting phase diagram shows large regions of unidirectional vortex core switching where the switching threshold is modulated due to resonant amplification of azimuthal spin waves. The switching behavior changes dramatically depending on whether the first pulse is applied in the x- or the y-direction. This asymmetry can be reproduced by three-dimensional micromagnetic simulations but not by two-dimensional simulations. This behavior demonstrates that in contrast to the previous experiments on vortex core reversal, the three-dimensionality in the dynamics is essential here. [ABSTRACT FROM AUTHOR]

Filtering antennas or filtennas realize both antennas and filter functions in a single structure with the sole purpose of reducing losses and size in the design of a radio system. This article presents a comprehensive view of various filtennas design techniques and their types, satisfying the requirements of different wireless communication standards. The codesign and synthesis approaches and multilayer structure, slot/slit, and parasitic elements are the frequently used techniques reported in the literature to date for the filtennas design. The codesign and synthesis approaches need an extra filter in the filtenna's design, thus making it more complex. Whereas multilayer, slot, and parasitic elements do not require additional filters, and help the radio design become compact in size. The individual elements, that is, filter and antenna used in the filtenna are designed by cutting slots, stacking, resonators, or metamaterial structures. There are two broad categories of filtennas, namely, planar and nonplanar. Most of the papers covered in this article are planar, whereas, under the nonplanar category, horn filtennas are analyzed. Performance in each case is compared in terms of size, complexity, and cost. Considering filtennas frequency of operation, they are classified as a single band, multiband, ultra‐wideband, and their MIMO configurations are analyzed to improve the reliability of the wireless radio system. Finally, various types of filtennas are compared, and the design guidelines are elaborated, mainly focusing on their application aspect to achieve more compact radio design solutions. The different techniques related to designing filtennas have been compared, and their glance details are provided for a more realistic assessment of individual techniques used till date. Thus, the authors believe that this review article presents a helpful guiding platform for researchers working on filtenna design. [ABSTRACT FROM AUTHOR]

Antidot lattices have proven to be a powerful tool for spin wave band structure manipulation. Utilizing time-resolved scanning transmission x-ray microscopy, we are able to experimentally image edge-localized spin wave modes in an antidot lattice with a lateral confinement down to < 80 nm × 130 nm. At higher frequencies, spin wave dragonfly patterns formed by the demagnetizing structures of the antidot lattice are excited. Evaluating their relative phase with respect to the propagating mode within the antidot channel reveals that the dragonfly modes are not directly excited by the antenna but need the propagating mode as an energy mediator. Furthermore, micromagnetic simulations reveal that additional dispersion branches exist for a tilted external field geometry. These branches correspond to asymmetric spin wave modes that cannot be excited in a non-tilted field geometry due to the symmetry restriction. In addition to the band having a negative slope, these asymmetric modes also cause an unexpected transformation of the band structure, slightly reaching into the otherwise empty bandgap between the low frequency edge modes and the fundamental mode. The presented phase resolved investigation of spin waves is a crucial step for spin wave manipulation in magnonic crystals. [ABSTRACT FROM AUTHOR]

Abstract: In this article, a folded monopole antenna loaded with complementary split ring resonators (CSRRs) is proposed for tri‐band applications. The two bands of the antenna are generated by the folding of the monopole (first band is caused due to fundamental mode and another due to higher order mode) and CSRRs have been responsible for the third band. By using four such antennas in form of an array, the gain and bandwidth have been improved. The prototype of the antenna array is developed on FR4 substrate and simulated results are experimentally validated. A simplified equivalent circuit model of the antenna has been developed and analyzed to quantify the power loss due to input impedance mismatch at each resonance frequency. By using the EM model of the proposed antenna in the Keysight ADS verification test bench its suitability to operate in the system environment has been confirmed. [ABSTRACT FROM AUTHOR]

We have studied vortex dynamics excited by the spin-transfer-torque effect and find that the direction of the vortex state can be detected electrically using the homodyne voltage signal generated due to the anisotropic magnetoresistance (AMR) effect. An external in-plane dc magnetic field is required to break the cylindrical symmetry in order to obtain a dc response of the homodyne signal. The sign of this rectified voltage changes with the handedness of the vortex state, which makes it a promising method to detect the vortex state electrically. Vortex dynamics is also observed by direct imaging in a scanning transmission x-ray microscope, allowing verification of the measured AMR signal in the correct power and frequency range. The results of micromagnetic simulations are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate that unidirectional reversal of the magnetic vortex core polarity is possible by excitation with sub-100-ps-long orthogonal monopolar magnetic pulse sequences in a wide range of pulse lengths and amplitudes. The application of such short digital pulses is a favorable excitation scheme for technological applications. Measured phase diagrams of this unidirectional, spin-wave mediated vortex core reversal are in good qualitative agreement with phase diagrams obtained from micromagnetic simulations. The time dependence of the reversal process, observed by time-resolved scanning transmission x-ray microscopy indicates a switching time of 100 ps and fits well with our simulations. The origin of the asymmetric response to clockwise and counterclockwise excitation which is a prerequisite for reliable unidirectional switching is discussed, based on the gyromode-spin-wave coupling. Situations are found in which a three-dimensional dynamics is important, because a vortex-antivortex pair starts to form close to the core of the original vortex in the lower part of the disk without completing the formation across the whole thickness so that it dissolves later on and does not lead to switching of the original vortex core. [ABSTRACT FROM AUTHOR]

We show, by experiments and micromagnetic simulations in vortex structures, that an active "dual frequency" excitation of both the sub-GHz vortex gyromode and multi-GHz spin waves considerably changes the frequency response of spin wave mediated vortex core reversal. Besides additional minima in the switching threshold, a significant broadband reduction of the switching amplitudes is observed, which can be explained by non-linear interaction between the vortex gyromode and the spin waves. We conclude that the well known frequency spectra of azimuthal spin waves in vortex structures are altered substantially, when the vortex gyromode is actively excited simultaneously. [ABSTRACT FROM AUTHOR]

Three-dimensional linear spin-wave eigenmodes of a vortex-state Permalloy disk are studied by micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation. The simulations confirm that the increase of the disk thickness leads to the appearance of additional exchange-dominated so-called gyrotropic flexure modes having nodes along the disk thickness, and eigenfrequencies that decrease when the thickness is increased. We observe the formation of a gap in the mode spectrum caused by the hybridization of the first flexure mode with one of the azimuthal spin-wave modes of the disk. A qualitative change of the transverse profile of this azimuthal mode is found, demonstrating that in a thick vortex-state disk the influence of the "transverse" and the "azimuthal" coordinates cannot be separated. The three-dimensional character of the eigenmodes is essential to explain the recently observed asymmetries in an experimentally obtained phase diagram of vortex-core reversal in relatively thick Permalloy disks. [ABSTRACT FROM AUTHOR]