In the emerging field of magnonics, spin waves are foreseen as signal carriers for future spintronic information processing and communication devices, owing to both the very low power losses and a high device miniaturization potential predicted for short-wavelength spin waves. Yet, the efficient excitation and controlled propagation of nanoscale spin waves remains a severe challenge. Here, we report the observation of high-amplitude, ultrashort dipole-exchange spin waves (down to 80 nm wavelength at 10 GHz frequency) in a ferromagnetic single layer system, coherently excited by the driven dynamics of a spin vortex core. We used time-resolved x-ray microscopy to directly image such propagating spin waves and their excitation over a wide range of frequencies. By further analysis, we found that these waves exhibit a heterosymmetric mode profile, involving regions with anti-Larmor precession sense and purely linear magnetic oscillation. In particular, this mode profile consists of dynamic vortices with laterally alternating helicity, leading to a partial magnetic flux closure over the film thickness, which is explained by a strong and unexpected mode hybridization. This spin-wave phenomenon observed is a general effect inherent to the dynamics of sufficiently thick ferromagnetic single layer films, independent of the specific excitation method employed.

The magnetic vortex structure, an important ground state configuration in micron and sub-micron sized ferromagnetic thin film platelets, is characterized by a curling in-plane magnetization and, in the center, a minuscule region with out-of-plane magnetization, the vortex core, which points either up or down. It has already been demonstrated that the vortex core polarity can be reversed with external AC magnetic fields, frequency-tuned to the (sub-GHz) gyrotropic eigenmode or to (multi-GHz) azimuthal spin wave modes, where reversal times in the sub-ns regime can be realized. This fast vortex core switching may also be of technological interest as the vortex core polarity can be regarded as one data bit. Here we experimentally demonstrate that unidirectional vortex core reversal by excitation with sub-100 ps long orthogonal monopolar magnetic pulse sequences is possible in a wide range of pulse lengths and amplitudes. The application of such short digital pulses is the favourable 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 counter clockwise excitation which is a prerequisite for reliable unidirectional switching is discussed, based on the gyromode -spin wave coupling.

Short wavelength exchange-dominated propagating spin waves will enable magnonic devices to operate at higher frequencies and higher data transmission rates. While giant magnetoresistance (GMR)-based magnetic nanocontacts are efficient injectors of propagating spin waves, the generated wavelengths are 2.6 times the nano-contact diameter, and the electrical signal strength remains too weak for applications. Here we demonstrate nano-contact-based spin wave generation in magnetic tunnel junctions and observe large-frequency steps consistent with the hitherto ignored possibility of second- and third-order propagating spin waves with wavelengths of 120 and 74 nm, i.e., much smaller than the 150-nm nanocontact. Mutual synchronization is also observed on all three propagating modes. These higher-order propagating spin waves will enable magnonic devices to operate at much higher frequencies and greatly increase their transmission rates and spin wave propagating lengths, both proportional to the much higher group velocity.

We present an experimental investigation of radial spin-wave modes in magnetic nano-disks with a vortex ground state. The spin-wave amplitude was measured using a frequency-resolved magneto optical network analyzer, allowing for high-resolution resonance curves to be recorded. It was found that with increasing excitation amplitude up to about 10 mT, the lowest-order mode behaves strongly non-linearly as the mode frequency redshifts and the resonance peak strongly deforms. This behavior was quantitatively reproduced by micromagnetic simulations. At higher excitation the spinwaves are transformed into a soliton by self-focusing, and collapse onto the vortex core, dispersing the energy in short-wavelength spinwaves. Additionally, this process can lead to switching of the vortex polarization through the injection of a Bloch point.The study of the static and dynamic properties of micron and sub-micron sized magnetic platelets is not only necessary for the development of new technological applications, but is also interesting for fundamental reasons, as they can be simple model systems to investigate magnetic interactions and the complex dynamics involved. This is especially true for platelets with a magnetic vortex ground state configuration. It is the simplest, non-trivial configuration in a magnet and a building block of more complex states. Such vortex configurations know several modes of excitation. The lowest frequency mode is the gyrotropic mode (typically in the sub-GHz range) 1,2 , and corresponds to cyclic motion of the vortex around the center of the structure. This mode has already been studied extensively, especially because its excitation can lead to switching of the vortex core 3-5 . At higher frequency (typically several GHz), azimuthal modes exist 6,7 . These modes hardly move the vortex core, but the magnetization is excited out-of-plane with a periodicity in function of the azimuthal angle and the wave vector k parallel to the magnetization m. Because of azimuthal symmetry, this mode is divided in clockwise (CW) and counter clockwise (CCW) modes. However, the coupling with the vortex core, lifts the degeneration between these modes and can result in uni-directional switching of the vortex core 8 . The third excitation mode is the radial spin wave mode. Here the out-of-plane excursion amplitude of the magnetization is now a function of the radius. As the wave vector is perpendicular to the magnetization direction, this is a Damon-Eschbach mode. Until now, experimental work has only focused on the low excitation regime where the response is linear 9-12 , but recent numerical studies predict that at higher excitation levels vortex core switching 13,15 and non-linear phenomena can appear 15,16 . Here, we present an experimental investigation of the non-linear regime of the radial spinwave mode and further elaborate on the non-linear phenomena a) Electronic mail: Mathias.Helsen@UGent.be appearing near the core switching threshold.Our samples consisted of 1µm Permalloy discs, 10nm in thickness located in ...