Constriction-based spin Hall nano-oscillators (SHNOs) show great promise for application as highly tunable microwave sources with straightforward scalability toward large coupled networks. However, details of the magnetization dynamics within SHNOs have thus far not been addressed experimentally, due to the minute time and length scales involved. In this work, we present direct imaging of the magnetization dynamics within a single CoFeB-based SHNO using time-resolved scanning transmission x-ray microscopy (TR-STXM). Our measurements reveal that the magnon amplitude is strongest at the two constriction edges, with a pronounced asymmetry favoring one edge, and that the emitted spin waves (SWs) exhibit strongly anisotropic propagation. Micromagnetic simulations suggest that grain boundaries and the Dzyaloshinskii-Moriya interaction (DMI) play a key role in both effects. Furthermore, the magnetodynamics changed during measurement, indicating that the CoFeB/MgO interface may be more susceptible to x-ray-induced modifications than previously recognized, challenging its presumed radiation hardness.

We investigate the effect of focused-ion-beam (FIB) irradiation on spin waves with sub-micron wavelengths in yttrium-iron-garnet films. Time-resolved scanning transmission x-ray microscopy was used to image the spin waves in irradiated regions and deduce corresponding changes in the magnetic parameters of the film. We find that the changes of Ga+irradiation can be understood by assuming a few percent change in the effective magnetizationMeffof the film due to a trade-off between changes in anisotropy and effective film thickness. Our results demonstrate that FIB irradiation can be used to locally alter the dispersion relation and the effective refractive indexneffof the film, even for submicron wavelengths. To achieve the same change innefffor shorter wavelengths, a higher dose is required, but no significant deterioration of spin wave propagation length in the irradiated regions was observed, even at the highest applied doses.

Two-dimensional (2D) van der Waals (vdW) magnets have recently emerged as novel skyrmion hosts. This discovery has opened a new material platform for tuning the properties of topological spin textures, such as by exploiting proximity effects induced by stacking of 2D materials into heterostructures, or by directly manipulating the structural composition of the host material. Previous works have considered the effect of varied composition in the bulk crystals of the vdW magnet Fe3-xGeTe2 , but so far the effects on the hosted spin textures have not been thoroughly investigated. In this work, real-space x-ray microscopy is utilized to image magnetic stripe domain, skyrmion and composite skyrmion states in exfoliated flakes of Fe3-xGeTe2 with varying Fe deficiency x. In combination with supporting mean-field and micromagnetic simulations, the significant alterations in the magnetic phase diagrams of the flakes, and thus the stability of the observed spin textures, are revealed. These arise as a result of the varying temperature dependence of the fundamental magnetic properties, which are greater than can be explained by the removal of spins, and are consistent with previously reported changes in the electronic band structure via the Fe deficiency.

Interlayer exchange coupling has been intensively studied for over 30 years and has been successfully incorporated into almost all magnetic thin-film devices. In this study we find that antiferromagnetic interlayer exchange coupling can be achieved across, and improved by, spacer layers containing over 60 at.% of magnetic material. Measurements of sputtered Fe-doped spacer layers with x-ray magnetic circular dichroism reveal that the magnetic atoms in such spacer layers retain a large magnetic moment. The addition of magnetic atoms to the spacer layer is shown to double the coupling strength, leading to the largest-ever-observed antiferromagnetic bilinear coupling strength in magnetic multilayers deposited by magnetron sputtering, which is the industrial technique of choice. Electronic structure calculations in this work predict an experimentally obtained dependence of interlayer exchange coupling on both the magnetic material concentration and the spacer-layer thickness, in contrast with the prevailing understanding of interlayer exchange coupling.

The realization of above room-temperature ferromagnetism in the two-dimensional (2D) magnet Fe5GeTe2 represents a major advance for the use of van der Waals (vdW) materials in practical spintronic applications. In particular, observations of magnetic skyrmions and related states within exfoliated flakes of this material provide a pathway to the fine-tuning of topological spin textures via 2D material heterostructure engineering. However, there are conflicting reports as to the nature of the magnetic structures in Fe5GeTe2. The matter is further complicated by the study of two types of Fe5GeTe2 crystals with markedly different structural and magnetic properties, distinguished by their specific fabrication procedure: whether they are slowly cooled or rapidly quenched from the growth temperature. In this work, we combine X-ray and electron microscopy to observe the formation of magnetic stripe domains, skyrmion-like type-I, and topologically trivial type-II bubbles, within exfoliated flakes of Fe5GeTe2. The results reveal the influence of the magnetic ordering of the Fe1 sublattice below 150 K, which dramatically alters the magnetocrystalline anisotropy and leads to a complex magnetic phase diagram and a sudden change of the stability of the magnetic textures. In addition, we highlight the significant differences in the magnetic structures intrinsic to slow-cooled and quenched Fe5GeTe2 flakes.

