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Spin-wave based computing requires materials with low Gilbert damping, such as Ni 80 Fe 20 (Permalloy) or yttrium iron garnet, in order to allow for spin-wave propagation on a length scale comparable to the device size. Many devices, especially those that rely on spin–orbit effects for operation, are subject to intense Joule heating, which can exacerbate electromigration and induce local phase changes. Here, the effect of annealing on the Gilbert damping coefficient α of 36 nm Py thin films grown on a Si substrate is examined. Ferromagnetic resonance measurements, high resolution transmission electron microscopy, as well as energy dispersive x-ray spectroscopy have been employed to determine α while also studying structural changes in the thin films. The Gilbert damping parameter was found to increase sixfold when annealed at 350 ° C, which was linked to the diffusion of Ni atoms into the Si substrate on a length scale of up to 50 nm. The results demonstrate that magnonic devices have to be treated with caution when Joule heating occurs due to its detrimental effects on the magnonic properties, but the effect can potentially be exploited in the fabrication of magnonic devices by selectively modifying the magnonic damping locally. [ABSTRACT FROM AUTHOR]

Being able to accurately control the interaction of spin waves is a crucial challenge for magnonics in order to offer an alternative wave-based computing scheme for certain technological applications. Especially in neural networks and neuromorphic computing, wave-based approaches can offer significant advantages over traditional CMOS-based binary computing schemes with regard to performance and power consumption. In this work, we demonstrate precise modulation of phase- and amplitude-sensitive interference of coherent spin waves in a yttrium–iron–garnet based magnonic analog adder device, while also showing the feasibility of frequency-division multiplexing. Using time-resolved scanning transmission x-ray microscopy, the interference was directly observed, giving an important proof of concept for this kind of analog computing device and its underlying working principle. This constitutes a step toward wave-based analog computing using magnons as an information carrier. [ABSTRACT FROM AUTHOR]

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]

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]

Magnonics is a potential candidate for beyond CMOS and neuromorphic computing technologies with advanced phase encoded logic. However, nanoscale imaging of spin waves with full phase and magnetization amplitude information is a challenge. We show a generalized scanning transmission x-ray microscopy platform to get a complete understanding of spin waves, including the k-vector, phase, and absolute magnetization deflection angle. As an example, this is demonstrated using a 50 nm thin permalloy film where we find a maximum deflection angle of 1.5° and good agreement with the k-vector dispersion previously reported in the literature. With a spatial resolution approximately ten times better than any other methods for spin wave imaging, x-ray microscopy opens a vast range of possibilities for the observation of spin waves and various magnetic structures. [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]

For understanding magnonic materials the fundamental characterization of their frequency response is essential. However, determining full dispersion relations and real space wavelength measurements are challenging and time-consuming tasks. We present an approach for spin wave excitation by a modified Sinc pulse, which combines a cosine signal with a conventional Sinc function. The resulting adjustable frequency bands lead to a broadband spin wave excitation at uniform power levels. Subsequently, time resolved scanning transmission X-ray microscopy is used for direct imaging of all excited spin waves in real space. To demonstrate the capabilities of this approach, a modified Sinc excitation of an ultra-thin yttrium-iron-garnet film is shown that simultaneously reveals phase, amplitude, and k-space information from a single measurement. Consequently, this approach allows a fast and thorough access to the full dispersion relation including spatial maps of the individual spin wave modes, enabling complete characterization of magnonic materials down to the nanoscale in real and reciprocal space. [ABSTRACT FROM AUTHOR]

Spin waves with wavelengths in the nanometre range could serve as data carriers in future magnonic logic or signal processing devices. We investigate the interference of spin waves emitted from magnetic vortices in two exchange-coupled vortex stacks. The spin-wave dynamics are studied using scanning transmission X-ray microscopy and micromagnetic simulations. Stacks of vortices provide an excellent controllability of spin-wave properties including a tunable wavelength in the 100 nm regime and manipulation of their propagation direction via the magnetisation configuration. Furthermore, interference gives rise to amplified or reduced spin-wave amplitudes in distinct areas of the structure providing controlled confinement crucial for future applications of spin waves. Spin waves are promising candidates as a building block for future magnonic devices. The authors present a combined numerical and experimental study of spin-wave interferences in stacks of magnetic vortices that are efficient spin-wave emitters in the nanometre regime. [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]

Spin-wave-mediated vortex core reversal is investigated using x-ray magnetic circular dichroism in a time-resolved scanning transmission x-ray microscope in combination with micromagnetic simulations. The evolution of magnetization dynamics in Permalloy disks is imaged after excitation with one-period rotating rf-field bursts. Selective unidirectional switching of the vortex polarity is achieved due to different switching thresholds for clockwise and counterclockwise excitations. A lower limit of about 200 ps for the unidirectional switching time after the onset of a one-period burst is found for the geometry of our disks (1.6 µM in diameter and 50 nm in thickness). [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]