Magnetism in reduced dimensionalities is of great fundamental interest while also providing perspectives for applications of materials with novel functionalities. In particular, spin dynamics in two dimensions (2D) have become a focus of recent research. Here, we report the observation of coherent propagating spin-wave dynamics in a ∼30 nm thick flake of 2D van der Waals ferromagnet Fe5GeTe2 using X-ray microscopy. Both phase and amplitude information were obtained by direct imaging below TC for frequencies from 2.77 to 3.84 GHz, and the corresponding spin-wave wavelengths were measured to be between 1.5 and 0.5 μm. Thus, parts of the magnonic dispersion relation were determined despite a relatively high magnetic damping of the material. Numerically solving an analytic multilayer model allowed us to corroborate the experimental dispersion relation and predict the influence of changes in the saturation magnetization or interlayer coupling, which could be exploited in future applications by temperature control or stacking of 2D-heterostructures.

Topological charge plays a significant role in a range of physical systems. In particular, observations of real-space topological objects in magnetic materials have been largely limited to skyrmions - states with a unitary topological charge. Recently, more exotic states with varying topology, such as antiskyrmions, merons, or bimerons and 3D states such as skyrmion strings, chiral bobbers, and hopfions, have been experimentally reported. Along these lines, the realization of states with higher-order topology has the potential to open new avenues of research in topological magnetism and its spintronic applications. Here, real-space imaging of such spin textures, including skyrmion, skyrmionium, skyrmion bag, and skyrmion sack states, observed in exfoliated flakes of the van der Waals magnet Fe3-xGeTe2 (FGT) is reported. These composite skyrmions may emerge from seeded, loop-like states condensed into the stripe domain structure, demonstrating the possibility to realize spin textures with arbitrary integer topological charge within exfoliated flakes of 2D magnets. The general nature of the formation mechanism motivates the search for composite skyrmion states in both well-known and new magnetic materials, which may yet reveal an even richer spectrum of higher-order topological objects.

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.

We report the imaging of short-wavelength spin waves in a continuous nanometer-thick YIG film with a Co stripe patterned on top. Dynamic dipolar coupling between the YIG film and the Co stripe lowers the spin-wave wavelength when spin waves enter the YIG/Co bilayer region from the bare YIG film, causing partial reflection at the YIG/Co edge. We use time-resolved scanning transmission x-ray microscopy to image the mode conversion process down to a wavelength of 280 nm and extract the spin-wave dispersion, decay length, and magnetic damping in the YIG/Co bilayer. We also analyze spin-wave reflection from the YIG/Co edge and its dependence on the wavelength of incoming and transmitted spin waves.

While the frequencies accessible by signal generators steadily rise, the synthesization of complex and arbitrary waveforms with high frequency components remains challenging, especially when restricted by an external reference clock. In this article, we present a comprehensive software package combined with state-of-the-art hardware as a solution for the generation of highly sampled, arbitrary radio frequency waveforms. The software can be used to conduct both synchronous and heterodyne pump-probe experiments due to a variety of different synchronization modules. While both kinds of modules allow for standard waveforms, such as sines, pulses, and bursts, as well as any arbitrary signal, the heterodyne modules additionally are not restricted by the reference clock frequency. Both the output and the synchronization module can be adapted to support additional measurement devices. Due to the modular software structure, individual classes can be exchanged while maintaining all functionalities. The software provides a user friendly graphical interface that allows us to compose, save, and load complex arbitrary waveforms within only a few steps. The frequency selectivity provided by the software-hardware combination allows us to directly target specific excitation states of physical systems. Conducting a heterodyne scanning transmission x-ray microscopy experiment, we are able to demonstrate the capabilities of the software when paired with a high sample rate arbitrary waveform generator. The heterodyne synchronization modules allow for unlimited flexibility leveraging arbitrary waveform generation to their full power. By solving the challenges of synthesizing highly complex electromagnetic waves, the software enables a large variety of experiments to be performed more conveniently.

Research into practical applications of magnetic skyrmions, nanoscale solitons with interesting topological and transport properties, has traditionally focused on two dimensional (2D) thin-film systems. However, the recent observation of novel three dimensional (3D) skyrmion-like structures, such as hopfions, skyrmion strings (SkS), skyrmion bundles, and skyrmion braids, motivates the investigation of new designs, aiming to exploit the third spatial dimension for more compact and higher performance spintronic devices in 3D or curvilinear geometries. A crucial requirement of such device schemes is the control of the 3D magnetic structures via charge or spin currents, which has yet to be experimentally observed. In this work, we utilise real-space imaging to investigate the dynamics of a 3D SkS within a nanowire of Co8Zn9Mn3 at room temperature. Utilising single current pulses, we demonstrate current-induced nucleation of a single SkS, and a toggle-like positional switching of an individual Bloch point at the end of a SkS. The observations highlight the possibility to locally manipulate 3D topological spin textures, opening up a range of design concepts for future 3D spintronic devices.

Spin-wave based computing requires materials with low Gilbert damping, such as Ni80Fe20 (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.